z accuracy, and low end

[BigoleRob]

so after running tests, i have found my PRT accurate to a (human error included) 0.0055 in the Z axis.

now that is not my problem infact im very happy about that. i have noticed tho that often times using straight flute endmills that, the machine will leave onion skin on the bottom side even when im cutting on a newly surfaced vac. table, as well as one end (particularly after about x60+ ) seems to be a "low end" of the table. that it will exhibit, onionskinning when the x60- will have scored the table. this difference must be around .01 or less but I havent been able to get a good grasp on what it is doing.

any sujestions.

im just trying to render the highest precision i can with this little machine.

thanks

[Brady Watson]

How long ago did you flatten the table? Depending on what your table materials are, relative temp & humidity, the deck can fluctuate a lot more than that just over the course of one day. Everything has a coefficient of expansion. It is important to flatten the spoilboard right before any job with critical Z thickness. Those are very good numbers for a PRT. Keep in mind that ShopBot advertised .015" repeatability on that tool - which is plenty good for woodworking....unless you are building cabinets for NASA :D

-B

[BigoleRob]

thats pretty funny.

yeah i resurface about once a week when we are running flat out.

i suppose that makes sense, on its accruacy. and i often find most material is only consistant in thickness (the cheap stuff 0.1 - 0.2 ish) and nice stuff is a bit better.

its just very consistant that the 60+ end seems to be "deeper". just seems odd how it works out.

I almost think sometimes that the final layer (onion skin) will ride underneath the bit, like it is being pushed underneath the bit. even on a resurfaced table.