How do I drill small holes straight? (I made a suppressor adapter)

[strantor]

It could be your drilling technique. I start a hole with a center drill that I continue to drill with until I'm as deep with it as I can go. If the hole is larger than 1/8", I start there. It's a PITA, because you have to clear it FREQUENTLY. But this creates a straight hole that I can now put a drill WITH THE SAME DIAMETER as the center and continue drilling. The hole created by your center will perfectly guide your twist drill and minimize wondering. Peck drill a hole with no thru pilot. Clear the bit frequently. Use adequate oil. If you need a larger hole, step drill in small increments up to close to your finished size. Bore to final size, if straight is your goal (and it usually is).

That's the technique I use for drilling (mostly steel).

I did use a center drill first. On the first two blanks, just to make a divot to start the twist drill in the center. On the second two, thinking along your same lines, I decided to take the center drill as deep as I could. Broke two center drills. I think I did not evacuate chips often enough and they bound up.

 

[strantor]

This also assume your lathe is all squared up and wants to drill straight holes, sometimes they don't, which is a PITA but fixable usually.

If I suspected my lathe was at fault, how would I confirm/fix?

 

[rgray]

The gun drill will need a hole to enter to work.
I use an end mill for that purpose. stiff and makes straight start hole.
Gun drill machines use a guide bushing for that purpose.
Lathe alignment is still needed as the start hole needs to be straight. The gun drill will follow the start hole and continue in the same direction.

For doing it with out a straight hole, you could drill the hole then use a tapered mandrel between centers to do the outer machining and threading.
That way the hole is now straight with the body of the part.
You would have to be real sure the part didn't slip on the mandrel. That would make a mess of you're threads.
I use super glue sometimes. It holds well and releases easy with heat. Downside of the releases easy with heat is if you heat it up in the machining process it will release. I use flood coolant to avoid that.
If I don't have the right size mandrel I just make one for the part. But then I have a cylindrical grinder for that.

 

[strantor]

For doing it with out a straight hole, you could drill the hole then use a tapered mandrel between centers to do the outer machining and threading.
That way the hole is now straight with the body of the part.

I'm glad you mentioned that. I also have been planning to build a spider to align the barrel better, get more accurate threads there, and I've been looking at these range rods to indicate the bore. They are tapered as well. That seems odd to me, as does your suggestion of a tapered mandrel. Why tapered? It seems to me that you want them uniform. it seems to me that a taper would leave room for slop on one end or the other. At the risk of sounding totally stupid and unqualified; the bore of a firearm barrel isn't tapered.... right?

 

[nnam]

I know everyone here are more experienced machining than I am. But may I add.

Taper probably gives a center alignment, like tail stock center.

You can machine a press fit arbor and press it in to hold if you can make a good straight part for that instead.

 

[strantor]

I know everyone here are more experienced machining than I am. But may I add.

Taper probably gives a center alignment, like tail stock center.

You can machine a press fit arbor and press it in to hold if you can make a good straight part for that instead.

The difference (as I see it; still waiting for confirmation) between inserting a tapered mandrel into a straight gun bore, and inserting a tapered tool holder into a tapered tailstock is, the tapered tailstock is tapered just like the tapered tool holder. As an extreme example, you know those conical paper cups used at the water cooler or for sno-cones? take one of them and nest into another conical paper cup (tailstock) - self aligning. Now take the paper cup and nest it into a toilet paper roll - self centering on one end, and free to flop around on the other.

I know I'm missing something here; tapered mandrels/range rods wouldn't be the recommended method if I knew what I was talking about. Would like to know where I'm going wrong in the logic.

 

[SLK001]

I did use a center drill first. On the first two blanks, just to make a divot to start the twist drill in the center. On the second two, thinking along your same lines, I decided to take the center drill as deep as I could. Broke two center drills. I think I did not evacuate chips often enough and they bound up.

If you broke two center drills, you need to clear the chips more often. At MAX, you can probably get 0.050" before you need to clear. I know that it takes time, but it will drill a very straight hole. It will also provide the guide for the twist drill OF THE SAME SIZE.

 

[strantor]

If you broke two center drills, you need to clear the chips more often. At MAX, you can probably get 0.050" before you need to clear. I know that it takes time, but it will drill a very straight hole. It will also provide the guide for the twist drill OF THE SAME SIZE.

I will try to muster some more patience next time

But what about one of these options? Might one of them achieve the same straightness with less patience required?

1/4" Carbide Tipped Die Drill

www.msdiscounttool.com

1/4" Solid Carbide Straight Flute Drill - USA

www.msdiscounttool.com

Single End - - 1/4" Carbide 4FL Single End Mill - Extra Long Length

www.msdiscounttool.com

 

[rgray]

At the risk of sounding totally stupid and unqualified; the bore of a firearm barrel isn't tapered.... right?

My closest mandrel is .2185 on the small end and .2205 on the large end with the tapered part being approx 3 inches long and overall length is 3.75.
The ends are smaller and have a flat for the driving dog.
smallest mandrel I have is .1245 small and .126 large end. Taper on it is 1.9 inch long.
It's a very slight taper and a part that fits wedges on tightly.
It's very easy to miss the size and your part almost tightens up, In that case I have used superglue or built my own mandrel to fit.

I built my first mandrel on my lathe. So don't be afraid to do that.

No taper in barrels.
Range rods I think used to be built with slight taper changing to more taper to wedge in the barrel.
Now (pacific tool) is building them with replaceable bushings on the end. You find the correct bushing that fits your barrel and it goes
on the end of the rod and when pushed in it bottoms on a slight taper so is centered in the bore.

If you use a mandrel without superglue (or maybe even with) your cutting pressure should be in the direction of forcing the part
farther onto the taper as opposed to pushing it off the taper.

Purchased mandrels are hardened and may even be HSS . The part is driven onto the mandrel with a deadblow hammer.
Usually I'm driving the mandrel into the part actually.

 

[pacifica]

A friend brought me this cheap little .22lr pistol, wanted me to come up with a way to adapt a suppressor to it. This is what I came up with. Turned down the end of the barrel and threaded it 7/16-20, then made this adapter with a 7/16-20 internal thread and a 1/2-28 outer thread. Worked well, after going through 4 failed blanks where the bore was crooked. Drill bit wander. How do I get better than a 20% success rate? I have somehow avoided learning this lesson; everything I've drilled/bored thus far has been larger diameter. The bigger bits don't wander as bad, and I'm usually going in with a boring bar afterwards anyway so it doesn't matter. I plan to do this again and I want to do it better, more predictable outcome. Is there a different tool I should be using other than a drill bit? I know a better ground drill drills a straighter hole but what drills a straight hole?

I've good success with a gun drill, (try ebay for good deals) also start with a spot drill and drill from both ends to control deviation and drift.
If you put the drill in your tool post holder you can perfectly center it on the hole you want drilled(don't trust your tailstock for precision).
Under drill by .01 and ream to desired diameter if you don't want to use a boring bar.