Rounding corners on small counter sink and precision slip fit.

I am making a tool for reloading to help uniform bullets. They are called Meplat trimmers which are commercially available for about 100$, but they are very simple and just aren't offered by enough people to make costs go down. Plus I work on my free time after school in a machine shop so its free even with materials except for time.

But my one issue im still having is how to round the inside of the hole past the counter bore. I cant run a deburring tool on it because it will be .75 to 1 inch deep in a hole thats only .2855 in diameter. I know you can buy corner rounding end mills, but they are much to big to fit all the way into the .2855 hole so that I can round the smaller one.

The hole that needs to be rounded will be roughly .15 or so in diameter, but I havent decided for sure. I need to look at the bullet geometry. After everything is reamed and drilled, it really needs to be rounded. I dont want that bullet marred up or scored at all by sharp edges. I could run a chamfer pretty easily, but you still have a 45 degree sharp edge. Im trying to avoid that if possible. The idea is that the bullet will slide into the jig past the bearing surface and the smaller diameter hole will force the ogive to be dead center. So I will be pushing it with my finger firmly while I bring in a .125 endmill from the other side to true up the tip as well as deburring.


Also, I mentioned running the counter bore to around .2855. The bullet diameter is .284. I need a slip fit that wont grip the bullet, but wont let it slop around. I looked at a couple hand books and it seems like a 1.5 thou overbore would be good for a slip fit without noticeable slop since its supported on the ogive as well as the bearing surface on the bullet.

The ideal way is to make a form tool out of a reamer/drill blank etc.

I wonder if you couldn't just spin a hardened mandrel in the end (with light pressure applied) to smooth the edges of a conventional chamfer. You'd simply make a form tool with a step which you would could carefully form as a small radius.

What about turning a HSS cutter with a radiused carbide bit? Then I could get whatever profile I wanted by grinding the carbide negative.

PoorMan said:

What about turning a HSS cutter with a radiused carbide bit? Then I could get whatever profile I wanted by grinding the carbide negative.

Click to expand...

For a simple 1 off, turn a piece of 01 drillrod to a snug running fit in C/B

Turn the mirror image of the profile you want on one end.

Reduce that end to a D section on centreline +/- 0.003 max

File / grind away the trailing side so you are left with a single lip cutter.

With a Swiss file CAREFULLY put a small relief clearance back from the cutting edge LEAVING AN ABOUT 0.005'' WITNESS MARK.

Stone up to a sharp edge on the flat only.

Harden and temper to ''medium straw'' another rub with the stone on the flat and you are good to go.

Use plenty of neat cutting oil and low speed say 50rpm & the job's a goodun.


NB, the quality of the finish on the radius will be determined by the finish on the tool.

It's a..mazing what a fellow can do with craytex points.

Mount one on a mandrel, scrubb to size, and shove it down the hole.

A dremel tool is all it takes.