Skipping steps and changing location

[srwtlc]

Hi all,

I have a problem that I'm also working with ShopBot on but I thought I would post it here also to get some input.
To make a long story as short as I can here is what I'm trying to do and the resulting problem.
I have a holding jig that holds 20 5" x 8" x 3/4" walnut blanks in a 5 by 4 array. Half of these blanks get a 1/8" deep recess and the other half get a 1/2" deep recess using a 1-1/4" bowl cutter.
I'm doing the 1/2" recess in four passes 1/8" at a time. These are done with a CR w/pocketing. While doing these recesses the motors are skipping steps resulting in a recess that gets further and further off center of the blank and a corresponding change of location. This happens only while cutting. I'm using a fresh bit and have tried various move speeds (1.75,1.5,1,.75,.5)
and router speeds (16,000-19,000) all with the same results. My jig is anchored well and not moving. I can rest my hands on the XY carriages and feel when it happens. This is a PRT96. I've had this skipping happen with a new 3/4" straight bit also making 3/8" deep cuts in 3/4" ply. I can't believe that the motors are that weak that they won't maintain a 1/8" deep cut without skipping. Anybody have any ideas?

Scott Worden
Timber Lake Creations

[peterk]

check your wires to the motors scott.i had a similar problem with one wire had a break in it that would only happen intermittently.could also be a motor.

[omnival@wctc.net]

If this machine is a rack and pinion, check the set screws on the pinion gears

[srwtlc]

I've checked all of those things once, twice, thrice, and then some. It's seemingly a force issue. I'm going to try another new bit from a different manufacture and see what happens.
Does anyone have a good way of checking the strength of the motors? What should the voltage/amperage be at the motors? What wires would you check that on with a multimeter?

[davidallen]

are you sure the pinons are still fully engaged in the racks? I've noticed that mine tend to move over time. I finally adopted the new method of tensioning the motors with a turnbuckle and that seems to work well.

da

[NeonBob200@cs.com]

Scott,
I'm having the same problem. Both my X and Y axis mis-step intermittantly even when cutting "air." It seems to always only fail when doing a jog move. Shopbot has sent me a new control box, a new driver circuit board for my new control box, a new Y-axis motor and a line conditioner. The motor was needed as it had gone bad, but my tool still mis-steps. I have a PRT96 which is only two months old.

Frank, at Shopbot, has told me there are 4 - 6 others that are having similar problems. I wonder if there are more?

I'm beginning to think there is an endemic problem in the driver board.

I have put a lift scale on my Y-axis and force tested it to 50-60 pounds in "move" mode and 20-30 pounds in "jog" mode before it fails. When I catch it failing in the middle of a file I can hold it back with one finger (about 2 pounds of force). It seems as if the drivers are not sending a constant flow of voltage to the motors.

Anyone else having a similar problem?

[edcoleman]

Scott:

I had a similar problem, except my Z axis was skipping. It turned out that it was a mechanical problem. Check all of your rails for squareness to make sure that nothing is binding up on you.

-Ed

[carol]

While we are on the subject of skipping......

Yesterday I bent a drill bit in the drill motor mounted in the A axis by raising it insufficiently to clear a hold down. Now the A axis does not raise and lower, except for a small amount at the begining and the end of the attempted move. I tried with both a MA and a JA command.

What did I hurt?

[srwtlc]

Hi Bob and Ed,

I finally checked back in here and noticed your messages. I've been so swamped with work and trying to get my ShopBot to work right (along with the guys at Shopbot)that I overlooked the forum a bit. I'll first answer Ed because that's the shortest one. Mine is square and free. Told you it was short. ;^) Now for the long one. Bob, I've done all the things you have and then some, short of taking the tool to a different location(more on that later). Mine only jumps under a heavier load(a 1-1/4" bowl cutter cutting 1/8" deep in 4 reps down to 1/2" deep making a pocketed rectangle in walnut). It always jumps to the positive Y when traveling X, and to the positive X when traveling Y eventually moving my recess further off center of my blank. I've tried all the things you have and also running it solely off of a battery backup unplugged from the shop circuits, and in the dark to see if the florescent lights might cause some electrical noise. Ted sent me some shielded cables to try and they acted like antennas and made things even worse. I could click the end of the cables to the ground at the back of the control box and make the tool jump and jerk across the table in positive values and topping the Z axis out. Talk about strange, I put those cables back in the box and covered them in a corner of my shop for fear they may get out and sneak up on me and strangle me when I had my back turned on them. I could pick them up and shake them out and get the tool to jump. Which leads me to the location issue. My shop is located approximately 150' from some "HIGH" tension power lines that go across the state of WI that emit quite a bit of stray voltage. If I could pick my ShopBot up under one arm and take it to a different location I would've tried that too. This alone makes my situation unique and it will do it every time I run the file. I can aircut all day long though and it won't happen, or if I'm using a smaller bit and cutting a file that both axis are moving simultaneously. You may be right about that driver board thought, though I'm sure that the guys at ShopBot know more about those boards than most of us ever will, but Ted also sent me a modified board that I put in last night and tried. Now I'm not a superstitious man, but I'm crossing all body parts, as I ran my file for this product last night and for the first time I made it through twenty blanks without a hint of a jump and seemingly very robust. I bumped the move speed up to 1.75"/sec with a router speed of 19,000. I ran out of walnut lumber and have more coming Monday so I'll put it to the test then. As for now I feel I might be able to hold my breath. Keep me posted as to how your doing, I think we need to work together on this one as at times I've had the guys at ShopBot rather perplexed. I've at least been the subject of conversation(or a cuss word) at the lunch table since mid December. ;^) That's the long of it.

