Accuracy in pocketing.

[RobH]

Hello,
I am having a problem with milling using Mach 3. The finished part is consistenly under size (about .5mm in 35mm). I use Lazy Cam to generate the G-code.
I have tried all the obvious culprits, backlash in leadscrews, Tool sizes in tools, and have checked axis movement with a DTI against DRO in the program and all are OK. The round pockets a perfectly round which seems to eliminate any suggestion of a backlash problem. Could there be a problem with the software?. I have tried the backlash setting both on and off with the same result. The error is consistant and I can fiddle the dimensions to get the sizes I want but would like to be able to get predictable results.
Any help gratefully recieved.
Rob

[BobWarfield]

How certain are you that the endmill is actually the advertised size?

I had some inexpensive but good endmills I got off eBay. They were name brand, but I discovered they were pretty far off in terms of diameter. I don't think they were as bad as 0.5mm, but by the time you are off on both sides of the pocket, maybe so.

I wound up measuring test cuts to see what the actual diameter was.

The other culprit to consider is tool deflection. Are you doing a very light finish pass? If you run it again does it still cut chips?

Best,

BW

[fatal-exception]

Is there a 'finishing allowance' setting in Lazy CAM? If its on, it will leave a specified amount of material for taking off with a finish path. I would check for that.

Paul

[RobH]

Hello Bob,
Thanks forthe reply, I have just measured the mills, I tried two, They seem spot on. (They are Dormer SKF).
I will check the tool deflection although I had the same result in plywood.
Rob.

[TDA]

Are you climb cutting? I've never heard of it causing that much of a mismatch but you might want to try a conventional cut cleanup pass.

[RobH]

No I am not climb milling, I will try the cleanup pass. Thanks.