Gun-drilled axles

Gun drilling is done with a gun drill in a gun drilling machine. The latter is basically a special purpose lathe which turns the piece being drilled. The former is a steel tube with a cutter head brazed on the end. In use, high pressure oil is pumped down the tube as the cutter is advanced into the work. The cutter head is a single point tool. The head bears on the already drilled part of the hole. There's a longitudinal groove the length of the cutter head and the oil tube. Chips are forced out of the hole down this groove by oil pressure.

Gary

For years I worked in a shop that did gundrilling as a major part of the business. Some gundrill machines spin the part. All the types we had, the most common, spin the drill. Our drills ranged in size from .060" x 24" to 3" x 48". The high pressure oil fed down the inside of the drill actually supported the smaller diameter drills. The larger drills needed oil only to blow the chips out. The part is held against a drill bushing that guides the drill when it first starts. After the hole is started the hole itself supports the drill. The drill point is ground such that a cross section of the hole at the bottom forms a "W" shape instead of the point a normal drill forms. There is a hole in the point that the oil flows through. The part to be drilled is supported such that it can be adjusted for location at both ends. It is easy to hold .005" concentricity oner 24". Since the part didn't rotate in our machines we were able to drill shapes that were not round. Such as the landing gear for 747 jets. There are gundrills that rotate the part and the drill at the same time. These machines are able to hold concentricity easier. They are also more expensive and less versatile. ERS

See

Thanks for all the replies, Very interesting. Looks like I won't be doing any of this on my lathe ;)

Texas Parts Guy

By using a Gun Drill.

Need one? Ive got a very nice Eldorado for sale.

Or I can turn you on to a gun drilling shop. Might be cheaper for a one off

Here are some links of how they work

Interesting stuff in all the links on this thread. I just can't understand what keeps the drill from wandering off the axis of the hole when it's drilling that deep.

Garrett Fulton

The drills don't ride on the hole they chew through the center like a twist drill. Instead, the leading edge of the cutter is off on one side of the hole. The force from cutting this way pushes the drill away from the cutting edge and causes the trailing edge to ride on the hole that has already been drilled. The trailing is pretty much a polished half cylinder of carbide running in oil so it follows the hole pretty accurately.

It is pretty important that the hole is started straight or the drill will follow the crooked hole just fine

Several years ago, I copied and posted to the dropbox a short article on precision drilling from a 1962 ASTME collected papers, vol 62, book one.

It has some good basic information on accurate drilling and why gundrills work.

Paul K. Dickman