Cutter Stop Rings

5 Bears Home Homebrew CNC bench mill

The bulk of milling with the KaVo spindle, at least for the near future, will be with 1/8" diameter cutters. I plan on buying collets for 1/4" shank cutters, and at least one other, probably 4 or 5 mm. I've picked up 1/8" carbide end mills in bulk, from several good companies, notably American Carbide. HSS is a poor choice for this type of milling, especially in these diameters.

One way to accurately repeat in Z with these cutters is with the use of a plastic stop ring, available in quantity from Drill Bit City.

A collection of carbide 1/8" diameter cutters. The bulk of the bits in the lower left of the tray are commercial carbide drills and other unusual cutters. Those to the right are a collection of 2 and 4-flute regular and ball end mills.

The bit tray was a simple exercise for the CNC mill. The holes were cut with a small carbide 1/8" drill. They ended up just enough oversized to allow the shanks to enter with a tight, near hydraulic fit.

The original ring setter I used for a couple of months. I thought that I was setting them accurately. The collet to the right would grip the shank. Inserted into the hole in the aluminum block, the tip would bear on the indicator, which would set the depth of cut below the ring. The problem was this: the rings varied slightly in thickness. The dimension of interest is the total distance from the tip of the cutter to the top of the ring, which is what rests against the KaVo collet. Thus, for utmost accuracy, I needed to modify the device so as to create a fixed distance between cutter tip and the top, or backside, of the stop ring.
Here is the modification to the stop-ring device... the next photo shows the fixed-distance "finger" in better detail. To set a ring, the cutter and ring are installed into the collet of the device, and forcibly inserted into the fixture block. The indicator, barely seen beyond the block, measures the tip extension relative to the stop ring. The cutter is injected well beyond the desired distance, then, the retaining finger is pivoted clockwise.
The finger stop is secured hard against the block with the knurled thumb nut, and the cutter + ring is then backed out from the block. The finger retains the ring, and the cutter is withdrawn until the indicator reads as desired. When the process is repeated between cutters, I gain a consistent Z for the cutters, probably to within 0.0003" or so.
To test the overall Z-offset accuracy, I prepared a few cutter of hopefully identical stop-distance, and installed a block of aluminum in the mill vise. Using simple jog commands, I cut a series of 1/8" channels, spaced by 0.100" for a continuous planed area. Between individual passes of the X axis, I installed/reinstalled various regular end mill cutters. To end the test, I installed a ball cutter, which should theoretically ever-so-slightly "kiss" the work on the very end of the cutter.

A quality 0.0001" indicator does the honors. I found that the vertical variation between the cut channels varied by no more than 0.0005", and that was worst-case, an "errant" cutter. Most channels, representing different cutters, matched each other better than that.

The "new and improved" ring-set device is a winner.