gantry control problem

[chadsami]

DHC with old software, worked for 12 years. Gantry motor sometimes stops. Won't return to same place... every trip top to bottom it is over half an inch off. I initialize... same thing. Cleaned connectors on the cables coming out of motor. Restarted computer. Still drifts 1/2". Called Pcam and they said I most likely have an encoder failure and that I should just buy the upgrade kit for 7000... This flu thing is killing business. No money. Any help PLEASE... thanks,
Chad

[rdj357]

The upgrade kit would be the cadillac solution for sure but like you say, this economy is going to be touch and go for a bit.

I agree that it is likely an encoder issue. You could order a replacement motor with encoder for the gantry for around $300 to see if that solved the issue. Unfortunately there is a slight possibility that it could be a wiring harness or controller issue. If the motor solved the problem and you decided to do the upgrade later you could swap your old motor back on when you send it back to them for the upgrade and keep the newer motor for a spare as it would work on the new setup as well. Just a thought.

[chadsami]

Thank you Robert for the reply. I will order a new motor, but before I do that, do you think it's possible to open up the old one and see if something is messed up? I did notice the wiring boot on it was loose, so dust may have been getting in it for a while.
Thanks for your help. Every penny matters at this point.

[adbuch]

If you have the expertise, it would certainly be worth the effort to take a look. But first I would consider swapping your carriage and gantry motor/encoder assemblies to see if the problem follows the suspect motor/encoder. If the problem follows the suspect motor/encoder, then I would agree with Robert that you most likely have a bad encoder. That could rule out a bad harness or bad controller. If you determine that you do indeed have a faulty encoder, you might consider replacing the encoder only.

optical encoder (w/ instructions)

500 cpr encoder with index pulse. Encoders are mounted on both the X and Y motors and report motor position to the control box. Encoders include allen wrench for installation. Replace if damaged or otherwise dysfunctional (not tracking motor position accurately or at all.) The old encoder is usually broken during removal.
Part #: D-194
Price: $98

David

[chadsami]

Thanks David, I'm very good at taking things apart and then throwing them away. Is that a part # from Plasmacam or another source?
When you say swap the assemblies, do you mean the entire motor or just the wires?

[adbuch]

I mean to physically move the motor/encoder assemblies. Remove both the gantry and carriage motor/encoder assemblies, then install the motor/encoder that you removed from the gantry into the carriage position, and install the motor/encoder you removed from the carriage position into the gantry position. You are not moving the wires (harness connectors) - only the motor/encoders. The idea is to see if when the "bad" motor/encoder is moved from the gantry to the carriage - does the gantry now work correctly and the carriage is the one with the problem. That is the part number from PlasmaCAM. It is available for purchase directly from the Owners Community Forum site. They list two different part numbers for the carriage motor and gantry motor. I'm not sure if there is any difference between them. Robert would probably know the answer. So before you actually swap the motors, I will check with Robert or John Derby on the PlasmaCAM Owners Community Forum to see if they are interchangeable.
David

DHC2 parts 1.jpg

DHC2 parts 2.jpg

[Joe Jones]

You have been running your table for 12 years?

First, check both drive belts, to see if they are tight and they have ALL of their teeth.

Next, leave the motors where they are, and swap the cables. Then test the machine. Does the problem move to the other motor, or does it remain a gantry control problem?

Encoders are a LOT cheaper when you don't buy them from PlasmaCam. You need the HEDS-5540-A06 encoder

https://www.mouser.com/ProductDetail/Br ... gL1S_D_BwE

The motor itself may be bad now. If that is the case, then PlasmaCam is (currently) the only place to get a replacement motor.

Also check to see that your rail gear racks are solid and not moving around in the rails.

Times are tough, and we are all dealing with this stupid coronavirus lunacy. However, I always advise people to keep spare parts on hand. A spare gantry motor, carriage motor, Z motor, both gantry and carriage pigtail harnesses, limit switches, cam followers, and even a spare main wiring harness. If you have the money, a spare controller box is also a very good idea.

Joe

[adbuch]

Hi Chad,
As was previously mentioned, and I think we all assumed that you had already done this: check the condition of your belts, pulleys, and gear rack (particularly the belts and pulleys). Also make sure the setscrews are in place and tight where the pulleys attach to the motor shafts. Check the larger pulleys as well for condition (damage, excess wear) and make sure they are tight. Check your belts for wear, cracks, or missing cogs. Swapping only the harnesses between motors will help determine if you have a bad harness or harness plug connection - but will not confirm specifically if you have a bad motor, encoder, gear rack, etc.

When you originally said: "Won't return to same place... every trip top to bottom it is over half an inch off. I initialize... same thing. Cleaned connectors on the cables coming out of motor. Restarted computer. Still drifts 1/2" - it is not out of the realm of possibility that this could be caused by something as simple as loose setscrews on the motor shaft pulley - causing it to slip (rotate on the motor shaft).

That could also be the encoder. An easy way to test the encoder for a DHC2 machine (and I would imagine DHC as well) is as follows:
1. Initialize the machine
2. Remove belt from gantry motor
3. Make sure the gear on the gantry motor is tight (both set screws)
4. Place a mark (use sharpie) on the outer perimeter of the pulley (see photos)
5. Record the current position reading for the Y axis as shown on your display
6. Make note of the position of your mark on the pulley, and then rotate the pulley counter clockwise by hand a total of 20 revolutions
7. Record the current position reading for the Y axis as shown on your display
8. Rotate the pulley clockwise by hand a total of 20 revolutions
9. Record the current position reading for the Y axis as shown on your display

Your display should change (increment) by 0.9" per revolution of the pulley. So whatever your initial display reading was, it should have decreased by approximately 18.00" as a result of 20 ccw rotations of the pulley/motor shaft. Whatever your display reading was after the 20 ccw pulley rotations, it should increase by approximately 18.00" after 20 cw rotations of the pulley and read very close to the first original recorded reading. If the results of this test do not agree with the above numbers, and you are seeing a discrepancy of 1/2" - then I would recommend replacing the encoder. If your results do agree with the above numbers, then it would seem that the encoder is working properly and you have (or had) another mechanical problem such as a loose motor pulley or loose driven pulley, worn teeth on pulley or pulleys, slipping or damaged belt, or worn gear rack.

David

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[rdj357]

The difference between the gantry and carriage motors is the orientation of the drive pulley. The set screw hub is on the outside of the gantry motor, the inside of the carriage.

As Joe said, for testing encoders swap the harness connections and use a hold test in diagnostics and/or the move out and back test to see if the encoders are working properly. As for replacing just the encoder, it's a real pain in the ass. It takes a .015" allen wrench and careful holding of the mouth and tongue. Not impossible but for the frustration, I decided I'd just replace as an assembly and save the pieces in case I ever wanted to mess with it.

[adbuch]

The difference between the gantry and carriage motors is the orientation of the drive pulley. The set screw hub is on the outside of the gantry motor, the inside of the carriage.

Looks like the same part. They just reversed the mounting arrangement. Flipped it over for the carriage motor. That's a good way to tell the difference between the motors if their not installed onto the machine (or if not sure which motor is which). Thanks for the tip.

David