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Methods for drilling flat bottom hole in aluminum gunsight

bootster

Plastic
Joined
Feb 5, 2007
Location
Maryland,USA
I need to drill a 11/64" pilot hole with a 1/4" flat bottomed hole to accommodate an 8/40 screw. What is my best route in doing so? TIA.

EDIT: I just found these in one of my shop drawers. The last three on the right are .249", .252, and one that's too small. Would they be of use? I don't see where I can add this attachment. Maybe I will open a new reply and try it that way.
 

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If you have a milling machine, McMaster Carr center cutting end mill 8876A34

Practice before you do it on the gunsight
 
One could drill to just shallow of flat bottom depth with a undersize drill first. That makes it easier plunging with the endmill.

Also protect from chips swirling and scratching anything. Pecking might be a good process.

For sure practice on material that is the same or close first until you get everything figured out well. Great rational advice.
 
If you have a milling machine, McMaster Carr center cutting end mill 8876A34

Practice before you do it on the gunsight

I have a 1947 Delta drill press that's not the combo unit that could be geared down for drilling in metal. It is the wood model. I am wanting to use a cutting end mill, but don't know whether the speed of the drill press would be up to the task. The 8876A34 is 11/64". I need 1/4". Am I wanting the 8876A17? Thanks for your help. Much appreciated.
 
I have a 1947 Delta drill press that's not the combo unit that could be geared down for drilling in metal. It is the wood model. I am wanting to use a cutting end mill, but don't know whether the speed of the drill press would be up to the task. The 8876A34 is 11/64". I need 1/4". Am I wanting the 8876A17? Thanks for your help. Much appreciated.

Speed isn't your problem. A 1/4" HSS endmill will plunge cut a piloted hole with decent results anywhere from 500-5000+ rpm. Your problem to do this job with a tool like that is going to be keeping everything rigid, on location, and concentric. Keeping speed below 1000 rpm will probably be ideal for you since your setup is likely
to be lacking in stiffness. Best of luck.
 
I love your idea of practicing on something first. My idea is to start with something in the 1/8" or less for the initial pilot, and then working my way up to 11/64". These end mill cutters are rather expensive for a one time gig, so I am probably going to have to buy at least two diameters. I guess I could go from 1/8" to 1/4"?

I guess I should mention that the two holes that I already have drilled are holding the scope mount on rather tightly as it is. The gunsmith who installed the scope mount 30 years ago used a one piece mount that had the two holes in the rear, and he only used a single screw in the front of the mount due to the fact that there isn't any metal at all in the front of the receiver, and it's not a steel receiver. It may be aluminum, but there is only enough metal to tap into in the rear of the receiver.

Maybe I'm overthinking it? It worked with just the two holes in the rear for 30 years, so do I really need to add this extra screw? I am pretty amazed at how tight it is with just the two screws in the rear. Maybe a single screw in the front with some blue locktite on it would be sufficient? This extra hole will end up costing me over $50, but is the extra satisfaction of knowing it's there be the difference between a shot of a lifetime, or a failure due to the "Compromise"?
 
I love your idea of practicing on something first. My idea is to start with something in the 1/8" or less for the initial pilot, and then working my way up to 11/64". These end mill cutters are rather expensive for a one time gig, so I am probably going to have to buy at least two diameters. I guess I could go from 1/8" to 1/4"?

I guess I should mention that the two holes that I already have drilled are holding the scope mount on rather tightly as it is. The gunsmith who installed the scope mount 30 years ago used a one piece mount that had the two holes in the rear, and he only used a single screw in the front of the mount due to the fact that there isn't any metal at all in the front of the receiver, and it's not a steel receiver. It may be aluminum, but there is only enough metal to tap into in the rear of the receiver.

Maybe I'm overthinking it? It worked with just the two holes in the rear for 30 years, so do I really need to add this extra screw? I am pretty amazed at how tight it is with just the two screws in the rear. Maybe a single screw in the front with some blue locktite on it would be sufficient? This extra hole will end up costing me over $50, but is the extra satisfaction of knowing it's there be the difference between a shot of a lifetime, or a failure due to the "Compromise"?

Can you post pictures? Might be good to have a look at it before you use your drill press on it. I would not recommend that very highly as likely your drill press is not that tough and solid.
 
This countersink hole only needs to be about 1/8" deep. I could start with the smallest end mill I have posted a picture of, and then ream it out slowly with the .252 one. Am I going in the right direction here?

EDIT: I have to recreate one of these holes elsewhere in the scope mount.
 

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Correct me if I am wrong, but you only need one 1/4" endmill?

There's no need to use anything other than a regular drill for the through hole to start, then counterbore it up with the endmill without moving the part.
 
Buy a piloted countersink for the screw. Trying to plunge an end mill to get a flat bottom hole on an old drill press sounds like a risky task.
 
As I understand it, you already have two holes at the rear that are the proper size and you only need to drill one more hole at or near the front. And that front hole does not yet have any pilot or undersized hole drilled in that location. This last condition is important for what I am going to suggest.

The traditional way to make a counterbore for a screw that has a flat bottom head is to first drill the hole at the nominal clearance size for the screw and then use a counterbore with a pilot that fits the clearance hole. That provides the needed guidance for the counterbore. Another way is to use a flat bottom milling cutter IN A VERTICAL MILLING MACHINE. A counterbore must have the guidance provided by a close fitting pilot and an end mill requires that the work be held firmly in one and only one position below the quill. Otherwise either of these cutters will wander off center while cutting and a broken cutter is the likely result. Counterbores are expensive and usually only have one pilot size available for each CB size. Vertical milling machines are even more expensive.

