Thought I would repost this thread (har-har-har...get it? thread! :rotf
for some of you new folks who are thinking about building your own gun.
Most if not all of the "box of parts" style of kits require drilling and tapping holes for the lock and the barrel tang bolts so hopefully you will find this post helpful.
Now that I've read thru the old thread I realize I really didn't say much about the actual threading process for making a tapped hole.
Of course the first thing to do after locating where the tapped hole is to drill the hole.
If the hole doesn't go all of the way thru (blind hole) the part, it must still be deeper than the needed depth of the threads. This is to accommodate the "lead" (pronounced leed) on the tap. This is an area where the cutting threads have been ground off to make a tapered area that will start into the tap drilled hole. Most of the taps you will find at the store are "taper taps" and this lead area will be quite long.
If the hole is blind and you need threads going all of the way to the bottom (like for a nipple in a drum) you will need to buy a "bottoming tap" or make your own as I described above.
After drilling the tap drilled hole you need to "start" the tap into the hole.
This is the most critical part of the process. The tap must be held exactly parallel with the tap drilled hole.
With the tap mounted in the tap wrench, align it with the hole and give it about 3/4 - 1 1/2 turns. You should feel it cutting without too much resistance.
Add a little oil and back the tap out 1/4-3/4 of a turn, add some more oil and screw it back in. Then following the instructions I gave up above finish tapping the hole.
If I'm tapping a hole that is for a screw or bolt that is going thru some wood, like a lock bolt or a tang bolt I use the hole in the wood to guide the tap.
To do this, I first drill the hole thru the wood and the metal part using the TAP DRILL.
That way the metal part is ready for the threads to be cut.
I then remove the metal part and use a larger drill that is the same size or slightly larger than the finished thread size to enlarge the hole in the wood so that it will clear (and guide) the tap. It will also clear the screw or bolt.
For instance, for a #8-32 UNF tang screw/bolt (.164 diameter) I will use a #29 (.136) diameter drill to drill thru the tang, the wood and the trigger plate.
Then removing the trigger plate which I want to thread, I enlarge the hole thru the tang and the wood using a 11/64 (.1719) or 3/16 (.1875) diameter drill. This will provide the clearance for the screw and the tap.
Removing the barrel and reinstalling the trigger plate I use the clearance hole to align the tap with the hole in the trigger plate and "start" the tap into the metal as I described above.
Usually, I run the tap in about 1 turn, back it off 1/2 turn, then run it in another 1 turn.
Removing the tap and the trigger plate I then can start the tap into the "started" hole, add the thread cutting oil and finish tapping the hole thru the trigger plate.
Doing it this way the tap will be lined up with the tap drilled hole even if it is on an angle and the thread cutting oil will not get all over the wood.
I would talk more about using a die to cut external threads but most of the thread cutting involved in building a rifle doesn't require cutting external threads.
The only exception might be a need to "chase" some finished size threads further onto a part which doesn't have sufficient threads on it.
The hardest part of this is starting the die onto the part without crossthreading it.
The rule to remember here is you should be able to screw a die onto a threaded part without needing any force at all until it gets to the partially threaded area.
Have fun!
