The blankI made a pattern for the screw and axle holes by gluing a square piece of 3mm mild sheet steel to the lock plate pattern and drilling out the holes.
This pattern was then glued to the bridle blank, which is a 9,5mm thick piece of mild steel and the holes drilled out. Notice the hole for the tumbler axle is not drilled and the screw holes are drilled with the tapping drill size.
The pattern was removed, each screw hole enlarged with a 3.1mm clearance drill and counterbored with a 4.0mm endmill to a depth of 2mm for the screw heads, before moving to the next hole. This insures the counterbore is centered on the screw hole.
Next, I flipped the blank, put #4 screw blanks from Brownell in all the screw holes and milled away as much as I could of the waste material down 6,1mm with a 6mm and a 3mm endmill.
Shaping the posts.First, I tried making a hollow mill from the same drill rod I used for the square broch. It dulled and stopped cutting after one and a half posts. I think I had annealed it too soft. I tried tempering it again, but it warped. I gave up on the hollow mill because making them is labor intensive and they are a pain to resharpen. Sorry, I forgot to take pictures of this attempt.
Next, I tried using the round table and a 3mm endmill. I mounted and centered the round table on the mill, as shown previously. I mounted a piece of 10mm thick mild steel on the table and drilled and tapped a 4-40 screw hole with both axis on the mill table locked.
The bridle blank was then mounted to the steel plate with #4-40 screw and clamped.
The head of the screw is 5,7mm in diameter. The X-axis is off set so the 3mm end mill barely clears the head when the table is rotated. I got nice round posts that were 6mm in diameter, but not centered on the screw holes. Something must have shifted.
I don’t think this would have impaired the function of the bridles, but it looks sloppy. I decided to recut the posts and make them slimer. I found this tool from ISCAR that worked like a dream. It’s a cutting insert for making round grooves for gaskets. Here is the link
https://www.iscar.com/eCatalog/Family.aspx?fnum=2546&mapp=TG&GFSTYP=M&rel=S&rcat=2800897&item2=&IC= I made a holder for the insert from 30mm axel rod.
The tool fits in my 16mm collet for the mill and cuts pillars 4,7mm in diameter. The blank is held in a mill vice and the placement of the pillar in relation to the screw hole can be easily adjusted using the x and y axis on the mill table. The recutting of the posts got rid of most of the offset and slimed them down.
Next, I drilled the hole in the blank for tumbler axel. I made a brass bushing from round stock. First, I center drilled it in the lathe from both ends with a 2mm drill before drilling through the whole piece with a 4mm drill. This gets rid of runoff. One end is turned down to 6,5mm so it fits in the tumbler hole in the lock plate.
The bridle blank is mounted on lock plate. This holds the blank level in the vice when drilling the axel hole.
I made a brass pattern for the bridle plate. The pattern is held in place by the bridle screws, the blank painted with dykem and scribed around, and drilled out with a 2mm drill.
Saw out the bridle and clean up with needle files.
Bridle screws.I’ve tried several alternatives and have not found one jet that completely happy with. The best one so far is to use Brownell’s #4 screw blanks. They are way to long and the heads are too big. The blanks are very soft and carbide turning inserts only mushes it instead of cutting.
In this thread
https://americanlongrifles.org/forum/index.php?topic=72984.0 , I was recommended to try high-speed steel turning tools. This worked like dream after honing the tool to a sharp point and polishing to 800 grit.
First, I turn the head down to 3.9mm, reverse the blank in the chuck with screw head behind the jaws so they only clamp on the shank.
I then cut the shank sticking out of the chuck down to 10mm and thread it in the lathe. The screw is still way long, but it is not possible to hold a shorter screw in the chuck. I need to start the threads in the lathe to insure them are not started crooked.
The rest of the threading is cut by hand while clamping it in a sjarn jig. The shank is very soft and prone to bending. It must be threaded in portions, max 10mm at a time.
The screw is mounted in the bridle/lock plate and cut to size.
Here is a couple of pictures of the finished bridle.
Next, I hope to start on the tumbler.
Best regards
Rolf
