Need Help!- Machine cutting oversize?

[factory5150]

I have a problem that is leaving me stumped. I have calibrated my motors in mach and they are within .0005 on the dial indicator. The drawing I am trying to cut was created in autocad. It has been checked and rechecked and all the lengths are where they need to be. I bring the dxf into vcarve pro and nothing has changed, the drawing is still to scale and on size. I make my tool paths and output it to the machine. I zero out the machine and regen the toolpath. Everything looks kosher until I measure it with my template and I see that it is .050-.060 too long. I am making fret slots for a guitar and this operation is pretty critical. The length is supposed to be 18.344 and the my length on the machine is 18.400 give or take. So each of my slots is out of place making my scale off. What am I missing here?

Any help would be much appreciated!
Chris

[xspyke]

I assume you are cutting a line.

stupid question. did you take into account of the diameter of the bit?

[ger21]

I have calibrated my motors in mach and they are within .0005 on the dial indicator.

How are you doing this?

If you cut, say, a 6" square, is it 6"?

[factory5150]

Unless I was wrong, stuck a dial indicator on my tabletop and used the steps calibration utility under "settings" in mach. Ran it back and fourth an inch until it read zero on all axis'

[ger21]

Just because it's accurate to .0005 in that 1" of the machine, doesn't mean it'll be over the length of the table. The larger the size you can calibrate it to, the more accurate it should be overall (in theory).

I'd recommend cutting some test squares and measure them. Cut them in different places on the table, too. You may find that it cuts slightly different sizes in different places.

[factory5150]

Sounds like a good idea. What do I do if my square is oversize at the end of the table and undersize at the beginning of the table? How do I offset the difference?

[mactec54]

factory5150

What type of screws are you using & what grade of screw, Like C5 or C7, this can make a big difference per 12in of travel, But for setting up you want to do at least 6in, in your case 12in would be better for your test set up in Mach

[factory5150]

I think they are C5 ballscrews. My machine is one of Georges (gio666) so I don't really have a whole lot of information on it.

[mactec54]

factory5150

Yes Georges are C5 which is about .0005 per 12in max so you should be ok with the machine, unless your fixed/thrust end of the screw is moving in it's bearing housing,

Next it could only be in the control set up, or your programing, are you running it to fast in rapid moves & losing steps

[factory5150]

Ill check the thrust bearing housing when I get home. I doubt its my programming because it was all done in vcarve pro so I didn't write anything. I know the drawing is correct because I checked and rechecked it. So my thoughts were the machine and its calibration.

My rapids and cutting speeds are the same, but right now I am cutting small inlay at 30ipm. I could crank up the rapids but I haven't really had time to mess with the inner workings of mach. When I cut my 12" square, and I find out its not cutting 12" do I divide the difference by 2 since its cutting 2 faces? And where would I put in the new number at? I dont know the specs of my motors or ballscrews so I cant figure it out the number the hard way. Sorry for the seemingly stupid questions, I am just a total noobie to DIY CNC.

[mactec54]

factory5150

It just seems like you have some ajustments to do, to get a better idea, Hoss has a video on the zone showing how to set Mach up, what you did when you first set your machine was correct what you did,but 1in is not enough, just do the same over 12 in

[stevespo]

I briefly played with the demo version of VCarve, but that's about it. It sounds like the AutoCAD file (dxf) starts out correct. Maybe you already answered this - but are you certain that the generated gcode has the correct values you're looking for?

It sounds like a machine calibration issue, but just wanted to be sure that you've ruled out VCarve doing something funky.

Steve