View Full Version : How is the breech face machined on a Browning type auto?

February 23, 2005, 02:22 PM
I can see how just about everything is machined in a typical auto pistol, with the exception of the vertical face of the breech. I just don't see how a mill could be set up to pass from top to bottom with only the space inside the ejection port to work with.


James K
February 23, 2005, 02:49 PM
End mill from the muzzle.


Dave Sample
February 23, 2005, 04:03 PM
Ditto, Jim

February 23, 2005, 07:13 PM
If not too technical, could you explain how they can make a perpendicular cut up to the top of the slide if the hole in the muzzle doesn't go as far? If the cutting shaft is parallel to the slide top (to make a perpendicular cut on the breechface), that shaft is going to stop against the top inside surface of the slide before the sides of the cutting tool can cut the breechface all the way up to the top of the slide.

And if the tool is canted, then the breechface won't be perfectly flat.

So how do you achieve a top to bottom cut in this manner? :confused: Thanks.

February 23, 2005, 07:56 PM
The BHP has [at least my 1970 had] a barrel bushing which is threaded into the slide .It is not designed to be removed but I made a new one to accurize the gun.Anyway there is a fair amount of room to put a mill without the bushing.

Dave Sample
February 24, 2005, 02:12 PM
The slide is held straight up in a vice and the flat end mill goes right in there and cuts the breech face. There is no "muzzle". The slide is machined inside with dozens of different tool set ups and that cut is just one of the easy ones. The end mill has a long shank so it can reach in and cut it. I would also guess it is a carbide end mill.

February 24, 2005, 03:33 PM
Do not forget that a custom end mill can have a cuter of a larger diameter than the shank it is mounted on. This allows a wider cut than the hole the tool was passed through.

Jim Watson
February 24, 2005, 04:15 PM
There is at least one maker of a 1911 mutant who advertises EDM cut breechfaces. But that was obviously not part of the original design. I suspect the top of the breechface where the barrel hood rests was cut with a shaper and the actual surface where the casehead sets was meant to be done with a long end cutting mill.

February 24, 2005, 04:44 PM
Maybe Jim just answered my question, talking about a "shaper", unless that was for the EDM process.

Dave, I said "muzzle" when I should have said the muzzle end or bushing hole. So to try and restate the problem:

If the breechface is being cut with a circular mill tool that is .50" wide, and the shaft of the cutting too is .25" in wide, and the shaft runs parallel to the top surface of the slide, and the slide is .20" thick on the top surface, how is the cutting tool able to rise high enough up the breech face to cut the vertical edges of the face all the way up to the top of the slide? It would seem that the top part of the slide would stop the shaft before the two sides of the cutter can get to the top of the slide.

All I can think of is that the top cut is made in two different ways, or the breechface isn't actually perfectly flat, but is actually concave and the mill pivots around a point somewhere ahead of the muzzle hole.


Dave Sample
February 24, 2005, 06:49 PM
They have CNC mills now that can do the impossible. But you know what? I don't care how they do it. Just so they keep doing it. You are going to sprain your brain mulling these type of things over and over.

February 24, 2005, 08:34 PM

I have all the machining design work pretty much done for a complete pistol. I don't think I'm going to sprain anything learning how to do a cut you refer to as one of the "easy ones".

If you don't know, that's fine.

February 24, 2005, 10:09 PM
For those who don't believe that the BHP has a bushing I still have mine and will send a photo for someone to post if anyone is interested !! :p

James K
February 24, 2005, 11:47 PM
The BHP slide certainly does have a bushing and it is used precisely because the larger hole makes the machining of the interior of the slide a lot easier than if the front hole were only barrel size. The Radom has a bushing for the same reason. My answer was correct, I just didn't go into details.

For front sight attachment, the hole for the tenon was drilled right through the bushing, which also helped fix the bushing in place. For the dovetail front sights, the same thing is accomplished by a rivet driven through a hole that extends from the bottom of the dovetail through the bushing; the head of the rivet is covered by the front sight.

(Unlike the 1911 type, the bushings on the BHP and Radom are permanently assembled by swaging and further fixed by the sight tenon and are not meant to be removed except at the factory level. One illustration taken from a Canadian army manual shows the bushing threaded in, but that is almost certainly an error, as I have never seen a threaded bushing and the parts diagram and other illustrations all show an unthreaded bushing.)

I never worked at FN, but I have no doubt they first drilled the slide forging almost all the way from the front, then did the basic breech face machining with an end mill the size of the hole in the front of the slide. The same tool may have been stepped to make the round cut for the head of the cartridge case. Then they used two other mills which they were also inserted from the front, but which were smaller diameter than the hole. The first cut the cartridge base hole (or maybe cut it through) and the second smaller one cut the half moon at the top for the small alignment lug on the barrel. The slide would probably have been moved against the cutters rather than vice versa, as this is easier to set up. I am sure guides were used to make sure the smaller cutters didn't whip.



February 25, 2005, 12:46 AM
That's a very good description of that type of slide machining, Jim.

But do you have any idea how it is done on 1911s, Glocks and Sigs with the top to bottom cut I described?

