suicide prevention help neede

[endgrainguy]

I"m having massive problems with my new shop built CNC. I'm getting inconsistant results from day to day, hour to hour. I "discoverered" yesterday that my x axis screw, (a THK 20 MM d. 20 MM lead, four start, bought used in apparently good shape) was apparently mapped incorrectly, as determined by measuring a 24" run called for in MIDI screen. I had assumed it was in fact a 20 mm lead, as labeled. It appeared to be off by alot --about 2.7 percent. I did the math, adjusted the steps in motor tuning, tested it a dozen times with good results, and figured I had solved some mysterious issues i was having. Ran my first set of parts, with lousy results. Tool paths seemed inconsistant -a second program for a finish pass calced from same drawing didn't match up, and I was left pretty confused. All individual tool paths were fine, producing the desired size pocket. The problem was in initial referencing. I rereferenced beween programs, which may have been a mistake, but I should be able to do that, right? Today I checked the x axis again, and results were again way off -this time in the other direction, and I got it back to going the distance asked for by undoing most , but not all ,of the change I made yesterday. So now it seems I've had three different values work, and I"m sure this will stop working as well.
Another issue is that the reff X axis behaviour changes radically with speed settings of the axis. At higher speeds (200 IPM) it doesn't do its return trip -just clicks off and stops, at what would be an obviously very different position than if it were traveling slower. This I"ve found to be related to trying a higher debounce setting, which I did because my limit switches were tripping at moments of high resistance to the cutter.
Could I have a corrupted version of MAch (it came on a refurbished PC sold by an Ebay CNC stuff vender). Could the PC itself be the problem? I'm so frustrated and confused, and I have no experience with this stuff beyond the misery I've put myself through these past couple months trying to build a functional machine.
If anyone who is expert in these issues lives near Greenfield , Massacusetts, (I"m actually a few minutes away from there, in Colrain, I"d be happy to pay for a consultation to try to figure out my problems. I'm a full time woodworker, and all other work has ceased as I"m consumed with getting this beast producing for me. Thanks!

[MrWild]

Sounds like your lead screw is floating. Moving in the bearings, bearings moving in the housing, or your nut is floating.

[rgoldner]

It's possible that you do not have your motors tuned correctly and are missing steps. It could also be noise adding steps.

[andy_ck87028]

Endgrainguy

Being in Australia, I'm a long way from you.

Are there any men's forums in your neck of the woods?

Rather than CNC expertize, your problems might well be solvable with the help of a bloke with a solid background in general engineering problems.

Being in the company of someone who you can share the problem solving with will take a lot of the emotional intensity away.

Take good care of Endgrain and the CNC problems will get solved.

Andy

[ger21]

How are you homing your machine at 200ipm??

Homing must be done at slow speeds for good repeatability, and preferably without using debounce. Debounce causes a delay in switch activation, which can cause inconsistency as well.

From what you've described, though, I'd also guess that you have a mechanical sloppiness somewhere.

[Gir]

There's probably nothing wrong with your PC. Double check to make sure the lead screws are moving back and forth, and don't run your machine so fast that it misses steps. Try running between 100-150 instead of 200. Also, to see if it's an alignment issue that's making you miss steps, decouple the motors from the screw and try moving the axis back forth. If you feel it dragging along in certain areas, your screw is a bit screwed up (budump, cha), and if it's at the ends it's an alignment issue.

Worst comes to worst, maybe you should try buying a new screw and nut. Sorry to hear about your problems, I'm sure you'll get it figured out soon!

[endgrainguy]

I guess it seems more likely to be electronic than mechanical. If it were mechanical, then somehow the effective lead of the screw was changing every so often, but was very stable until changed again, or at least stable for awhile. That doesn't seem possible.I'm running the axis way slower than its capable of, jogging it at around 100 IPM, when it runs smoothly at up to arround 300, at least -- (with a 31" travel there's not much use for high speeds). I am stepping at 1/25th microstep, because it's such a coarse lead (20 mm). I'm using Keling drivers and steppers and BOB, the 640 OZ/inch motors and KL-6050 driver on x and y, with the KL-8078 on Z.
Now noise, which I have no real understanding of, seems more likely. It's as if the pulse rate is slowly, or periodically changing by some definite percentage of the expected rate, around 2 or 3 percent. I did use sheilded cable for limit and home switches and for steppers though, and the x axis stepper doesn't run in a cable carrier at all. Any suggestions on how to diagnose "noise' issues? If it were lost steps, again, I don't see how I could get consistant results at times. Lost steps is more random, right? My profile tool paths meet up nicely, and I can send out a dozen sets of GOX0, GOX21, commands, and it gives perfect results on both ends. And then hours later, they're way off. I'll check my other axises. The Y has a funky used ball screw I'm replacing anyway, but Z is very nice fine pitch ball screw.

