TTalma,
I'm curious.  Why do you want this in RPM and not in degrees?  Is it to figure out a good feedrate for your file?

Chad, the problem is, you only have one feed rate at the time.
So say you have a line of Gcode that all axis are doing a synchronized feed move on (G1 F5 X5.000 Y5.000 Z5.000 A360.000).  All axis are going to end at the location requested at the same time.

Without the A axis this move would take 1 minute. But because of the A axis this line will take 360 degrees /5 degrees per minute = 72 minutes to complete.

However, using the same axes and values (G1 F5 X5.000 Y5.000 Z5.000 A360.000) but replacing the G1 with G93 and the F5 with F1 (G93 F1 X5.000 Y5.000 Z5.000 A360.000) it would finish in one minute............ assuming motor tuning would allow it to finish in the time of course. Now if any of the axis can't move the requested distance in the requested time (due to motor tuning) the time it would take will be extended to the fastest it can possibly happen in.

Hope that makes sense.

Brett,
That's pretty cool.  I get that, and this can actually be pretty useful on a couple machines we have here in the shop.  
I was just wondering why TTalma wanted RPM over Degrees.  
He wants to make Mach read in RPM no Degrees.  
I was curious why, exactly, does he want to do that.

He wants to make Mach read in RPM no Degrees.  I was curious why, exactly, does he want to do that.

Because he has a hard time relating to it and so do a lot of other folks.

IE;
Walk into candy store in a metric nation, request a  1/4 # of candy,
the waiter  says back to you ..... How many grams of candy? .......
DAHHHHHHHHHHHH you say 1750 grains......,
the  waiter replies how many  grams please .....
DAhhhhhhhhhhhhhh  you say .1 kilos.....,
waiter replies the scale is in grams only and again asks how many grams............,
DAhhhhhhhhhhhhhhh, frustrated you say.....  just fill my hand with candy and weigh it !

ie; Another one
You buy a car and the tach reads in degrees per min!
What would be the first words that would appear in the bubble above a persons head?  

RICH

Thanks for the updates. I need to change my way of thinking, when I do that it makes sense.

The main reason I want RPM's is because what I'm making (spiral patterns) the degrees don't really matter, I'm looking for how far my X axis will move for each revolution. I also plan to "wrap" flat models around the cylinder. So the circumference in inches matters to me as well.

And on that note after some more fiddling I was able to get my file working correctly (while thinking in degrees per min)

Thanks for all of the help.

Well, I hesitate putting this up because it is a simple fix but comes with other issues. If you want the rotational axis to display and work in RPM instead of DPM just change your steps per in motor tuning to Current Steps Per X 360. This will break other things though like toolpath preview and most CAM programs wont generate code for it.

I maintain the proper fix is to use G93. And change your way of thinking of course. The above may be a fix for you but anyone reading this needs to make sure they understand that the real issue is your way of thinking. Just because you can do something doesn't mean you should. Mach like all other software is code. The code can be written to adapt to certain things but to an incorrect way of thinking is not a goal of any programmer I know. This is why standards and following them is so important.

No one should hate having to change their way of thinking to be on par with standards. No shame in it and you will be smarter for changing. Be happy that you are smarter than the machines today. That will not be the case much longer IMO. Today you can adapt your way of thinking to overcome obstacles keeping you from the desired end result. Most machines can't do that now. But when they can (and they will)....... we become useless eaters. So, take pride in your advantage today. It may not exist tomorrow.

HAHA.  Thanks Rich.  I mean I understand the difference in how you think about things.  I didn't know if it was that issue or if he NEEDED the RPM for something.  But I do like the analogies used.

I would also hesitate re-calibrating the motor.  Instead, I like Brett's idea of G93.  Elegant and accurate.

But, just for fun.  What would it entail to add a DRO to translate rotational RPM into RPM.  I think this could easily handled by the PLC Script.

What would it entail to add a DRO to translate rotational RPM into RPM

Not sure what you meant here. RPM = RPM Maybe you meant DPM to RPM? Or maybe RPM to IPM?

Ha!   Didn't realize I did that
Yeah, the rotational feed into rpm.   The example earlier was 21,600 DPM = 60 RPM
Or IPM to RPM

Hi,
now get real and use radians per second like a proper physicist...

6.282 radians/sec =60 rpm LOL

Jokes aside, radians have value when you consider that the ratio of the circumference of the arc traversed to the cricumference
of the entire  circle is the angle in RADIANS.

Thus for OPs problem 1 inch on the surface of his 4 inch diameter job:
1/4 x pi=0.07959 radians which at 57 degree per radian=4.5 degrees.

So radians, an angular measurement has a direct correlation to surface circumference, its just the numbers look a bit weird.
Like 360 degrees = 2 x pi, 90 degrees= pi/2 and so on.

Craig