This afternoon I machined the top surface text into the four model railroad throttle cases I’ve resumed work on.
I quasi-kinematically located them on the threaded table with two Lock Stops along the left edge and another on the bottom edge, with Gator Clamps opposing each. (That way I could swap in the new workpieces just by loosening the Gator Clamps, without having to rezero X and Y on each one.) This was my first use of either, and I found I had to really carefully tighten the button head capscrews on the Lock Stops to prevent twisting them.
I decided to machine some little washers from .025" brass sheet to go under the heads of the BHCS as a buffer (similar to what I do at work when positioning a precision mechanism). I used Carbide Create to draw the inside and outside diameters of the washers, and to the OD I added two diametrically-opposed .010" tabs (CC won’t allow thinner tabs, and good thing as it turns out).
For the washers themselves, I screwed a wasteboard to the threaded table and used tape-and-superglue to attach the 2.5 x 2" brass stock to it. I eyeballed the SW corner zero and touched down the tool on a piece of Zig-Zag paper (.001" thick so if it’s pinched, the tool is at zero) to zero the Z.
I used a .010" stepdown on a 5/64" flat endmill (I could not find a 1/16" in my stash) and the washers did not quite cut to the bottom. I had not bothered skimming the wasteboard, and the tape-and-superglue will have some thickness variation so I was not worried.
I duplicated the CC job and redefined the ID and OD cuts to be .030" deep with a .030" stepdown, so it would just take a cleanup pass comfortably below the bottom of the stock.
To my surprise, the OD toolpath skimmed down the top surface of the tabs as it progressed. This got me to wondering. After I finished the cut (and this is why the .010" tab thickness minimum turned out to be a good thing, because there was still .005" tab left), I re-opened the cleanup CC file and redefined the cut and stepdown to be .040". The tabs disappeared from the simulation.
So the conceptual error in defining the tabs, is that they seem to be keyed to the maximum cut depth, and not the bottom of the stock. With a .025" stock and a .010" tab thickness, the top of the tab should be .015" below the top surface of the stock, even if I decide on a whim to make my outlining cut 1/8" deep.
Logically there can’t be a tab below the bottom of the stock, so the tab should be keyed to the bottom of the stock. For years I used SheetCam to make 2.5D parts and the tabs worked this logical way, and I would like CC to act the same because I’m making a good-faith effort to use it in lieu of SC…
