While the Tap & Die go hand in hand as threading tools, they are very different in the way they work. They seem very similar, just inside out, but that makes it so there are different tricks for success. So, we will talk success tips for making external threads with a die.
The Tap and Die are both cutters for making threads. Taps have teeth on the outside for making threads on the inside. Dies are the opposite with teeth on the inside for making threads on the outside. For typical DIY, taps are much more common, and are easier to use properly. Dies are less used (fortunately), and come to failures more often (unfortunately). Mostly, we buy external threads in the form of a bolt, however, there are times when we do need to create them. One way is with a die.
What Is A Thread Die?
Remember this picture? It comes from our article on tapping for internal threads. The long straight parts in this image are taps – for cutting threads on the internal surfaces of a hole. The round cylindrical parts in the image are dies – for cutting threads on the external surfaces of a cylinder.
For reference, the “inside” story on threads – tapping internal threads – is covered in this other article “Get Thread Tapping Straight“.
Today, we are talking about dies, and tricks for making good external threads.
In DIY there are at least 2 really good uses for dies. The First, for chasing existing threads (cleaning up damaged threads); and the Second, for creating new threads.
The first use, chasing or clearing existing external threads with a die, is the simpler case, so we can start there.
Anatomy Of The Tool
A die is a pretty simple looking tool. It is like a nut, but with interrupted internal threads. The edges of the interrupted threads are the cutting teeth.
Thread dies are often round on the outside like in the picture, but they sometimes have a hex (or other) shape on the exterior. Like the one in the video below. The round ones usually have a divet or 2 around the perimeter which the tool holder uses to drive the die. This can be seen in the image above. Using a special tool holder is the first trick for successful external threads with a die.
Just like a tap, they come in different configurations like with 3 or 4 or 5 or more columns of threading teeth. Some also have a pinch screw which allows you to adjust how tight the thread is.
While specialty dies come with special teeth shapes for cutting particular materials, general purpose dies are usually good enough for DIY.
Thread dies have a front and a back – or more accurately, an entry side and an exit side. You can tell the entry side because it has a relief in the cutting threads – with reduced height teeth for starting the cut. The relief in the entry side also helps center the die on the cylinder to begin the thread.
Update: As pointed out in the comments, many dies have a relief on both sides, allowing the die to be started from either side.
Clearing External Threads With A Die
Have you ever tried to turn a nut onto a bolt and found there are burrs in the threads making it difficult or impossible for the nut to pass? There are many ways to conquer this, and one way is with a die.
There are 3 caveats for this trick.
- First, if the bolt is hard, like grade 8 or something similar, the die may not cut a serious burr. The bolt may damage the die.
- Second, depending on how the bolt threads were originally created, a die may peel all the threads, which will cause problems. Bolts often have “roll formed threads”, with a significant radius at the root. If the die creates a sharp root (typical of external threads with a die), then it will cut that into the bolt.
- Third, if original external threads are super accurate (“tight” fitting), a general purpose die may cut them less accurate – looser. It is not often a problem, but worth mentioning.
In other words, if the die does not want to go nicely on the bolt, then clearing external threads with a die is not the best choice for that bolt.
Clearing threads with a die is a cool trick when it works. The idea is to simply thread the die onto the shaft, like a nut. As you turn the die, it trims off burrs and damage leaving a nice thread again. Obviously, it cannot replace material, but it can cut off burrs to clean it up.
The Tricks To Success:
- Make sure the die goes on straight. Easier said than done.
- Make sure the die comes nicely onto the existing threads. If it does not align correct, it will cut new threads, on top of the old ones, and ruin the part. “Cross Threading.”
Anyway, take care with alignment, then using a thread die can help fix the damaged threads.
Creating New Threads
When we think of a thread die most of the time, we think of cutting new threads. Effectively making a bolt. If that is what you need, then for sure, the die is a great tool to accomplish it.
The concept is simple – place the die on the end of a shaft, then start turning the die. As the die turns, it cuts new external threads. Right? Yes, that is the concept. Doing it right is often a very different story.
The first few times I tried to put threads on a shaft, I ended up with a mess. Turns out it is really hard to get the die straight on the shaft when you are doing it by hand. I ended up with a new thread that was slightly cattywompus to the shaft. I do not have a picture, but imagine the threads going at a slight angle to the shaft – where further down the shaft, the threads get deeper on one side and shallower on the other. Eventually the die will not turn anymore, and the part ends up as scrap metal. The unfortunate thing – there is no way to correct the issue once the threads are cutting, and you cannot tell you have it wrong until some threads are there.
The trick is to use something that forces the threads to start straight. Here is a video of how I do it now.
Video: Cutting New External Threads With A Die
Here is a quick video showing one technique for cutting good external threads with a die. Just to make it illustrate the success, we show the technique with a difficult subject. This video uses a thin wall stainless steel tube, welded seam.
Thin wall is a challenge because if the dies is off a little, the tube will break. The welded seam is also challenging, because the weld has different properties (more brittle) making that spot harder to cut. It wants to make the die tilt. Finally, stainless steel is not the most friendly material to work with. Some alloys like Stainless 303 are not so bad to machine, but most stainless steel is not so nice for machining.
Enjoy the video.
So if you have ever had trouble – like I have – with getting a die to start truly straight, the above video illustrates one good technique. There are not so many times when a thread die is really needed, but when you do need it, then it is sure nice to have one. Like the tube in this video – I did not want pipe threads (and they will not fit that diameter anyway). So, a thread die does the trick.
The key take-away from the video is to use other common equipment to assist in making external threads with a die straight. Straight along the axis of the part we are creating the threads on. The drill press works well, but there are other good tools as well. If your drill press is not big enough, then get creative to be successful making threads with a die.
Instead Of A Thread Die . . .
While the die is the right tool for many jobs, there are substitutes that can come in handy instead of the die. The first, and most beautiful is a friend with a lathe. While that is not always available, how about variations using bolts?
On many occasions I have cut the head off a bolt to use it for something. Bolts are cheap comparatively, and they are much stronger than the typical material we can cut threads on. One example is in the image. I needed pegs with threads, so instead of starting with a shaft and adding threads, I just cut the heads off some bolts, then rounded the other end with an “essential” grinder. Works perfectly.
Other options include using bolts or threaded rod, or something similar. For instance, welding a bolt sticking out of a chunk of tube – which keeps me from needing to create the external threads with a die.
The possibilities are endless for substitutes, just be creative. Anyway, now you know a good trick for creating external threads with a die, you do not always have to find a work-around.
Good Luck With All Your External Threads!
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