Drilling the center hole in a Monocore

[aresworeaviators]

Hey guys, I'm trying to drill the center hole in a monocore. I bought a long 8" drill bit in 17/64". However by the time it gets 7" in, it seems to be a few thousandths off center. How do you guys drill that hole? Thanks!

[Bowen1911]

how much is a few thousandths? .003? .053?

Maybe you should step your bore so it gets bigger as you go

[maccrazy2]

I would suggest a gun drill before machining the baffles or maybe a wire edm machine. I have not made a mono so these are just suggestions.

[SRM]

Drill the hole, then put between centers to work the OD down to size. Instantly centered hole!

[Historian]

Hey guys, I'm trying to drill the center hole in a monocore. I bought a long 8" drill bit in 17/64". However by the time it gets 7" in, it seems to be a few thousandths off center. How do you guys drill that hole? Thanks!

Do you drill half way in and then reverse and come in from the other end, as they did when they
dug the Chunnel from both England and France?

Also, drill a smaller diameter hole and the bore out to your final dimension.

[dvanncvann]

Drill the hole, then put between centers to work the OD down to size. Instantly centered hole!

this.

[Hoop]

Boring bar....

[82nd airborne]

Boring bar....

17/64"x8"? I dont have a boring bar like that!

[ghostdog662]

Drill the hole, then put between centers to work the OD down to size. Instantly centered hole!

Assuming your lathe is perfectly level that is a great way to get the hole centered.

Just make sure you use a center drill first.

[este]

9 People and only one even mentions the correct tool to use. A gun drill.

The OP never mentions material. If he's using Ti or a good stainless, there is a damn good chance of practically melting a carbide bit inside the material. Is he using coolant? Is it even getting that far in? Are we talking a lathe for fixture or garage-style with a drill press?

While just ramming it in, then turning down the outside may work... A gun drill is the correct tool to use. I have only ever seen one though, years ago at a machine shop I lived near. I think they may be rare-ish.

[sconagher]

I called Thompson gun drill in CA as I worked for a company that used them when our gun drill went down. I asked them how much $ and how close the holes would be to center in a piece of stainless 8". It was $75.00 set up charge and almost $1 per inch. Tolerance was going to be within .02! Told them to go pound sand, I could do way better than that with the jacked up lathe I had access to at that time or on a mill.
Using a drill a little smaller than finish, take small pecks going half the length, turn around and repeat. Use your finishing drill (reamer if you can) and finish the hole to size. Place between center and turn the od. It will give you practice aligning the tailstock on the lathe. Most of them I've seen are always off a little.
Hope this help.

[Homer]

G'Day Fella's,

The correct procedure to drill any hole in a lathe, is to initially set up the material/Metal running true.
Then Face Off the end of the material/metal with the lathe cutting tool.
Then use a Center Drill to confirm the starting point of the hole to be drilled.
Then drill a Pilot Hole with an under size/smaller drill bit first.
Then finally, drill the correct size hole that you need to finish up with!

Hope that helps
Johno

[rcrdps]

Assuming we don't all have access to gun drills,... and assuming you're doing it on the lathe.

Make sure your lathe chuck is mounted true. I just had this problem about an hour ago. Mine has the camlock spindle, and whoever built the backing plate only used 3 of the 6 available cam locks, so it's really sensitive to how much pressure you use on the cams. So the bore of the chuck isn't true with the bore of the spindle.

So, after you indicate for concentricity, run the indicator down the length of your workpiece and make sure nothing changes there.

Gene

[flip]

I did mine on my chicom enco lathe. 4 jaw, get run out to .0005, square the end, center drill the end to start, peck drill with 1/8" bit as far in as it would go (2-3"), then use a 1/4" extended length (12") cobalt bit and peck and clear. Took about 2 hours total on 304 SS using cutting oil as the lube. So far no scuffs or strikes.

ETA: Tornado Tech threaded the Ruger Mark III the can is mated to.

[SRM]

I did mine on my chicom enco lathe. 4 jaw, get run out to .0005, square the end, center drill the end to start, peck drill with 1/8" bit as far in as it would go (2-3"), then use a 1/4" extended length (12") cobalt bit and peck and clear. Took about 2 hours total on 304 SS using cutting oil as the lube. So far no scuffs or strikes.

