Warning !!!!

[Pdvdh]

I've used them from Albion to Pratt Whitneys, and the all pretty work the same way.
gary

How difficult is it for the operator to determine whether the hole has been drilled straight, or crooked?

 

[Trickymissfit]

Very good post and description Gary !!!!

I have watched the process but never operated a gun drill.

I also believe that the set up and the operator has a major bearing on the quality of the drilling.

Your comment about watching the chips is also correct and applies to all metal cutting.

Thanks

J E CUSTOM

I learned most everything I know bits and pieces at a time. At first everything just seemed wrong, but once you see what each part does it all falls in place. One thing I couldn't discuss here (even if I was an expert) is the grind on the gun drill tip. Very secretive and the coolant isn't anybetter. What I will tell you is that they replace the coolant every week like clockwork. Most of the better coolant setups had three sets of redundant filters that were switched over by the hours on them. Your looking at nine filters and the large conveyor filter to catch the big stuff. Some of them have a chiller added to the mix as well.
The guys that ran these machines ran nothing else, and sometimes there would be a cell with three or four completely different machines. Normally he only ran one at a time as these are something you have to stay ontop of all the time.
The guide bushing box is rebuilt often, and I had at least a half dozen all the time to rebuild. We never used the factory box much, as they usually were not good enough. Yet there were a few out there in some very odd situations. Everything is hard piped as a quick disconnect won't flow enough coolant. Even then the connectors are mil spec.
In a rifle barrel, we are looking at small holes. They are hard to get the chips to pass thru the flutes. The cutting pressure is very stiff for a reason. A lot of steels will work harden, and you always want to cut virgin metal. The bad part is when something goes wrong. The drill will twist like a wire cable, and you literally have to cut it pieces to get it out. Part is junk as well as the bearing box. Operator has about five seconds to get the drill retracted.
Making machine parts and some other things, I had about five or six jobs on gun drills all the time. Takes the operator three hours to do a setup, and an hour to drill the hole. If your lucky. Drills are bought and for the most part ready to use, but they go strait to an area that puts their own grind on them. I see little difference, but the end user does. They are constantly checked on a Shadowgraph to make them cut right.
the whole process is expensive
gary

 

[Trickymissfit]

How difficult is it for the operator to determine whether the hole has been drilled straight, or crooked?

How difficult is it for the operator to determine whether the hole has been drilled straight, or crooked?

when he's drilling the hole he has no idea, and a lot of it is with his gauging equipment. One outfit I saw was a bar that had maybe sixty thousandths clearance all the way around. It then had these probes that were about every two inches. Everything comes up on a screen. Another was a rod with a single ruby tip that traced the entire bore. To use an air gauge, the bore must have a very fine finish. We almost always used a Mapol Reamer for things checked via an air gauge. Very expensive, but also extremely accurate.

I have heard that rifle barrels were checked with a ball headed rod that actually rubbed against the bore. The barrel was rotated, and the error showed up on the other end as well. I've not seen the setup in person even though I worked for the same corporation.

The bearing box I spoke of before has everything to do with a strait bore, and bore size. The drill goes thru it, and is then loaded hydraulically with extremely high pressure. Some bearing boxes have a rotating bushing in there as well. What happens is sensitive (intellectual property) so I'm sorta glossing over much of it.
gary

 

[J E Custom]

Thanks Again Gary, Nothing like having someone that has been there and done that explain the process. It also explains why a barrel can be bad if not thoroughly inspected.

It's interesting that the process is similar to cutting a chamber in that spindle speed and feed pressure are all very important to prevent burnishing/Hardening of the barrel material And to get a proper finish.
(Also adds to the life of a reamer).

J E CUSTOM

 

[Trickymissfit]

An issue with gun drilling is knowing the stock you are cutting. I made a lot of prototype parts, and used nothing but certified lots of steel. A little more money, but you know what your getting. I've seen piles of bad drill jobs that were traced to a lot of steel (production lines). When your drill, there might be a soft spot or even a hard spot deep in the hole. This is quite common with Asian steels, and some European brands. You cannot normalize the steel because you often buy it in a prehard state. Rifle barrels are probably bought in a prehard state as well. You save a ton of money, an also time involved.

I got involved in burnishing processes a few years before I retired. New to me, and an interesting process. Never thought of it as being precision, but I learned it can be down to +/- .0002" if you know what your doing. I've always felt that the perfect process for cutting a chamber would be with a Mapol reamer (say a .308 chamber), but would never work with a custom chamber design. Here's a process that would cut a chamber within a +/-.00015" window and maybe take five minutes. Finish comes out very smooth with a satin look. (probably a single digit micro)
gary

 

[jrock]

JE,
In your process, do you recut the center holes or use the manufacturers?

Seems that re-cutting the center holes true to the bore at each end could change the readings. Probably a slight change.

 

[J E Custom]

JE,
In your process, do you recut the center holes or use the manufacturers?

Seems that re-cutting the center holes true to the bore at each end could change the readings. Probably a slight change.

Yes. And often because of the poor quality of the countersink. When I do I go in these steps.

First I inspect the countersink to see if it is uniform. if it looks ok I will do a preliminary center to center check, Then when I get ready to do a final straightness check, I face each end off square and seat the centers in the bore using the tops of the lands to be as accurate as possible. This will give me very good readings, But voids the manufacturer's warranty Because it removes his barrel ID. so you must be sure that you can use it before this step.

I do this to determine the need for re contouring for straightness and uniform wall thickness.

J E CUSTOM