Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: warhorse on June 12, 2010, 11:44:57 PM

Hi Guys,
Im running a dual spindle router with two 4th axis rotaries. Everything seems to be working fine except one major detail.
Determination of steps per inch for the rotaries....
There does not seem to be a constant steps per inch, to maintain accuracy, for every cylinder I turn.
I ran a test where I turned several stepped cylinders. There were 6 steps in the stepped cyliinder. each bigger than the other by .125 or .25.
If I was able to determine steps per inch to make one cylinder accurate, the others would be off.
and this carried through all cylinder tests.
Is there a setting somewhere in Mach3 that is controlling how mach interprets or translates, to make a single constant good for all geometries?
Thanks in advance.

Hi Warhorse  Maybe I'm misunderstanding your question  but if not  what you want is physically impossible.
Cheers
Ian

The best solution, maybe, is to work in degrees  they are constant (steps/degree) no matter what diameter/circumference cylinders you are working with.
Tweakie.

Rotary axis are steps/ degree, not steps per inch.

Thanks for the input guys. Im new to 4th axis, and fairly new to the mach3 controller, so Im on a steep learning curve.
When we first started using the 4th axis it was set in degrees, and we had trouble with the chuck spinning around around instead of simply stepping some small distance.
we changed it to distance, and of course are looking for a constant that doesnt exist.
could some one be kind enough to briefly explain how we must set this up to work in steps per degree? What settings must be changed and how do I find the constant of steps per degree that I need?
Thanks much in advance,
WH

say for example you're using 200 steps per rev stepper motors and 8 microstep drivers. It would take 1600 steps to turn your motor 1 rev or 360 degrees. So 1600/360=4.4444444444444444444444444444444 steps per degree. So that's what you'd enter into "steps per" in motor tuning for your rotary axis.
Then if you command say G1 A360 your motor will turn exactly one rev or 360 degrees by Mach sending out 1600 pulses. Of course you may spot that the steps per is not an integer, therefore if you commanded say G1 A1 to move 1 degree, Mach could only send out either 4 pulses or 5 (actually 4) so it would be as near as it could get to 1 degree but not spot on.
Cheers
Ian

Ian, thanks for the input.
These rotaries are geared so I am assuming that must also be calculated?
Im trying to contact the engineering department of the device manufacturer to determine the constants of the motors and controllers.
Thanks,
wh

Hi wh,
Just a thought, and with a bit of lateral thinking on this topic for operating a rotary axis in linear mode.
If you were to calculate the steps per one revolution of a rotary axis and divide the answer by pi and then enter the result into the ‘steps per’ for the A axis  it would then have the correct linear movement on the surface of a 1 inch diameter cylinder (fitted into the A axis).
Using Screen4 (or Screen Designer) add the A Axis Scale DRO (OEM62) and LED (OEM44) which for some reason have been removed.
Now for any diameter of work fitted in the A axis, perform the calculation 1/diameter and enter the result into the A Axis DRO.
Then the linear movement on the surface of the work will be correct for that diameter. :\
I have not actually tried this but I will do.
Tweakie.

HI tweakie,
thanks for that thought. I know that would work, if we were carving cylinders or carvings.
Unfortunately we are carving irregular nurbs surfaces. So within a few inches of length of a part there may be an oval, and then it may
loft down to an irregular shaped square then loft to an egg shape in section.
this is where our real problems show up. I think for the parts we are doing, that degrees are neccesary.
I had a local consultant tell me distance was what we needed to do, but that was obviously wrong.
all of your help and advice is greatly appreciated.
WH :)

You can do some wierd stuff by changing between linear ( ie; using steps per inch ) and rotary. I posted some info on an attachment for my Atlas Lathe conversion. Never finished exploring the use, saving that for when i retire i guess. ;)
Some day when i get energetic may even finish the rotary write up i started. ::)
RICH

I am not sure that this will be of any help,but here is what I have done with my rotary. I have set it up so that one revolution is equal to 36" . That allows me to use degrees by simply subtracting a decimal place from the inches. In other words, 90 degrees would be 9" and 275degrees would be27.5 inches. One degree would be .1" 10 degrees is one inch. Half a degree would be .05"
Does that make since? Helpful?

Does that make since? Helpful?
Hmm....stay with STEPS / DEGREE for indexing as suggested. You want to move 90 degrees you code G0 (or G01 with a feed rate) A90.0 ...whats so hard about that.
It "could" make sense if you were slaving the rotary axis and making coordinated linear moves around a 36" piece.
If you use a stepper you will find that the feed rate will vary / won't be true / is not linear......
I wouldn't make simple indexing complicated, but ,.....do as ya wish ;)
RICH

Rich: I guess that I was in a hurry to get the rotary set up to do a job and did it that way so that I could keep up with where I was with the axis DRO for a multiple pass. That was the only job that I have needed the rotary axis so far.
Will the axis DRO indicate position in degrees? I have to admit that learning Gcode for me is in a "as needed" format. I find myself having to relearn a lot of it if there is an extended period of time between useage. I have to relearn my cad programs if I don't use them often. It is a pain. Age has not been kind to me. I can remember the way I set my rotary up but would need to study to do it differently(the right way). I do need to set it up properly and write everything down so that I can pick it up again . The feedrate issue would be a good reason to do that. I may call on you for help later on. You are in Gainesville?

Mr C, your with your method, you could get some strange feedrate issues. Using steps/degree, and setting the part radius, you'll get a consistent feedrate regardless of the radius of the part.