# Should I keep Hartford or Bridgeport?



## Susquatch (Oct 5, 2021)

Well, it's decision time. As many here will know, I was originally looking to buy a bigger more rigid mill to replace my old mill/drill. As luck would have it, I ended up buying two - both at a great price. Rather than just flip one, I decided to repair both for the purpose of making them work well enough to be able to decide which one of the two I wanted to keep and which one I would sell. Neither one worked as received. Several months have transpired and I've now repaired the essentials on both mills and I have used them both to make a few parts. One is a Hartford 2HP 3ph step pulley clone, and the other is a Bridgeport 2HP 3ph VariSpeed. With some significant differences, either one will meet my needs. So which is the better choice?

For those who have not been following along, and wish to review the background, here are links to the two threads that detail each mill's basic problems and all the repairs I have made to date:

https://canadianhobbymetalworkers.com/threads/new-to-me-hartford-mill.3490/

https://canadianhobbymetalworkers.com/threads/new-to-me-bridgeport-mill.3491/

Here are the highlights:

*Hartford Step Pulley Mill*
.. New 2HP VFD rated 3ph motor
.. Teco L510 VFD - 10 to 120Hz
.... Can go to zero but why?
.. All new belts
.. 9x42 Bed in good condition
.... A few peck marks
.. Backlash on x & y axis
.... Under 10 thou after adjusting
.. Gibbs - all excellent
.. Only 3 speeds on pulley
.... 1 sacrificed to fit motor shaft
.... VFD easily compensates
.. Auto down feed not working
.. Motor & drive smooth & quiet
.... Some minor clatter at startup
.. Additional Work Required
.... Repair auto down feed
.. Future Parts availability
....Most parts same as Bridgeport
.... Some parts are hard to find
.... A few are not available at all
.. Storage compartment in base
.. Extremely smooth quiet machine
.. Very good appearance
.. Overall Condition "Very Good"


*Bridgeport VariSpeed Mill*
.. Rewound 2HP VFD 3ph motor
.... Not VFD Rated
.. Teco L510 VFD - 30 - 75Hz
.... Might go slower with a fan
.. 9x42 Bed in fair condition
.... Lots of peck marks
.... But no high spots
.. Backlash on x & y axis
.... 80 - 90 thou after adjusting
.... No adjustment left
.. Gibbs - worn out
.... No adjustment left
.. Vari Spd belt noisy
.... Probably needs replacing
.. Auto down feed not working
.. Motor and drive a bit noisy
.... Minor noise at most spds
.... A few sweet spots
.. Additional Work Required
.... Repair auto down feed
.... Replace both leade screws
.... Replace all gibbs
.. Future Parts availability
.... All parts available @ H&W
.. No storage compartment in base
.. Appearance is rough
.... Machine has been well used
.... Needs rust removal
.... Needs a good cleaning
.... Needs a paint job
.. Overall Condition - Moderate
.... But still quite usable

*Summary of Decision Factors*
.. Hartford Pros
.... New VFD Rated Motor
........ Full speed 15 to 120Hz
.... Very Low Backlash
.... Very smooth, quiet, and solid
.... Great condition & appearance
.. Hartford Cons
.... Availability of parts
.... Not a genuine bridgeport
.... Loss of one pulley position
.... Not a varispeed

.. Bridgeport Pros
.... Genuine Bridgeport Mill
.... All parts are still available
.... Mill works as is but rough
.. Bridgeport Cons
.... Leade screws are worn out
.... Gibbs are worn out
.... Motor is not VFD Rated

Power down feed on both mills needs repair.

*My thoughts:*

I am old and God willing, at best I'll get 10 years use out of either mill. However, I believe either one will outlast me and do everything I need it to do with it over that time.

I really like the idea of owning a genuine Bridgeport. However, when I stand back and think about which girl I want to marry and which one I want to live with for the rest of my milling life, I realize how much I really love how smooth, quiet, solid, and good looking the Hartford mill is, and I don't like the idea of replacing the leade screws, split nuts, and gibbs on the Bridgeport. Yet, if I don't replace them, I will curse them every time I use the machine.

So, all things considered, I favour keeping the Hartford. But a wise man is never afraid to change his mind. Unless you guys change my mind, I plan to clean up the Bridgeport and then sell it - someone else can replace the leade screws, split nuts, and gibbs.

Anyone disagree? If so, please tell me why. And please don't hold back. You can't hurt my feelings.


----------



## Mcgyver (Oct 5, 2021)

Spindle tapers the same?

you know if all that is badly worn on the bport, so are the bearing surfaces.  Replacing that stuff without re scraping it will be expensive and might not accomplish much.  otoh imo a VFD is poor speed control - they deliver constant toque but you want is constant HP.   Overall I'd go with the one in better condition, but its imo sub optimal to lose the low speeds and high torque on a varispeed drive.

Put the bport head on Hartford?

What does a VFD rated motor mean?  Marketing or is it a real difference.  Guess I wasn't paying attention   I've never had a problem putting a VFD on any old 3P motor


----------



## John Conroy (Oct 5, 2021)

I think Mcgyver has a great idea. Putting the Bport head on the Hartford would save all the work of getting the sliding surfaces back in shape of the Bport chassis and you would have a varispeed head. Of course you would loose the re-sale value of the Bport if that is a concern for you. If it were me I'd keep the Harford and fix the few thing that it needs and then sell the Bport as it should bring good money.


----------



## John Conroy (Oct 5, 2021)

This is a drawing of the power down feed if you haven't seen one before. There is a slip clutch under the round cover on the left side of the head that's not shown here.


----------



## Susquatch (Oct 5, 2021)

Mcgyver said:


> Spindle tapers the same?



Yes, they are both R8. The taper on the Bridgeport is worn a bit though. It had no key when I got it and the previous owner probably undertightened it so it spun a few times.



Mcgyver said:


> A VFD is poor speed control - they deliver constant torque but you want constant HP. Overall I'd go with the one in better condition, but its imo sub optimal to lose the low speeds and high torque on a varispeed drive.



I guess I don't really understand this point. I have heard this before and didn't understand it then either. Perhaps you can help me "get it".

Torque is a rotational force independant of time or distance. Work is force x distance. Hp is the rate at which work is done (work done per unit time). So for any given load and rpm shouldn't either rating hold the same numbers?

But in any event, unless I am mistaken, my VFD doesn't control force or HP. It controls rpm. As long as it holds rpm, as the load increases, it will increase both torque and hp until its limit is reached.




Mcgyver said:


> Put the bport head on Hartford?



Hmmmmmm, I have thought about swapping tables, but I never thought of that one! I will have to give this some more thought.....



Mcgyver said:


> What does a VFD rated motor mean? Marketing or is it a real difference. Guess I wasn't paying attention  I've never had a problem putting a VFD on any old 3P motor



Nor did I. While I think your tongue is in cheek a bit,  my VFD worked just fine on both Motors too. However, my understanding of the difference goes like this.

