January 12, 2011
In working on firearms, you'll often see damaged or worn screw heads. While the threads will generally be in good shape, the slot in the head can be worn or disfigured.
One of the great things about having a column in Shooting Times is the opportunity to get to know a lot of folks in the firearms community. Not too long ago I got a letter from John Hayslip down in Alvin, Texas. John saw a piece in which I slotted a screw head with a hacksaw. In the course of his gun work, John had come up with a very nice fixture to slot screws with more precision and consistency than I could ever achieve using my modified hacksaw. Best of all, the only piece of machine equipment John used with his fixture was a drill press. I was so impressed with John's fixture I decided to put together a similar outfit. I also contacted him and asked if he would mind if I shared this with other Shooting Times readers. John, being the nice guy that he is, readily agreed.
In working on firearms, you'll often see damaged or worn screw heads. While the threads will generally be in good shape, the slot in the head can be worn or disfigured. Generally, this is due to use of a screwdriver blade that's too small or doesn't fit the slot properly. Ideally, you want the screwdriver to provide 100 percent contact to the inside of the slot--top to bottom, side to side, and from end to end. Normally, you'll get this only with properly ground and fitted gunsmith screwdrivers. The standard hardware-store screwdriver has a tapered head that seldom ever fits a gun screw properly.
That tapered head is what generally causes damage to gun screws. The head tends to ride up and out of the screw slot when it's twisted. As it rides up and out, it'll often peel off metal from the top edge of the screw slot. Even if that doesn't happen, you'll frequently have the edge of the slot battered or pushed over, and it just gets worse with time.
There are a number of ways a screw can be repaired. Sometimes, if you're lucky, you can re-cut the slot, make it a bit wider, and remove the damage. If the screw is really chewed up, you'll have to weld up the head, shape it, and cut a new slot. I've often done this when replacement screws were not available or the threads were just so doggone odd I couldn't reproduce them on my lathe.
In the past, I've used a number of specially designed files to cut or repair the screw slot. These imported files tend to be very thin, fragile, and expensive. A single file can set you back about $40 or so. Needless to say, I looked for alternative techniques and came up with using a modified hacksaw blade. The teeth on most hacksaw blades project out to the right and left side of the blade. This is called "set" and makes the slot wider than the blade to prevent binding for smoother, easier cutting. The problem, as John noted, is getting the slot perfectly centered in the screw head. Even if you're off just a bit, it'll stand out like a sore thumb.
John uses a slotting blade mounted in a simple holding fixture in his drill press. These circular blades come in a variety of thicknesses and are available at most machine tool supply houses. I picked mine up from Enco, and they cost about $10 a piece. That's a lot cheaper than those imported European screw slot files!
To hold the screw, a simple replacement drill chuck was mounted on a 4-inch angle plate. I paid $15 for the chuck at the local hardware store and about $18 for the "economy grade" angle plate from Enco. My keyless drill chuck was threaded 3/8 inch x 24 so it was a simple matter to mount it on the plate with a machine screw.
The Fixture In Use
Using the fixture is simplicity itself. The slotting blade is mounted in the holding fixture and this is placed in the drill press chuck. A home-made pointer or indicator is then placed in the chuck that is mounted on the angle plate. The height of the drill press chuck is adjusted so that the sharp end of the pointer is centered on the edge of the slotting blade.
The pointer is then removed and the welded-up and reshaped screw is placed in the keyless chuck. At this point the slotting blade is perfectly centered on the screw head. That's great, but the next problem is how to control the depth of the cut with the slotting blade.
That's also really simple. The fixture with the screw in place is positioned against the edge of the slotting blade. A drill whose diameter is equal to the depth of the slot you want is placed flat against the bottom edge of the angle plate. Next, a metal ruler or straight strip of steel is laid against the other side of the drill and clamped to the drill press table. The angle plate is pulled back, and the drill is removed.
The drill press is set up to turn about 500 to 600 rpm. A drop or two of cutting oil is applied to the head of the screw. Don't forget to wear safety glasses. The saw will throw out tiny shards of metal, and you darn well don't want any getting into your eyes. Also, watch your fingers. The saw has no guards, so be careful.
With the drill press on, the head of the screw is slowly and carefully fed into the slotting saw. Take your time and don't force it. Initially, the screw head is fed directly into the saw blade, and then the fixture is moved back and forth along the strip of steel that acts as a fence stop to get an even, uniform slot through the screw head. It's simple, easy, and precise.
Until next time, good luck and good gunsmithing!