Monday, May 18, 2015

Guest Post: Cutting Threads with Taps & Dies

by Firehand
(Editor's note: Firehand is a blogger and a part-time blacksmith. He is a frequent contributor to this blog and his previous articles may be found here.)

Have a machine screw/ bolt/ threaded fitting whose threads have gotten dinged-up? You can use a tap or die to clean up (the technical term is "chase") the damaged threads. In fact, being able to modify (or, if need be, make) a bolt, nut or threaded fitting can be a pretty handy skill to have.

Modifying a sling swivel stud for a rifle is good skill practice, and it makes something useful when finished.

Note: I'm no machinist, so this isn't expert advice. This is "stuff picked up while making knives, working on guns, and generally fixing or modifying crap and the experience directly related to that."*. It's possible an actual machinist might read something here, grab their hair and scream "NO!"

Well, I'm writing this, so deal with it.

Left: tap. Right: die.
Depending on what you're cutting, you'll need either a tap or a die, and a wrench to make use of them.

A tap is designed to cut threads into a hole (internal, or "female" threads). A die is designed to cut threads into a shaft or rod (external, or "male" threads).

Both are made of tool steel that can be hardened: the threads are cut into a blank, flutes (taps) or holes (dies) are machined in, and then the piece is heat-treated.

The sharp remainder of the threads are the cutters: you rotate the tool on/in the piece and they do the cutting. The flutes or holes serve the purpose of giving the chips cut out of the metal somewhere to go; otherwise they either clog up the works and get in the way, or cause the tool to jam (this is absolutely something to be avoided, as a jam can easily result in a broken tap or work piece, thus ruining the project).
Left: tap wrench. Right: die wrench.
There are different styles of tap and die wrenches; these are the ones I have. 
Not shown, but something you'll definitely need: cutting oil. In some places it may be difficult to find the specialized stuff, but be reassured that a small bottle will work for a lot of cutting. If you can't find it, or don't have the cash, remember that it's same as a gun: any lube is better than none.

Cutting oil serves two purposes:
  1. It lubricates the cutting teeth, making the job easier and helping them stay sharp longer.
  2. In the case of threading a deep hole, or a long piece of rod, it helps carry heat away. This is rarely a real with done-by-hand jobs, but is something to remember.

The Project
I've been working on a AR-10 rifle. It's almost finished, but it  needs a sling stud on the handguard. The handguard company makes one for the quick-detach sling swivels; however, the buttstock is an A2, which has a standard sling loop. so I wanted a standard swivel stud that I could hook a regular detachable sling loop onto.

So off to the boxes of parts and "I might need this sometime" stuff, and lo! An old set of Uncle Mike's swivels and studs was located, the stud for the front having machine-screw threads, and the threads are the same as the holes in the handguard: 10-32, which means "Size 10 screw body, cut with 32 threads per inch".
Yes, I started this before I thought "I should take pictures!"
So imagine the last 1/8" or so of the screw being unthreaded.
Two problems, though:
  1. The threaded portion is way too long (easy to fix, though)
  2. The threads don't run all the way down to the stud part(or head, or whatever), which would mean it wouldn't fit flush against the handguard. This is where "cutting the threads" comes in.
Cutting the Threads

1) First get a 10-32 die**.

2) Fold some paper a few times to protect the stud from the vise jaws and clamp it in the vise.

3) Put a couple of drops of oil on the threads, then run the tap onto the screw. When it reaches the unthreaded area, start cutting.

Important point: one side of a die is designed to be started onto the piece, the opening is funnel-shaped to make it easy to start.

4) The usual method is to turn the tool 1/3 to 1/2 turn, then back off 1/4 turn; this helps move chips into the flutes to clear them from the cutting teeth.

5) Continue until the die bottoms out on the stud and can't turn further.

6) Spin it all the way back off.

7) You'll find that this leaves a short distance of the screw still unthreaded, or only partially threaded (how much depends upon the die). However, I need them to run all the way.

Remember that 'start from this side' important point? Now you turn the tap over in the wrench, clean any chips off the piece and out of the die, add a drop or two of oil, and start cutting again. The cutters on that side of the die are (depending on the die) for the final diameter or may have a slight flare, so you can use it to cut the threads into the last portion of the screw.

8) Stop when the tap bottoms out; do NOT try to force it further. Back it all the way off and inspect the workpiece. You should now have nice, clean threads all the way. Wipe or spray them clean (I used brake cleaner) to get rid of the cutting oil and any chips.

Cutting It to Length

First, figure out just how long it has to be. You can measure, but I use a counting method: Starting by running the screw into the hole until it touches the barrel, I then back it out, counting how many turns it takes to go from touching the barrel to out (seven, in this case). Then I do it again just to be sure.

My method to mark where to cut: I take a 10-32 nut and run it all the way down, then I back the stud out the proper number of turns. The inside edge of the nut marks the max length.

Do that twice, too.

Now you can use a hacksaw or whatever else to cut the screw, I'm using a jeweler's saw, which is basically a hacksaw that uses very fine blades. Pick the right one and it will cut steel.

 Clamp the exposed end of the screw into a vise, and start the saw cutting right against the nut. You can back the nut off a further one-half or one turn if you want a bit of fiddle space, which is what I did. Cut slowly and don't put pressure on the saw blade(see 'fine' above, which translates to 'a big delicate'). Just let the weight of the saw do the pushing, and do use a drop or two of oil.

That'll give you a clean end.

Clamp the stud in the vise (remember the padding!) and use a fine file to square the end, and then use it at an angle to give a slight taper( chamfer, to be technical) at the very end.

Clean it all up and try it. If you need you can always file a bit more off the end.

You now have the stud you needed. Pick your hole, set it in place with a bit of threadlocker, and it's done.

I'd suggest flushing the cutting oil and chips off the die, then put on a little regular oil to protect from rust.

Bonus How-To!
You just might want a washer of some kind to fit between the stud and handguard (I did). If you've got some leather punches, that's easy: find something suitable (I used a piece of innertube) and start punching.

You say you don't have leather punches? Do you have some fired cartridge cases and a deburring tool?

Use the deburring tool (you could also sand or file) to get an edge on the case mouth.

 Put the material on a wood block or something else suitable and rap it with a hammer or mallet.

Take the cut piece, center the smaller case on it, and rap.


*I read somewhere the definition of 'experience' is "What you learned when you were expecting something else."
** I USED to have one, but it's disappeared so one had to be bought; about $7.

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