Standard Motors are designed to run at a fixed speed. They have internal cooling systems that can handle the heat generated at the rated power level at the rated speed. But at lower speeds, the fan doesn't work as effectively and supplemental cooling is required. Similarly, standard Motors have bearings that can handle the standard speed. If you turn the motor too fast, the bearings can fry. The general rule of thumb that I found was to keep non-vfd rated motor speed within 50% of the rated speed.

On the other hand, VFD rated Motors can run at much lower speeds without overheating and have bearings that can handle the higher speeds too. My VFD rated motor is 1800 RPM at 60 hz. But it is rated to handle anything from 0 rpm to 3600rpm.


----------



## Susquatch (Oct 5, 2021)

John Conroy said:


> I think Mcgyver has a great idea. Putting the Bport head on the Hartford would save all the work of getting the sliding surfaces back in shape of the Bport chassis and you would have a varispeed head. Of course you would loose the re-sale value of the Bport if that is a concern for you. If it were me I'd keep the Harford and fix the few thing that it needs and then sell the Bport as it should bring good money.



I'm going to give the idea of swapping heads some more thought. More on that later. 

But I agree that doing that will create two clones instead of one and it will probably affect selling price significantly.


----------



## Susquatch (Oct 5, 2021)

John Conroy said:


> This is a drawing of the power down feed if you haven't seen one before. There is a slip clutch under the round cover on the left side of the head that's not shown here.
> 
> View attachment 17542



There are a few pictures in this graphic missing.

I repaired the center gear system on that graphic on my Bridgeport already. That part works fine on the Hartford.

In both cases, I think the clutch system got sticky and didn't release. I expect I will find a broken or stripped worm gear or bevel gear.

In any case, I don't plan to fix that till this winter some time. Or maybe later.


----------



## whydontu (Oct 5, 2021)

If it was me I’d keep the Hartford. Having Bridgeport on the side of a machine is irrelevant if it doesn’t work. If the intent is to make chips, it sounds like the Hartford will be able to make chips with much less hassle.


----------



## 6.5 Fan (Oct 5, 2021)

I'm in the Hartford camp. Bridgeport is just a name and it looks to require a lot more work.


----------



## Tom O (Oct 5, 2021)

Hartford camp also.


----------



## YYCHM (Oct 5, 2021)

Hmmm..... I wonder how you can add a survey to this thread?  @Janger can you shed some light?

In any event I vote keep the Hartford, ditch the Bridgeport.


----------



## Tom Kitta (Oct 5, 2021)

I would not worry too much about VFD rated motor or not - usually that just means better insulation. Older motors are over build and one can say most are also VFD rated for that reason. Bearings in both motors are almost always the same & for most smaller motors they have no issues running even at 5000 rpm or more. I would think the issue would more be of balancing. Main issue is heat. 

Since VFD rated motors do not have any extra cooling their fans also move at a slow speed of non "VFD rated" motor. 

Also a lot of things are rated at full load or even at service factor - not an issue for most pp as most pp do not run their mill close to full load continuously. 

In short do not worry about it - just make sure you do not smell insulation burning in your motor and you are fine. Whatever it is VFD rated or not.

As for replacing lead screws - you can just go for rolled ball screws - very cheap from China. Main problem would be adding adapters to the setup but the benefit would be sub 0.001 backlash. Then again - it is a project.

*The main question is, do you want to fix the BP? If yes, you can keep her, if not then you can sell her.*

Have you considered keeping both? Or is it space / $ issue?


----------



## kevin.decelles (Oct 5, 2021)

Adopt me, keep both, leave me the BP in your will.

I too would keep the one in best shape.  Kind of like having a 64 1/2 Mustang, and some other muscle car isn't as sexy but puts down more HP....... some would keep the Mustang because it is iconic......  others would take the muscle.....

Either way you've got better machines than I -- good posts/pictures/journey!  Thanks for posting!


----------



## Susquatch (Oct 5, 2021)

Tom Kitta said:


> *The main question is, do you want to fix the BP? If yes, you can keep her, if not then you can sell her.*



I am done fixing Mills for a while. I want to move on to making and fixing other things  So I'll take your vote as one to keep the Hartford and sell the Bridgeport.



Tom Kitta said:


> Have you considered keeping both? Or is it space / $ issue?



I have lots of space. But I got myself into a hot tub with a crazy lady. When this all started, I gave my bride my savings so she could buy an industrial sewing machine. That put a mill on hold for a while. Then along came this great deal on a Hartford and she agreed to dip into our regular spending (no steak for a year) to take advantage of the opportunity. Then along came a deal on a Bridgeport and I borrowed the money on our power line because it was an investment that would get paid back - no matter which one I ended up keeping.

So ya, I can make room for it. I'll just move my tractors around a bit. My shop is 40x80 (actually 40x60 because I have a room built into it). But I don't really want to sleep out there. I like my bed and the warm lady I share it with. So no matter how much I'd like to keep both, one of those mills has to go regardless of whether or not I have room.


----------



## Susquatch (Oct 5, 2021)

kevin.decelles said:


> Adopt me, keep both, leave me the BP in your will.
> 
> I too would keep the one in best shape.  Kind of like having a 64 1/2 Mustang, and some other muscle car isn't as sexy but puts down more HP....... some would keep the Mustang because it is iconic......  others would take the muscle.....
> 
> Either way you've got better machines than I -- good posts/pictures/journey!  Thanks for posting!



I have 4 kids and 9 grandkids and a possible great grandchild on the way already. So...... much as I might like you, I have this feeling you would cost more than all of them put together. Not happening! Sorry Kevin.


----------



## Brent H (Oct 5, 2021)

@Susquatch: from all the work you have done to the machines, you appear to have poured more time and effort into getting the Hartford running to your satisfaction: it needs less to give you back more at this point.  
You have the perfect motor for the Hartford and thus the pulley system is irrelevant- speed control is yours to command.  
For the Bridgeport- new gibs are about $325 USD plus shipping, new x and y axis screws about $350 USD and new nuts for those are about $70 USD.   Then you have any wear issues to also look into and the power feed.   You are in another $1500 CDN and your time.  Plus you say the quill has a worn collet holder - probably needs new bearings all around etc etc.  
I think your decision was made when you made your first chips on the Hartford.


----------



## Susquatch (Oct 5, 2021)

Brent H said:


> from all the work you have done to the machines, you appear to have poured more time and effort into getting the Hartford running to your satisfaction: it needs less to give you back more at this point.



I think they are pretty even in terms of research, work, and passion. Lots of little stuff required doing on the BP. But I did have to buy a motor for the Hartford and even though it was a great deal it was not free. 



Brent H said:


> For the Bridgeport- new gibs are about $325 USD plus shipping, new x and y axis screws about $350 USD and new nuts for those are about $70 USD. Then you have any wear issues to also look into and the power feed. You are in another $1500 CDN and your time. Plus you say the quill has a worn collet holder - probably needs new bearings all around etc etc.



Wow...... That's way more than I figured. I think that's a show stopper. 



Brent H said:


> I think your decision was made when you made your first chips on the Hartford.



You have great insight. When that cutter hit first steel, a bull alaskan moose couldn't have licked the smile off my face...... I couldn't believe my eyes or my ears......


