Hi Guys,
I have to mount a picatinny rail to a modified M17 for a scope. I thought I'd create the necessary radii for the front and rear receiver ring with the boring head, holding the blank vertically.... But I don't have the height without much stuffing around.
So I thought I might do it by machining 3 flats tangential to the receiver ring radii.
My question is:
1) Is this a done thing? and
2)should I shoot for 3 points of contact of just 2 as they do in action Vee-bedding blocks?

Cheers
Tom

Personally I'd make a fixture to hold the rail and machine one end then flip it and machine the other. But if I had to machine tangent flats i'd go with two only but then i'd also bed the mount for full contact

Similar things are done, and as Stott has noted, the gaps are filled with steel filled bedding compound.

When I do this, I tilt the head 45 degrees and use a fly cutter. The rail is left long on each end, I clamp it flat to the table, mill the radius and cut off the clamping ends.

Jeremy

Either tilt the head and use the bottom side of the end mill to cut an eclipse to form the raduis. You can search for the formula of the cutter diameter vs the head angle.
BUT remember to dial the center of the part back in after you tilt the head.
Also the center of the cut can change depending on which way you are cutting due to deflection.

Or in days past when I had a lot of the bases to cut, I had made a new fixture, shaped like the ring dovetail section complete with a clamp. Then could clamp that on the top of the table to allow the base to hang over the side of the table.
Then dial in its center, rough out all of the bases, reset with the finish cutter and mill the bases again.

JW

Yes that's the elegant way but you wanted to use your 3 flats idea, that would work too.
I do this sort of thing on my lathe with the cross slide vise. Using end mills and boring bars use as fly cutters.