[brian257]

I finally have my Bridgeport CNC conversion with Ethernet Smoothstepper and servo motors running good and I need to do some higher precision work than I have done in the past. I am pocketing some interpolated holes for round lenses and they need to be accurate to about .001", but I am having a little difficulty.

I have checked the backlash on the machine and I have 0 on the X axis and .001" on the Y which I have taken out with Mach 3. I have measured the accuracy of movements and they are now perfect. I am using a very good Promax 3 flute 1/2" carbide mill that cuts like nothing I have ever seen. Just ran some test holes in aluminum at 1.044" diameter and 3/8" deep. I ran a roughing pass with .025" stock left then a finish pass at five times around with .005" stepover on each pass (damn Rhinocam does not make it easy to do a prefinish pass at .020 then a finish at .005 without doing it in two operations). I am running it at 3,000 RPM and 25 inches per minute and climb milling.

What I am getting is slightly slightly scalloped looking and feeling edges and the hole is measuring a few thousandths undersized. Taking a second run of the same finish operation it takes a little bit more off and cleans up the scallops a little bit and gets the dimensions about a half thousandh undersized. A third run does a tiny bit more and removes a hair more of the scallops.

What does not make much sense to me is that I was doing some much more aggressive cutting of .640" diameter holes earlier and those are so smooth they look like they were polished.

I have been doing manual machining for a long time and a .005" finish cut on a very rigid machine is what I am used to. Should I be leaving only one or two thousandths for my finish cuts? Or is this the Smoothstepper not making smooth and coordinated enough movements to cut without the scallops? Is my feed rate too fast? According to Gwizard I should be feeding this cut at almost 60 inches per minute, but I am slowing down and seeing better looking results.

Any suggestions on what to try would be appreciated.

[brian257]

Oh, and I do not have Rhinocam set up to cut curves as line segments so it is making G03 arcs.