[Surfie]

Hi All,

I recently built my hobby cnc machine and have been using ti to make some brackets. However, I have found that the machine tends to never cut the same line twice...

I want to cut a piece of 3 mm thick aluminium in a particular shape (see the "Bracket" image). I have set it to cut many small layers which works great (ie there is no gummy up of the bit). The interesting thing is that I have noticed with each layer cut the y zxis cut is off by about 0.5 mm (see the "Cut" image). From the wonderfully straight lines I can tell that the x axis is cutting fine. I thought it may have to do with my clamping. However that does not really explain why it is perfectly straight in one direction. My first hypothesis is that the coupler between motor and ballscrew is flexing too much as the bit moves through the metal, resulting in the stepper thinking it has gone the entire length when it hasn't. But I have the same couplers on all axes.

Any thoughts as to why it would be doing this and help on how to fix it? I dont really want to have to invest in a closed loop system (I doubt this would solve it anyway if it is flex in the coupler)

Thanks in advance.

[Jim Dawson]

It looks like the coupling is slipping, the stepper is loosing steps, or the steps / mm is not correct.

[awerby]

It could also be deflection of the tool, combined with some flex in your system. It's only the most rigid CNC routers that are able to cut aluminum reliably. How thick is your tool, and how far does it stick out from the collet? If you put a dial indicator against your spindle and lean hard on the other side of it, when the machine's powered up, is there any change on the dial? What happens if you run the pattern in something a lot softer, like machinable wax?

[jahnj0584]

Thats definitely just backlash in your system. Deflection of the tool or machine look much differently.

[Surfie]

Thanks for all your comments.
I finally got around to trying a few things. Turns out the coupler was slipping. The screws had worked loose. After tightening those it cut beautifully! !

Thanks again