Looking forward to your farquharson build Bob!

Interested in building one of these someday too... That's gonna be a beauty!
Cheers,
Jeff P

In number 3 of the English gun books sold by Tom Row, they state that the area between the trigger guard front frame mountings was challenging to machine.  The arrows in the first picture shows this area. The second picture show the trigger guard and how it should fit.  At the time I thought, how hard can it really be!  For a real machinist probably not a problem, but for me another story!  It requires a .312 (5/15) radius and there is no way, that I knew, to get in there to machine for the clearance needed.  After talking with a couple other folks and not getting an answer here is what I came up with.

It would be interesting to hear from others how they did or would do this.
Keep in mind the .312 radius is centered on the screw hole.

I started by machine the area with a straight end mill, just to clean it up somewhat and help me get centered on the breech block mortice for the next operation.  This can also be seen in the first picture.  There was no way to machine the complete area needed this way or machine the radiuses for the clearance of the trigger guard. 

First, wanting to be very precise on the alignment between the holes for the screw, and the square of breech block mortice, I bored the two hole with a tiny boring bar. Each side is a different sizes.

Once the holes were completed I made an arbor that fit the two hole with just a slip fit.  The larger size was made longer so it went through to the far side of the frame.  I then made a .625 (5/8) blank with a hole fitted to the arbor.  This blank was made from O-1 tool steel.  See picture 3 of this assembly in the frame.

Picture 4 shows cutting the teeth into each side to the the blank.  I think by now you can see where this is going.

Once the teeth were done, the the cutter was heat treated.
Bob

Very nicely done.  I’m going to have to remember that trick with the cutter on the arbor.

You created what we called a "reverse counter bore" in the aircraft industry. They come in handy for just such an occasion.



   JMH

You can buy that tooling at your better tooling supply houses. I used one inside of bulkheads where the nut of a fastener must sit flat. The hole breaks out in a radius so we use that type of tool to create a flat for the nut to sit on. I think we got them from US Tools maybe? Been a long time so don't quote me. 




JMH

Steve,
I would agree the British did it the same.  One can tell by the shape of the original frame with the step just behind this area.  Unfortunately this frame will not have the step because the casting breech block hole had to be straightened up and I now machine everything from the center of that mortice.  In order to get this area centered I had to eliminate the step and will need some cosmetic welding to clean up the way it looks.

Check you pm’s please.
Bob