Drilling holes for scope mounts

[Double J]

--I'm looking for a better way to drill and tap recievers. I have a fixture from Brownell's that works nice, but my concerns are with my drill press. It just isn't consistant enough to trust. So I ordered a micro-mill that is supposed to be accurate to within .002". Any thoughts on useing the mill?

[srtrax]

I have used a table top mill for years, and used it for drilling scope mounts. I have the Forerster jig that i use. I put my jig in the vise and use it for clamping the jig in place. If you'll indicate the back jaw of the vise with the quill of the mill, get as close as you can to zero runout on vise, should be no problem. But once that is done it would'nt hurt to put jig in vise and run an indicator across it too! To check for runout. I don't have to remove vise to make diffrent set-ups, and saves me time this way. I don't have and wish i did, is a digital readout, thoes are worth there weight in gold, planning to get one soon, have one on a big mill and love it! You will wonder how you ever scoped a gun with a drill press after you use the mill.

[Double J]

--I'm hoping the little mill will have enough clearance. Didn't consider that when I ordered it. I was only worried about being able to get a consistent depth. Also, will a digital readout fit on the micro- mills? That is a good thought.

[srtrax]

Sorry but i really don't know much about the micro stuff, hope it works for you. As far as a digital readout goes if its available i do recommmmend it. With a DRO. if you can get your reciever set right, and know the hole spacings, then its just a matter of drilling them.
With the Forester jig i get the reciever rails 90* to the mill, and set the bbl. in the v-blocks and drill, knowing the spacing of the scope mount holes, move it as needed. The DRO measures only table movement and not any backlash in the machine, so there nice to work with!

[brickeyee]

"With a DRO. if you can get your reciever set right, and know the hole spacings, then its just a matter of drilling them."

How did we ever survive without them????
Either note position of dial or zero.
Rotate dial counting turns (this is why you want a 0.100 per turn feed) then any fraction of a turn.
The table is in the correct position.

Zeroing and aligning take longer than anything.

[Clark]

I have a B Square drill and tap fixture for bolt actions.
It has different bushings for the drill and the tap.
I can do a rifle in a few minutes on the drill press.


It takes me a long time on the mill to drill and tap.

With pistols, not meant for scope mounts, I have to use the mill to drill and tap on the mill.

I always use the fixture on the drill press when I can.

[Double J]

--Thanks guys. Just ran a couple practice runs with the new mill on octagon barrel and round barrel blank. Sweet! :)

[mikikanazawa]

Congrats on the mini mill. You can do a ton of receiver work on them. A proper setup will take a fair bit of time, but once you have everything true and clamped down the cutting only takes minutes and you will have the confidence and pride of a clean and correct job.

You should practice on a lot of scrap before you begin the real work... there are a myriad of mistakes to be made, heh.

[Harry Bonar]

Dear Sir:
Always - always - use a smaller dril for your hole than the specified drill.
For instance, you'll be using 6x48s mostly so use about a 35 drill and then ream with a 31 - that gives you a real accurate 31 hole. Drilling with the "tap drill" ALWAYS will give you over a 31 hole! Threads on 6x48 or 8x40 aren't very deep threaded anyway - you need every thou' to get good holding screws.
Also, after tightening down use a driver and "TAP" the screw - this seats it with the base and you'll be able to tighten a smooch more.
Harry B.

[Clark]

I usually drill a larger hole that specified on the Starrett drill and tap chart on the wall in my shop. This makes it easier to tap. And I use Tap Magic to lubricate the tape. I cool the drill with Cool Mist mixed with water in a plant sprayer bottle.

But for 6-48 holes for scope mounts [not a standard size and so not on the chart], I use the drill that small #31.

I like to pull the barrel on Mausers, so I am not drilling a blind hole in the large ring.


If I am not in drill and tap fixture with drill bushings, I start the hole with a short stout spotter drill.

[James K]

Hi, Clark,

Sorry, but Harry is right, never use a larger tap drill than that specified. Sure, the tapping will be easier, but the threads won't be as deep and can easily strip when the screw is tightened, or the screw will work loose easily under recoil.

As for the B-Square jig, its purpose is not only to get the right screw spacing but to make up for any looseness in the drill press since the drill bit is guided by the jig, not the drill press. If a mill is available and getting the correct spacing is no problem, OK, but the jig will work fine and eliminate a lot of guesswork.

(BTW, a No. 31 is the right tap drill for a 6x48 screw.)

Jim

[Clark]

The guy I learned that trick from worked at Boeing for 30 years on a mill.

When you fly on a Boeing plane, you are flying with some holes that were easier to tap.

When I mount a barrel vise on a 3/8" steel plate bench with 1/2-13 bolts, there is little risk that I will strip them out if the holes are bigger.

The force on the scope mount is shear in front and rear, compression in front, and tension in the rear.

The threads that might get pulled out in recoil are in the rear.
I have been unable to pull any 6-48 out in the rear even with very hard recoiling rifles.

I do think they can be stripped out with over tightening on installation.

I have drilled and tapped ~75 rifles with 6-48, expecting to replace with 8-40 if they strip out.

So far none have every stripped out.

I have had a tap break.
That is a mess.

[kirbythegunsmith]

I post this for the non-machinists in the audience:

Tapping a hole of normal size relationships of drill hole, etc., you have roughly all of your retention strength at the level of screw breakage strength once your hole reaches a depth equivalent to the diameter of the screw thread. This only counts when screw and hole material are similar, not a steel screw in aluminum.

Going deeper in the hole does not gain materially in the strength, but gives more surface for contact, loctite retention grip (you can use a lesser grade, maybe), and may be OK for longer screw hole depth to have a more opened hole.

Short holes always have a need for strength retention, and oversize holes there are not wanted. I might consider going one size down on drill size when tapping a short hole, but maybe one size up for normal depth holes in very tough to drill materials. You can also just partly drill with the larger size, and finish with a regular size drill, so the tap has a better chance to align and start in the tough stuff. Consider doing that through your pilot hole that is undersized, not put a #30 a little way into a #31 hole, for example.

Drill press, etc. usage may have the drill pull itself in by way of the helix grabbing and yanking the drill in like a self-feeding screw, and snapping off drills or slipping the shank in the chuck are other bad times to avoid. That possibility also exists during break-through in an existing hole, especially when the under surface is not flat. A curved interior has 2 "high" spots to catch the drill tip at breakthrough point, so go gingerly there, OK?

Hope this info. is helpful.

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[Harry Bonar]

Sirs:
Technically on a hole and taping a 75% thread will hold 90% of what a 100% will - but I prefer 100% - but - when you use a 31 drill and a 6x48 tap you're only getting a 75% thread.
As the smithy has earlier said (I think) four threads will hold all the screw will in parent steel.
Harry B.

[brickeyee]

Machinery's Handbook has the thread depth tables of you want to adjust thread depth, but remeber the rule about drill bits ALWAYS producing a larger hole than the bit diameter. Usually only a few thousandths, but that can be a lot for fine threads that are not very deep to begin with.

The other mechanical rule is that no mater how many threads deep a hole is tapped, only about 3 threads actually do the work.
Even thread pitch has a tolerance on it.

Course threads are prefered in soft materials, while finer threads can be used in harder materials.

[alexander hamilton]

good info

[Harry Bonar]

Sirs:
The only way to get 100% thread is to drill your tap hole to the MINOR diameter in your book - that will give you as close to 100% as possible - most thread guages on charts are for 75% threads.
Harry B.