ETA: Tornado Tech threaded the Ruger Mark III the can is mated to.

Did you measure the runout after the hole was done?

[SRM]

I vote gun drill as well. The difference between a drill bit of the best quality and a gun drill is like apples and steak.

No average Joe that runs a lathe has access to a gun drill.

This IS one of the processes of machining that seperates the men from the boys I guesss.
Six one, half a dozen the other.

[Capt. Link.]

Center it face it spot it drill it half way turn it around and ream it to final size.The reamer will make the hole round and straight.
A gun drill is best but a lathe is the wrong tool for a gun drill.The feed is way to fast and you must have HP cutting fluid pumped through the drill.
PS a half round drill will cut the deepest hole with the least run-out 5D or so.

[57fairlane]

SRM brought up an extremely good way of doing a centered hole pretty easy.

I have access to CNC equipment here at work so it goes a lot faster than you guys without high pressure coolant but basically I setup some soft jaws in the mill, drill the hole 6" (uses 2" starter drill bit) then put it in a lathe. 3-jaw clamp on maybe 3/8-1/2 material, live center in the other end and turn the end with the center in it enough to clean up and make an uninteruppted cut. Flip it around and grip it on the turned end and use the center on the other end.

I am drilling 1" bar stock and usually the holes are within .005" right off the bat.

Doing it this way leaves the holes within .0005-.001" in 6" long pieces and takes 30 minutes or so start to finish to cut a raw piece and have a finished piece ready for a monocore.

[Hoop]

Boring bar....

17/64"x8"? I dont have a boring bar like that!

My point was to drill it from both ends best you can then indicate it off the OD and bore to size.

I built a barrel removal tool for a Beretta Storm this way. It needed to be 13" long with a hole through it about .685" best I can remember. I took a CM steel 1" bar and drilled it from both ends then bored it to size. I made the last 3" removable and built a few ends to use with different calibers. A set screw keeps the end adapters from spinning off.

When I was done, the through hole had no detectable runout.

[flip]

I did mine on my chicom enco lathe. 4 jaw, get run out to .0005, square the end, center drill the end to start, peck drill with 1/8" bit as far in as it would go (2-3"), then use a 1/4" extended length (12") cobalt bit and peck and clear. Took about 2 hours total on 304 SS using cutting oil as the lube. So far no scuffs or strikes.

ETA: Tornado Tech threaded the Ruger Mark III the can is mated to.

Did you measure the runout after the hole was done?

Nope, only measured hole dia., did the look down the bore test with the can on. I put some machinists dye on the last three spots where the bullet passes through and no scuffs after 10 shots. That was my runout test.

[Bowen1911]

Nope, only measured hole dia., did the look down the bore test with the can on. I put some machinists dye on the last three spots where the bullet passes through and no scuffs after 10 shots. That was my runout test.

And that is all that is needed

[aresworeaviators]

HOLY COW! I didn't get an email so I didn't know anybody had responded! Thanks everyone for the overwhelming amount of great replies. I have an Enco 12x36 lathe, and I drilled the hole in aluminum (7075).

As far as the gun drill - I'm not even sure what that is, and it sounds as if most people don't have access to one. I really like the idea of using centers on the holes, which would make the hole centered. I was planning on leaving the full aluminum stock (about 24" long) attached to the monocore until I was ready to part it off. I guess I'll part it off and then use centers, but I will have to make sure that my lathe is perfectly centered (I just got the lathe recently, haven't done any real precision work on it yet). I also like the idea of drilling from both sides, but I already drilled the hole and did a bunch of machining on the monocore on my bridgeport, I'd have to do that all over again.

How close do my tolerances REALLY have to be if the hole is 17/64 and this is for a .22? It seems like that should be plenty of "error" room.

[rcrdps]

You're probably good to go. You'll get more error in the several very likely times that your supressor gets a little loose. If we all drilled perfect holes, and our supressors never got loose, we wouldn't be needing .260 on a .224 bullet. I doubt your hole is drilled off by 40 thousandths. That'd be drill press quality.

Gene

[ghostdog662]

At 17/64 you would have enough room for .223, so .22 would be more than adequate assuming your lathe isn't setup on hillside.

[aresworeaviators]

Thanks guys, that is sort of what I was thinking (and hoping). Time to make some more chips!