----------



## Mcgyver (Oct 5, 2021)

Susquatch said:


> I guess I don't really understand this point. I have heard this before and didn't understand it then either. Perhaps you can help me "get it".
> .



real simple, power = torque x angular velocity.  Power, HP, is a direct function of torque and speed.  Because of cutting speeds, we have to run big cutters more slowly, right?  So if speed (RPM) is reduced, torque has to increase to equal the same power.  

Think if you put a VFD on a 1800 rpm motor.  slow it to 180.  You can (depending motor size) probably stop it by hand - you need torque to go up as speed goes down to maintain the same HP.

Another example is conveyors.  It takes about the same torque at an speed so VFD's work well for speed control.  So if the 1800 rpm motor runs it a 40 feet minute, and you slow it down to 900 via the VFD, torque is the same, but you're gonna a pile of say stone off the conveyor half the size.  Torque stayed the same, speed cut in half, the power got halved (half work was done in a given period of time)

VFD's might claim to offer constant torque, but none offer constant HP.   When guys say it works just fine for them, what they're saying at whatever lower speed they're running, they only need a small fraction of the full speed HP.  Thats fine, but its still a big built in compromise imo



Susquatch said:


> On the other hand, VFD rated Motors can run at much lower speeds without overheating and have bearings that can handle the higher speeds too. My VFD rated motor is 1800 RPM at 60 hz. But it is rated to handle anything from 0 rpm to 3600rpm.



that makes sense, powered fans vs the blades on the rotor.


----------



## Susquatch (Oct 5, 2021)

Mcgyver said:


> real simple, power = torque x angular velocity. Power, HP, is a direct function of torque and speed. Because of cutting speeds, we have to run big cutters more slowly, right? So if speed (RPM) is reduced, torque has to increase to equal the same power.



Right on. Torque is force, rpm is speed distance. A totally linear relationship. 

Yup, torque has to go up to equal same hp at lower rpm. Linear math. 



Mcgyver said:


> Think if you put a VFD on a 1800 rpm motor. slow it to 180. You can (depending motor size) probably stop it by hand - you need torque to go up as speed goes down to maintain the same HP.



I think braking by hand confuses the issue. I doubt very much that I could stop my spindle by hand. It isn't a matter of hp. It's pure torque. My hand is a big strong one. But I cannot generate the torque required to stop the spindle. 

Here is a graph of the relationship. You can see the linear nature of the two factors in the two straight lines. The torque stays constant below rated rpm and hp stays constant above. One gets traded for the other because the third variable (rpm) changes. (I believe the slight uptick in the torque above rated rpm is the result of lower losses in the motor.) 








Mcgyver said:


> Another example is conveyors. It takes about the same torque at an speed so VFD's work well for speed control. So if the 1800 rpm motor runs it a 40 feet minute, and you slow it down to 900 via the VFD, torque is the same, but you're gonna a pile of say stone off the conveyor half the size. Torque stayed the same, speed cut in half, the power got halved (half work was done in a given period of time)



I don't know anything about conveyors but I accept what you say. 



Mcgyver said:


> VFD's might claim to offer constant torque, but none offer constant HP. When guys say it works just fine for them, what they're saying at whatever lower speed they're running, they only need a small fraction of the full speed HP. Thats fine, but its still a big built in compromise imo



I have never seen any VFD claim anything other than as shown above in the graph. Constant torque below rated rpm and constant hp above. Its a linear relationship. As rpm changes, one has to give to maintain the other. 

I think that you are probably correct about what people say. But since hp is a measure of the rate at which work is being done. Lower rpm almost always means less work so no surprise there. 

I also think that most of the stuff I have read make the assumption that a 2hp motor always produces 2hp when its running at its rated rpm.  But I don't agree. It only produces that much when it is working at its peak capacity. If it has no load, I just sits there spinning at virtually zero hp because it isn't doing any work. Maybe that's what you were getting at in your conveyor example. 

I'm not really sure, but I think perhaps we may be talking in circles here - saying the same thing from different perspectives. Perhaps you think HP is more important than torque and I feel they are both important because they are so closely related. Or perhaps you are above the curve and I'm below it..... LOL!!


----------



## Brent H (Oct 5, 2021)

Lads:

For some clarity: torque being the force and horsepower being the rate at which that force is done. The difference is torque is doing the work, while horsepower is how fast that work is being done.
Depending on the load being applied an electric motor only needs to develop enough Hp to overcome the applied load(resistive force) or counter torque.  So if no load is applied to the electric motor only the Hp to keep it rotating will be developed and very little torque.  A 2 Hp motor can develop up to 2 Hp at the rated RPM but may run at rated RPM at much lower Hp.


----------



## Janger (Oct 5, 2021)

YYCHM said:


> Hmmm..... I wonder how you can add a survey to this thread?  @Janger can you shed some light?
> 
> In any event I vote keep the Hartford, ditch the Bridgeport.








Visual answer Craig. -john


----------



## Susquatch (Oct 5, 2021)

Brent H said:


> Lads:
> 
> For some clarity: torque being the force and horsepower being the rate at which that force is done. The difference is torque is doing the work, while horsepower is how fast that work is being done.
> Depending on the load being applied an electric motor only needs to develop enough Hp to overcome the applied load(resistive force) or counter torque.  So if no load is applied to the electric motor only the Hp to keep it rotating will be developed and very little torque.  A 2 Hp motor can develop up to 2 Hp at the rated RPM but may run at rated RPM at much lower Hp.



Right on @Brent H. Nicely put as always.


----------



## Mcgyver (Oct 6, 2021)

Susquatch said:


> Right on. Torque is force, rpm is speed distance. A totally linear relationship.
> Yup, torque has to go up to equal same hp at lower rpm. Linear math.
> !



yup, thats all it is.  P=T x Angular Velocity (speed).  Of course power is the ability to do work, we don't want power going down as we slow the machine.  i.e. if we want 1hp on our lathe at 2000 rpm, no ones going to be happy with 0.05 HP at 100 rpm.  Same with a mill.   If you are driving some big 4" fly cutter at 100 rpm, or a slab mill cutting in a horizontal or a large diameter piece in the lathe, its going to stall if you only have 1/20th the HP at normal motor speed @ 60 Hz.  You need torque to go up as speed reduces to maintain the same power (ability to do work).   A mechanical transmission does that, a VFD does not.

(all in the context of clarifying why VFD speed reduction is inferior to mechanical speed reduction)



Susquatch said:


> Perhaps you think HP is more important than torque and I feel they are both important



its not that one is more important, they are interdependent.  Its what happens when speed changes that is the issue but its HP that does work.   If the speed reduction is not able to increase torque as speed goes down ()i.e. maintain power) its rather sub-optimal - loss of power.

It may not be noticeable if you're going from say 1800 rpm to 1500, few of us drive machines much of the time if ever at capacity.  However if the VFD is means of speed control, drop the speed to 90 and you won't have enough power left to make a cut.  That's were my poor anecdote of trying stop it with your hand comes in, I shouldn't encourage doing so, but with just a pulley on a motor on a bench you can stop it that way at 1/20 of its 60Hz rpm - there is barely enough power to turn itself over.  Ignoring losses, that 2hp is now acting like 1/10 HP motor.  Reduce speed via mechanical means and your torque is now 20x what it was at 1800 (again ignoring losses)....exactly what you need to power a big diameter cutter or large diameter piece of work in the lathe.

