Finished today the fourth attempt at making an English style square breech plug and I think this one is acceptable with the correct dimensions. I’d like to thank Bob Roller and Curtis for the help with measurements.
I drew the plug in Qcad, which has tools to take precise measurements of angles, radii, and lengths from the drawing. These are needed for the various steps in shaping the percussion bolster. I don’t think I could have managed without it. I had hoped to show the different steps in shaping with Qcad drawings, but the print screen function in windows 11 doesn’t work with Qcad.
The plug blank starts as a piece of mild steel round stock 50mm (2”) diameter 55mm long, weighing 845gram (1.9 lb).
I have a small hobby lathe (80kg) and mill (120kg, Chinese) The main challenge was finding the right sequence of steps to tackle such a heavy lump in the lath chuck, later holding it securely in the mill vice while shaping.
When hacksawing by hand such thick stock, my cuts are always crooked. I had to add 0.25” -0.5” to the length to compensate. The cuts had to be trued up in the mill. Trying to true up the ends in the lath either broke the cutter or knocked the blank out the chuck. Make sure the blank is mounted square in both axis(x/y) before milling.
Mount the blank in the lathe, indexed against the face of the lathe. Turn the sides round and true up the end that’s sticking out of the jaw. You have now a perfect cylinder that’s centered in the chuck.
Turn down the end of the blank to a cylinder that’s 19.5mm in diameter and 14mm in length. This is going to be milled into the hook.
The spindle hole in my chuck is 20mm. Reverse the blank with the hook portion in the spindle hole and the large cylinder indexed against the face of chuck. This reduces the length of the blank sticking out of the chuck jaws. Turn down a new cylinder that’s 19.5mm in diameter and at least 14 mm long. This going to be the threaded portion. Make sure the large cylinder is 23.5 long. This is going to be the body of the bolster/plug. Next relieve the “last threads” before cutting the threads. I did this with a small parting tool(2mm). If I tried to do this after threading, the tool broke.
The threads are ¾”-28, cut with a die and started in the lathe to insure squareness. This die is too big for my lathe, and it was hard to cut the threads cleanly. The rest of the threads were cut holding the blank in a vice and turning the die holder. This gives more leverage and breaking the shavings every few degrees gives cleaner threads.
Put the blank in the lathe the hook in the spindle hole and the large cylinder indexed to the face of the chuck. Turn the length of the threaded portion to a hair over 0.5” and center drill a hole 3mm (1/8”). Continue this hole until it protrudes 10mm into the large cylinder body. This will be the fire channel for the powder chamber. Use a 12mm round nose mill in the lathe to drill out the powder chamber.
Do not drill the powder chamber deeper than the threaded part of the blank. Polish the inside with a Dremel tool.
Next fit the plug to the barrel using Prussian blue, before doing any work on the body/bolster. Doing this now, saves you the hassle of having to index the barrel and plug to match up with the correct barrel flats.
Screw the blank into the barrel, mount in mill vice with the top indexed flush against the top of the vice jaws. Mill down as far as you dare without marring the barrel.
Do not rotate the barrel and mill down the next flat. Each flat milled down will reduce the surface area clamped in the vice. Sooner og later the mill will pull the barrel out of the vice.
You will notice that the top and bottom flat are not completely parallel. This goes for the other pairs of barrel flats. To clamp the blank securely in the vice for the rest of the shaping the sides must be parallel.
Paint the blank with dykem and scribe around the barrel flats. Mount the blank in the vice and mill down the bottom flat to a hair above the scribed line.
This is going to be a left-handed plug, so the percussion bolster must be on the left side. Mount the blank in the vice with the left side down, make sure it’s square in the vice, mill down to a hair above the scribed line for the right flat.
Mount in the vice with the right side down and mill the left side down to 7,5mm above the scribed line for the left flat. This material is needed for the percussion bolster.
Do not attempt to mill the diagonal flats. Keep the blank square. Mount the blank in the vice the threaded portion down. Mill the hook blank square while adjusting the length of the bolster blank to 23mm, it’s finished length.
The next step is to mill the nipple face. Paint right side dykem, strike a horizontal line that’s in the center of the fire channel in the powder chamber. Strike a vertical line that’s 10mm from the front of the bolster. Where they cross is where the fire channel ends. The nipple face is slanted 36degrees. Tilt the mill head 36 degrees, mount a stubb of a 2mm drill in the mill. Adjust x-axis on the mill table so the drill points to the center of the cross. Lock the x-axis.
Hoist the mill head up to it barely clears the top the blank. According to the Qcad drawing the center of the nipple hole is 24,9mm from the right side of the plug. Strike a line that’s parallel to the right side at 24,9 mm. Shift the y-axis of the mill table so the drill points to this line. Lock the y-axis.
Mount a 12mm end mill and mill out the nipple face to a depth of 5,3 mm. If you had milled the diagonal flats first, you would not be able to hold the blank securely enough to do this.
Drill and tap the nipple hole now before shifting anything to a depth of 6,2mm. This ensures the nipple is in the center of the face and threaded squarely.
Unlock the mill table and mill out the hook.
Mount the blank in the vice with the right up. Remember the cross we scribe here? Center drill here until the drill breaks into the threaded nipple hole. Counter sink the hole. Thread the hole for a M5 screw. Turn a M5 screw out of the same material as the blank. Put a 3mm drill in the fire channel, 620 green locktite on the screw thread. Screw it into the blank until it hits the drill. Carefully pull out the drill and dry overnight.
Cut off the screw and pean it into the countersink.
Mount in vice right side up. Tilt mills head 45 degrees and mill the diagonal flats on the right side a hair over the scribed lines. Repeat for the bottom right diagonal
Return head to vertical. Mill a shelf that’s 10,8mm wide and 3,7mm deep. This shelf touches the middle of the lower diagonal flat and is the bottom flat of the bolster.
Tilt the head 45 degrees and mill out what left of the diagonal flat.
Take out of the vise, paint the top flat with dykem, strike a line that’s parallel with the front edge of the cube, 7,5mm from the edge. The inner surface of the front shield is a cone where the base tangents this line and rough out with stones in a Dremel.
Shape the bolster with needle files, elbow grease and patience. Last step is to file the plug flats flush with the barrel flats.
Will be starting on the standing breech tomorrow.
Questions:
If you look at the last picture showing the right side of the plug, see two blemishes, I’d like to correct.
The blue arrow points to a ding on a barrel corner. Is there a simple way to fix this?
The red arrow points to the screw that fills the excess plug channel. When filing flush after peening the screw thread became visible again. One option would be to make a new screw with a head, heat the plug to 200 degrees Celsius to burn out the locktite and try to remove the screw without messing up the plug. Mount the new screw and pean the head into the countersink.
The safest option might be to leave it be. The barrel and plug is going to rust blued and this might make the blemishes less visible. Any advice?
Hope I havn’t put you to sleep
Best regards
Rolf
