garageFab.cc

Here come more mini-router tests - mostly milling soft wood, trying some Delrin. I've run into several issues along the way - they slow down the progress some, but on the other hand they are good learning experiences. 

Dremel runout 

First, the grooves that I initially milled with high speed cutters were about 0.02" wider than the tools. I googled "dremel runout" and found various bits of information, ranging from "Dremel is no good because of runout", to "Runout is much better with chuck as opposed to collet".

I don't have a dial caliper, so I used 0.01" drill bits to visually estimate the runout. The more they wobble, the more they become transparent. I also bought Dremel chuck to see if it will help, but didn't see any improvement - if anything, runout was worse because the chuck is longer than the collet. The problem was, of course, overtightening of the collet nut - I was doing it with the small wrench supplied in the Dremel kit, while it should only be finger tight. This by the way is mentioned in the Dremel manual. Oh well, RTFM.

Noise? Stepper resonance?

After the problem with runout was out of the way, I decided to mill some test pockets in 3/8" thick plywood. The GCode files for the test pockets actually come from the new shaft mount design, intended to be milled from 1/2" thick Delrin. I spent a couple of evenings repeating the same job as X axis would stall in unpredictable places. I experimented with feedrate, cut depth, turned up the current on the board trimpot (up to 3 hours), with little effect. It puzzled me that the same sequence ran just fine with Dremel off and without workpiece. I almost gave up and decided that some mix of insufficient high-RPM torque+vibrations from Dremel+stepper resonance is to blame.

One day I decided to clean up the sawdust before turning on the computer (and Arduino). To remove the dust, I use a small 12V car vacuum that is powered from the same PSU as the stepper drivers. When I turned it on, I noticed how steppers started to slowly rotate - and then I realized just what might be the culprit of all these unpredictable stalls.

Indeed, with the vacuum off, the next pocketing job ran fine until the end. I used Dremel #194 high-speed cutter bit (1/8" dia), 15 IPM feedrate, 1/32" cut depth and speed setting 8 (I think that corresponds to about 18 kRPM). Lower feedrates resulted in more smoke and I don't think the router can do higher feedrates reliably.

Milling Delrin with Dremel 

Since the shaft mount was meant to be made from 1/2" thick Delrin, I was going to use the piece that I bought for making ACME nuts. However it measured about 0.025-0.030" thicker than 1/2", so I thought I'd use larger high-speed cutter bit (5/16" dia) to thin it down. Prior to that, I tried the same #194 bit on Delrin at cut depths from 0.01" to 0.06" and found that 1/32" was about as much as this bit could handle at 15 IPM, i.e. similar setting to wood, except lower spindle speed (setting 4). I then used the 5/16" high-speed cutter bit at the same depth but slower feedrates (7-10 IPM) to shave some material off the Delrin bar. As can be seen in the pictures, the result is basically ridges and valleys varying in height up to 0.01". Looks like I will need to run a lot more tests with different bits/cut parameters to find combinations that work for Delrin. A quick google search did not turn up much usable info on milling Delrin with Dremel bits, and what little I read on milling Delrin in general, did not have a lot of specifics, explaining it by dependence on a particular material at hand. I think I'll start compiling a table of Dremel bits and cut parameters I used on different materials.