What mostly would solve this electronic transmission issue would be using a servo motor - it feedbacks RPM and the controller increases current as needed to maintain RPM.  The little consew motors are a great example.  three magnets and a hall sensor.  3/4 HP (claimed), dial it down to 200 rpm and you cannot stop that motor by hand.  I don't know of options in larger sizes as all the servoes I've seen are expensive and the feedback is intended for precision motion applications, not just simple feed back of rpm


----------



## Susquatch (Oct 6, 2021)

The tone of your reply is GREAT. I always worry when I have discussions like this that I am upsetting the other party. But I love such debates because they help me see and understand things better than I would otherwise.

The main thing I am getting from your points is that perhaps you don't realize that a modern VFD actually does have feedback just like a servo motor does. A VFD contains a microcontroller with all kinds of smarts in it. It uses its own knowledge of the timing and magnitude of the current flowing in the three circuits to know exactly where the motor is and how much load it is seeing. As the motor load goes up, the motor tries to slow down, but the VFD throws more current at it which causes more torque, to maintain the RPM. So a modern VFD is very much like the servo motor in your example. In fact, I'm pretty sure the very definition of a servo would include a modern VFD because a modern VFD uses feedback to provide correction control. Anyway, I digress.



Mcgyver said:


> yup, thats all it is. P=T x Angular Velocity (speed). Of course power is the ability to do work, we don't want power going down as we slow the machine. i.e. if we want 1hp on our lathe at 2000 rpm, no ones going to be happy with 0.05 HP at 100 rpm. Same with a mill. If you are driving some big 4" fly cutter at 100 rpm, or a slab mill cutting in a horizontal or a large diameter piece in the lathe, its going to stall if you only have 1/20th the HP at normal motor speed @ 60 Hz. You want torque to go up as speed reduces to maintain the same power. A mechanical transmission does that, a VFD does not.



I both agree and disagree.

If I am running THE SAME CUTTER at half speed, I am automatically and inherently doing half the work too. So I only need half the power I would need at full speed.

But I think your example helps me see the point that you are maybe getting at. We all know that there is an ideal cutting tip speed for a given tool tip and metallurgy. For large cutters, we need to slow the mill down in order to maintain the tip surface cutting speed. But if tip speed and depth of cut is staying the same, the work done at the tip is also staying the same.

Again, I think that the difference in our perspective in this case is that I would say you need more torque to keep the tip going, and you have said that you want more power.

The two are intimately related so I don't think this difference of perspective matters much.

What does matter is that bigger diameter cutters do the same work at lower rpm.

The point that @Brent H made so much better than I did is that this doesn't matter either as long as the AVAILABLE TORQUE is enough to handle the load. Bigger cutters put more torque on the spindle exactly the same as a longer wrench on a nut does. So the motor needs to be able to apply that same higher torque from the other end.

Up to a point, it can. But the "point" IS THE POINT. (LOL!!!) The whole issue crumbles when that point is reached. If there isn't enough available torque, the motor and spindle will stall and bad things can happen.

If this is the point you have been making all along, then I AGREE!

On the other hand, the "point" I have been trying to make is that this is no problem as long as the AVAILABLE torque is sufficient. My point is really just that as the demand for more torque goes up with bigger cutters the VFD DOES SUPPLY more current to increase torque (up to its limit) to meet the demand.

This is also why bigger mills with bigger Motors can handle bigger cutters.

Implicit in your point, assuming I understand it now, is that you don't believe that there is ENOUGH AVAILABLE TORQUE to drive the cutter when the cutter is bigger and the shaft has to turn slower to maintain the cutting tip speed.

I think your perspective applies equally well to big, medium, and small mills. The smaller the mill, the less available torque it will have and of course that means the lower the available hp too.

So, coming full circle to your original point (which I think I understand now), my mill has a big 2hp motor on it with a crapload of available torque. Therefore, I am of the view that it will have sufficient available torque to drive any cutter that I am likely to use at the VFD set speed that such a cutter wants to be driven at. That might not be the case with a smaller mill and smaller motor though. My own smaller mill/drill did work with a 3 inch fly cutter, but it used gearing to achieve the desired torque (as you described) and it bounced and chattered itself crazy. This low rigidity is the main reason I wanted a bigger mill.

Also to the point I think you are making, it's important for me to keep in mind that I also still have 3 pulley grooves and a back gear (6 speed ratios) in my pulley head "transmission" (to use your words) to achieve more torque if I do ever need it. But for most work with 1/4 and 1/2 inch end mills, I believe it will do just fine driven by the VFD. The nice thing is that I have the VFD to do most of the speed tuning and the belt and back gear choices to generate humungous torques for those few times when I might actually need it.



Mcgyver said:


> It may not be noticeable if you're going from say 1800 rpm to 1500, few of us drive machines much of the time if ever at capacity. However if the VFD is means of speed control, drop the speed to 90 and you won't have enough power left to make a cut. That's were my poor anecdote of trying stop it with your hand comes in, I shouldn't encourage doing so, but with just a pulley on a motor on a bench you can stop it that way at 1/20 of its 60Hz rpm - there is barely enough power to turn itself over. Ignoring losses, that 2hp is now acting like 1/10 HP motor. Reduce speed via mechanical means and your torque is now 20x what it was at 1800 (again ignoring losses)....exactly what you need to power a big diameter cutter or large diameter piece of work in the lathe.



I'm gunna chuck a big bar in my mill, turn the speed down to 80rpm and see if I can stall it by grabbing the bar with my hand.....

<JAnger interrupts>Moderator tears burning hair out of head omg don't do this kids.<JAnger>



Mcgyver said:


> What mostly would solve this electronic transmission issue would be using a servo motor - it feedbacks RPM and the controller increases current as needed to maintain RPM. The little consew motors are a great example. three magnets and a hall sensor. 3/4 HP (claimed), dial it down to 200 rpm and you cannot stop that motor by hand. I don't know of options in larger sizes as all the servoes I've seen are expensive and the feedback is intended for precision motion applications, not just simple feed back of rpm



As noted above, that is EXACTLY what a modern VFD like mine does. It knows the motor rpm and it adjusts current flow to increase torque as required to maintain the rpm up to the limits of the VFD & motor.

My VFD Is a TECO L510. I really like it.


----------



## Brent H (Oct 6, 2021)

@Susquatch :  since you have a VFD rated motor on your Hartford you will be fine with any speed you choose to operate at. You will get more consistent torque at lower speeds and be fine running at higher speeds.
For NON VFD rated motors - just a standard type motor, it is designed for rated Hp at the RPM stated on the name plate and the duty cycle limitations.  Using a VFD to control a standard motor above it’s rated speed and subjecting it to full load can cause issues with excessive heat. This can also occur at lower RPM’s than rated and trying to use all the available HP.   Most of us will not use devices in this manner so a VFD on a Standard motor will be fine.  
As a work around some folks will increase the Hp of the standard motor so it can perform more adequately for the application.  
As you stated in your above comments, the VFD rated motor will behave along the lines of a servo type motor by delivering more consistent torque/Hp over a wide range of speeds.


