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where do I start??
« on: October 01, 2007, 05:52:53 PM »
ok this machine has got me stumped. lets just start out that I have a little compact5 cnc Ive been tinkering with for about a week now. I have it up and running on a hobby cnc driver bored and two nema 23 japan servo, step motors. It is a metric machine with 8mm diam leadscrews. not so sure of the pitch because all attempts to calculate the pitch leave me way off mark with actual movement. now to get the machine to move reliably, exactly one inch. at 1/4 stepping with a 2.5/1 pully ratio. Ive ended up with 20360 steps per inch on the X axis and 20325 on the Y. dont ask me why. its not back lash because all measurements are going in 1 direction. Now that my first mystery. nto really a problem because if it moves the distance I tell it too than fine.
Now my big problem is this. I touch off my part. set the part zeros. run a program and before my first opperation is done Im like 1/10th of an inch off tolerance. and not in the direction thats common. my diameters are ending up smaller so by all accounts Im gaining steps. now Ive heard of loosing steps but how is my tool moving further than it is supposed too. Ive run axis test to move slide all over to see if I loose my position and nothing. it only seems that under load somehow my position in the X axis shifts about -.05''
where do I start in trying to figuring this one out.

vmax549

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Re: where do I start??
« Reply #1 on: October 01, 2007, 06:32:38 PM »
Looks like the bed screw is 10TPI. Can't see the traverse screw to tell.

So 200x4=800, x2.5 = 2000 x10 = 20000 steps per inch.


Then set up an indicator and confirm a gcode move of 1" equals 1" on the indicator. Do it for both axis to confirm the steps are correct. make sure that x axis is in the correct mode.   If all is well there then look at noise causing gained steps.

(;-) TP
« Last Edit: October 01, 2007, 06:37:36 PM by vmax549 »
Re: where do I start??
« Reply #2 on: October 01, 2007, 07:30:00 PM »
well If its a 10tpi leadscrew it sure is an inacurate one. when I set my controller to 20000 steps per. I get a total of .981'' of movement on the Z when I tell it to move an inch.
as for the noise. My motor wires are sheilded and grounded but the hobby cnc board has the breakout and drivers all in one unit so I dont really have any low voltage step direction cables to sheild. where else could I be getting noise. should I move my DC Motor controller and power supply for that matter to another box. to a seperate box? what else is there to try and eliminate noise. or figure out if that really is my problem


vmax549

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Re: where do I start??
« Reply #3 on: October 01, 2007, 08:03:27 PM »
Are you sure of the reduction, did you count the teeth on each pulley???? The screw I have on mine measures 10 to the inch or extremely close. Are you seeing the error on all axis or just x.

So you are saying when set to 20360 steps per inch you get a movement of 1 inch

Set up a test block and zero the indicator. Make 10 1" moves back and forth by way of gcode then check if the indicator comes back to zero. Then do 20 1" moves back and forth and recheck. Is is gaining , loosing or stay the same??

(;-) TP
Re: where do I start??
« Reply #4 on: October 01, 2007, 08:43:09 PM »
yep Im sure. like I said. same pulleys on both axis( a 16 tooth and a 40 tooth) same screws and I have 20360 steps to get it to move 1'' on X and 20325 on Z. Ive been testing all sort of test programs with my dial indicator. moves perfectly 1 inch in any direction. also I cant get it to lose or gain any steps, motor on or off. that is until I run it under load to actually cut a part. then I end up about .05 off position on the X. but Im  only moving in a negetive direction for the program and somehow am ending up .05 too far not short? so Im gaining like 1000 steps. dont really understand it at this time. perhaps it will come to me in my sleep. as for the feed screws. I dont know what the deal is there unless they're very worn. but again I dont have any backlash problems so that cant be it either. ???

Offline docltf

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Re: where do I start??
« Reply #5 on: October 01, 2007, 10:07:51 PM »
Guyname

          i think i know what you are up against.i had weird backlash 2 weeks ago on a machine that was good for 10 months.
          screamed for help ,no info.my problem was in the drivers ,even though everything looked perfect.set x steps perf
          set y steps perf cut square no good.check your set screws on your gears first then come with the run down on
          on your comput.pci,agp onboard stuff added cards you know the routine.

bill
Re: where do I start??
« Reply #6 on: October 01, 2007, 10:14:47 PM »
well as far as backlash is concerned I have 0 on the X and like .005 on the z(tolerable). but as for everything else you just abreviated the heck out of I have no idea. what was wrong with your drivers?

Offline docltf

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Re: where do I start??
« Reply #7 on: October 01, 2007, 10:34:11 PM »
i have backlash so i have to use the backlash feature.but your missing what i am talking about.i had my x set so it would
travel 0 to 1' and back perf and the same for the y axis.but when i cut out a square x was .006 and y was .004 off.the
problem was in the device drivers. i had to uninstall them and then reinstall them in safe mode.not the regular way.

bill
Re: where do I start??
« Reply #8 on: October 02, 2007, 07:13:30 AM »
ok so you are saying it was your software then. not the stepdrivers themselves. so you what reinstalled mach3 in safe mode on your computer. I should definately look into the comutor then anyway. Im running a pretty old pc that can only run mach at 25000khz or else i start tripping driver watch dogs and things start to get real weird. so it could be that. I thought it would be ok to run this dinky little lathe but I guess not. will check that out.

Offline jimpinder

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Re: where do I start??
« Reply #9 on: October 02, 2007, 08:00:46 AM »
I don't think  you are missing or gaining steps. You seem to have done just about every check in the book, and got everything sorted. Dont worry about the number of steps per inch -  35 in 20,000 is a tiny percentage. I am not sure whether you are talking about a lathe or miller, when you talk about the diameter being 1/10th inch out.

I think your problem, with respect, is that you are just not getting zeroed properly at the start.
With a lathe, I take a facing cut, and zero the Z axis. I then move the tool along the work a little and jog the tool until the tip just touches the work. I then measure the diameter and set the X axis to half of that ( I am in radius mode).

With the miller, touching the work in both axis is fine, but you must then set the axis to the diameter of the tool, not to zero, then bring the tip down and touch the top of the work to set z (usually to 0). If you are using a 1/4 inch cutter this could account for the problem.

Make sure on your tool table that any reference to diameters and lengths are erased and none of the check boxes are checked for tool offsets.

I cannot see that your computer can be at fault, because if everything is OK while the thing is running around, then it is not going to change just because you are cutting something - how does the computer know ???

If you are doing what I have said, then I apologise - and have no idea where the fault lies.
Not me driving the engine - I'm better looking.