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This article is a continuation on my "All about screws" article. If you haven't read that one, please do now.
What is tapping?
Tapping is the process of cutting a matching set of threads into a piece of metal, plastic, acrylic, or wood. The process of tapping uses two separate tools: the tap which cuts the threads on the inner diameter of the hole, and the die which cuts the threads on the outer diameter of the bolt or screw.
The process of cutting the threads using a tap is called tapping, and the process of using a die is called threading.
It all sounds complicated and harder than what a beginner would want to deal with, but I'm here to tell you otherwise! With a little practice you will be able to drill and tap your own holes, and cut your own threads! It's really not all that hard.
What you'll probably need:
If you're just getting started you probably don't have a tapping set. These will come in either imperial or metric and can run the range from fairly cheap to uber expensive. You get what you pay for, and a quality set will break less and last you longer. For a beginner, get the cheap set and break them and learn what you can and can't do. It's going to happen anyway, so don't throw your money away!
You'll also need some sort of cutting fluid. Without this you're going to have a bad time of it. Lubrication is the word of the day. Buy some cutting fluid. Trust me.
Step 1: Types of Taps and Their Uses
A tap cuts threads on the inner diameter of a hole, like a nut. There are a few main types of taps:
Plug Taps (or Second Taps)
These are the most commonly used taps as they are easiest to line up and start your threads. The have a gentle taper in the business end of the tap, which is used to center itself in the hole you are threading. The number of threads in the taper is typically 3 to 5.
These types of taps allow you to cut all the way down into a blind hole (a hole that doesn't go clear through your material) and have threads to the bottom surface. This type of tap is usually used after you've already partially threaded your hole with a tapered tap. The straight edges at the beginning of the tap make it very difficult to accurately line up the tap with the hole you are tapping.
These taps are similar to plug taps as they have a taper in the head of the tap. A tapered tap has a more pronounced taper, and this allows for a less aggressive cut for cutting threads. The number of threads in the taper is typically 8 to 10.
While the previous taps were the types that you would use manually, there is also a type of tap that you'd use in a lathe, a mill, or a drill press. These powered taps, commonly called a "spiral point" tap are usually used in holes which go all the way through your working material as they break and eject the chips forward, into the hole.
The above tap types are cutting taps, where they actively carve a groove into their working material. The forming tap pushes the material back into itself to form the threads. These taps, since they do no cutting work, do not have flutes (cutting grooves). These types of taps are utilized in forming threads in blind hole since they do not create chips and do not need to be backed out. Since the method of forming threads is by pushing the material into itself, this method is generally only used in softer materials like mild steel, brass, and aluminum.
Step 2: About Dies
Dies are used to cut the outside threads which will mate into a threaded hole or nut. This is what cuts the "male" part of a thread. Dies are generally made in two types: solid and adjustable. The solid ones are a set size, and the adjustable ones have a screw or set of screws which allows you to make the threads slightly tighter or looser depending on your need.
Dies are used with a die wrench, which fits around them and gives you leverage over the cutting process. These wrenches generally have a centered hole for the die to fit into, and two handles for holding on to and cutting.
Chasers or Die Nuts
There are a second, related set of die-types which are called chasers or die nuts. These are used for cleaning up old threads which have become worn from use or corrosion. Chasers can not be used to cut new threads! The generally come in a set size and have a hexagonal shape for use with a wrench.
Step 3: Tapping Fluids
At this point I want to talk a little bit about tapping fluid. As I mentioned earlier, it is critical that you use some sort of lubricant. What you use depends on what you are cutting into.
For mild steel, use a petroleum or synthetic cutting oil.
For alloy steel or stainless, use a petroleum cutting oil and mix it with kerosene or mineral spirits, 9 parts oil and 1 part diluter.
Do not use a lubricant on cast iron! You can clear out the chips with pressurized air.
On Aluminum use kerosene or mineral spirits with a small amount of petrolum cutting oil. WD-40 is often a decent substitute for tapping aluminum.
When tapping brass, use kerosene or mineral spirits without anything else.
Finally, with bronze, use kerosene or mineral spirits with a small amount of petrolum cutting oil.
Step 4: How to Tap Threads
Start by defining the correct size. If you have a screw or bolt that you are targeting, look up the size and required hole for tapping. Wikipedia has a good chart that can help you figure out what size your hole should be. Many taps will have the required hole written on the side of the tap.
Drill your hole, trying to keep it perpendicular to the work piece. A drill press is a great way to do this. If you don't have a drill press handy, you should look at using a jig.
Countersink the mouth of the hole. This will help guide the tap into the center of the hole and help ease the flutes into starting the tapping process.
Once you're ready, you can grab your tap and tap handle. Unscrew it until your tap fits in, and screw it down until it is tight, but not so tight that you won't be able to get it open again. Everything in moderation :)
Apply some tapping fluid to your tap. Check the section on which fluid to use for your material.
Place the tip of your tap into your hole. Keep the tap as perpendicular as you can and apply downward pressure. Start turn your tap slowly clockwise. The teeth of the flutes will catch and you'll start cutting your threads. Every few turns carefully back out your tap and clear the chips.
I can't stress this enough: go slowly and clear your chips. The most common reason for breaking a tap is that a chip will get wedged in between the tap and the wall of the hole you are trying to thread. The tap is not strong enough to withstand the shearing forces and will snap. Trust me that you don't want to have to deal with getting a broken tap out of a hole you critically need.
Keep this process up (twist, untwist, clear chips) until you've worked your way all the way down. Apply more fluid if you think that it has run all away or if you've had to wipe it away because you were clearing chips.
Test your newly tapped hole by screwing in a mated screw or bolt. If this is a custom hole size, you'll need to first use a die to cut a matching thread.
Step 5: Using a Dead Center to Make Your Life Easier
If you are already somewhat accustomed to working with a tap and have found that getting things lined up just right is a pain, I highly suggest using a dead center in the chuck of a drill press to make sure that you have even pressure applied perpendicular to your work piece.
Take a look at one of my favorite blacksmiths doing it here.
Hope that helps someone :)
Step 6: How to Thread Using a Die
The process of threading using a die is very similar to tapping using a tap.
Prepare your stock by selecting an appropriately over-sized rod. Gently taper the end. I like to do this in a drill press on low, using a file to create an even taper.
Clamp down your rod, or use a vice to hold it. I prefer a vice because it lets me work vertically. You can work horizontally, but I've found that it is much harder to maintain a perpendicular cutting angle.
Set your selected die inside the die holder, and secure it in place. Apply some cutting fluid according to your material type.
Place the die on the end of the rod and push down with reasonable pressure. Don't push down too hard as you may bend your rod if it is thin, or you may slip and hurt yourself. A dab of pressure will do ya.
In the same way that you cut your threads with your tap, go in slowly and evenly a quarter turn and then back out and clear your chips. Keep going as far down as you need your threads to be.
Step 7: Practice, Practice, Practice
Tapping and threading is something that gets much easier once you get a knack for it. Watch some videos on YouTube, or have a machinist show you a few tricks. The best way to get better is to do it yourself and feel what is the right speed, pressure, and methodology that works for you.
Have fun and thanks for reading!