----------



## RobinHood (Oct 6, 2021)

Sorry late to the party…



Susquatch said:


> It knows the motor rpm and it adjusts current flow to increase torque as required to maintain the rpm up to the limits of the VFD & motor.



Actually, I believe, a VFD does not know the RPM of an induction (asynchronous) motor. It only knows the rpm of the rotating magnetic field in the stator windings it is sending to the motor. It “assumes” the rotor is keeping up with that rotation.

The only way a controller knows the rpm of a rotor is if you have a sensor on the rotating element and feed it back to the controller.

I believe that is how servo motors and servo drives work.

Here is a little blurb on HP vs Torque: (Of special interest is the last paragraph).

https://www.kurz.com/variable-frequency-drive-torque-vs-hp

And another bit about VFD concepts & myths:

https://www.controleng.com/articles/the-truth-about-five-common-vfd-myths/

Which mill to keep:

From all the posts on your various threads, I would go with the Hartford. I would make use of the available belt change ratios you have available and only use the VFD for fine tuning the speeds in between (Like you would do with the VariDrive on the BP). It will increase the life of your inverter duty rated motor.


----------



## Susquatch (Oct 6, 2021)

Brent H said:


> @Susquatch :  since you have a VFD rated motor on your Hartford you will be fine with any speed you choose to operate at. You will get more consistent torque at lower speeds and be fine running at higher speeds.
> For NON VFD rated motors - just a standard type motor, it is designed for rated Hp at the RPM stated on the name plate and the duty cycle limitations.  Using a VFD to control a standard motor above it’s rated speed and subjecting it to full load can cause issues with excessive heat. This can also occur at lower RPM’s than rated and trying to use all the available HP.   Most of us will not use devices in this manner so a VFD on a Standard motor will be fine.
> As a work around some folks will increase the Hp of the standard motor so it can perform more adequately for the application.
> As you stated in your above comments, the VFD rated motor will behave along the lines of a servo type motor by delivering more consistent torque/Hp over a wide range of speeds.



I recognize that my knowledge base does not include a lot of milling experience. So I have to pay close (and sometimes overly debative) attention to the wisdom and experience of others.

That said, my career before farming was automotive and therefore I believe that I do understand torque and horsepower quite well. Experience can be awesome but it can also be a cross when it blinds you to the details of different ways of looking at things than I am used to.

I might also add that I have a heavy electronics background (primarily also in automotive) which is yet another cross in my efforts to gain a good working knowledge of milling.

So, I'm ever so grateful to guys like you and a bunch of others like @Mcgyver & @Dabbler (to name just two others among many on here) who have taken the time to set me straight, argue with me, put up with me, and also tell me when I'm thinking correctly.

I'm very comfortable with my choice thanks to everyone here. And as you said earlier, my mind was actually pretty much made up the minute I first used my Hartford. It really is a thing of a beauty!

Thank you for helping me get here.


----------



## Mcgyver (Oct 6, 2021)

Susquatch said:


> The tone of your reply is GREAT. I always worry when I have discussions like this that I am upsetting the other party. .



Thanks and likewise.  I am hard to offend, and in turn, if it seems like badgering or flogging the dead horse, its just an attempt at trying to explain myself.  A course once said 95% of communication is body language, tone, inflection etc....we'll have to muddle through with the 5%!



Susquatch said:


> The main thing I am getting from your points is that perhaps you don't realize that a modern VFD actually does have feedback just like a servo motor does. A VFD contains a microcontroller with all kinds of smarts in it. It uses its own knowledge of the timing and magnitude of the current flowing in the three circuits to know exactly where the motor is and how much load it is seeing. As the motor load goes up, the motor tries to slow down, but the VFD throws more current at it which causes more torque, to maintain the RPM.



I could quickly end up in territory that through lack of knowledge I have no business talking about, however its my understanding that they do this with a bit of extra current for awhile at or around the motor's design speed.  I would be impossible to take a say 1800 motor and have the VFD maintain speed @ 90 RPM by putting more current through so that it would deliver 2HP or remotely perform like a transmission.  it would need 20x the current.  This Teco drives you mentioned say on the brochure they are constant torque, not constant HP (which would be impossible)



Susquatch said:


> If I am running THE SAME CUTTER at half speed, I am automatically and inherently doing half the work too.



if at the same DOC and feed.  On a machine tool, work is essentially the removal rate - i.e. cubic inches per minute.



Susquatch said:


> Again, I think that the difference in our perspective in this case is that I would say you need more torque to keep the tip going, and you have said that you want more power.



Actually, I've said you want the same power - e.g. say 2HP over the speed range.  To get the same power, torque must go when speed goes down.   Thats what a VFD doesn't do that mechanical transmission does



Susquatch said:


> The point that @Brent H made so much better than I did is that this doesn't matter either as long as the AVAILABLE TORQUE is enough to handle the load. Bigger cutters put more torque on the spindle exactly the same as a longer wrench on a nut does. So the motor needs to be able to apply that same higher torque from the other end.



100%,  a big cutter needs higher torque.  However it also needs lower speeds.  The VFD cannot increase torque as speed is reduced




Susquatch said:


> Implicit in your point, assuming I understand it now, is that you don't believe that there is ENOUGH AVAILABLE TORQUE to drive the cutter when the cutter is bigger and the shaft has to turn slower to maintain the cutting tip speed.



If in whatever scenario, if there is enough torque to do the job, everybody is happy, its a moot. However with a VFD, torque does not increase as speed is reduced, which has to happen for large cutters.  lets look at a real world example (ignoring friction, slippage etc) My mill goes from 3600 to 100 rpm.  If I have 2HP at 3600 rpm (60 Hz) and electronically reduce to the 100 rpm, this is 1/36 of speed at motors speed @ 60Hz.  That means at 100 rpm, I will get about 1/18 of a HP.  Who thinks that going to adequate?  If I use the mechanical transmission I'll have all of the 2 hp.....because torque will be 36x higher at 100 than at 3600

Its simply power = torque x angular velocity.  And (afaik) VFD's are not able appreciably increase torque as speed is reduced (I just looked at the brochure of one you mention, it talks about constant torque).




