Dropping the Z - Gouging the table

[Jerry In Maine]

I have a bit of a problem that's seemed to come from nowhere. Not blaming it on VCP4 - just asking here and in the MACH3 forums.

Running VCP4 and MACH3, and a HobbyCNC controller and 125 oz. steppers. Machine is homebrew but has been working well with no problems.

Was cutting block text with a 60 degree bit. Had another toolpath set up to follow and clean up with a .125 end mill.


I was 6 letters into the string of text and suddenly the Z dropped what looked about -.5" and went through the work. Letter was a B and it looked as though the X&Y were taking the bit to the lower-left corner of the letter.

Reset everything to 0 and tried again - this time it happened at another place in the sign. Same symptoms - Z dropped about -.5". Note sure if the machine meant to do this or it suddenly lost enough steps to put it that far behind.

I was running slow. Have run considerably ster with no step loss. Cables seem tight, etc.

Using the MACH 2/3 ATC post - no reason to think the problem is here though.

So - I have the type of problem I HATE to try to troubleshoot: Random and intermittent.

Any suggestions?

[darde]

check for static. I had a similar problem ever time the dust hose contacted the gantry frame. It would step down the spindle speed 2 or 3 times during a job.

[js11110]

Also, i've seen this problem with electrical interference. The CNC mills at work would stop in the middle of a cut when dripfeeding a long program whenever we were welding aluminum (which takes alot of juice) in the bay next door. We solved the problem by putting these "barrels" on the cord that went from the computer to the machine. I can't remember what they're called.

Jason

[GripUs]

If you have your computer connected to a network (like I do) or the internet, try disabling the network before you start to cut. I had a problem with my "Z" running down through the table. Techno-Isel tech services suggested that fix and it worked.

Joe

[GroBru]

I have had the same problem but my x or y axis cut a path across my work for no reason and the longer the program the worse it got my problem was the usb key i was using. I downloaded the g code directly from the usb key or flash disk if you prefer to the routing table (Lexar usb key) and now i transfer the file on the computer running the program and no more problem.

[CRFultz]

Jerry make sure it's not in the code first before you start chasing stray electrons.

Also your computer needs to be optimized to run mach3...or at least it should be.

There is a optimize file on the Mach website...at the bottom of the downloads if i remember right.

If all thats good then you would need to isolate the interference problem.

make sure your stepper wiring is at least 1 inch away from any AC line.

Shielded wiring is best, grounded at the drive end only.

Use a line suppressor on your AC line into the power supply....if you can get your router on a different AC circuit then the power supply, that would help as well.

Hope this helps...
Chuck

[Jerry In Maine]

Ok -
Tried the file again and it gagged at the same spot. I'll attach it to this post.

When cutting a 'B' it looks like it was about to make a rapid movement to the lower-left hand corner of the letter. Instead of lifting the Z it just cut across the letter. This happend at/about line 3051. I let it go since the work was already spoiled. At/about line 3050 it plunged deep, and at line 3067 it made an odd move across the letter again.

As mentioned - thius was the second time the problem happened at/about the same location on the same file. Another similar incident happened yesterday on another file.

The machine has run great until now. I have ferrite beads on the spindle leads to help suppress glitches. Also have them on the motor leads.

Removed the vac hose (cough-cough) and no change.

Loud cursing has no effect outside of scaring the cat.

I was running slow but have run faster prior w/o any lost steps.

I do have the machine on a network and will try disabling it to see if it helps.

Damn...

[Tony Mac]

Hi Jerry,

The CNC toolpath code looks fine and we have run it through a 3rd party product to simulate the moves, and it runs correctly.

The Feedrate in the file looks very quick - 12 inches / sec and this could well be causing the problem?

If you run the job at a slower feedrate does this make any difference?

This type is problem is often related to static or PC > Controller communication issues, as the other guys have mentioned.

Tony

[lockeyone]

I had this happening in my Y axis and what I finally did was to tune (slow done) my motors slower in MachIII. I was trying to push my motors too much I guess. Hasn't happened since and I really haven't noticed the machine running any slower.

[garyb]

Jerry, I ran your file it runs just fine even at 47 mins, I would look for mechanical problems such as loose pinnons etc or electrical interference as the others had suggested.

This is not a feedrate problem, feedrates in mach are IPM or RPM not IPS, Jerry's file is running dead slow at 12" per minute.
Don't believe this is a network problem either, else you would get a script error at file start up rather than loosing your z axis in the middle of a run. Good luck with it
g.

[wdenis]

Hi Jerry,

Hope you don't mind a relative newbie chiming in but your problem brought back a bitter memory that might apply. While cutting a large poem and almost done (200.000 lines of code) my screen saver came on and my X axis cut the most beauyiful .020" furrow right across the poem. What I created that morning was scrap and a lesson well taught. I set the screen saver to "none" and that problem was no more.

Wayne

[lockeyone]

Maybe I should explain a little more on the issue I had with my Y axis. I could still run a file slower and have the issue. I think it had to do with my motor tuning in the effect that when it went to do a rapid move I would loose .5 inch in my Y. After retuning my motors never saw the issue again.

[Jerry In Maine]

Ran the file a few more times - allowing the bot to retrace the original kerf in the stock.

First time the Z dipped inthe same spot as the prior two times. Second time it dipped in the letter prior to that - but just before it did I heard a quick buzz. I think this was the Z motor to feedscrew coupling spinning. Not sure - tonight I plan to take the Z axis apart - but I think it spun until I caught the setscrew again and this accoounts for the DRO indicating +.5 when the bit was buried into the work well into negative territory.

Odd that it kept happening at the same spot initially - maybe it was a combination of movements that caused a bit of a bind - at least enough to cause the couplings hold to not be suficient to keep the motors shaft from breaking loose


Feed was ceded to be 12 in/Sec? Wow. Didn't realize but will fix. I usually set the speed via MACH3s interface - and it was at 12 in / min.

[Jerry In Maine]

Yep - loose Z coupling.

It would apparently spin a few turns under heavy load - but didn't apear loose on quick examination. Set screws were tight, but when I removed the couplings I saw that they had cut slight threads in their opposing surfaces.

This was the only joint where I didn't grind flats on both the leadscrew and motor shaft (forgot )

So I added the flats - dripped on some threadlocker, tightened everything up and everything is fine now here in Downeast Maine.

Moral(s) of the story:
Things can be loose on the inside even though they seem tight on the outside.

Setscrews can't be relied upon unless they push against a flat - so if you don't have flats ground on your shafts you should consider adding them. An abrasive wheel on your Dremel makes quick work of this.

Thanks for all the help!

Jerry In (partly cloudy and temps in 70s) Maine