I have successfully used another way to make counterbores in screw holes. I use the DeWalt Pilot Point drill bits to FIRST drill the counterbore. These pilot point drill bits produce a near flat bottom for the full diameter step and a small, pilot hole below that level. When I say "near flat" I mean a slightly convex surface which the screw head will seat on near the thread diameter and the outer edge of that screw head will be just a few thousandths above that "flat" bottom. With just a little bit of torque, that "flat" surface will be brought into complete contact with the screw head as the metal of both cold flows just a little bit.

DeWalt pilot point drill bit 1/4" - Bing - Shopping

Select the "DeWalt pilot point drill bit 1/4" " from that page to see what I am talking about.

Then after the counterbore has been created I use a body/clearance sized drill bit to open that pilot hole left by the pilot point bit to the full clearance size for the screw.

If you use this combination of drill bits to create a hole with a counterbore it is necessary to do the larger, pilot point drill FIRST so that it's small, pilot section will provide the centering action that is needed. This can be done on a drill press. A milling machine is not needed.

I have used this technique for screws and bolts that see far greater loads than your scope mount will ever see.
 
Thank you EPAIII for that response, and the rest who posted responses as well. I used to have a set of those bits, about 25 years ago, that were called, "Bullet Bits". They were made by Black and Decker at that time I believe. They have the little pilot bit sticking out from the main size. I think I have resharpened all of them to a split point, or whatever I had set on my drill sharpener. I will get one and do as you suggest, as I can see this being a very good way to do it.

Thanks to all who have offered help here.
 
When I need a flat bottom hole like hole in a rifle barrel to mount a scope or iron sight, I first drill with the appropriate tap drill, then with the same size tap drill that I have ground to drill a flat bottom hole. I use only a drill press to do this. When drilling through bases like in your picture, I make drill bushings in the lathe that fit the counterbores in the base and use them to ensure that the hole is centered and in the right place.
 
Is it really necessary to have a flat bottom hole in this case?
I mean if there is no break through because of the tapering drill point why would it be necessary? I mean one could just take a little material off the end of the screw not much but just enough to clear.

Sometimes the drill hole for a tap will not allow anything but a smaller endmill interpolating the bottom not taking away material needed for the tap threads.

One can even put a taper on the screw close to the angle of the drill itself. Thanks for the picture I see a existing hole and counterbore and grooves on the surface which you will put in a tapped hole with what you want being a flat bottom.

Cutting a spot on that ridged surface is needed before drilling that so that your drill does not walk and gouge what you already have. Since it is a rifle machinists will go the extra mile/s to insure nothing is screwed up. No pressure right? Someone has to do it why not us.
 
I have a good selection of drills that have had the bottom ground flat. You drill the hole normally and then switch to the flat bottom drill. Unlike an endmill the flanks of the drill won't cut so there isn't much walking around when flatting the hole.

4C14EDB1-90B1-4E68-AE90-87F47C88415D.jpg

If you have steady hands you can take the counterbore drill and grind a step in it for the thru hole diameter and punch it in one go. I have a t&c grinder but have spun them with a cordless drill against a bench grinder before. Add a little relief to the cutting edges and it works fine.
 
Trueturning, I made it clear that I am just replicating the hole in the scope base. I am not going to be tapping anything in this process. There is no mention of anything tapered in my OP. The 8/40 screws are flat at the bottom, so the scope base fits tight to the receiver, that's why a flat bottom is needed. I will drill a new hole in the receiver, and tap it after I get the new countersunk hole in the scope base, but that's not what I came here to ask how to do. I'm going to do it myself. Thanks for the reply.

Nmbmxer, thanks for that tip. It's much appreciated.

Thanks Gazz. This is a clearance hole for the screw. The tapped hole will go all the way through the receiver. I don't want the hole in the scope base to be tapped at all, as it's just a hole for the screw to go into a tapped hole. If you are talking about sending the tap drill through the original hole for locating it, and reaming the scope base out afterwards for the screw to clear, then I see what you mean. I just need to replicate the hole in the picture. It's a countersunk clearance hole for a tapped screw, yes, but I am just interested in drilling out the scope base for the tapped screw to clear. I got the specs for the hole from the scope base manufacturer. Thanks again.

EDIT: I meant to say that the scope base isn't going to be tapped, and I said "Receiver". The scope base will have a clearance hole, and the receiver will be tapped for the 8/40 screw. I have fixed the mistake.
 
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Buy a piloted countersink for the screw. Trying to plunge an end mill to get a flat bottom hole on an old drill press sounds like a risky task.

Do you have a link to a bit that would do that? I need an 11/64" pilot with a 1/4" flat bottomed countersunk hole. I have looked all over, without finding one. Thanks for the reply.
 
Do you have a link to a bit that would do that? I need an 11/64" pilot with a 1/4" flat bottomed countersunk hole. I have looked all over, without finding one. Thanks for the reply.

McMaster-Carr

$25 or so on McMaster. Most people have plenty of 1/4" endmills hanging around, which is why that was the first suggestion if you have the body clearance hole drilled already. Try contacting user Bluechipx, I know he had a ton of surplus 1/4 inchers, was selling them a buck a piece. Might still have an active thread in the for sale section here.
 
It is entirely possible to make a drill bit with a pilot in a home garage or small shop. I needed one to cunterbore an existing hole so my Bullet drill trick would not work. Here is the one that I made while holding the drill bit in a hand held drill and using a Dremel to grind the tip down. After I had the flat bottom and the pilot section, I sharpened the flutes, again with the Dremel and by hand.

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And this photo shows the set up on my workbench.

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IIRC, this was an old, broken 3/8" drill bit that I started with.



Do you have a link to a bit that would do that? I need an 11/64" pilot with a 1/4" flat bottomed countersunk hole. I have looked all over, without finding one. Thanks for the reply.
 








 
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