[Gerald D]

Our PRT 96 worked fine since assembling it last year. However, last week it started to kick out of position when the router was switched on/off. Then I recalled reading about the electrical grounding somewhere and set about installing a good ground system (from my milspec navy days).

Drilled and tapped the z-slide, the y-gantry and the steel table (with x-rails). Connected all these points in series, as well as finally to the stud on the back of the ShopBot control box - total of 3 ground cables. Each cable had about 5 sq.mm copper area, with lugs crimped at each end. the paint was scraped away at each connection point. Now the "kick" is gone.

My reasoning is that the v-rollers had become dirty/corroded and were no longer giving a good electrical path (for which they are not intended.

The paint on the ShopBot is very good, and one cannot assume that the parts are all in electrical contact. In my case, we tack-welded the whole machine together, after all the alignments and calibration, and so we have definite grounding of all the parts.

Are all the problem machines grounded properly (especially Scott's near the power lines)?

[Gerald D]

Small omission! Drilled and tapped y-car as well - therefore 4 cables in series.

[NeonBob200@cs.com]

Scott,

Thanks for your input. Maybe the modified board is what I need. I have experienced your jumping and jerking problem but I think it is not related to my mis-stepping problem. I have my shopbot located in my neon shop. Whenever I use my neon leak detector tool it causes my machine to move in positive directions too. This will only happen if the machine and computer are both on and connected. My neon leak detector is essentially a hi-voltage cattle-prod that discharges a 10,000 volt arc over the glass tube. For however long I discharge the arc my shopbot will move - even though it is 30 feet away!

My mis-stepping problem is not related because it is always the case that I lose steps as if something is holding back the car, not adding steps as what happens when I use the leak detector (I never use my neon equipment when cutting files anyway).

My X-axis rail and control box are well grounded. Perhaps I should ground my Y and Z axis as suggested by Gerald.

[Ted Hall, ShopBot Tools]

This is an interesting thread ... and Scott and Bob's problems are ones that have been on our mind for the last month or so. In fact, we have about 6 tools that seem to be experiencing electrical noise problems to one degree or another. This is a bit of a progress report.

Our initial thought was that the problems were similar and that we merely needed to identify the source of the noise in one of tools, and the solution would fall into place. Unfortunately, as usually turns out to be true with an obstinate problem, it is beginning to look like each of these problems is different. Several were solved by improving grounding or adding a line filter. Others have not been. In my experience, tough problems usually also turn out to be multiple problems ... which means a good bit of diagnostic effort is required. Our additional problem in this case, is that we cannot duplicate the problem here and have not experienced it with our own tools. We've gone to the nearest tool with a noise problem, but it was not severe enough to provide much data.

One point for those who are following this thread. Electrical noise as an issue is only likely to occur with new PRT tools. These tools have a microstepping-motor-driver module which we purchase. It is a neat little driver, but because it is a standard package, it's control is via a method usually referred to as 'step and direction'. This basically means that when a signal is received on the 'Step' line, the motor will take a step. Because these drivers are designed to handle a range of speeds, they are very sensitive. The 'Step' line is basically a hair trigger, just waiting to fire a step off. The least little electrical bounce on this line will cause the tool to take a step. If you take a look at the board, you will see that there is filtering in place to try and rule out errant spikes. If the filtering were too aggressive, then true step signals would also be filtered out.

