Screws and bolts are simple options for fastening two pieces of material together. As you might have guessed, the secret is in the threads, but how would you go about making your own? In this section, we’ll go over the tap and die, a set of tools that allows you to make your own threads wherever you need them.
A tap is a tool that cuts threads into a hole so that a bolt can be screwed into it. While they look similar to bolts, taps are typically made of high-speed steel and have long channels ground into the sides, leaving gaps in the threads. As the tap turns into a hole, these channels allow the chips that are carved out of the material to break free and be ejected.
SELECTING A TAP
Taps come in three main styles:
Taper — starts narrow and tapers to full thread width. This means it starts easy when threading into a hole, but they need to be turned farther in order to form a full thread in a hole.
Plug — the most common tap for general purposes. It has a slight taper, but allows for threading almost to the bottom of a blind hole.
Bottoming — for forming threads in the full length of a hole that has a bottom. It’s recommended to use a taper or plug tap to form the initial threads.
For every bolt, there’s a corresponding tap with a matching diameter and number of threads per inch. Make sure that you choose the right one! Similarly, for each tap, there is a corresponding drill bit size that you should use to drill the initial hole. These relationships are typically printed on the packaging for the tap, or they can be found through a quick internet search.
If you’re not sure what thread count a bolt is, you can use a screw-pitch gauge (Figure A) to match one of its blades to the thread profile.
TIP: If you find yourself using taps often, it’s a good idea to keep each tap with its corresponding drill bit. I keep duplicate drill bits just for my taps. That way, when I need to put a ¼-20 bolt somewhere, my tools are ready to go.
SECURE THE PIECE TO BE TAPPED
This is one of the most important parts. Taps are very brittle, so if your tap or your material moves too much, it will break the tap off in the hole. To avoid that, it’s a good idea to use clamps and align the hole so that it’s either directly below you, or directly in front of you. This will help the tap stay true when you’re turning it into the hole.
SELECT THE TAP WRENCH
Since taps are just bits, they need a tool to turn them. The trusty tap wrench is specifically made for this (Figure B). For tapping holes ¼” or less in diameter, a 5-7″ tap wrench will do, but for anything larger, you’ll want a longer handle. Insert the square end of the tap into the adjustable jaws of the tap wrench, then twist the handle that tightens the jaws.
If you don’t have enough clearance for the handle, you can use a T-wrench (Figure C). These smaller alternatives are great anywhere a normal tap wrench won’t fit.
Here comes the tricky part. Line up the tap with the hole, making sure that it’s completely perpendicular to the material (it can be helpful to have another person sight it from the side while you do this). Apply slight pressure to the tap, and begin to turn the wrench (Figure D). You should feel the cutting threads begin to catch, so double check that the tap is straight, and do not force the tap or it may break. After you’ve made a couple full revolutions into the material, you can stop applying pressure, and give the tap a small turn backwards after each complete revolution to break the chips off and clean out the thread. Once you’re done, turn the tap out, blow the chips out of the hole with compressed air, and test it with a bolt!
TIP: If you’re tapping a soft material like aluminum, brass, or cast iron, you won’t need lubrication, but if you’re tapping steel, some cutting oil will help you out.
A die (Figure E) is basically the inverse of a tap: It cuts threads into the outside diameter of a rod. The rod’s diameter will dictate which die you use.
PREPARE THE ROD
To prepare the rod for threading, just file or grind a bevel onto the end so that it’s easier to turn the die onto it (Figure F).
PLACE THE DIE IN DIE STOCK
After you’ve selected your die, you’ll need to put the die into the die stock, which is similar to a tap wrench, but has a socket that the die fits into. Most die stocks have a set screw on the side, which is lined up with the dimple on the die, and screwed in to tighten the die into the stock.
CLAMP THE ROD IN PLACE
Rather than try and spin the rod into the die, we’ll spin the die onto the rod. That means that the rod will have to be held securely, ideally in a vise. This also helps keep the rod straight while spinning the die onto the end.
CUT THE THREAD
Cutting the thread is almost identical to the tapping process. Make sure that the die is aligned with the rod and apply pressure, then turn it onto the rod. Once the initial threads have been formed, and the die has “caught,” you can remove pressure and continue turning (Figure G), remembering to give it a small backward turn once every revolution to break the chips. Once you’ve threaded the rod to the desired length, give it a test!