Gunsmithing drill bit and tap for scope base screws
- Thread starter bbhank
- Start date
Re: drill bit and tap for scope base screws
I have drilled and tapped ~100 military rifles, including Arisakas.
I used to use a Wheeler Engineering clone of the B-Square drill and tap fixture on a drill press:
http://www.midwayusa.com/viewProduct/?productNumber=149632
Yes, it is possible to sink lower than B-Square.
I have now given up on the fixture, and do it manually with vertical mill.
The edge finder technique can get within .001" accuracy, while the bushinged mandrel in the bolt bore system of the fixtures is prone to larger errors.
The goal is to have a Weaver or Picatinny rail that is normal or 20moa tipped down relative to normal to the receiver face.
That is checked with a scope adjusted to the center in half rings. The scope, when turned in the half rings, should keep the cross hairs on the same spot in the image. The scope's cross hairs are now concentric with the scope tube.
That scope is then used to test that the rail is normal to the receiver face. The adjusted scope should bore sight to a distant target or 20 moa below it.
This is all easy to do with a one piece rail, but harder to do with a two piece mount.
I got a new Marlin 17V out of the box, and in the box were two Weaver mounts. Those mounts could be screwed onto the receiver with all kind of angles.
Rings are made with great precision.
The upper part of a Weaver or Picatinny rail are made with great precision.
The mount to receiver fit has slop.
The goal is to get those two piece Weaver mounts in the same plane on top and on the same line along their edges. That can be checked with a flat surface.
To get the mounts aligned [or one could sacrifice the rings to lapping or buy Burris Signature rings] I fixture them while epoxy cures between the mount and the receiver.
If a chip gets in a drill and tap fixture and the holes are drilled off center of the receiver, I prefer to alter a Weaver mount, rather than weld over and re drill or re drill elsewhere.
To modify the Weaver mount, I waller over the mount screw hole and the countersink on the mill. I waller over the concave radius on the bottom of the mount.
To hold the Weaver rail while wallering over a hole or changing the height or concave radius on the bottom with a boring bar, I fixture the Weaver rail in the mill vise with modified V blocks.
I have drilled and tapped ~100 military rifles, including Arisakas.
I used to use a Wheeler Engineering clone of the B-Square drill and tap fixture on a drill press:
http://www.midwayusa.com/viewProduct/?productNumber=149632
Yes, it is possible to sink lower than B-Square.
I have now given up on the fixture, and do it manually with vertical mill.
The edge finder technique can get within .001" accuracy, while the bushinged mandrel in the bolt bore system of the fixtures is prone to larger errors.
The goal is to have a Weaver or Picatinny rail that is normal or 20moa tipped down relative to normal to the receiver face.
That is checked with a scope adjusted to the center in half rings. The scope, when turned in the half rings, should keep the cross hairs on the same spot in the image. The scope's cross hairs are now concentric with the scope tube.
That scope is then used to test that the rail is normal to the receiver face. The adjusted scope should bore sight to a distant target or 20 moa below it.
This is all easy to do with a one piece rail, but harder to do with a two piece mount.
I got a new Marlin 17V out of the box, and in the box were two Weaver mounts. Those mounts could be screwed onto the receiver with all kind of angles.
Rings are made with great precision.
The upper part of a Weaver or Picatinny rail are made with great precision.
The mount to receiver fit has slop.
The goal is to get those two piece Weaver mounts in the same plane on top and on the same line along their edges. That can be checked with a flat surface.
To get the mounts aligned [or one could sacrifice the rings to lapping or buy Burris Signature rings] I fixture them while epoxy cures between the mount and the receiver.
If a chip gets in a drill and tap fixture and the holes are drilled off center of the receiver, I prefer to alter a Weaver mount, rather than weld over and re drill or re drill elsewhere.
To modify the Weaver mount, I waller over the mount screw hole and the countersink on the mill. I waller over the concave radius on the bottom of the mount.
To hold the Weaver rail while wallering over a hole or changing the height or concave radius on the bottom with a boring bar, I fixture the Weaver rail in the mill vise with modified V blocks.
Re: drill bit and tap for scope base screws
If it were me:
Skip Brownells unless you enjoy a small selection and grossly elevated prices.
Any big name industrial supply can give you all the tooling you want.
MSC
McMaster
Rexx
I'd purchase a high cobalt, split point, screw machine length drill. #1 rule in any machining operation is rigid work holding and the shortest tool for the job. It means better finishes and better accuracy.
If your holes are blind, consider a skip tooth helical tap that evacuates the chip out of the hole instead of pushing it ahead of the tool. Think of it this way: If shoveling snow doesn't it make more sense to pile the snow to the sides or behind you instead of directly in front of what your trying to plow through?
For starting the hole use a standard gun tap with a long lead. It'll help reduce the load on the blind hole tap.
Guns are generally made from tool grade steels in the high 30 to low 40 Rockwell C scale for hardness. This means cheaping out on low quality tooling isn't a wise decision.
Mitsubishi, OSG, Hanita, and Cleveland are all names of good tap manufacturers. Mitsubishi makes some of the best taps out there.
I like Castrol Moly D tapping fluid. I've been at this stuff for over 15 years (machining) and it's the best I've found yet.
It's best to start your holes with a C drill.
good luck.
C
If it were me:
Skip Brownells unless you enjoy a small selection and grossly elevated prices.
Any big name industrial supply can give you all the tooling you want.
MSC
McMaster
Rexx
I'd purchase a high cobalt, split point, screw machine length drill. #1 rule in any machining operation is rigid work holding and the shortest tool for the job. It means better finishes and better accuracy.
If your holes are blind, consider a skip tooth helical tap that evacuates the chip out of the hole instead of pushing it ahead of the tool. Think of it this way: If shoveling snow doesn't it make more sense to pile the snow to the sides or behind you instead of directly in front of what your trying to plow through?
For starting the hole use a standard gun tap with a long lead. It'll help reduce the load on the blind hole tap.
Guns are generally made from tool grade steels in the high 30 to low 40 Rockwell C scale for hardness. This means cheaping out on low quality tooling isn't a wise decision.
Mitsubishi, OSG, Hanita, and Cleveland are all names of good tap manufacturers. Mitsubishi makes some of the best taps out there.
I like Castrol Moly D tapping fluid. I've been at this stuff for over 15 years (machining) and it's the best I've found yet.
It's best to start your holes with a C drill.
good luck.
C
