[machinewbie]

I have a homebrew router, 2X2 cutting area. It is stepper driven by means of Gecko 201s. The steppers are old compumotors by Parker. The control is DeskCNC. I have a large knee mill and plasma table that are under DeskCNC control also. I have had a problem that I have been working on for a couple of months off and on. I first thought it was electronic in nature and rebuilt from that standpoint. It was still problematic. I then rebuilt the mechanics. It is still not performing very well. Sorry about the long lead in to the problem.


The problem is in the Z axis. It always errors incrementally in Z in the positive direction over time. If there are few moves in a project, there is little Z error. If there are a lot, like in the stl file that I have been trying to cut, the error becomes so excessive that in a couple of hours the machine has risen above the work and is cutting air.


The stepper motors are old, I am guessing 80's vintage. I have swapped motors and drivers with other axis and have not been able to nail it down.


I was thinking about swapping cables but I cannot think how that could be uni-directional.

Any ideas would be greatly appreciated!!

[HuFlungDung]

Obviously you are losing steps, I would imagine.

Is there any reason to expect that more torque is required to drive the Z down than to raise it? How does everything feel mechanically when you turn the axis by hand?

Are you plunging into the work? What kind of materials do you cut?

Have you run trials where you simply run the Z axis up and down in the air for a while? Does it lose position?

Have you tried homing the machine after every Z plunge?

Sorry, I can only ask questions That dilemma is why I would never run open loop feedback on a machine tool.

[Crevice Reamer]

Hi MN! Check that your Z axis travel IS what the software calls for. IE: If software calls for 3 inches up, does the Z actually MOVE 3 inches? If not, then you will need to adjust Steps per inch until it does.

CR.

[machinewbie]

Hi Hu,

I have tried dry runs where there is no load associated with a cut, or electrical interference with the router motor or the dust collection system. Mechanically it feels very good, it is geared down low to eliminate the possiblity of the work overpowering the Z holding ability. The motors are 1200 oz units. They are heavy and tend to get hot in operation. I have tried large fans on the motors for cooling and that did not help. The temp was around 140F. They might have run hotter than that, I do not think they have run so hot as to damage them. I do have the proper current limiting resisters on the Gecko drives for the motors. I have purposely slowed down the feedrate to a crawl. I agree with your opinion of a stupid machine. The mill is servo with encoder feedback. The plasma table is dumb as well. I have not had any problems with it yet. The runs for plasma are a fraction of the time compared to that of a router.

I do not have homing capability on any of the tools. I have always referenced the piece. I do use a tool setter on the mill. I will look into integrating it into the router and using it to periodically set the Z height. This might be the easiest and least expensive way to get around this problem.


Hi CR,
I have indicated the accuracy of the movement and it is good. I am sure there is some backlash. But if I understand backlash correctly it is repeatible from one move direction to the other. I did not think it was cummulative.


I did have a problem on the mill where it would wander. The error was erratic and was not unidirectional. It turned out to be the couplers that connected the encoders to the ballscrews were slipping.

[BobF]

I had a loose slipping coupler issue on my machine also. check it.

[TOTALLYRC]

I am running Deskcnc on my router too and it has been working well for me.
One the z axis, you are running 1200oz/in motor with gear reduction?
What do you have for a max steps per secong on the z? What is the tpi of the screw and what is the gearing? PS voltage.
With a G201 and round motors on mine, I had to run the machine quite slow as to keep it from stalling and losing steps. I went to 425 nema 23 motors on 2 axis and it is much faster and more predictable. I will be swapping out the third motor soon.


Mike

[dertsap]

your lucky that the z raises as time moves on rather than what normally happens which is the opposite , it sounds to be possibly an accel/decel issue ,
i had an issue with mine a while back where i was loosing steps and i couldn't figure out what the problem was , until i plugged in a different 5v power supply for the logic , the logic supply was the problem

[Clockwork]

1. Wiring from the parallel port to your G201s should be isolated from any 120V wiring and the wires that go to the motors. If they're too close, noise can be induced into the step/dir wires with inpredictable results. You said that this is only happening when the machine is moving and never while it was stopped.

2. If possible, remove the connectors from the G201s (with wires attached) and exchange them with another axis. You could also try alternate wiring of the step/dir pins for the Z with associated changes in your program settings.

3. Some printer driver software (especially HP) periodically sends a qwery string out to the parallel port. This could be interpreted as positive Z steps by your drives.

It's not the motors, steppers don't move by themselves.

Backlash is never an issue on a vertical Z axis. Gravity is your friend.