Slight step over on second/third pass

[Dan Sj0]

Hi all,

Consider yourself prethanked for looking at this post! I appreciate any feedback assistance you might offer.

THE SETUP
Rhino/Visual Mill/Mach 3

MACHINE
Rebuilt by a friend (far away, so I can't throttle him) with Geckos etc, Digital Tool frame, servos, 3 1/4 PC router, belt drive

THE APPROACH
I cut a lot of 3/4" furniture grade plywood. I do that in essentially two steps.
1. 1/4" downcut spiral bit cuts to 1/4" depth.
2. 1/4" upcut spiral bit cuts to 1/2" then 3/4+ penetrating the piece.
Note: Downcut runs clockwise to the piece, upcut runs counterclockwise.

THE PROBLEM
The upcut runs cut slightly offset from the downcut, usually in one axis. In small parts spread across the table the offset grows incrementally. You can imagine the problems: dimensions, edge finishing, fitting ETC ETC ETC!

TESTS
I cut two squares to test the cut direction setup and the down/CW up/CC approach yielded the best results. Smallest step, square piece, 1/32 off in one direction. NOTE *** Never been trained on CNC. Just bought this thing from my buddy and started messing around. Don't know jack about Gcode and just work the VM the best I can. Is this CW/CC thing even right???

THOUGHTS
Is this a backlash problem? I've got belts, not a gearbox. But everything drives on tracks...
On the physical setup, I've tighted all excentrics, replaced the router (it was time), checked/matched belt tension as much as I am able.

Seems like the bits are driving off course, pulling out of line. It does seem fairly consistent in one axis, but inside and outside curves are problems.

NEXT STEPS
...that I've thought of: backlash compensation. Any clues on setting that up? Standard tests?

Or, selling the router and getting out of the 'making stuff for people' business.

Your thoughts/comments are so appreciated. I am-out-of-my-gourd frustrated with this and it has been going on for too long.

THANKS!

DAN

[MetalHead]

Is this a MechMate? Can we see some photos of the problem? Also more details about the machine. Servo's? Rebuilt Machine?

[Alan_c]

Is this machine a MechMate? This site is aimed at MechMate builders and users. There may be various causes for the results you describe but not knowing the type of system you are using will make it difficult to tie it down to a particular issue. Try over at the CNCZone.

Using two different cutters may be the main source of your troubles - I suggest getting a combined up/down cutter. Set your program to cut almost all the way through (in as many passes as you may require), offset away from the finished size by 0.5mm. Then do a final cut (all with the same cutter) on size and depth set to cut through. That will remove any chance of getting tide marks or offset paths as you will be doing one final pass on size. This will not fix any backlash or setup problems you might have in your system. If the finished parts are at the correct size then the problem is probably not in the mechanicals.