FYI ... the older PR rack and pinion tools have a ShopBot built stepper driver. This driver is not as smart or clever as the new one. However, because it requires continuous control and activation, it is not vulnerable to electrical noise. Thus, electrical noise has never been a problem with these tools (severe electrical noise can disrupt serial communication and a communication error will occur ... but no spurious steps). [BTW ... Carol, your accessory problem sounds more like the driver is bad ... better call in for a new one.]

What seems to have worked so far for Scott is a control board with more agressive filtering and some other fussing. However, Bob, I believe that we have sent you this same board (as well as a line filter) and it did not work for you?? I believe that we are going to have to push a little harder here to rule out your interference problem.

I find Gerald's discussion of multiple ground points interesting. It is indeed the case that the rolling axes may not be well grounded just through the the tool. Indeed, momentary breaks in the ground connection as the tool is moving around may contribute to electrical chatter. We'll take a closer look at this one ...

More generally, the overall problem is identifying the source of the noise. Typically it gets in over the power and ground lines and is a result of motors elsewhere going off and on. UPSs and line filters (that include ground filters) are helpful here. However, you will also note that a ShopBot is a moving antenna and it is possible for field transients to be picked up. Shielding is only partly effective here, and as you noted in Scott's case, not at all helpful for him.

[dan@burningimpressions.ca]

Just a thought ted maybe he should look for RF interference, close by such as CB, fire hall,Taxi,
any such with a transmitter or tranciever..

[dmdraper]

If Scott is that close to 765 kV "high lines", he has some serious circulating current in his shop (do your florescent lights stay on when you flip the switch off?:-). My neighbor has a short-wave antenna about 30 yards from my shop - I can tell every time he transmits because my shop to house intercom buzzes.

I agree with Ted's analysis - there is a lot of stray EMFs floating around these days. A single "cause" may not be leading to this effect.

[NeonBon200@cs.com]

Just so no one mis-reads my frustration:

The people at ShopBot have been very responsive to my calls for help. I know my problem is just as frustrating to them. I am sure we will find the cause(s) soon.

Today my tool ran a 92 minute part file without a hitch! Maybe I just need to stroke her a few times before I start a file :o)

[srwtlc]

I can agree with that! ;^) No mis-reads here either please.

[ajohns707@aol.com]

I too have a problem with one of the x motors jumping or skipping only durning a jog this has produced an interesting work station table. I have a surge protector and added grounding to the table. I'm going to mark the gear to see if it is slipping but I don't think that is the case. Interesting forum and problem. It has been fun building this and learning some new phrases.

[garbob]

Cell Phone Problem

I had a customer in last night while I was cutting a sign and he received a call on his cell phone. The sign was almost finished and as it was cutting the last letter it jumped down on the Y axis about an inch and a half and continued to cut the top of the S on the bottom of the S. I let it run anyway to see what would happen. When it came to finish by putting a vee groove inside the outside cut line it still worked okay and put the border in the right spot and at the right offset.
I also have had similar problems, but to a way lesser extent. This was caused by my next door neighbour was using his CB radio.

[wdusttodawn@aol.com]

I have been having a problem with z positioning on the new prt96.
the z axis keeps getting deeper every time it moves to the rapid plane and then back to the cutting surface. ex. cutting 3 pockets .5 deep
taking .2 apass first pocket will go .05 to deep
second pocket will go .1 to deep and the third pocket will cut .150 to deep. I have read the trouble shooting section in the manual and have checked all machanical on the z carriage and rack and pinion and all is working fine. then I looked into unit values. The z axis has a 24 tooth pinion. The unit values from shopbot are 733.386
When I use the unit values from shopbot, I test the z positioning with an indicator and jo blocks,
ex. I zero the z axis on the table, I program a move command z2.0 and run my jo block under the indicator. the current location will say z 2.0 but the actual distance will only be 1.981. then I Program a move command z0, the current command will say z0. but the actual will be -.2 and will keep getting deeper every time I repeat this exercise. so I changed the unit values untill I got the z axis to move the programmed distance.
the new unit values for the z are 743.0. now the z will move the progamed distance one to two times under the same test as before. then will float all over the place. It seems to move the z origin. I have run all the test in the book and tried some of my own and still cant solve the problem. If any one has an answer for this problem Please help
Thank you
Dusty

[jerryk]

Dusty,
I had a similar problem when I first set up my machine. Take a close loor at the jog and move speed of your z axis. I exceeded the jog speed of the z axis and this caused the z axis to incrementally go deeper. Slowing down the z axis jog speed solved the problem

[dusty]

thank you Gerald that did the trick