[AussieNate]

Hi all,
Long time viewer first time poster, I have recently made my own CNC router from pretty rough parts and made extremely cheap.
It is made from MDF using drawer runnings and a DIY stepper driver board using GRBL (Arduino thingy)
I am attempting to cut some 3mm acrylic sheeting. the machine is performing pretty good for something slapped together but I have run into an issue.
The cut is one pass with a 3mm HSS endmill. the finish it good but on opposite arcs of the arm are the same but the one next to it is different.
See attached photo and the gcode.

So I guess my question after all that is my problem the machine or my gcode.

I hope all that makes sense. Please help

Thank you

Nathan

[ger21]

The g-code looks fine here.

[AussieNate]

So that makes it my machine.

Thank you for the quick reply

[awerby]

It sounds like you're losing steps; this happens when the forces required to do your job exceed the torque that your motors deliver. It could be that you can fix that by tinkering with the acceleration settings or by going slower, giving your machine a little more time to do the job. Another option would be to take the cut in two passes instead of one. That would require less torque, and probably result in a cleaner cut.

[nlancaster]

I don't think it is losing steps. I would expect a large offset at the start/stop point if it was losing steps. I would look more to the rigidity of your machine. Part of the reason I say this, looking at your part it is skewed around the z-axis looking down at the part. And that can be caused by the cutter pulling the machine around the part, and if your machine is not rigid enough the skew would be in the direction of rotation of the bit vs the direction it is cutting.

[MI370]

Why don't you make a simple adapter to hold a fine tip marker and run the program (with the setting ON the line) on an uncut workpiece drawing the shape profile out. Then on the same piece run it with your end mill and see if / where any in-accuracys are occuring or where you may be loosing steps.

[ger21]

Looks like i might be a loose coupling or setscrew.

[wizard]

I have to lean towards the guys suggesting that you have a stiffness problem or an issue with your couplings. Towards the end it almost looks like you gained steps which could mean the tool is pulling one or more axis along either due to looseness or actually loosing steps. It would be interesting to know your order of cuts as the right hand side looks to be the worst making me wonder if it was the last cut.

One thing that might help here is to take an extremely light cut a few thousands deep to see how well you track the intended cut and then one half the depth you are currently cutting. The idea here is similar to using the pen described above. That is to get an idea of what you get with minimal load or varying loads. If you pay close attention to the parts orientation you ought to be able to zero in on the worst axis to investigate that one first. Depending upon he geometry of the machine it doesn't take much deflection at all in the slides to show a big error at the tool.

As someone else mentioned it doesn't look like you are loosing steps at first. You can verify this by establishing a reference portion to return to after the entire cut is finished. Make sure you drive to this reference position in the same manner that you established it in the first place. The idea here is that if you let the tool bore a shallow hole when establishing the reference point, the machine should return exactly to that hole at the end of your cut. The idea about approaching the hole in the same manner as it was established is that we initially are looking to see if the machine lost steps so you don't want backlash to be a factor. You could also use the hole to check for backlash but one thing at a time here. The other thing with loosing steps is that often you can hear it as the machine complains loudly. Still you want to eliminate one problem at a time.

I'm asking here about your drivers more out of curiosity as I don't think you are loosing steps, at least not initially. Are the stepper drives completely discrete parts or are you using one of the integrated indexer chips? I ask because I'm wondering how you set winding currents. Obviously this is only a problem if you are losing steps, still it is interesting to know what your drives do.

Some where in the net a guy went on at length how he "adjusted" drawer slides to remove much of the slop in them for a device he was building. It might be worth hunting for. However I would be surprised if you ever got close to linear slide performance. If your slides are the problem you will have to deal,with it sooner or later. Your machine design might also be a factor in how the drawer slides perform so it might help to have a few pictures posted. Even if the machine looks like hell you might get some pointers as what to address.