How To Tap A Hole

By Steve Hill, CO

Oh I hate it when this happens!

It’s four o’clock in the afternoon. You have to attach the NYUCB footplate (and a bunch of other stuff) on a KAFO that must be fit to the patient in half an hour and the stirrups aren’t even tapped yet!

You finally find your favorite 10-32 tap on your bench (it was hiding under a work order, as usual) and begin the job at hand. You need to tap these stirrups so that they can be screwed onto the plastic footplate. With one hand you firmly grasp the stirrup and with the other hand you press the head of the tap to the hole to be threaded. It’s not easy to get the tap to begin its long, arduous journey to through the hole, but after some fumbling it starts in. Halfway through you are starting to feel the elation of a job almost finished when suddenly “SNAP”! The tap breaks of in the hole. Damn. Now you have to try to get the broken tap out and hope you don’t destroy the threads you do have in the process. There must be a better way…. And there is.

There are three ways to tap a hole. The first way, described above, is time consuming and will eventually lead to a broken tap or, at the very least, a lot of wasted time. Here are a couple of other options you might want to consider.

The Vice and Drill Method

One of the problems that leads to the disastrous conclusion above is the fact that your hand cannot hold both tap and stirrup at perfect right angles to one another. This results in the tap entering the hole at a slight angle. That’s okay at the beginning of the hole, but once you get to the center, or just past, the tap is meeting too much resistance binding it up, breaking an otherwise good tap.

To rectify this situation, you must clamp the stirrup in a vise or similarly stable platform. This allows the hand with the tap to enter straight or at least a more controlled angle. You don’t have to worry about keeping both hands steady.

The other part of the problem is the jerky motion of trying to spin a tap by hand. I don’t care how good you think you are, at some point you have to let go of the tap to spin and re-grip, and this is where the problem lies.

The solution is to have the tap spin automatically, freeing your hand to simply try and hold it straight toward the hole. This can be easily accomplished using a tool you already have laying around. Your drill gun. Simply mount the tap in the drill gun and use the speed control to slowly spin the tap to a successful conclusion. This method won’t keep you from breaking taps, but it will greatly reduce the problem.

The Automatic Tapping Machine

Now here’s my favorite solution. It’s not cheap, but it will save you a lot of time in the short run and a lot of money from broken taps in the long run. It’s a tool called an Automatic Tapping Machine. There are several brand names that you can select from, but the best one, in my opinion, is called the TapMatic.

What is an automatic tapping machine you ask? It’s a device with a built-in clutch that you can attach to a drill press and use to tap any material. It works like this.

The TapMatic is chucked into a drill press. You use a drill press because it’s a stable platform that assures the tap will spin true and not move back and forth. Set the speed of the drill press on low, as slow as it will go. Apply a nice sharp tap to the receptacle on the TapMatic. It’s an odd kind of attachment that uses a rubber bushing to assure the tap isn’t too rigid in its mooring.

Now you’re ready to tap a hole. Turn on the drill press and make sure the tap is spinning slowly and in a clockwise direction. Clamp the stirrup firmly with a pair of vice grips (to make sure the stirrup isn’t yanked out of your hand by the tap) and align the hole with the spinning tap.

The tap will quickly grab a hold of the stirrup and begin to cut threads. As soon as the tap makes its way through the metal, pull sharply downward on the stirrup. This engages the internal clutch and reverses direction of the tap, spinning it back out of the now threaded hole. Be very careful not to tap too far as this will result in the tap running past its threads and reaming the hole out to an uncomfortable size.

Each hole takes approximately three seconds to tap. That’s right, seconds, not minutes or hours. But most importantly is the tap will not break on every other hole. Or any hole, for that matter. The two dedicated TapMatics we have running in the shop now have had the same taps in them for well over a month and have tapped thousands of holes.

Helpful Hints

Here are a few helpful tips on taping holes. First, always use tapping fluid. Always. The taps will stay sharp longer and cut better if you use proper lubrication. Second, always tap at right angles to the hole. Even a slight angle going into the holes can result in catastrophic failure of the tap, i.e. breakage. Third, and most important, always use tapping fluid. I know I said it twice but it’s really important.

Tapping holes in metal is an important part of fabrication and doesn’t have to be a nightmare. Proper technique and a few good tools and you’ll be an expert in no time.