Susquatch said:


> It knows the motor rpm and it adjusts current flow to increase torque as required to maintain the rpm up to the limits of the VFD & motor.
> 
> My VFD Is a TECO L510. I really like it.



hmmmm, I would say its not doing that except very close to its natural speed (@ 60Hz).    If I'm wrong (always a possibility), there's been some major break through in VFDs!   I'm not an electrical designer but do see a fair number of them (usually SEW Eurodrive)...certainly doesn't mean I know everything but I wouldn't i was out of the loop so to speak.   I looked at those drives you mentioned and what I read they are constant torque, not power.  You could not possibly pump through enough current to maintain power when the speed is reduced substantially like is done with a mechanical transmission

The other way to create a electric speed control that performs adequately over a range is with cubic inches.  I (and others have done the same) put a 5hp motor in my 10ee, grossly overpowering 10x20 lathe, so that I'd get a decent amount of power at slower speed.  This is a common with electronic speed control spindles - overpower them so they'll still have enough umph at low speed.  I laid on a bit of hyperbole on the suggested servo solution - while there is direct feedback, and performance is hugely improved, they too have to avoid the laws of physics.   A lot of CNC's for example don't have transmissions and drive the spindle from a servo.  They solve the problem of low HP at lower speeds by starting with a lot more - use big bloody servoes so they'll have at least some umph at low speeds.  I've got a small/med CNC with a 15HP servo spindle for example....but there is no way at 100 rpm it could drive a big cutter on a big cut like my 5HP horizontal can, probably not even 1/10 the removal rate of the 5HP  horizontal @ 100 rpm


----------



## Susquatch (Oct 6, 2021)

RobinHood said:


> Actually, I believe, a VFD does not know the RPM of an induction (asynchronous) motor. It only knows the rpm of the rotating magnetic field in the stator windings it is sending to the motor. It “assumes” the rotor is keeping up with that rotation.
> 
> The only way a controller knows the rpm of a rotor is if you have a sensor on the rotating element and feed it back to the controller.



I really don't think that is correct @RobinHood. Because the position of the rotor does affect the current flowing in the stator, the VFD can and does sense, and can calculate, and can control the position and speed of the rotor in a 3phase induction motor. 

It is called sensorless vector control mode. Not all VFD's have sensorless vector control. The better ones do. 

Here is a link to an EE article that explains it. If you google sensor less vector control you will find lots of others. 

https://www.eeweb.com/sensorless-vector-control-and-torque-control-vfd/

If you still disagree, PLEASE tell me why. I am eager to learn and the worst that can happen if neither of us is willing to change our minds is to agree to disagree. That said, I change my mind quite easily..... 

I love the other articles you referenced. In fact, as @whydontu would have said, "the internet is smaller than we think". If you look at the hp/torque/rpm graph I provided earlier, you will find that it comes from that same hp torque article you linked to. Small world eh! 

I also found the VFD facts and myths very interesting. Thanks for that. 

As you may have read already, I am planning to keep the Hartford. Nobody has provided any advice to the contrary. Thanks for your vote too! It means a lot!


----------



## Mcgyver (Oct 6, 2021)

Susquatch said:


> It is called sensorless vector control mode. Not all VFD's have sensorless vector control. The better ones do.



I think what sensorless vector drives do is achieve constant torque over the speed range, including zero.   You need that for cranes or conveyors but it doesn't address the challenges of a machine drive where you want torque to increase as speed slows (constant power).   I never seen and can't find any claim that any VFD can do so.


----------



## RobinHood (Oct 6, 2021)

I do agree with you that sensorless vector controlled VFDs have a better ability to control the rpm of the rotor. Not all VFDs sold today use that technology. Certainly 5+ years ago, only the high end manufacturers offered that option for premium $$s.
I probably have not kept up with what’s on the market today and at what price as 6+ years ago I switched over to a RPC for my 3 phase requirements and have never looked back.

I am a “manual” type of drive guy. Gears, belts, drive lines, etc. are easier on my brain than electrons running around in wires. I do have to say though that I am very impressed on how much more user friendly electronics have become. Example of the fellow using a stepper controlled 4th axis to hob gears on his mill (video linked in another thread on this Forum). That gives so much more options, all at the push of a button. I can foresee something like that in my future.

Too bad the BP had such a hard life and needs so many $s worth of work. So if you can sell it at a price that covers your expenses to someone that can bring it back to its former glory, I think you still have done very well.


----------



## Dabbler (Oct 6, 2021)

So there's a lot to unpack here...

The torque/HP discussion is great as an academic exercise, but, with respect, is irrelevant in a hobby context.  Unless you are a very experienced machinist, wanting to optimize your time on a piece of work, a hobby machinist can always take smaller cuts to minimize wear and stress on your machine. That said, the Hartford can easily handle a 2 1/2 inch face mill that has 6 inserts with a reasonable DOC and chip.  Something like .003 tooth engagement and .080 DOC.  I would be very leery of going more than that.

If you are a 'must have' kind of guy, you can up-size your motor, but the abuse your spindle is going to get isn't worth it IMO.



RobinHood said:


> only use the VFD for fine tuning the speeds


 +1 from me

As a rule of thumb, the avilable torque drops off as you deviate from the 60 HZ design frequency.  It is unnoticeable between -50% and +150%  As you drop near 25% speed, you have about half torque (just experience here, not actual testing) and at 10%  (my minimum) it is about 1/4 of the torque.  At 200% (my maximum) the torque is less than a third.  My VFD is also a TECO 510 in SV mode - a very fine VFD indeed.



Mcgyver said:


> I would say its not doing that except very close to its natural speed (@ 60Hz)



What it is doing at, say 10hz is to supply as much energy at the rated voltage possible until the coils saturate.  Effectively it means delivering as much current as the leade length and coil wire size will absorb.  The VFD measures current flow and dynamically calculates the energy needed to try to preserve troque, so the 10Hz signal is much like a 100% duty cycle square wave.  The motor heats up quite a bit a 10Hz and an external cooling fan is strongly recommended for all but the shortest 'on' time.  I regularly use my non-inverter motor at 20Hz for long cuts, and get some, but not dangerous, heating of the motor (and VFD) BTW.



Brent H said:


> since you have a VFD rated motor on your Hartford you will be fine with any speed you choose to operate at. You will get more consistent torque at lower speeds and be fine running at higher speeds.



Agree completely - however torque drop off still follows the pattern I mentioned above.  The VFD rated motor has strongly bonded coils (usually High temperature/high heat coefficient epoxy).

Some of the discussion rely on extreme examples:  In my experience, a TECO VFD on a 2HP pancake motor on a Bridgeport style mill using a 1/2 inch cutter can easily go down to 180 RPM without changing belts or using the back gear...  I tend to take light cuts, however.


----------



## Susquatch (Oct 6, 2021)

What @Dabbler said gets a +1 from me with just one small (prolly "academic") quibble.  LOL! 

I still maintain that torque does NOT drop off as rpm goes down. I drank the VFD coolaide, and after suitable digestion, came to believe that the "available" torque really does stay constant below rated rpm. The keyword is "available" as per what @Brent H explained earlier. 

However, it may well be that the available torque (which is as high as it ever got at the rated rpm) still isn't enough to do the required work at low rpms which might easily give the impression that it has gone down when it really hasn't. Just my thoughts for whatever that is worth. 

Btw @Dabbler, I think your explanation of how the VFD saturates the coils in a way that a normal 3phase supply could never do is excellent. 

What's important in all this HP torque discussion is that I'm still happy with my decision to keep the Hartford! And I LOVE my VFD!  I wish all you guys could be here to celebrate with me and witness  how quietly and smoothly that baby makes chips out of solid steel. For me it's a thing of pure bliss that I wish I could share with others who would appreciate it as much as I do.


----------



## Susquatch (Oct 6, 2021)

Mcgyver said:


> On a machine tool, work is essentially the removal rate - i.e. cubic inches per minute.



