We had a number of people ask for an O-scale version of the gas truck kit that I made, so I decided to start working on it. Everything was going just fine until near the end of the milling. At this point, the end mill started to wander, gouging the mold, and then the cutter broke:
You can see the problem in the center of the photo. I was using a long reach 1/32” diameter cutter to sharpen the end of the bands on the tank. The band detail was originally cut with a 1/16” diameter cutter. So I was going back with a finer cutter to make the bands sharper. Unfortunately, the long reach 1/32” cutter was much too flexible, so it got off track (it curved to the right on one pass). And then on the next pass, it curved to the left and broke. Grump. The thin part of this cutter is a little more than .4” long so it can reach down to the bottom of the band at the very right. But that long, thin shaft is not stiff enough.
I could fix this mold by removing about .010” from the top of the mold and then milling the cavity again. However I also realized I should be user a thicker master. This block of aluminum is 1/2” thick, and the cavity is .04” deep, which doesn’t leave much room on the back to resist injection pressure. So I think I’ll order some 1” thick stock and start over.