Hi All,
I wanted to pick up this thread that had so much helpful information. Sorry for my slow response. In short, school started, everything got super busy, time on the shopbot was slashed, and we got it to work much better quickly.
Joe, you were right! Sure enough we had some real piece movement on some of our cuts with the original text cuts. The thing was that we screwed it down just to be sure after cutting about half of what we had done. The vacuum was just not strong enough with the small pieces. We had a newly leveled bed, but it couldn't do it. Now we are doing both vacuum and screw down. I have to say I don't like the screw down - and I don't like cutting through to a sacrificial board, but with tabs it makes it work and 1 sheet of mdf for every 10-20 cutting sessions is workable.
Also, we are now using the pre-mask on the surface of the mirror, thanks for this suggestion as well.
And thanks as well to those who suggested the two pass system. We are running the oflute 1/8" first over each of the toolpaths and then chasing it with the tapered end mill (no more ball nose). It is producing a cleaner cut now. Both o-flute and end mill are going at 14k and .5ips now. IT's accurate, it seems to still wobble a bit if we speed up much beyond this, but we don't really need to. Still makes me think we may have something a bit loose in the Shopbot, but it certainly was not the cause of the big wobbles that originated this thread.
Thanks to all for the help and suggestions. Sorry for the delay in response.
Peter