Metal lathe options

[VicHobbyGuy]

So far, your report card comment would read "Student is willing to learn" Nothing wrong with that. "Coach-ability" is a very important quality! Thanks for taking the time to delve into this.

Don't 'take this the wrong way', please! The only thing you are learning here are the limitations of the 7x lathe ! Learning the real machining stuff is all still on MY 'ToDo' list.

 

[VicHobbyGuy]

I think part of the key to this, is the Vever lathe seems to have poor slower spindle speed operation and I need a think set change, to operate at much higher spindle speeds by myself, when I am used to the ability to turn much slower when machining difficult materials. And threading could be interesting also.

Absolutely correct, IMO.
People look at the watt ratings of the DC motors in the 7x lathes and relate those to the AC motors on older 9x and 10x lathes (like my Logan 210 with the 1/2 HP AC motor). An AC motor with speed reducing belts and pulleys vs a DC motor with a small (fragile) controller board.....they are far from the same in the 'muscle' they can deliver at low speeds.
Yes, threading coarser pitches can be exciting with a 7x which doesn't like to run very slow.
Some skilled 7x lathe users have fitted hand cranks to use for threading coarse pitches.

 

[Susquatch]

Increasing the feed would cause more problems in cutting hard materials with a 7x lathe, I think.

Unless I missed something, @Chris Cramer isnt cutting hard stuff any more. My last read was that he had switched to cutting Stainless Steel and this last discussion was about that.

You have selectable -via a lever- feed on your lathe? What feed rate do you usually use?

Stainless doesn't like high speeds. For 1 inch OD generic SS, I'd be shooting for someplace around 100 rpm. But for now my lathe will only do 70 or 200 so I'd go with 70 and might try 200 if that didn't pan out. 70 is as slow as my lathe will currently go. I'll be installing a 3ph motor and VFD later this spring - I'm working on the wiring right now. Then I'll be able to cut at whatever speed I want. I definitely want slower speeds for threading. I wouldn't be surprised if this particular Stainless cuts really well as low as 40 rpm or as fast as 300.

I'd shoot for between 5 & 10 thou/rev feed rate ( 0.2 mm/rev), and then adjust faster or slower based on how it cuts.

Similarly, I'd start at around 20thou depth of cut (0.6mm) and see how it goes. I wouldn't be afraid to go to 30 thou if it likes that and it's not at all uncommon for me to shave stainless a thou at a time with Sharp HSS.

 

[VicHobbyGuy]

Unless I missed something, @Chris Cramer isnt cutting hard stuff any more. My last read was that he had switched to cutting Stainless Steel and this last discussion was about that.

Believe me, stainless isn't a soft material for a 7x lathe. You big lathe guys are hilarious!

 

[Susquatch]

Some skilled 7x lathe users have fitted hand cranks to use for threading coarse pitches.

I've thought about adding a hand crank, mostly for threading to a shoulder, but for now all I do is stick a chuck key into the chuck and use the key as a handle to crank the chuck.

 

[Susquatch]

Believe me, stainless isn't a soft material for a 7x lathe. You big lathe guys are hilarious!

Maybe, but stainless is still like cutting butter compared to hardened 4140 or a grade 8 bolt.

 

[VicHobbyGuy]

Stainless doesn't like high speeds. For 1 inch OD generic SS, I'd be shooting for someplace around 100 rpm. But for now my lathe will only do 70 or 200 so I'd go with 70 and might try 200 if that didn't pan out. 70 is as slow as my lathe will currently go. I'll be installing a 3ph motor and VFD later this spring - I'm working on the wiring right now. Then I'll be able to cut at whatever speed I want. I definitely want slower speeds for threading. I wouldn't be surprised if this particular Stainless cuts really well as low as 40 rpm or as fast as 300.

I'd shoot for between 5 & 10 thou/rev feed rate ( 0.2 mm/rev), and then adjust faster or slower based on how it cuts.

Similarly, I'd start at around 20thou depth of cut (0.6mm) and see how it goes. I wouldn't be afraid to go to 30 thou if it likes that and it's not at all uncommon for me to shave stainless a thou at a time with Sharp HSS.

Excellent guidelines; thanks! That low speed region is the troublesome one for 7x lathes.

 

[VicHobbyGuy]

I've thought about adding a hand crank, mostly for threading to a shoulder, but for now all I do is stick a chuck key into the chuck and use the key as a handle to crank the chuck.

I hadn't thought of that for single point threading; thanks.
I do that with taps and dies, but with those it's only 'one pass and done'.
Repeat passes, single pointing would be a lot like real work.

 

[Susquatch]

That low speed region is the troublesome one for 7x lathes.

I'd like to understand that better than I do. Is the problem lack of torque or insufficient rigidity?

 

[RobinHood]

Perhaps look into tangential tools.

Holders can be shop made and the cutting tool can be carbide blanks cut easily with a diamond wheel and then honed on a lap. The geometry is also easy as it gives all clearance angles by design. Cutting forces are also reduced because of its geometry.

