Machine scope base to fit receiver

[sask3500]

I have a mill drill and lathe at the shop, just wondering how gunsmiths cut a radius in the bottom of a rail or mount to fit a round receiver.

Specifically a flat Weaver rail to a round begara rimfire action at 0 MOA or cut the factory 30 MOA rail to 0 MOA.

Thanks.

 

[Jahnj0584]

I milled my receiver flat, it's a lot easier than trying to make that undercut and trying to match radii and create that undercut. A form tool would be king.

 

[Potashminer]

My only attempt to make a base curve fit to a rifle was with hand files. Can be done, but really challenging to do! I found is very hard to keep the filing to be straight, and discovered that epoxy bedding the scope base to the receiver, with little shims, was actually "my friend"!!

In mounting many bases, I prefer ones that seem to have slightly smaller radius than the radius of the receiver - so the edges of the base touch the receiver - torquing the mounting screws often deforms the base enough that it sits snug to the receiver. If the scope base radius is larger - get those edges floating out in the air - no support - torque the screws down and still not much to hold against side-to-side wiggle . So if you get a radius cutter, want to get it a bit smaller than the radius of the receiver - I would suggest. Some years ago I down loaded a chart - perhaps from Brownell's website - shows the underside radius that Weaver claimed to make - on the various Weaver bases - and hole spacing, height, etc. Is not much help about which scope base number goes on which rifle, but you can pretty much get the exact specs for a Weaver #36 base, for example.

 

[Potashminer]

Here is that chart, if it helps: You will likely have to "zoom in" to read it.

 

[Scotty454]

Short of having a cutter with the same radius to match the top of the receiver, you could set up a fly cutter and adjust it to match the radius. Gotta be able to actually measure the radius first though.

You could also just machine a V profile into the base. It wouldn't sit as nice as a matching radius, but it would accomplish the straightness/parallelism part, and be a hell of a lot better than flat.

 

[sask3500]

That's the line of thinking I had, to set it up vertically and make a tool like a flycutter to go in the collet chuck. Because it's a mill drill there's enough spindle travel to cut it.

I still need to shoot the gun today. Theoretically there is enough adjustment in the scope I mounted to accommodate a 30 MOA rail but we'll see...

 

[375 RUM]

8”Horizontal Vertical Rotary Table Rotary table

 

[gaff]

run an end mill on a piece of scrap. angle the end mill to achieve the correct radius. once you get the right radius cut the bases.

 

[sask3500]

That's probably the cleverest approach, even keeping track of the tilt angle and endmill size would get in the ballpark again. I eyeballed it through the bore and sighted in at lunchtime. It zeroed out with the 30 MOA rail Bergara provides so I don't have to modify anything.

 

[trevj]

I have done this with a single point cutter in a boring head. Toe clamp the base to an angle plate, and feed the boring head down across it once you have got it set to the correct radius.

 

[I Dont Care About You]

............

 

[cuslog]

I've done it on a lathe. Mount the rail horizontal in a tool holder on the cross slide. Cross drill a piece of round bar so you can secure a piece of HSS tool steel, grind a suitable cutting edge, amount of stick-out creates the radius. Round bar / cutter in the chuck.
Keep that tool you made - might use it again !

 

[bearhunter]

run an end mill on a piece of scrap. angle the end mill to achieve the correct radius. once you get the right radius cut the bases.

This, but I would grind the endmill to the profile of the receiver. This can get expensive if the end mill is of good quality or quite large.

 

[Butcherbill]

I might be tempted to try the old fashioned way first, especially on an aluminum rail/base. I’ve done this with steel rifle sights to fit them to a shotgun barrel prior to soldering, worked well. Probably took less time than it would’ve trying to setup and do test pieces on a mill or modify a cutter etc etc.

 

[gaff]

If you can't tilt the head on your mill. It's also possible to cut the desired radius with a small fly cutter with the base vertical.
simular to what cuslog suggested.

 

[guntech]

Many gunsmiths do not have a mill but most have a lathe... Me for one.

I would hold the base in the lathes milling vice and use a fly cutter held in the chuck. Did that many times over the years...