Get Thread Tapping Straight
A Tap and Die set is a near must for a DIY fab shop. While the thread creating functions are pretty easy to visualize, sometimes getting the tread to start straight and run true is a different matter. The thread tapping functions are easiest, so let’s talk about that first. Dies are are less frequent, and tend to be more difficult to start perfectly straight. Since that’s really another topic, if you’re interested, there’s this article about Successful Threads With A Die. In the meantime, let’s get back to internal thread tapping.
Threads For Bolts
Invention of the thread has to be one of the greatest accomplishments. Conceptually, it’s just a ramp wrapped around a rod that allows you to slide another ramp (equal and opposite) wrapped inside a hole until something causes them to stop — namely, the bolt gets tight. With one inside the other, the continuous ramps are in contact with angles that keep them from sliding back. That is, while under load the friction overcomes the sliding forces. (Until, of course, other forces are present — like vibration, or you turning it with a wrench.)
That’s the theory, and the way we accomplish it is to cut (or roll) the ramp (thread) into the two pieces, male and female. Hence, the tap and die. A tap puts internal threads in a hole (like a nut). A die puts external threads on a shaft (like a bolt).
For this article, we’ll focus on the tap for cutting internal threads. These are far more common for DIY simply because we can so easily buy the external threads in the form of a bolt. Yes, we can also buy nuts (internal thread), but we sometimes need those internal threads in the part rather than adding a nut. That’s where the tap comes in.
Manufacturing facilities have complex machinery to put threads in parts with ease. They have things set perpendicular to run perfectly — usually with dedicated machinery. For DIY we don’t have that convenience nearly as much. We need to rely more on our own setup and skill.
Getting It Straight
Conceptually, threading a hole is easy. Just drill the right size hole, then put the tap end in the hole and start twisting. But what if the tap does not enter the hole straight? What if you need to tap a hole right to the bottom? What if the tap binds in the hole?
There is a lot written and a lot on video about thread tapping, so I’ll skip the details. However, I will review just a couple things that help to get it right.
- Drill the right size hole. Pick the bolt you want, which determines the tap, then select the drill to match. It’s worth buying the right drill for the tap you want. (I found a cool app for my phone “Tap And Drill Chart Calculator“)
- Use some sort of lubrication while tapping. Tap Magic, or any other machining oil. I’ve even used motor oil in a pinch.
- Start carefully. Make sure things align, then do your best to make things stay that way.
- Break the chips. This can be a point of discussion, but while the tap is continuously moving it’s not necessary to stop to break chips, but if you stop motion, then you should break chips occasionally. You’ll see that in the videos below.
- When tapping blind holes particularly, it’s good to back the tap partially out to remove chips. DON’T take the tap out completely, because it’s tricky to find the right threads to put it back exactly. Turn it mostly out, and blow away the chips you can.
Thread Tapping Tools
There are several versions of taps available. Just a quick look online will show straight taps, spiral taps, bottom taps and chip clearing taps (just to name a few). There are a lot of specialty tools, and specialty options, so don’t worry too much about all the details.
Tap tool ends can be different based on what you’re trying to do. Here are a few below. The more threads that in the chamfer, the easier it is to get the tap to start straight, but the deeper you have to tap to get the same number of full threads. The “Plug Chamfer” is the most common. (Images from McMaster.com – best website ever.)
If you’ve ever cross threaded a nut, then you know the issues that can happen if you don’t start the thread in straight. That can be exaggerated with a tap (and more so with a die). If you don’t get the tool straight with the hole to begin, it can make a bit of a mess. That said, taps can be fairly forgiving and just follow the hole if you’re careful.
Simple Tapping Video Example
Here is a procedure I like to use that pretty much guarantees good alignment of the hole and the tap.
The video shows a simple way to align things when thread tapping. Using the same set-up as when you drill the hole, leverage the location and vertical position of the drill press to hold the tap straight.
Thread Tapping Woes
One of the things I dislike most about tapping threads is the repetitive and time consuming nature of it. Twist down, then back it just a little to break the chips, then twist down again. Over and over for as many threads as you need. If you need to go deep in a blind hole, then you should probably back it out (mostly), blow the chips away, then twist it down to the bottom again. For fine thread bolts, that can be a lot of twisting over and over. If I have to do several, it’s just annoying.
Sometimes, doing it all by hand (without the drill press), the tap will go in slightly angled, and that messes up the start of the hole. That is more true when using bottoming taps. Lots of care in the start is really important.
Finally, what if the tap binds in the hole? This can happen if the chips get too bungled up, or if the tap is off at an angle. The things to avoid it are the 5 things in the list in the “Getting It Straight” section above.
Using Machines to Help
So, what about using machinery to assist? Sure, that can be done too. But, do it really carefully. Here’s a quick video of thread tapping into the ends of some aluminum extrusions. The part not shown in the video is the vise attached to the press table, hanging over the edge of the table to allow drilling into the end of the long work-piece. (Sorry, I should have shown that.)
The aluminum extrusion is not the important part, it’s using the drill press to do the thread tapping. Note that while the spindle is turning continuously, there is no chip breaking. After the spindle stops, we need to do the chip breaking to avoid getting the tap stuck in the work-piece.
It is important to note that with aluminum this is less risky, but with steel, depending on the alloy, the forces internal to the tap can become really high. Other materials, especially when the tapping is deep or through thick pieces, need careful attention. Be super careful with this technique, because it’s a big hassle to dig out a broken tap.
Wrap Up On Thread Tapping
That’s about it. There is a ton of additional information about the tapping tools and about tapping in different metals. Just search on-line and you’ll find as much detail as you want. To me, it’s fascinating to look at the different cutting geometry for various metals, and the many different cutting fluids available. Those are probably important for production environments, but for us in DIY, a good General Purpose tap with some sort of oil seems to work well enough for just about everything.