I’m new to cnc, but have got far enough to know that I have a problem that is beyond my knowledge, or of knowing where to look.

Using Mach3 on a home made machine, a Hobby CNC controller with steppers that I’m running at over voltage but at under their rated amperage. I am running them at full step and they move very smoothly when jogging them or running g-code, Except. ----- I converted a dfx file of lettering, using Mach3 and each letter starts being routed in the middle of the left side. Almost every time I’ve run it, at some point, at a random letter, when it plunges to start the next letter at the correct point (the middle of left side) it will start routing as if that point is the top of the letter and offsets the remainder of the letters down ½ a letter. It is not a g-code problem because it is random at which letter it does this using the same file. I also hear no skipping of steps of the motor. And it plunges in the correct spot according to the code but looking at the Y DRO, after I see it has messed up, it is off by about 1 ½ “ (routing 3” letters). So, it plunges in the correct spot, but then moves –Y about 1 ½” to get to where it’s supposed to be to start the letter, and continues from there.
It does this 2 out of three times.
Any suggestion of what to look far would be appreciated.

Here’s a section of code typical of where it stops one letter and starts another:
G1 X7.3352 Y1.3604
G0 Z0.250000
G0 X7.8158 Y2.5113
F15.000G1 Z-0.1250
F12.000G1 X8.1944 Y2.4754
G1 X8.2350 Y2.5113

The letter height is from apx... Y1.0922, to Y3.9274

Louis

Louis:

  Do you m,ean the Y DRO is showing one thing , but the axis is not there when you say the Y DRO is meesed up? Is the Y DRo accurate??
This is an important distiction.. Are you running full step because you have to ?


Thanks
Art

Art,

I don’t think the Y DRO is as messed up, as much as it’s switching to a different reading. I don’t know if it’s instantaneous or over a few seconds as I’m watching the router to see if it’s moving correctly. It’s like it (the router ) is at say X2Y2.5 (actually and DRO reading), there’s a Z+ to raise the cutter followed by a move to X3 followed by a Z- plunge. At this point it make a –Y move, say to Y1.100 as the g-code says. It is at this point that when it reaches the Y1.100 that it continues in a Y- direction and I look at the DRO and it shows it is at say Y2.something (below 2.5) and is continuing on in a Y- direction to a new Y1.1 which is about 1.5 “ lower than it started out as.

As I said in my first post, I hear no skipped steps from the motor. In fact, there is no pause in the motion of the motor.
 I’m running full step because it seems to run smoothly set as such. Previously I had it at 1/8 step, but it just seems to run better at this setting (plus I burnt out a controller chip while running at 1/8 step and I’m superstitious). Should I try half step or more?

Louis

Louis:

  Please send me the code in full so I can check it out. Does it do this evry run through it?

Thanks
Art

Louis,

I have that same controller except it's positon monitored with encoders. If you want me to check the file, I'd be happy to do so.

Send to:
rogersmach@yahoo.com

Ron

Art and Ron, I emailed you both the code.

Louis

Louis,
 
I ran it here 5 or 6 times and it ran perfectly. I'm using a closed loop system with encoders on this "HobbyCNC" board and if the stepper went elsewhere the interface would halt all motion even if the toolpath didn't reflect it. Anyways, the only thing that I can see is where the F value is entered. F values are usually entered at the end of the string. If you zoom in on the path that is Z- to feed into the part and rotate the path dynamically it looks like the path is a G1 and G0 together (blue line and red dashed line when rotated). That might be causing the trouble. At any rate, it ran perfectly fine here.
 
Ron

I notice that you are trying to use a feedrate with a G0 rapid, modally. I've run a lot of Fanuc machines and would never try this. Not really sure if it has anything to do with your problem, it's just all I see there that doesen't look right.

Have you tried removing the G00 and replacing them with fast feeds, say 1/4 of your rapid speed, this will show you of you are getting skids.

Graham