Standard involute gear cutter in a Bridgeport on a 7/8" arbor. For those not familiar with gear cutter nomenclature, this one is No.7 (in a series of 8) with 20 Diametrical Pitch (abbreviated as P here and as DP on other gear cutters) and it cuts gears with 14 to 16 teeth. The full depth of cut is 0.108". The pressure angle is not marked on this cutter. We had to cut a gear with the cutter to determine what pressure angle the cutter was. We got lucky and this one was the correct 14.5º which is what we needed. In my collection I just happened to have this single cutter, acquired some time over the last 30 or so years from an unknown source. The arbor holding the gear blank is unusually long because there is a bad bearing in the dividing head causing runout. By using a long arbor and keeping the cutting near the tailstock, we've reduced the runout of the arbor. I'm still shopping the used market for the #4 cutter in the 20DP series to do the 32 tooth gear my friend needs.
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Thank you for the photo, from the shot that you had of the gears sitting on your mill it looked like some type of endmill was being used. Do you have any issues using the (milling) arbor with the unsupported end? When I cut some 4DP gears on the horizontal mill, I had to do everything I could to increase rigidity. On the 4dp cutters you are going just over 1/2" depth of cut.