This is an excellent way of looking at it. That insight of yours takes into account both torque and hp. I think it also highlights the difference in the way each of us had been looking at this. Basically, I submit that torque is what takes the metal off, but hp is what dictates the "rate" at which that torque can take it off. That's because power is a rate of doing work, and torque is a force. 

I might be wrong, but I think our minds just met on some very common ground..... (insert big happy smile here!)


----------



## whydontu (Oct 6, 2021)

as many an old drag racer will tell you, horsepower sells cars, torque wins races.


----------



## Susquatch (Oct 6, 2021)

whydontu said:


> as many an old drag racer will tell you, horsepower sells cars, torque wins races.



LOL! I'll second that one! 

Maybe that's also why I'm so tuned into torque over hp. I did do a bit of drag racing in my day. I also ended up in an automotive career, was part of several engine design programs, and I'm definitely old now.......

But ya, to your point, the marketing guys always wanted to know if we could squeeze out a little more hp and we kept trying to brag about the torque. I didn't enjoy those challenges then........ but I miss them now......

Thanks for the memories....


----------



## Chicken lights (Oct 6, 2021)

whydontu said:


> as many an old drag racer will tell you, horsepower sells cars, torque wins races.


that’s not quite true but I understand the point you’re making 

light car with a high revving small mill can dominate in bracket racing versus big blocks in heavy cars 

it’s all in the setup to get across that line


----------



## Susquatch (Oct 7, 2021)

Chicken lights said:


> that’s not quite true but I understand the point you’re making
> 
> light car with a high revving small mill can dominate in bracket racing versus big blocks in heavy cars
> 
> it’s all in the setup to get across that line



Speaking as a guy who raced bracket, and who later was involved in the design of engines for passenger cars and trucks.....

You are right. 

But as you probably well know, bracket is a special case. There is a lot more to it than just big blocks in heavy cars VS small cars with high rev engines. 

All that aside, @whydontu's point is still fundamentally right. The whole world loves HP and that's why the marketing folks push it. Everyone loves to talk about it, but very few really understand it. 

Since you brought it up though, I would add that one of the holy grails in engine design is the best of both ie a high torque, light rotating mass, high reving engine in the lightest weight body and frame possible. What you really want is instant response high torque. That's totally different than machining which is mostly a steady state process. In any acceleration race what you really want is a rotating system (crank rods pistons, Cams, chains, flywheel, driveshaft axle wheels and tires) that weigh nothing and therefore have no inertia and can go from 600rpm to 6000 instantly! 
  ...... and then you need to have some magic way of maintaining traction.  

Anyway, that field is huge and I'm sure an entire library of books could be written on it. 

More importantly, it doesn't change the fundamental importance of torque or my choice of mills.

Wanna grab a case of beer, join me beside my mill, and swap war stories? Actually, you can all just show up - I have lots of beer for everyone and I feel like sharing!


----------



## Mcgyver (Oct 7, 2021)

Susquatch said:


> I still maintain that torque does NOT drop off as rpm goes down. .



You right, 100%  - torque does not drop off.  Never argued it did.  I argued, and will to the end time or until basics mechanics is proven wrong, that HP drops when speed is dropped unless torque is raised - what a manual transmission does.  Cut your speed to one 1/10 and you must increase torque 10x to have the same HP.   I'll try and chat with one of SEW's engineers (we buy a lot of drives from them) to confirm the point of a vector drive only means its better at delivering constant torque (i.e. does not/cannot maintain HP)

As I said, you might not notice it going from say 1800 to 1500  or even less rpms (because you weren't using all the torque available).  But what if I took out the transmission on my mill and tried to go 3600 to 100?  at 100 You'd have just over 1/20th of a HP.  Now if someone considers that academic or theoretical and as such of no real importance, man, that is some light use of the mill!  Its about as theoretical as gravity - just try to ignore it in practice 

Just for the record, this was never about telling someone what to do.   It was answering your question why a transmission is superior to a VFD as a way to reduce speed.  That is a true statement, not withstand the fact it might be moot for someone who either never uses low speeds or only takes very light cuts.  However put a 6" round bar in the lathe or power up the horizontal with a 6" diameter cutter, scrap the transmission, replace it with a VFD  and crank the speed down to achieve the correct sfm.....how well do think you're going to do with the say 0.1 HP you now have?

I could do a video of slowing a small lathe in low speed driven via belts, then slowing it down only by VFD.  I'n both both cases the torque at the motor will be the same (constant torque over a high quality drives frequency range from 0 - 60Hz), but I'll be able to stop the spindle by hand when only using the VFD (and could possibly do so when the belt speed reduction is used).  Does that not illustrate the point? Its an inferior way to reduce speed.  What you want in a speed reduction device is for torque to increase as speed is reduced to maintain HP (ignoring losses) - which is what a mechanical transmissions does.

I'm just repeating at this point, so come on over with that beer.....we'll find a new horse to flog


----------



## Chicken lights (Oct 7, 2021)

Susquatch said:


> Speaking as a guy who raced bracket, and who later was involved in the design of engines for passenger cars and trucks.....
> 
> You are right.
> 
> ...


How far are you from Chatham?
asking for a friend


----------



## Susquatch (Oct 7, 2021)

Chicken lights said:


> How far are you from Chatham?
> asking for a friend



About 20 minutes south. My farm overlooks Lake Erie!  Does your friend like machines?


----------



## Chicken lights (Oct 7, 2021)

Susquatch said:


> About 20 minutes south. My farm overlooks Lake Erie!  Does your friend like machines?


my friend likes machines, farming, trucks and small town Canada life, he tells me he’s in Chatham in a few days


----------



## Susquatch (Oct 7, 2021)

Mcgyver said:


> You right, 100%  - torque does not drop off.  Never argued it did.  I argued, and will to the end time or until basics mechanics is proven wrong, that HP drops when speed is dropped unless torque is raised - what a manual transmission does.
> 
> ........
> 
> I'm just repeating at this point, so come on over with that beer.....we'll find a new horse to flog



You are right we are both flogging a dead horse at this point. 

Let's just agree to disagree on whatever it is we disagree on and to be honest, I don't even think I know what it is that we disagree on anymore! 

I'll bring the beer  

Know anyone with a leer jet to get us all there?


----------



## Susquatch (Oct 7, 2021)

Chicken lights said:


> my friend likes machines, farming, trucks and small town Canada life, he tells me he’s in Chatham in a few days



Hope it's for longer than just the weekend. I'm headed to Barrie in a few minutes for Thanksgiving with my Sister and her family.

PM me for details!


----------



## Dabbler (Oct 7, 2021)

Susquatch said:


> Know anyone with a leer jet to get us all there?



as a matter of fact...  but not available to any of us pleebs.


----------



## whydontu (Oct 7, 2021)

My business partner just chartered a Pilatus turboprop to fly my wife and I up to his private lake 300km north of Vancouver. (yes, he and a few buddies own a lake, fishing lodge, and 12,000 acres of ranch land). I’ll see what I can do. No promises.


----------



## Brent H (Oct 7, 2021)

Hey @Susquatch - if your heading to Barrie I am only 15 minutes north!  We could meet up for a Beertini !


