accuracy

[alexS]

Hello,
I noticed sinds a while that somehow we have precision issues on pour shopbot. Say we mill a pocket or a profil job, it's sometimes more than a milimeter off, mostly less than the actual drawing. We are absolutley precise with milling bit definitions, so that can't be the problem.

Is there anything I could check in the code or the like?

Thanks for help

Best,

Alex

[knight_toolworks]

there is slop in the machine and you have to learn to deal with it. The best way I found is using a cleanup pass. So either cut most of the way with a climb cut or offset a little bit then the final cut in normal direction with no offset will take care of it pretty well. pockets cutting them slow helps or offsetting them and doing a light cleanup pass. bit defection can be part of it but that depends on the size of the bit.
Now has the cuts been accurate on the same file and nothing has changed? if so then it is not just machine slop.

[jerry_stanek]

There also could be bit deflection the bit is bending just a little. We even get that on our Biesse rover that is over $250000.00 USD

[michael_schwartz]

spindle runout, and the actual diameter of the bit are also possible factors.

If the parts are undersized by 1mm, it sounds like the bit is deflecting towards the cut line, and pulling the machine,causing it to flex even further in that direction.
I would try climb cutting first to deflect the bit away, and then follow up with a single cleanup pass, set to conventional, at full depth. You probably won't even need to offset the first (roughing) tool path. Just duplicate the tollbooth, and change from climb, to conventional and change the cut depth, for the bit. Note that there will be some additional steps involved if your using tabs.

There are a lot of things, that can affect tolerances and when you see a significant error like 1mm there are likely several contributing factors.

[alexS]

Thanks all for the hints.
I also got a lot responses from the fablab community.
Will go through many checks and post results here

Thank you again!

[rhfurniture]

Hi from UK.
1. Check pinions for wear, if over 1 year old on heavy use, change anyway.
2. If the error is consistent along an axis, and all mechanical and software issues have been eliminated, you can cut 2 holes along that axis, 1 meter appart, accurately measure the distance between, and micro-adjust the unit value until it cuts the holes the right distance appart. (make a note of the original values first).

All best from sunny Cheltenham,
R.