Making a fly that works is by far the part of this project I’ve struggled most with. This was due to my lack of my understanding how the sear, tumbler, and fly function together. I’d like to thank ALR members for answering my questions in the gun building forum. I think I understand how the fly functions.
Short discussion on fly functionThe fly’s function is to keep the sear nose out of the half cock notch on the tumbler when the lock is fired with a set trigger. If you are not going to use a set trigger, skip the fly.
The tip of the fly is a triangle with corners A, B and C.
In the next pictures I’ve removed the bridle so you can see how the fly moves in relation to the sear and tumbler.
When the lock is set, the fly is in the forward position, side AB functions as a ramp that keeps the sear out of the half cock notch when fired.
When cocking the lock, the tip of the sear must engage with side BC to move the fly to the rear position, and side BC of the fly must be parallel or below with the bottom of the half cock notch to clear it.
If the sear only engages B, tip of the fly, it will hang up on the fly and not move to the rear. If this happens, increase the curvature of the tumbler in front of the fly cutout (red area). This increases the angel of attack of the sear nose, so it applies pressure to the side BC and not the tip B.
When cocking to full cock, the tip of the sear must press down on side AB of the fly to move it to the front position to clear the full cock notch. To do this, B (the tip of the fly) must be below the bottom of the half cock notch. If not, the sear will hang up on B (the tip of the fly).
Locating and drilling the hole for the fly tit(axel) on the tumbler.The center of the tit hole is located on a radius that starts at the center of the small tumbler axel and intersects the midpoint between the half cock and full cock notches. The tit hole is located as close to the small tumbler axel as possible.
The easiest way to do this, is to make a brass gage. I used 3mm thick brass rectangle from sheet stock.
Strike a line and drill two holes centered on the line. 4mm hole for the tumbler axel and 2mm for the fly tit.
Mill away the brass down to the line, keep the holes intact.
Paint the tumbler with dykem, mark the mid-point between the tumbler notches.
Superglue the gage to the tumbler and align with the mid-point. Drill out the tit hole 4,5mm deep (the tumbler is 5mm thick).
Milling the fly cut-outThe fly cut-out is a wedge 1,9 mm forward the half cock notch and 1,9mm reward the full cock notch and tangents the tit hole.
Mark off these points on the tumbler, put a drill bit in the tit hole, draw the tangents.
Mill out to a depth of 1,8mm with a 2mm end mill. Go slow and careful. These mills break easy.
Clean up the bottom with files and stones.
Making the flyThe fly was made from 1075 spring steel. First step is to cut a square blank 5mm x 6mm x 60mm.
Mill out the tits 3mmx3mmx3mm.
I rounded tits with a hollow mill I made to 2mm in diameter. Notice the tits are place all the way at the edge of the blank. The flies are wedge shaped and done this way there only one side that has to be cut.
Cut out the edge and round the point so it fits in the tumbler cut-out.
Make two brass caps to clamp the fly in place on the tumbler.
Clamp the fly in the rear position in the cut-out and trace a line that fallows the bottom of the half-cock notch.
File down to this line. The tip of the fly must be somewhere along the line C-B*.
The next step is to file off wedges along D-C until B*is a hair above the top of the half-cock notch. Then file the rear facet of the fly.
The rest is just decorative filing, hardening and heat bluing. Sometimes even when the fly looks correct, it sometimes fails to clear the half-cock notch when cocking the lock. This is usually easy fixed by cycling the through fired- half cock -full cock with the harden fly mounted in the tumbler. The fly removes high spot on the sear that caused the sticking.
Here is a couple of pictures of the project so far.
The next step will be the main spring and the stirrup. Any suggestions on how to locate where the hole for the main spring should be?
Best regards
Rolf
Links to the other parts:
left-handed Alex Henry/Staton/Bob Roller lock. Part 1. Lock plate and Hammer
https://americanlongrifles.org/forum/index.php?topic=73225.0left-handed Alex Henry/Staton/Bob Roller lock.Part2. Four post bridle and screws
https://americanlongrifles.org/forum/index.php?topic=73497.0left-handed Alex Henry/Staton/Bob Roller lock. Part3 The tumbler.
https://americanlongrifles.org/forum/index.php?topic=73935.msg736278#msg736278left-handed Alex Henry/Stanton/Bob Roller lock. Part4. The sear and searspring.
https://americanlongrifles.org/forum/index.php?topic=75495.msg750459#msg750459
