Need Help!- Random Z movement during job

[Lindzr]

Hello - I hope someone may be able to help with a problem I am having with my quasi home built machine

Issue: when cutting a job the machine will be cutting fine and suddenly move randomly (down) on the z axis messing up the job. Has been happening intermittently for months but much more often lately. Issue seems to be random, as I can use the same file and manage sometimes to cut the job complete, and even if it does mess up, it does not seem to do so in the same place. See image attached. Item is the bottom of a skateboard mold - you can see on the left my rough cut with 1/8 stepover, and the right is 1/32" stepover finish pass with 4 visible screwups. All are from the same file. I have been trying for 2 days to cut this part with no success and I am going crazy.

Machine was originally a Practical CNC 4 x 8 servo router. It never really worked consistently since I got it a few years ago (this random issue) - Piece of junk really but I was a noob when I bought it and they are out of business so there is no support either. Most of my cutting jobs are simple 2d MDF ski and snowboard templates and I managed to limp my way through in spite of the finicky machine. But I have recently started to cut skateboard molds which take a lot longer on the machine (some are 2 or 3 hours) and the random z issue happens almost every time I try and cut now, making it impossible for me to complete the jobs anymore.

I read and learned as much as I could trying to find likely causes and came to the conclusion it was probably an interference issue since it was random and not repeatable in the same place.

First try was to re-cable the whole machine with top quality cables. Didn't help. I then replaced all of the connectors on the control box in case of a weak connection there. Didn't help. Over time I came to feel the machine was useless for any job longer than a couple of minutes - even some really small jobs would mess up sometimes so it pretty much sat there gathering dust as I felt it was unlikely to get through a job without fuc#$ng up

I got pissed off and decided to replace the controller and all the motors/cables, use Mach 3 software instead of the WINCNC pci card the old system used etc. Basically replaced everything except the frame and gantry which are also crap but that project is for another day. Because I was still convinced it was likely some sort of interference issue I opted to build it with stepper motors this time with the understanding they would likely be less prone to interference (someone correct me if I am wrong here).

I know that is somewhat of a downgrade but don't need a fancy machine - just one that will cut consistently without these random screw ups and I thought this would lick the interference issue

WRONG - the machine still does the same thing much to my disappointment (severe understatement #$%$&*^!@)

Yesterday I swapped to a different computer in case my computer at the shop has some sort of random issue but the other computer did not help.

I'm at wits end and have no idea what to do now so I thought I would post for ideas. I have literally replaced the whole machine now pretty much and still have the problem.

I suppose it could be the g-code but I cannot understand why it would not be a repeatable problem in the same part of the code if it was? Any thoughts about this?

FYI - the files are drawn and g-code generated in Bobcad 21. Simulate fine in Bobcad and do not appear to have any obvious funky geometry when I "generate geometry from g-code" which I often do as a quick check to see if any obvious errors show up.

Anyway, I would appreciate any input. I can post the .tap or .cad files if someone wants to look

thanks in advance!!

Lindsay

[Alex_Cole]

upload your gcode file as we can check that pretty quick and rule that out for you. When you setup your cables for the encoder and the stepper motors did you ground the shield in the cable to one end, both ends, or at all?

If you grounded your motor/encoder cables make sure you ground them only at one end as in servo system especially you can make a ground issue worse if you only ground it at one end.....at least from my experience anyhow.


AC

[austin.mn]

Is there any chance you are running a VFD for your spindle?, or even in the shop?

If so, and it isn't isolated you will run into this type of intermittent issue. I had similar issues the first time I ran a VFD in the shop. It drove me nuts with all the intermittent issues.

If that is indeed the case, you can get around it by putting the VFD on an isolation transformer.

[BobF]

What type of steppers and drives? Some have had issues.
I had an issue on mine which was a slipping motor coupler. Check that.
Do you have limit switches? Do they ever trigger on there own? Likely a noise issue. Somtimes you can fix this with a small cap as a filter.

[Lindzr]

tap file attached. Thanks for looking at it and making any suggestions. Was generated in Bobcad using the "generate tool path" command on a surface I created with the contours of the skateboard mold

Cables were pre-fab from the vendor I bought retrofit kit from. http://www.lowcostcncretrofits.com/7...persystem.html. I am not an electronics guy so I wanted a plug and play system. This kit seems pretty solid. Well organized, clear wiring instructions etc. I am assuming someone on the other end knows their grounding and whatever interference issues would crop up since they sell them all day long and I don't know my electronics well enough to second guess anyone.