----------



## Susquatch (Oct 7, 2021)

Brent H said:


> Hey @Susquatch - if your heading to Barrie I am only 15 minutes north!  We could meet up for a Beertini !



PM sent.


----------



## Susquatch (Oct 21, 2021)

Sometimes you just have to try things. 

I was making a big mandrel in my lathe today when I suddenly realized that the mandrel would be perfect to see if I could stall my mill motor at its lowest VFD setting of 5 hz by hand. 

So I chucked the mandrel and tried it. Here are two videos one with the back gear and one without. Both are the slowest belt position. Both are only 5hz on the motor. 

https://www.dropbox.com/s/7farsammtpsozd1/20211021_141224_1.mp4?dl=0

https://www.dropbox.com/s/9zvb41u3dkk3f03/20211021_140437_1.mp4?dl=0

Bottom line - the motor didn't slow down at all. In fact, it didn't even grunt! It just kept going like as though I didn't even touch it. I am a big guy with very strong hands. The shaking in the videos is the result of me giving it everything I had. 

After that I tried a 20 thou cut with the biggest fly Cutter I have. That didn't flinch it either but it sure did bounce! I believe the bouncing was mostly because I don't have an R8 fly Cutter and had to use an R8 to MT3 adapter which destroyed the rigidity. The motor didn't grunt with the fly Cutter either. It ran smooth as silk and didn't lose any speed at all. 

All this goes to show that a good VFD and VFD ready motor are a great combination. I'm very pleased with how they perform.


----------



## Brent H (Oct 21, 2021)

@Susquatch : throw in 1/2” x 4 flute end mill and take a 50 thou cut off the face of something.  
Maybe it will stall out - what ever 

My cousin is a machinist, he was by a few years ago and I needed a flat on a boring bar - I just got my mill.   No problem he says - zing - off goes 0.050” in one pass.  Hmmmm …. Says I.   After that … let it rip and see what she (ol’ Bridget) can do !! LOL


----------



## Susquatch (Oct 22, 2021)

Your wish is my command! I'll try that later today. 

I think that slow speed machining is an interesting exercise in machine dynamics and material properties. In my minds eye, I can actually see the cut happening. Slowing down the Cutter contributes to the visual. 

I am thinking that the primary difference between high and low speed cutting is tearing VS slicing. Higher speeds might contribute more inertia to keep a slice going. Slower speeds might cause the material to yield in small time increments before giving in to the force and breaking off. The difference might even explain the surface finish relationship. 

Even so, I find it easy to visualize a slow motion cut VS a high speed one. Maybe I'm just kidding myself into believing what I see in my mind. But I don't think so. It takes roughly the same force (overall torque / tip vector force) to yield and cut the material regardless of cutting speed. But the amount of material removed in a unit of time changes the HP requirement. As tip speed is reduced, feed rate must be reduced proportionately. 

Torque demand is roughly the same, but hp demand goes down. It all makes sense. 

It's a bit like finite element design or integral calculus. Slow it down and break it into smaller increments analysed one tiny element at a time. Alternatively, it also works in my mind if I look at it like a static balanced force system, with a curl of material coming off the Cutter tip frozen in time. 

I wonder if that is the basis of the science and math behind all those cutting speed charts, curves, and tip designs.......


----------



## Susquatch (Oct 22, 2021)

Brent H said:


> @Susquatch : throw in 1/2” x 4 flute end mill and take a 50 thou cut off the face of something.
> Maybe it will stall out - what ever
> 
> My cousin is a machinist, he was by a few years ago and I needed a flat on a boring bar - I just got my mill.   No problem he says - zing - off goes 0.050” in one pass.  Hmmmm …. Says I.   After that … let it rip and see what she (ol’ Bridget) can do !! LOL



Here you go Brent. 50 thou at 5hz side milling 1 inch thick mild steel was zero problem.

The top of the attached photo was cut at 5 hz on the back gear, the rest was cut at 5hz in direct drive.

No way did that even load the motor let alone stall it. The only difference is the finish - plainly visible in the photo - as my discussion above suggested it probably would be.

Again, it's a cool insight into the material properties and mill function. And it's amazing to watch that end mill peeling off thread after thread of steel at the speed of smell.






The one lonely line just below the back gear cut is when I stopped cutting to get a better continuous grip on the hand wheel.


----------



## Susquatch (Oct 22, 2021)

Love my Hartford mill.


----------



## Brent H (Oct 22, 2021)

Beauty!


----------



## deleted_user (Mar 2, 2022)

RobinHood said:


> Sorry late to the party…
> 
> 
> 
> ...


I'm not sure about the most common VFD but many motor controllers can derive the rotor rpm by measuring the back emf produced by the motor.

very common technique in sensorless motors and controllers in ebikes. I'm sure that some higher end controllers must have the ability.

ps yes I know this is an old thread. I just feel chatty


----------



## Susquatch (Mar 3, 2022)

TorontoBuilder said:


> not sure if this was addressed elsewhere later, but the issue with bearings is not the speed but the arcing that is produced by voltage spikes from the high frequencies that VFDs operate at... the arcing causes pitting, like a tiny EDM machine ruining your shaft and bearing unless they are insulated or grounded as in a inverter duty motor.
> 
> this is why you want to use the lowest carrier frequency you can without nasty noise from the motor.



I would agree with that being an issue and you are right to point that out. 

But I don't think it's correct to say that it's that not the bearing rating. So let's just say that it's both. Many older motors have a low speed rating for the bearings and Rotor. In this case, it's the high rpm of the motor that causes damage. This happens no matter what the carrier frequency is because rotor speed itself is the killer not the carrier frequency. 

Bearing damage from a high frequency carrier happens no matter what the motor speed is. Bearing damage from high rpm happens no matter what the carrier speed is. Both are damaging.


----------



## Degen (Mar 3, 2022)

On VFD's I'm going to comment on what I learned about 28-30 years ago when I sold the first first 2 installations in Canada for my company.  Simple VFD's (now cheap today only control frequency and have current capacity overhead to supply what the motor requires).  This fries your motor and limits your power.  Good VFD's also vary voltage.  This way the current doesn't need vary but the voltage does in conjunction with the frequency keeping the wattage the same throughout the frequency (speed), no motor burn out (ideally).

Torque is determined by push which is amperage, Amperage in this cause is also heat, so at the upper end Voltage is used instead  of torque.

However if you can get your motor up to speed (sometimes that is the trick) rotational mass overcomes the need of torque at high speeds.

At low speeds we drop voltage to remain constant power but lower rotational mass messes again with perceived torque.

So knowing that use a VFD correctly and you can do almost anything, get that balance wrong and Balding is in your future.

BTW I use light belt tension on my mill to allow free spin on the motor (avoid torque rob by belt) and rely on the mass of the system spin to prevent load stall torque.  Works well.


----------



## Dabbler (Mar 5, 2022)

On the VFD front, bearing erosion is primarily from static discharge, not induced current or VFD frequency - for motors that any of us will see.  Make sure your grounds are good, and don't worry about VFD frequency selection on that front.


----------