 

[Susquatch]

I hadn't thought of that for single point threading; thanks.
I do that with taps and dies, but with those it's only 'one pass and done'.
Repeat passes, single pointing would be a lot like real work.

All you do is stop the lathe as you get close and then crank the chuck till you reach the shoulder. How far that ends up being is a matter of coordination - which I am losing as I get older. Hence the desire for slower spindle speeds.

 

[VicHobbyGuy]

I'd like to understand that better than I do. Is the problem lack of torque or insufficient rigidity?

Both: Lack of torque at low speeds and lack of rigidity are problems with 7x lathes. There's plenty of info about improving the rigidity of 7x lathes, but no easy solutions to the torque problem.

 

[Susquatch]

Perhaps look into tangential tools.

Holders can be shop made and the cutting tool can be carbide blanks cut easily with a diamond wheel and then honed on a lap. The geometry is also easy as it gives all clearance angles by design. Cutting forces are also reduced because of its geometry.

Do you mean shear tools?

If so, I use them all the time for shaving tiny amounts or improving finish.

But I thought Chris was trying to get metal off fairly fast.

 

[VicHobbyGuy]

All you do is stop the lathe as you get close and then crank the chuck till you reach the shoulder. How far that ends up being is a matter of coordination - which I am losing as I get older. Hence the desire for slower spindle speeds.

Understood; thanks.
(I really wish we had a 'thumbs up smiley in this forum....)

 

[VicHobbyGuy]

Perhaps look into tangential tools.

Holders can be shop made and the cutting tool can be carbide blanks cut easily with a diamond wheel and then honed on a lap. The geometry is also easy as it gives all clearance angles by design. Cutting forces are also reduced because of its geometry.

Thanks for that suggestion.
I bought one of those tangential tools, but I am 'underwhelmed' by it.
https://www.eccentricengineering.com.au/products/diamond-tool-holder/the-diamond-tool-holder

 

[Susquatch]

Both: Lack of torque at low speeds and lack of rigidity are problems with 7x lathes. There's plenty of info about improving the rigidity of 7x lathes, but no easy solutions to the torque problem.

I was afraid you would say that. You can buy torque with some gearing or a bigger 3PH motor run by a VFD. Since it's a DC Motor now, I'd prolly just try gearing. Maybe as simple as a smaller pulley on the existing motor.

My first shot at rigidity would be less stickout on the tool and the part, then I'd look at a live center in the tailstock, and/or a follow rest.

But if it's so common a problem I bet there are other members here who are better experienced with what works and what doesn't on smaller lathes. That's kind of outside my experience so I'd only be making recommendations based on my engineering background and experience.

It would be interesting for me to be able to use one like that for a while. My first lsthe was made in the mid 1880s. It has a small throw of around 7 inches but a very long bed of around 48 inches. It was originally run off of a line shaft, but the previous owner installed a very small 1/4hp 110V motor. I've done an amazing amount of work on that old girl over the course of prolly 40 years till I got the new lathe 10 years ago. I didn't have big problems cutting stainless with it.

 

[VicHobbyGuy]

Maybe as simple as a smaller pulley on the existing motor.

The pulley is pretty small already. About 25mm diameter cogged pulley on a 9 mm shaft.

My first shot at rigidity would be less stickout on the tool and the part, then I'd look at a live center in the tailstock, and/or a follow rest.

Definitely those would all be good things to address.
7x lathes suffer from looseness in the gib/dovetail on the compound and cross slide, and also poor attachment of the carriage to the ways.
Some of these things aren't such a problem with even a bit larger lathe, where added mass adds inertia.
It's instructive to put a steel bar in the toolholder and give it 'a wiggle'.
For many 7x owners, it's their first lathe so analyzing problems is difficult.
Also, our techniques can be pretty poor!
I see a lot of 7x lathe pictures with the compound still in the 'shipping position', parallel to the ways. Users get the compound 'hanging out' toward the headstock, and that doesn't help with the chatter and looseness.

But if it's so common a problem I bet there are other members here who are better experienced with what works and what doesn't.

Yes, they are probably hiding here somewhere!
That's why I recommend people to go where the other 7x sufferers hang out.

 

[Susquatch]

Thanks for that suggestion.
I bought one of those tangential tools, but I am 'underwhelmed' by it.
https://www.eccentricengineering.com.au/products/diamond-tool-holder/the-diamond-tool-holder

I could be wrong, but I don't think that's what @RobinHood was referring to.

Here is one link to a recent thread on them.

Thread 'Vertical shear lathe tool' https://canadianhobbymetalworkers.com/threads/vertical-shear-lathe-tool.7408/

You can also search the forum for "shear tool" because the subject comes up quite often.

I love shear tools. They work when nothing else does.

 

[DavidR8]

I bought one of those tangential tools, but I am 'underwhelmed' by it.
https://www.eccentricengineering.com.au/products/diamond-tool-holder/the-diamond-tool-holder

Is this sized for your mini-lathe?

 

[Susquatch]

The pulley is pretty small already. About 25mm diameter cogged pulley on a 9 mm shaft.

Can you squeeze in a bigger pulley on the other end? And are there any other intermediate gears that can be revised.