How do you avoid run-out when drilling a pistol barrel?

[Rolf]

I've found a source in Norway for gunmetal round stock (up to 2" diameter).
My brother in-law has a huge (2000lb) industrial lathe and is willing to try to make a couple of 12" smooth brass pistol barrels.

But he is worried about run-out when drilling the barrel. He thinks the drill migth come out of the barrel stock an 1/8" or more off center.

How do you avoid this?

Could  we, after drilling the barrel, mount it in the lathe with a cone(spindle?) in each end of the bore and turn the sides paraell with the bore?

Best regards

Rolfkt

[jerrywh]

Drill the bore first. Then, turn the exterior between centers for a perfect barrel.

[Dphariss]

Ideally you should use a gun drill type drill. Spin at high speed 1500 rpm at least and used a lot of cutting oil. Spin the barrel and not the drill.
Drill slow.
Twist drills tend to wander.
Still have to machine on centers to get the bore perfect at each end.

Dan

[David Rase]

The last 2 pistol barrels and the blunderbuss barrel I made were pilot drilled from each end, then followed up with a drill several thousandths of an inch undersize and then reamed.  No problems.
DMR

[Rolf]

I've got a 10mm twist drill thats long enough for a 12" barrel. Would this be suitable for a 0.40 caliber bore?

David, where can I get a reamer thats long enough for a 12" pistol barrel?

Best regards

Rolfkt

[rick landes]

Jerry is right on target (as is his norm  ).

When drilling out larger pieces (such as a cannon bore) we start with a smaller drill for the pilot hole then work up to the near (about .050 min undersize) final size. We use a 12 flute reamer to do the final cut and polish out the tube when done.

Squaring everything to the bore and cutting the outer dimensions is the final step.

A 12" hole will not have much walk out. Just take your time, cut slow w/o "horsing" the drill through the material, clean your chips out often and oil with a good cutting oil.

[bob in the woods]

I did a brass pistol barrel and used a square type reamer, like shown in "the gunsmith of Williamsburg"
The corners do the reaming, and it went pretty fast on the brass. My barrel is only 6 inches long and I did use a twist drill, but, found that a slower speed, not faster, worked better on the brass. It is pretty "gummy" stuff. A sharp bit, and clean often was the mthod I figured out [ by the time I was finished! ]

[Rolf]

Rick, you said you used a smaller drill to make a pilot hole.
I would have though, the smaller the drill the more run-out you'd get.
What size would you recomend for the pilot hole?
I'm aiming for a 10-11mm bore.

Can anyone recommend a good supplier for  twist drill suiteable for 12"-14" pistol barrels? Here in Norway I can't get anything larger than 10mm thats long enough.

Best regards

Rolfkt

[Acer Saccharum]

A bit must be sharpened specially for cutting brass to avoid grabbing. The first hole can be drilled with a standard sharpening, but if you are going to enlarge the hole with a bigger drill, you must sharpen your bit so it doesn't grab. 

Grind or stone the sharp leading edges of the drill lips to form a small flat that is parallel with the long axis of the drill. A small vertical flat on the leading edge will prevent the drill from pulling itself into the brass, oft breaking the drill, or stalling the machine, and ruining the bore.

Here's a case where an illustration would save a thousand words.

[JBlk]

In the Early sixties I worked in a large factory producting earth moving equipment.Next to the four spindle speed drill that I was operating was a rifle drilling machine.The unit had two spindles that drilled from the bottom up using a heavy volume of coolant which was water mixed with cutting oil. The drill spindles would drill for a few seconds and then withdraw from the hole which was flushed with the coolant.This cycle continued until the hole was completed.The drills that were used was the lenght of the desired hole with two two flutes and what was called a rifle grind.The cutting edge was flatter that a conventional drill and it had a chip breaker ground through the center of the point.This type of grind would cut much faster than a conventional drill point, and produced small chips instead of a long thread of metal.The material being drilled was in a fixture made to hold the material being drilled and a drill bushing guided the drill in and out of the hole.I assume that this drill was left over from the war years and the company adapted it to another use after the war.If I remember right the feed was very slow and the spindle speed was very high.This might not help you with the runout problem but thats how they accomplished the problem and held a very close tolerance.

[rick landes]

Rick, you said you used a smaller drill to make a pilot hole.
I would have though, the smaller the drill the more run-out you'd get.
What size would you recommend for the pilot hole?
I'm aiming for a 10-11mm bore.

Can anyone recommend a good supplier for twist drill suiteable for 12"-14" pistol barrels? Here in Norway I can't get anything larger than 10mm thats long enough.

Best regards

Rolfkt

With that small of a hole I would (after chucking the piece checking square and the like)start with a center drill and mark a good 3/4 of finished bore size starter hole. I would drill it with a bit very near the 8 mm mark and let the finish reamer clean up the remaining mil. The finish dia is quite small and you are exactly correct with the observation of the smaller bit tending to track more. However, the bits following the pilot will only follow the lead made by the pilot. Brass is very easy to machine compared to other metals. It does have a tendency to self feed so go slow on the boring and clean the chips from the flutes more than you might think you need to do.

 I had spoken to a good machinist about boring a pair of 3/16" touch holes in a 16" seacoast mortar (about a foot of boring!) He did it with a starting pilot and one pass...

http://www.mcmaster.com/ has long length extension bits that will meet your need. What I could not find in the 10-12 mil dia was a suitable finish reamer that was not in the high dollar price range...

Just a thought, perhaps since you are only doing 2 barrels do you have a machine shop that would bore/ream the barrel blanks for you? It may be more cost effective since bits and reamers of this length are quite pricey...

[T*O*F]

Rolf,
Here's how the backyard mechanic with no lathe handles the problem.



Drill your pilot hole.  Select a unibit that comes closest to the final size you want.  I think the smallest available progressively tapers UP to .50.  They are also available in .75 and 1.00.  Select the step that is closest to the final size you want and grind off everything above it.  Mount it in a 12" drill extension and slowly drill your hole.  Since the bit is tapered, it will follow the pilot hole.  Depending on the hole size, you may also have to reduce the the diameter of the bit extension somewhat.  Once done, hone the hole with plenty of oil until it's smooth.  Stones are available in a variety of grits.

Do not underestimate the hone.  When I raced motorcycles, we used to use a cylinder hone to bore out the barrel to fit the next oversize rings and/or piston, with great accuracy.  Highly modified, two stroke engines are prone to the frequent seizing of pistons due to heat transfer problems.

You can also use this to remove bad rifling from pistol barrels to make them smoothbored.  And it's a $#*! of a lot cheaper than buying all the expensive lathe tooling.

[halfmeasures]

TOF - I believe Rolfkt wants to avoid runout, not induce it. Jim

[T*O*F]

I believe Rolfkt wants to avoid runout, not induce it.

Jim,
Not everyone has a lathe at their disposal, nor to they want to invest in expensive tooling for one-time use.  I was simply pointing out that fine work can be done by more inexpensive methods.  The hole won't have any more run-out than if drilled by other methods.  In lieu of the hone, he could also make a reamer from round stock pretty easy.

Likewise, since it's brass, he could also run threaded rod thru the bore with a tapered brass ramrod guide on each end, chuck it in a drill press, and use a lathe file to turn the barrel and get rid of any runout......were it to be present.

Those who skin cats are often more adept than those who pay someone to do the skinning for them.