[jaysihn]

I built my first CNC router over the past few weeks.

So far all I've tried to do is draw some things with a pencil I mounted to it until I'm more confident with the software.

I've hit a roadblock that I want to get over before I move on to anything else. While the machine is running, it will randomly shoot off in a straight line, sometimes that line is bigger than the entire drawing, and then it will go back to drawing as if nothing happened. The glitches sometimes are in the X and sometimes in the Y. I've not seen them in the Z but since I'm only doing pencil drawings, I only have the 2 Z commands; I'm sure if I was doing some 3d carving it would show up in the z direction as well.

I've got the speeds dialed down really far to eliminate any missed step issues but it still has these random glitches. They are always in different spots on the same drawing.

I'm running mach3 to a hobbycnc 3 axis board. the power supply is 36V from a chinese supplier. the pc is an old Dell with windows XP. The parallel port is 3.3V. I've got 3/8" x 16 threaded rod for leadscrews and am only running the thing between 10 and 20 inches per minute.


Any ideas? I'm thinking its probably the computer...

[ger21]

When it goes "shooting off", do the DRO's show the actual position it's moving to? It sounds like your g-code is telling it to go there, as there really isn't anyway for the PC to make it move on it's own.

[jaysihn]

no, the random movements shift the zero points. if the random movement is 3 inches in the x direction I can tell the machine to go to zero at the end and it will be 3" to the right of the original zero.

The random movements are never in the same locations and I'm using the same gcode file each time.

[ger21]

Is it moving where it's not supposed to, or not moving when it should be? Are you hearing a "grinding like" noise?

[jaysihn]

Its moving where its not supposed to. My test drawing is about 4" x 4" and there have been cases where the random movement was a 6" or 7" line! If it was one of the motors not responding, I would expect to see a straight line as well but that straight line should be no bigger than the original drawing.

[ger21]

Can you post your g-code?

[jaysihn]

That will have to wait until after the work day. I've been using gcode I downloaded from a thread here and haven't modified it.

[whateg01]

If it isn't happening at the same place, and the machine is losing zero, I'd guess it's not the g-code. Make sure your grounding is all good.

This doesn't seem likely in your case, but... I don't know if the HobbyCNC board is susceptible to this or not, as mine doesn't have any trouble with it that I know of, but I do know that I have another machine that doesn't like the outputs from my PC. The threshold is too high for the parallel port outputs. So, check to see what the voltage is going to on the pins. 0 and +5?

Is your driver board (HobbyCNC) in a box? Or is it out in the open? What kind of supply are you using? Maybe there is some EMI causing the driver board to think the step line is toggling when it isn't.

[p38nut]

Are you sure that the Hobbycnc BOB is good for 3.3v inputs? Looks like it says 5v logic, to me. I could be wrong.

Worth checking. Mike

[jaysihn]

I agree with you that its not the gcode.

the board is not in a box right now, its out in the open.

the voltage coming out of the parallel port is only 3.3V. I was surprised to find this since its a 10 year old Desktop PC.

How do you think the grounding should be set up? Currently I just have an extension cord end that I cut wired to my power supply. the power supply has a 36V dc + and - that I ran to the hobbycnc board. there is about 8" of wire between the driver board and the power supply. The hobbycnc board doesn't have any other grounds tying back to the AC ground and its attached to a laminated shelf board right now.

A lot of this stuff isn't how I want the finished machine to look, I just wanted to get it up and running before I spent a lot of effort making a nice electronics box for it.

[jaysihn]

Not sure what the actual voltage threshold is. I hope that it is something that simple! Anyone have some resources for a good pci parallel port card thats known to work good with mach3?

[Lee DeRaud]

The only thing I can think of that is consistent with the symptoms is that one or more of the direction lines is intermittent. It will look random, but it probably isn't, in the sense that, if you look at the gcode where it goes walkabout, it was actually supposed to move that distance in the other direction on one or more axes.

(That may not sound particularly useful, but at least it cuts down on the number of possible errant signals.)

[whateg01]

You may need to use a breakout board. The HobbyCNC board isn't really a BOB. It's just a driver board. I had to get a BOB to control the torch on my plasma table and then take the output from there to the HobbyCNC board. I also had the problem with 3.3 V outputs driving another machine. I bought a CNC4PC BOB that will work on a 3.3 V input signal and drives the HobbyCNC board with a full +5 V output.

[jaysihn]

is the BOB's only purpose to bump the voltage from 3.3 to 5 volts? I was just going to get a PCI parallel card that is known to output 5V instead of 3.3 and install that in my PC.

[whateg01]

Here is the BOB I'm using. It's not opto-isolated, which is advised by many, but a lot of us aren't using opto-isolation.

CNC4PC

Good folks to work with, too. At least, that's been my experience.

[whateg01]

The HobbyCNC board only drives the steppers, so there is no way to control things like spindle using just the HobbyCNC board. The BOB has outputs that you can use to control relays for things like vacuum, spindle, etc. So, while it is useful for bumping the voltages up, it is also useful for adding functionality to the machine. It also has input pins for wiring up limit and home switches.

Arturo (CNC4PC) also suggested the C11 BOB as it has opto-isolation and an analog output that could be used to control the spindle speed. It also has a couple of relays built in.

[jaysihn]

Thanks for that info. I think its a no brainer then to buy a BOB rather than a new card for the PC.

[whateg01]

Yeah, since you are probably going to need, or at least want, the BOB anyway.

[Fastest1]

The Hobby Cnc Pro is a great board. You havent even confirmed what the issue is. It sounds as though it is missing steps. I use mine to control relays and spindle indexing for threading. It isnt optoisolated but that isnt a must, preferable but old pcs are cheap too. It is a BOB, what pins can you not access?

[Fastest1]

A BOB (break out board) allows access to input and output pins of a PCs parallel port. Usually desktops had 5 volt signals, laptops were said to have 3 volt. I have had luck with both. How well is the machine adjusted? Has it ever been running accurately?

is the BOB's only purpose to bump the voltage from 3.3 to 5 volts? I was just going to get a PCI parallel card that is known to output 5V instead of 3.3 and install that in my PC.