Cutting speeds (Im sure again, and again and again and again.....)

[TheLocalDrunk]

Im having an issue here.
Every time I think I have the right cutting speed figured out I bounce the number off of my machinist buddy and he laughs saying "that is way to fast" or"Way to slow".

Now it is easy enough to find a chart of cutting speeds for HSS. (Milling operation)
But, I am told that a carbide insert facing mill will want to cut at faster speeds. But I cannot find any chart that helps tell me what the CS is for carbide milling.

Am I missing something? or is it just my crazy pills?

(I would just be keying it into the (CSx12)/TDxπ) formula)

 

[Tom Kitta]

Well - as usual there is a lot of simplification present everywhere.

First of all the speeds and feeds in a simplified chart are given for few materials and are given usually for certain tool life / certain percent of max beyond which tool life is "unacceptable".

Thus electronic based software expands on the concept and actually asks you how hard you want to push your tool - you don't need to go 100% of the speed with HSS in your material - you can go say 50% - HSS or M42 is "forgiving" i.e. it will not cheap easily. Smaller drills may break etc if you go really slow.

The whole idea of going fast is for production use - where you optimise your tool life to part making speed.

Now with carbide it is a bit different as it is by far not as tough as HSS or HSS+Co. I.e. it can break and shatter on you. Thus you cannot exactly go dead slow (well you can but it may chip). Carbide is an evolution of HSS - High Speed Steel - which is evolution over Carbon Steel - i.e. for production you want to go fast, faster even faster - to make more parts & spend less in tooling maximising profit.

Some will tell you to figure out the grade of carbide insert and get manufacturer and check online - this is a bit time consuming but can be done. If you don't know the grade / manufacturer and you have no idea etc. The next limiting factors would be rigidness of your machine / depth of cut / cutter size -> carbide hates vibrations. But say you are well within your machine capability - vibration is not an issue etc. etc.

Then I would start at say 2x the speed of equivalent end mill in HSS for the material. See how it goes. If you chip the stuff you can go faster. Go 3x. At that speed it should not chip - through your inserts may burn if say you are using turning inserts for cast iron in hard steel milling. This is my personal experience - others probably have 100x more knowledge on this - I start with low ultra conservative speed. In production they would experiment with much faster speeds.

Feeds - treat each insert as a cutting edge and start with mid for HSS with same flutes - carbide usually doesn't like small cuts but that is again for "production" - its inefficient - you use more inserts to remove same metal amount with tiny cuts. For hobby use its not an issue. I personally go a lot with a feel method - if the machine is about to start walking out the door maybe you are too aggressive.

 

[TheLocalDrunk]

Thanks for the info.
I have heard and understand a good portion of that.
(So if I have this correct you are saying use a HSS guide and X2 to start?)

Let me reword my question.

"Cutting speed can very for carbide from 500 to 1000 depending on coatings and material it(s) used on"

How the hey-hoo did he come up with the number 500 to 1000 for carbide?
I just need a general starting point then I can let the metal and machine talk to me from there. I just dont know where to start which is why I am looking at a generic CS guide.

 

[Tom Kitta]

Lets say you have 1" HSS end mill and the material is mild steel. The speed for that should be around 250 rpm on the low end and about 350 on the high end. You can if you want go slower - even 150 rpm but surface finish will suffer & feed would need to be slow or you would be cutting too much with each teeth. https://www.custompartnet.com/calculator/milling-speed-and-feed

SFM for mild steel is around 100.

So lets say you are now using 1" carbide face mill or just indexed endmill with say 2 inserts. I would say 500 rpm would be very slow for it and it may chip. So say start at 750 rpm - still very slow. I run 1" non-indexed carbide end mill in mild steel (coated) at 1200-1400. It produces hot blue chips. The end mill is dull. Feed is around 4 (if I remember correctly).

As you can see https://littlemachineshop.com/reference/cuttingspeeds.php carbide is at least 3x more SFM then HSS.

A lot of professional shops have combinations dailed in - as you can see from the LMS diagram there is a lot of "variation" all over the place - the rule of the game is to run carbide at high enough speed so it doesn't chip. lets say at speed X it doesn't chip. Speed X+100 is a bonus to make parts faster or produce better finish.

