Problem- First cut problems: Problem Solved!!!

[JeLC]

I got my DIY system up and running yesterday and made my first cuts this morning. But I've got problems of which I'm not sure what they are nor how to resolve.

First I'll say I was running Linux/emc2 and had latency problems. Put in a new video card which helped a little, but not much. I changed to a Win2K system running the trial version of Mach3. I like Mach much better and will purchase it as soon as I need to cut over 500 lines.

But to my problem, I'm not getting anything close to a repeat cut between stepdown passes. I'm using 1/2-10 single start acme screws (and they came new with some backlash). I don't have any idea of what losing steps would look like on an actual cut. I also have no type of anti-backlash devices on my screws. (which I know I've got to fix)

By viewing the attached image, can anyone tell if it's backlash or what is my problem. I used an 1/4" upcut spiral endmill on MDF, so I know I used the wrong type bit, but the roughness of my cut is the least of my problems right now. I cut a simple area clear around a circle (in three passes), and then a profile cut around the outside (in four passes).

I also quit smoking about two weeks ago and I'm ornery as hell. And these CNC problems aren't helping.

[OSWALDO]

g'day mate,
I am no expert on cnc machines, have built one though, anyway, what speeds were you cutting at and what pass depth. Also check that backlash you need to fix it first befor cutting any more samples. Anyway i would at a guess say your motors are loosing steps. best way i have found is to zero the machine, cut in one direction with three passes or more, then send the machine home but disable the auto zero function, this will tell you if the machine is jumping around..
hope that helps...
oswaldo

[Hirudin]

Originally Posted by JeLC ... I don't have any idea of what losing steps would look like on an actual cut. ...

I think it looks a lot like your picture. It looks a lot like my first cut.
It looks like the machine is losing steps at some point when it's trying to move right. So it thinks the bit is farther to the right than it really is so it ends up cutting off the right edges of the part.

Keep in mind that I am also a beginner. I bet one of the things I suggest below will work, but my suggestions probably aren't the fastest way to fix your problem.

If I were you, the first thing I would try is to reduce the depth of cut. In fact, after I saw similar results from my first cut that is exactly what I did. I figured I'd play it safe and reduced the depth of cut to 25% of what it was before. So if you are doing 0.25" now try reducing it to 0.0625".

Maybe the acceleration on your motors is set too high. You could try reducing it a little bit and see what happens. I would personally go to about 75% of what it was before. So if it's at "5" now try reducing it to "3.75". I think it's actually possible the acceleration could also be set too low. It's counter intuitive but too low of an acceleration can also be a problem. I don't remember what it's called, but stepper motors have certain speeds where they have very little power (and thus lose steps). Using the correct acceleration can make your motors pass by these speeds quickly so they don't cause any problems.

The velocity of the motors could be too high too. I'd probably go to 50% of what it is now then work my way back up.

Another thing is that it might be sticking/binding somewhere. To find if this is the issue maybe you could try moving the axis back and forth at the highest speed you can. Go back and forth and keep increasing the speed until it either wont move at all or it gets stuck in certain places. If you can find a certain part of the axis that is sticky it might be easier to figure out what is happening. It's possible the screw(s) are just a little tighter at that spot. Making the machine move back and forth on that spot for a while (maybe about an hour?) would probably loosen it up.

[JeLC]

I did not read far enough into the documentation of Mach3 and I never did a 'step loss' test. I got everything running and couldn't wait to cut that first peice. When I set up my toolpaths, I did decrease the depth of cut. The software had the tool cutting .2" deep. I changed that to .1" on each pass.

Also when I set up my motors, I set them as high (velocity and accel) as the system would allow. After getting away from my machine and thinking about the problem, I believe I have both backlash and step loss. I'm going to put some type of antibacklash devices on it, and then retune my motors. I'll get there, it's just a matter of playing with settings and wasting a lot of MDF.

[ger21]

Originally Posted by JeLC Also when I set up my motors, I set them as high (velocity and accel) as the system would allow.

That sounds like the problem. Try doing a series of fast back and forth moves, and make sure it returns to the start point at the end of the series. Use this method to find a speed and accel where you don't miss steps, then back off another 15-20%.

[JeLC]

Turned out to be a loose set screw on the coupler between the motor shaft and the lead screw shaft on the X axis. The acme screw was doing some extra spinning when the motor reversed direction. I tightened it up, plus the ones on the other two axis, and now everything is cutting fine.

Chalk that up to lessons learned.

[BobF]

Glad you found it. Had I seen this thread earlier, I would have posted to check those, as that is exactly the problem I had.