Thursday, June 28, 2007

The one crack that doesn't mean a dead gun

A few weeks ago in "When not to buy a pistol" I wrote:
Oh, and a note for someone who isn't averse to a little gun smithing work, or a good bargain; there is one specific, common, very small crack that doesn't ruin a 1911:

If there is a small crack in the bridge of metal over the slide stop notch (not the pin hole, the notch in front of the plunger tube), and that crack doesn't extend beyond the corners of the notch; the metal can be machined out, and the frame saved.
Yesterday, I wrote about rebuilding Kommanders pistol... well... funnily enough, I now have a perfect example of the problem I mentioned above:

Before we go any further, let me just make a note about the pictures here. I've deliberately blown out the contrast and sharpness on these pics to highlight the problems; the metal in no way looks this roughly machined, pitted etc... The photographic enhancement I've done kida makes it look like the surface of the moon, when in actuality its no worse a final machines surface than any other gun. Also, sorry for the focus issues. They all looked properly focused to the eye, but I didn't use the right depth of field. I don't have a very good short lens, and should have shot these with aperture priority and manual focus.
Anyway, what you see here is a classic example of the "slide stop notch bridge rail" crack (yes, seriously, the little piece is called the "slide stop notch bridge rail").

That one little spot is the weakest point on a 1911 frame; and it's a load bearing part. Not only that, but it just happens to be the point of greatest torsion and flexion in the frame... and to top it all off, they take a big bit out of the supporting structure below it, making a natural stress rise...

Let's just say it's not surprising to see a crack here. In fact it's so common, that many 1911 manufacturers and custom gunsmiths simply modify the frame to remove that section of the rail by default.

In the case of this 1911, the crack has probably been there for a while, but it was never noticed before. These cracks generally start small and thin; and they aren't always apparent with a basic field strip under the lube and light dirt (even a clean gun still has some grease, lube, particulate etc...). Often, you need to clean the gun off thoroughly and view the rails under a bright white or green (higher contrast) light to even see the crack; and if you don't know what your looking for, you might not even notice it.

In this case, the process of refinishing (heating the gun up to 200, degreasing it, repeating that; then heating the gun up to 350 degrees to cure the epoxy) probably very slightly widened the crack; then shooting it chipped the coating off, highlighting the crack perfectly as you see above.

That's why I call it a perfect example.

Now... repairing it. Aye, there's the rub.

I mentioned in that same piece that one of the most critical factors in accuracy in a 1911 is the rail fit and geometry. Well, we're about to chop a section of rail out here; it is ABSOLUTELY CRITICAL that in doing so, we don't change the fit of the rails, the geometry, or the surface hardness.

If you cut or grind too hard or too fast on the rails, you are going to differentially heat them, which will induce torsional warping, as well as de-temper the areas around the cut. It could reduce the hardness of the rails is some areas, increase it in others; and can cause both inaccuracy, and the premature failure of the frame.

The way you avoid that is to work quickly, at as low speed as possible, while maintaining a good clean cut that doesn't bind or bog down. As the part heats up (before it gets too hot to touch) you stop, move off the work piece, and come back to it after letting it cool.

Before we continue, let me make my standard disclaimer:

If you bugger up your gun based on any advice or information I give you; you are an idiot and you deserve whatever you get. I wash my hands of it.

...Now, what tools to use?

You could do this job very quickly with a number of different tools. A bench grinder would do it, a band saw, a die grinder; even an angle grinder; but I find that they all are lacking in fine and precise control; and they don't fit into the angles I want to get into (reader Mark also points out that the proper tool for the job is an end mill. Yes Mark, it is; but I don't have a mill... yet...)

Also, I think that a conventional die grinder, or a bench grinder would heat the work piece too much too fast.

I have a die grinder, but really a micro die grinder is what I think is called for here. Unfortunately I don't have one of those (I DO have all the other tools I mentioned above, just not a micro die grinder). What I do have, is a couple of different Dremel tools.

For this job I used a Dremel XPR (variable speed moto-tool) with a flex shaft, and a reinforced abrasive cutoff wheel:

These particular pieces are the favorites of gun butchers everywhere; but if you are careful, slow, and deliberate, you can do a very good job with them. Professional gunsmiths almost all use Foredoms flex shaft grinders (or the Chinese copies thereof) for their heavy duty work; but I don't know a single gunsmith that doesn't have a few Dremel tools around.

As I said above, the proper technique is to have the work piece properly braced, and your hands and tools properly braced in relation to it. Then go as slow as you can while maintaining a good cut; without getting too much heating of the piece, and at as low an rpm as possible.

If you see any more sparking than this, you're going to fast:

For any heavier work you'll generally want to clamp the frame in a padded vise; but in this case you aren't removing a lot of metal, and you're going to need to reposition a lot, and get into some tight spaces and angles; so I'm risking life and limb, and bracing the frame on my thigh and knee (with a flame resistant leather pad under it) with a solid hand grip.

Obviously you can't see here, but I am wearing safety glasses. Any time you're using power tools you should wear safety glasses; but this is especially true when you're cutting or grinding metal, and doubly so again when you're using a tool that's spinning at high speed. Those cutoff wheels throw off tiny bits of abrasive at very high speed, and they sometimes shatter catastrophically; believe me you want to be wearing safety glasses for that.

You might want to wear gloves for something like this.. it's probably a good idea; but I think you lose too much feel for fine metal work if you wear gloves. Also, my hands are so scarred up, burned, and calloused anyway (cooking, welding, shooting, knife and sword work, unarmed martial arts, and playing guitar)... I'm not too concerned about it.

What you want to do is to cut out the entire portion of the rail above the frame opening; and then cut the rails back a bit at a slight angle on both sides. You're probably going to want to take the cut in a couple passes, one to create a larger hole to work the cutoff wheel in (which reduces the heating) and then a final cut on both sides. Here it is after the first rough cut before finish grinding:

I'm showing the rough cuts here, because the final finished ground and polished surfaces don't photograph well.

What you're trying to do is eliminate and prevent any stress risers; which means no sharp corners. Heres a couple views from above showing the first steps in chamfering the ends of the rails:

From these rough cuts, what you do is refine the angles and surface grinds, using a hard rubber or compounded abrasive impregnated stone like this:

You don't want to be using a carbide, carborundum, or silica type stone here; they are too hard and abrasive, and they will heat the piece too much, as well as removing too much material.

What I did with this stone, is bevel the notch back to a shallow keyhole shape; and then I put a bevel on each of the sides of the rail (you can see the start of the bevel above). I broke and relieved all the edges, lightly radiused the square corners, then wire brushed and polished the edges.

Oh, and while I was at it, I also chamfered all the edges of the mag well, top and bottom (as you can see in the first overhead pic above), and just for the heck of it, I did a full ramp and throat job for him. His stock ramp and throat were pretty rough, and I had the Dremel out already so...

When you do a throat job, unless the angles are really bad, you don't want to mess with them. In this case, I didn't change the geometry at all, I just used a soft polishing stone to properly smoothly radius the whole thing, 'til a fingernail wouldn't catch on the feed path. I usually finish up with a fiber polishing swab and compoundl but this one was so smooth I didn't bother:

When it's done properly this repair doesn't compromise the strength of the gun at all, and actually INCREASES reliability; so not only is the gun fixed, it's better than it was before.