"Cutting speed can very for carbide from 500 to 1000 depending on coatings and material it(s) used on"

I think this is SFM - that the 500 to 1000 is the SFM for carbide. It can actually be way more in aluminium where you multiply by 3 again. But you don't have to go 6000+ rpm or so - if say your machine is limited to 2500rpm you mill that aluminium with that carbide cutter of 1/2" size at 2500rpm - it is simply not working as fast as it could but I am sure it will not chip yet (depending on load per teeth - i.e. feed is not crazy).

I.e. he is talking the max speed -> the min speed is "the chip speed" + 1.

You compute your RPM from the size of the cutter and SFM for your material / cutter material combination.

 

[TheLocalDrunk]

I have the formulas
I dont have the proper SFM per material for carbide inserts.

( https://littlemachineshop.com/reference/cuttingspeeds.php is the best chart so far.... but that says its for turning, not milling. The milling chart doesnt offer carbide)

 

[Tom Kitta]

Here they give you 100-300 http://www.harveytool.com/cms/GeneralMachiningGuidelines_17.aspx
Here 100-500 http://www.endmill.com/pages/training/Speed and Feed - Carbide End Mills and Drills.pdf

So you should start at the lower end as there seems to be belief that carbide will not chip at 100 SFM in end mills that are made of carbide.

Here is example for inserts: http://www.mitsubishicarbide.net/contents/mmus/enus/manual/end_mills_AQX_INCH.pdf

As you can see they recommend 400-600 SFM for mild steel for one of the grades of their milling inserts.

Here is a thread of someone else asking for feeds and speeds: https://www.cnczone.com/forums/torm...-speeds-feeds-tts-indexable-mill-inserts.html

What does it all mean for mystery inserts? Well, you can try slow at 100 SFM and move up from there. If these are milling inserts in mild steel they should be able to handle 300 but I am no expert.

 

[TheLocalDrunk]

 

[PeterT]

The other thing to consider - you may well determine specs or range of speed & feed for a given material from a carbide/insert manufacturer. But it kind of presumes an industrial machine & maybe industrial process like flood coolant. There is no 'reduction factor' value to apply this to a small hobby machine where new factors come into play - casting rigidity, machine mounting, torque available at that rpm, slack in the saddle or quill or table. I think the guidelines are good start, but basically it seems to turn into a qualitative thing. What do the chips look, how is the machine sounding. Is it groaning under load or making vibration noises? Its not just the tool tip, its wear & tear on your drive line too. There are times when my carbide is way under HSS because if chip evacuation issues or its a finicky part to hold in the vise. Another example - you can push a roughing EM harder than a conventional EM. It makes nice efficient chips & is actually easier on your machine all other things equal. Even though they are for higher removal, they are kinder to low HP hobby machines, something that gets overlooked. So in this case a HSS rougher might actually exceed a conventional flute carbide EM.

 

[TheLocalDrunk]

The other thing to consider - you may well determine specs or range of speed & feed for a given material from a carbide/insert manufacturer. But it kind of presumes an industrial machine & maybe industrial process like flood coolant. There is no 'reduction factor' value to apply this to a small hobby machine where new factors come into play - casting rigidity, machine mounting, torque available at that rpm, slack in the saddle or quill or table. I think the guidelines are good start, but basically it seems to turn into a qualitative thing. What do the chips look, how is the machine sounding. Is it groaning under load or making vibration noises? Its not just the tool tip, its wear & tear on your drive line too. There are times when my carbide is way under HSS because if chip evacuation issues or its a finicky part to hold in the vise. Another example - you can push a roughing EM harder than a conventional EM. It makes nice efficient chips & is actually easier on your machine all other things equal. Even though they are for higher removal, they are kinder to low HP hobby machines, something that gets overlooked. So in this case a HSS rougher might actually exceed a conventional flute carbide EM.

Yes. This is the way I look at it. I just don’t know where to start in order to evaluate the chip

Seems like a lot of trial and error.