Turning RPM calculated from Turn Diameter or Stock Diameter?

# Thread: Turning RPM calculated from Turn Diameter or Stock Diameter?

1. ## Turning RPM calculated from Turn Diameter or Stock Diameter?

Back when i was working on a lathe, i always calculated RPM based on the actual diameter of the workpiece.
This is the way i was taught at school and have seen this suggested in all sorts of literature.

Now I am selling Speeds and Feeds software and this is the way RPM calculation is implemented there.
Every so often my customers report that RPM must be calculated using Turning Diameter.

But what is the "Industry standard" in this case?

*Stock Diameter?

or

*Turn Diameter?

2. If RPM were to be calculated using stock diameter instead of turn diameter, then what would be the reason for having G96 constant surface speed? My vote is for calculating RPM based on the actual diameter being turned.

3. RPM calculation on the machine is based where the tool tip is sitting in X.

It does not change the machine's calculation if you are taking off .01" or .500" per side, even though the SFM of the upper part of the .500" DOC will vary greatly as the overall diameter of the turned part is decreased.

4. Stainless
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Hold on, you are selling software to determine speeds and feeds for lathes, and have to ask that?

5. Originally Posted by Gobo
Hold on, you are selling software to determine speeds and feeds for lathes, and have to ask that?
I know that machines calculate cutting speed at the tip. But that is because the machine does not know the depth of cut it is taking.

Like noted above. When turning .2 dia at .1 doc, the real SFM at the outside will be double that of the tip....

Im my case (and if anyone cares about RPM) RPM it makes sense to calculate RPM at the stock dia.

6. Aluminum
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Is there that much difference between the tip of the tool diameter and the stock as far as RPM would be calculated?

What is usually the first part of an insert that fails? Tip? The top edge where the stock diameter is? I would say have the "correct" rpm at the tip...if it even made that much of a difference.

7. Originally Posted by lakey0
Is there that much difference between the tip of the tool diameter and the stock as far as RPM would be calculated?

What is usually the first part of an insert that fails? Tip? The top edge where the stock diameter is? I would say have the "correct" rpm at the tip...if it even made that much of a difference.
For large diameters it does not matter.
But closer to 0 RPM calculates to insanely large numbers. So i 'fixed' it by sticking to outside diameter at all cases

8. Cast Iron
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Which program covers turning? I have the FSwizard App on my iPhone. I recently bought a lathe and I was hoping that there was a turning tab hiding in there somewhere.

9. Originally Posted by Chris Attebery
Which program covers turning? I have the FSwizard App on my iPhone. I recently bought a lathe and I was hoping that there was a turning tab hiding in there somewhere.
All of my programs do.
There is 3 tool types for turning in the Tool Type drop-down.

10. Stainless
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A depth of cut of .1 at a diameter of .2 is unrealistic. Your scenario is invalid. Question- how much actual time have you spent operating lathes?

11. So, when facing from large diameter to small, you use the tooltip to calculate RPM, but when turning along Z you use the stock diameter? Could get confusing.

12. Originally Posted by Gobo
A depth of cut of .1 at a diameter of .2 is unrealistic. Your scenario is invalid. Question- how much actual time have you spent operating lathes?
0.4" stock
0.1" per side.
0.2" final diameter.

Whats so ingalid even for this HYPOTHETICAL scenario?

Of cource i only have spend 11 years machining.

13. Originally Posted by mrainey
So, when facing from large diameter to small, you use the tooltip to calculate RPM, but when turning along Z you use the stock diameter? Could get confusing.
I guess that settles it. Most people prefer sfm at tool tip.

I will just add an option so users can choose how to calculate with 'at tip' being default.

Thanks!

14. Stainless
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D.O.C. of .1 at a diameter of .2 = 0.

15. Originally Posted by Gobo
D.O.C. of .1 at a diameter of .2 = 0.
I meant the finished diameter being 0.2"
Sorry for misunderstanding then: )

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Understood.

17. Cast Iron
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Ah Ha! Thank you Sir!

Originally Posted by zero_divide
All of my programs do.
There is 3 tool types for turning in the Tool Type drop-down.

18. In my world I use tool tip for RPM calc.

19. Originally Posted by Nerdlinger
In my world I use tool tip for RPM calc.
The thing is. In my opinion you don't always need to know RPM at all. As mentioned by many people in G96 mode RPM varies with diameter at tip X coodrinate.

But when you do keed to know the RPM, you might aw well like to have it calculated from the stock diameter.

Regardless. I will just make that optional.

20. Titanium
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Originally Posted by zero_divide
When turning .2 dia at .1 doc, the real SFM at the outside will be double that of the tip....
Like already mentioned this wouldn't be true for facing. I've never heard anything like this before. I think It's always been the tip. Lol...

Brent