I’ve done one breech block cut in a receiver using a Bridgeport quill.  For the cutting tool, I used a square carbide insert and it worked well, and was much faster than I thought it would be.  However, I found I had to move the receiver away from the cutter on the upstroke to avoid chipping the insert.  One day I may actually finish the project!

Another really important thing, with square cuts, is to grind your cutter with a corner radius, as jhm suggested, to help with stress risers, it will also make the cutting edge last longer.

A corner radius as small as 1/64 will help, 1/32 is more than twice as strong. A excellent finish in the corner is also very desirable.

50 years ago when just starting my career in a mold shop and before we had our first tube type Elox EDM I used a Bridgeport Quillmaster to cut square pockets in the mill. A single flute cutter ground at a 90 degree and the Quillmaster set at 45 degrees did a slow but decent job to remove the radius from the corners of a pocket. I was curious and did a search, quite a bit of information on the internet.

I like the way the square corner cuts. Could I add a radius with a small round file?

frnkeore wrote on Sep 20^th, 2023 at 1:54am:


Sorry. I did not make it clear. I don't want to change the tool. I want to add the radius to the finished cut. The tiny radius would extend beyond the walls, but the corner would disappear!

I think the mortise corners on the #1 are cast that way. Why not? I appreciate your bringing it up.

Do as GT says and finish it with a thin piece of emery (about 220 grit) wrapped around the file, to try to take most of the scratches out, from filing.

The cutter using 1/2" HSS. The cutting point is on the center line of the quill. It is sharpened only on the end; not on the sides.


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It takes a rather generous 1/2" cut and can require more force than I like to enter the material. I resolve the issue by  filing a small relief angle at the beginning of the cut.

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I usually advance the cutter a couple thousands at a time. With the relief angle the cutter enters the material easily. With oil on the material I can feel the cutter action as it shaves. I continue to move the cutter up and down in that position until I no longer feel it shave. It is a learning experience. The receiver will be cut some in its shaping which removes the relief angle so there is no cosmetic concern. I have this hole ready for the final clean up cut.