Machining My Own QD Swivel Stud

Machining My Own QD Swivel Stud

I mentioned some days ago that the Harris QD swivel stud adapter that I purchased for my M1A did not quite fit, and I had been forced to make a replacement part for the washer that goes inside the stock. (If you have no idea what a QD swivel is--you can see one here. You press or pull to the side, and it quickly releases it from the gun stock. Hence: QD for Quick Detachable.)

I was not completely happy with the result. The reason is that the Harris bipod tightens up to the stock by turning a thumb screw. To work correctly, the base of the QD swivel stud must be screwed down to the material of the stock. The swivel stud adapter is about .750" long, and if I turned it in far enough to be screwed down to the stock, about .100" was sticking above the .110" thick rectangular part that I made to go inside the stock. The gun would go together, but it required force, and that's never a wise thing with a mechanical part--especially since the part the screw was hitting was the cylinder in which the operating rod resides.

I added several washers between the stud and the stock, getting the end of the stud's screw away from the operating rod assembly, and the gun would go together without force--but now the bipod thumbscrew would not allow it to be tightened enough to grip the stock firmly.

The obvious solution is to use the lathe to turn .100" off the end of the stud. But in spite of having a heck of a lot of taps and dies, I did not have a 10-32 die to recut the threads, which might be necessary afterwards. (This is on order, because you never know when you might need to rebuild civilization with the contents of your garage.) Besides, there's a sense of accomplishment in making a part like this from scratch!

1. I started with 1/2" diameter steel rod, because I had that lying around the shop. (It would have been faster to start with 3/8" rod.) I cut it with the chop saw to about .700", and trimmed it on the lathe to .650".

2. Then I turned the end that was going to have the hole in it for the quick detachable part to .368" diameter for a length of .350". (It actually came out more like .363" diameter.)

3. Then I turned it around, and turned the other .300" (which will be the threaded end) to .245" diameter.

4. I had a 1/4"-20 die, so I threaded the .245" end. (This is also a somewhat stronger thread than 10-32. As we computed previously, a 1/4"-20 screw requires about 7500 Newtons to shear. The stock will probably break before the screw does.)

5. Now I put the part in the vertical mill, and used a 4 flute end mill to flatten opposite sides of the cylinder to flats. The target was .300" wide, because that's the QD swivel stud width on the flat sides; I settled for .318" wide.

6. Now I measured the center point for the hole into which the QD part goes. I used a center drill to mark the spot, but the mill vise on the vertical mill doesn't have enough grip on a round part like this to do more than mark it.

7. Then I put the part in the drill press vise, and drilled a hole that the QD part goes through. It should be .143", but my .140" drill was the closest size, and it was a bit tight of a fit, so I moved up to the .168" drill, which speeds up connecting the QD--and still leaves enough steel that I'm not worried about it breaking. (Well, perhaps those manuevers where the aircraft with the tailhook grabs you off the runaway might be a problem, but I try not to do things like that.)

8. Last step: the rectangular washer that i made previously had a 10-32 hole. I enlarged it and tapped it to 1/4"-20.

Everything now goes together without force, and the bipod locks firmly in place against the bottom of the stock.