February 25, 2005, 06:02 AM
Jim , No it's really threaded, want a photo ??

February 25, 2005, 11:23 AM
Why don't you just post the damn photo and be done with it? Or you could start a new thread and stop stealing mine so I can get an answer.

Dave Sample
February 25, 2005, 08:41 PM
I just recieved 12 new slick slides from Caspian Arms this AM and Handy's question leaped out at me as I gave them a look see. I am really curious now to see how they do all of those neat cuts inside that slide. I have to admit that the machining process is not my thing, but it is fascinating to wonder how do they do it.

February 26, 2005, 01:11 AM
Breech face milled, barrel extension cut done with a broach.


February 26, 2005, 01:51 AM
Here's what Unforgiven said on the 1911forum:The breach face on 1911's was traditionaly cut with a shaper. Newer versions are advertised to be cut by edm which results in a much nicer finish. I suspect some of the cheaper guns are just investment cast breach face and all.

If Sam's "broach" is the same thing as a "shaper", I guess we have our answer.


Jim Watson
February 26, 2005, 07:42 AM
A broach is not the same thing as a shaper.
A shaper is a single point machine tool that moves a cutter back and forth in a straight line. Big industrial machines have automatic advance of the depth of the cut.
A broach is a multi-tooth cutter, each cutter slightly larger than the previous one until the final size of the cut is reached. Just make a pilot hole and pull the broach through and you are done. Simple but not easy. The precision broach was the machine tool innovation of the 1930s, comparable to EDM today. A history of the M1 describes how John Garand designed the rifle around the best tooling available at the time.
So it is likely the 1911 was at least originally made with the barrel hood relief of the breechface cut with a shaper. But a broach would be faster and EDM is getting common on a lot of products it would once have been too expensive for.

Agree with Dave, a tour of a gun plant would be fascinating. Preferably one a little out of date. You can't see what is going on in a CNC machining center like you can a mill or lathe.

James K
February 28, 2005, 06:50 PM
I want to correct what I said above and apologize for the error. The BHP bushing is screwed in. I was deceived by a bad memory and by some drawings that don't show it, but the threads are there.

I am indebted to Mete, who sent me a pic of a bushing removed from a Belgian BHP that certainly has threads. The Browning High Power Automatic Pistol, by Blake Stevens, has drawings that don't show the threads but pictures of cutaways and also the bushings that do. It is possible that production techniques changed at some point, but for now, I have no doubt the bushing was threaded and was screwed in.

Regardless, the BHP barrel bushing was considered a permanent assembly to the slide and was not designed to be removed by the customer. When I make statements about gun parts or assembly, I prefer to check them out myself. I did not do so this time for the simple reason that I could not do so without wrecking a BHP slide, something I did not want to do.

A point on the front sight. I said that the hole for the sight tenon was cut through the bushing. That is true when there was a full tenon, but for a time in the post-war manufacture, the front sight was just soldered in and had no tenon. The cut was deep enough that the sight still locked the bushing, but the cut did not go completely through the bushing.

On the subject of bushings, I might note that the BHP had the same problem with the breech face being impacted by extensive firing that cropped up with the M1911. This was solved in the same way, by screwing in a hardened bushing ("slide stud" or "rear slide bushing") around the firing pin hole. The problem was finally resolved in 1947, when FN went to a hardened slide. Colt had gone through the same process, and a bushing (they called it the "recoil plate") was used from 1935 by Colt and also by all the WWII contractors. Colt also went to hard slides in 1947. Very few owners of either pistol know that the rear bushing is there.


James K
March 1, 2005, 05:42 PM
Mete has kindly permitted me to post his photo of a BHP barrel bushing. It looks like this one is not drilled all the way through for the sight tenon, which would indicate it was on a pistol that had the soldered sight.


Harry Bonar
March 5, 2005, 08:34 PM
Dear Shooters.
I learned something.
I asked Novak how bushings were put in the H.P. He said they're silver soldered in. I asked, "have you had one out?" No!
Now I see by the pict :) ure that they're screwed in, maybe also silver soldered too; but I really learned something by that picture. Thanks
Harry B.
- I'm sure not taking mine out! :barf:

March 6, 2005, 07:06 PM
They may have changed since 1970. You could x-ray the slide to find out .It wasn't easy to get it out !! nor easy to machine the threads on the one I made !

James K
March 11, 2005, 04:08 PM
I have since dug around and found several pictures and books that show the bushing threaded in, so I don't think there is any doubt. There is also no doubt that they were intended to be a permanent assembly and not removed, and that they were used solely for manufacturing purposes, not for reasons of accuracy.


Robert Hairless
March 23, 2005, 03:27 AM
Not to complicate matters but I once shot out the bushing on an FM Detective upper that was an a Hi-Power lower. As everyone knows, FM is the Argentine company that makes a Hi-Power clone and its Detective model is a Compact version, roughly the size of a Colt Commander. My reason for volunteering this tedious information is that the bushing and slide were not threaded on the FM Detective. My recollection is that they were soldered, but evidently not well enough. I returned the mess to JLD Enterprises, from whom I bought it, for replacement with a new upper.