Gunsmithing Gunsmith technique question

[Killswitch Engage]

I'm curious when I hear about adjusting scope base holes in an action that are not "on axis" with the raceway. What technique is being used to "move" said holes to a new location in the action while basically putting them in the same hole? It's been my experience that pretty much a hole made bigger for a bigger fastener will simply follow the hole that already exists provided you are not using some type of micro boring bar to do this and If you could, how would you not end up with some form of eliptical hole. This is all provided the action holes aren't being welded shut and "re-found". So, let's have some of the colonels famous recipe...how you do it.

 

[Turd in the Pool]

Don't know about the colonels' but most actions are tapped for 6-48 screws. Going to an 8-40 will allow for correction in most cases. If it's way off you may have some original threads that don't clean up but nothing to worry about unless you over torque the screws.

Set the action up and indicate TDC and the x on the front screw. Assuming you have a good tight machine all you need is a jobber length carbide 29 drill to get the hole where it needs to be. You can run an 1/8" end mill first to clean things up. If you've made it this far the other 3 holes should be a breeze. Carbide won't follow a hole. It tends to break if you try to flex it.

 

[The King]

The one time I saw this done it was on a massively strong milling machine using an end mill to cut the hole and not a drill bit. The end mill allows the hole to not follow - needs to be "Center Cutting" rated flat end mill. I've also heard of people making a plug for the holes and loctite redding it in before drilling slowly with tons of coolant. You can use allthread or a set screw to make the plug and then grind it flush.

 

[Big Toby]

I'm curious when I hear about adjusting scope base holes in an action that are not "on axis" with the raceway. What technique is being used to "move" said holes to a new location in the action while basically putting them in the same hole? It's been my experience that pretty much a hole made bigger for a bigger fastener will simply follow the hole that already exists provided you are not using some type of micro boring bar to do this and If you could, how would you not end up with some form of eliptical hole. This is all provided the action holes aren't being welded shut and "re-found". So, let's have some of the colonels famous recipe...how you do it.

At LRI we helically bore them to 8-40 in our Kitamura CNC then thread mill them. That is to eliminate a drill bit deflecting trying to follow the old holes.

 

[LRI]

Drills like to follow an existing hole. So do taps.

If limited to a conventional mill then the best answer would likely be to find an endmill with a diameter appropriate for the 8/40 tap/drill size. Metric mills being your friend here.

Then you power tap as the tap will be less likely to wander.

For us (as john stated) its a different process. Holes are id bored using a 1/8 endmill in a process known as interpolation. Then the hole is thread milled. Assuming the machine is tight (meaning backlash is kept to an absolute minimum) your safe to assume the hole lactions are spot on.

Our kitamura has a runout of .00012" in each axis. This was verified with a Renshaw wireless barball test.

Hope this helps.

C.