That said, anyone with suggestions on what I might check to minimize any potential issues with cabling and grounding would be appreciated. I would rather way overdo any wiring and do whatever I can to eliminate possible issues

My machine uses a porter cable router, no spindle. That said, would any VFD or spindle settings in Mach 3 cause issues without the actual hardware attached? I more or less configured it using settings supplied by the vendor so perhaps there are things on or enabled that should not be. I assume you are suggesting the actual hardware could introduce noise. The guy a few doors down has a machine with a spindle - how close would the machine have to be to introduce whatever noise?

Not sure about the motors or drives as the system was pre-fab. Could probably find out easy enough. This is the motor code on the data sheet I have 60HS78DE076

I checked the coupler - seems OK and I have cranked it tight a few times. The amount of error on most of these screwups is 25mm or more down in the z axis so if it was just the coupler slipping I think it would have caused the it to completely disengage as it only grabs about 12mm on each end

Having been standing right at the machine when this random z issue happens its more like it just randomly plunges when it is not supposed to and I run for the emergency stop

I do have limit and home switches. Currently I am still testing the machine and though all of the switches are wired to the controller and enabled in Mach 3 only the x and y home switches are actually installed on the machine. The other cables/switches are just coiled up beside the control panel awaiting installation. So far there does not seem to be issues with the switches that I know of. I would think that if a limit was triggered the machine would stop? In my case the program keeps running but the z positioning just drops when it is not supposed to - the router keeps mangling the part until I stop it or it stalls. Would caps or other filters be warranted anyway?

As I say I would rather over do it and get rid of this issue

thanks again for the input

Lindsay

[jalessi]

Lindsay,

What brand stepper drives and breakout board are you using?

Have you optimized Mach3?

Do you have virus protection running in the background on the PC?

The Mach3 computer should be a bare bone version without virus protection or any other background services or applications running.

Jeff...

[Lindzr]

As I said below I am not sure on the brands of the motors etc as it was a prefab package.

I will investigate tomorrow when I am at the shop and report back for opinions on the components.

The computer is reasonably optimized - I am computer literate so it is pretty well set up. no virus software, minimum services running though I have not gone through and uninstalled every app/shut off every possible unnecessary process. Definitely not anything heavy running that I know of. I will go over that tomorrow as well.

[jalessi]

Some Stepper Drives require the Sherline 1/2 Pulse Mode to be enabled in Mach3.

Symptoms like you are describing have been cured with this fix.

See attached .JPG


Jeff...

[Lindzr]

Forgive my ignorance and these additional dumb questions on wiring and shielding. I had been under the impression a stepper system was less prone to noise but it seems like from some of the posts so far that may not necessarily the case -

what gauge and type of wire do people recommend for smallish stepper motors? From the posts I found it seems that 22 gauge is common. On my machine the Motors are 8 wire NEMA 23 (z) and 34 (X & Y) where they are paired and joined to 4 leads on the cable as per my suppliers instructions - will check on the motor brand tomorrow

Maximum cable length? - the supplied motor and limit cables are 22 gauge, shielded, and 20 feet from controller to motors to allow room to run through the flexible cable runs on the machine and place the computer a few feet from the machine (length was at my request slightly longer than they usually supply - I was told 20 ft should be OK but not to exceed 25 feet?). My serial cable is 25 feet - 6 foot was almost too short and I had a 25 footer so I used that - bad idea?

There is a splice in each motor cable about 8" from the motors where the cables are joined to the wire from the motors using a small terminal block that was on the supplied cables. I just wrapped that with electrical tape. Would it help to shield or change that configuration to better reject any potential noise.

the flexible cable run on the gantry that has the x and z axis motor cables and their limit switch cables also has the power cable from the router running through it - bad idea or is this OK?

How about the actual power source for the controller, computer and router? - all currently plugged into a basic power strip on the same 20 amp circuit - router is not plugged into the strip but is on the same circuit. Would separate circuits and or UPS power supplies etc be helpful?

thanks again

[Lindzr]

I will inquire with the vendor if I they suggest enabliing Sherline mode. I did see that mentioned in the Mach 3 setup video though did not think it applied to my machine as it seemed to be working ok on the initial simple jobs I tried

[jalessi]

I believe you meant 25 foot parallel port cable.

A 25 foot cable between the computer and stepper drive is 15 feet to long.

Jeff...

[jalessi]

The Sherline Mode only takes a second to enable and may surprise you.

It wont hurt that's for sure.