[Gir]

Noise is rarely a problem, but since you're using shielded cable It's extremely unlikely. Working for a while and then suddenly being off by a good bit seems like missed steps, especially since you described the clicking noise (those are missed steps). Try running it at 100 IPM for a while and see if you still get skewed results.

[endgrainguy]

Thanks for suggestions. I'm not in practice homing at high speeds , but I was trying to determine if the distance traveled in response to a command was changing in response to setting the axis speed higher or lower in motor tuning, and i homed the axis to start each check, which is how I noticed the erratic hopming at high speed with debounce up high (2000). I can understand why that would be the case. I ramped up that debounce setting to try to stop the limit switches from tripping when I was cutting, which they did every few minutes, finally disenabling them, and i forgot that i had experimented with debounce. Doesn't address my unstable travel issues though. You guys all seem to think it's mechanical, and I guess if y and Z are stable over time that would sem to point that way, but I still don't understand how a screw can demonstrate to have an effective lead of about 19.6 MM, and respond perfectly to a dozen 21" long travel commands, and then a few hours later, start behaving as if it were a 20.05 mm lead (these numbers just approximations, but close). Is it possible for missed steps to be so consistant?

[mcarvey]

I've only used servo's before so I can't say how common/consistent lost steps are. It is a little tough to diagnose your system since there are many places where things can go wrong. You need to systematically find out what is working correctly and only procede to the next step if you're 100% sure that it is working the way that it is supposed to. He is how I would procede if I had a similar problem.
1. Check the mechanicals. Mount a dial indicator on some place solid, and clamp a pair of vice grips (being careful not to chew up the surface) on the end of the ballscrew (or motor if you can do that) Make sure that one revolution of the ballscrew corresponds to a certain linear distance. Test it with a load on it. If there is any backlash it is coming from either the nut/screw combination, the coupling between the motor and the ballscrew (if you've clamped onto the motor), the AC bearing between the screw and your table, or a combination of some of the above. Once you are convinced that rotation of the motor can be consistantly be mapped into a linear travel you can cross this off as a source of your problems.
2. Make sure the electrical system is doing what it is supposed to. This is a little more difficult to do because you are testing the interaction between mach, the motor drivers, and the motor (and its loading conditions) Most people don't have a convenient way of making sure that mach is correctly outputting pulses so you may want to think about just wiping the computer, installing new OS, install fresh version of Mach. If you computer is not doing the correct thing, it will probably take more than 2 hours to diagnose and be very frustrating. Better to start fresh and not have to worry about that.
Once you have a fresh install, make the stage go back and forth at a slow speed (like 10 ipm or so). If it doesnt' work consistently at 10ipm, it probably won't work consistantly at 200. If it does work, start increasing the ipm to 20, then 30, etc. If it works consistantly at 60ipm, but not at 75ipm you'll have a much better idea of you're problem.
How have you grounded the shielded cable by the way?

Hope this helps
-Matt

[cut more]

Matt,
There are a number of guys in southern N.H. who have experience with Mach.
In fact there is going to be a meeting sunday I think. I will e-mail the head of the group to post the time under the mentor forum.
If you r machine is vibrating enough while cutting to trip your limit switches then either you have very weak springs on those switches or you are working the machine pretty hard, which could lead to lost steps. I can't explain the varying inaccuracy over time unless, over time something heats up and causes binding in the way/guides. You didn't say what type of machine it is (moving gantry/moving table) but does everything move freely? how about after the machine has been running for a while?

Just a couple thoughts,
Cutmore

[mactec54]

Hi endgrainguy
You said the limit switches are tripping how did you ground the sheild for your wiring if
they are tripping then you have a wiring problem they should not trip if done right
once a limit is tripped you should rehome the machine