Threads – Tapping and Thread Milling

Hey folks, today we are going to talk about threading in multiple forms. For the most part I am going to discuss my experiences with the different types of thread cutting/forming, so if you are looking for tons of technical information I apologize, but there are so many variables when it comes to the threading –perhaps I can write a more technical blog on each type of thread forming. For now, we are going to give a general overview of cutting threads based on my experiences and opinions. I know, opinions are like… well, you get it, just stay with me and hopefully I can provide some insight.

First and foremost we have cut taps. In my experience cut taps are the most widely used across most industries. Cut taps are reasonably cheap and very versatile. You have all seen the drill charts that give you the tap drill sizes required for different threads. Pretty straight forward – drill the hole to the right size and depth, countersink the hole, then tap it. Cut taps can be used by hand, on a drill press with a tapping head, on a knee mill, or rigid tapping on a CNC machine.

Creating Threads in Through Holes with a Cut Tap

Standard tap for creating threads with a CNC milling machine.

When determining which tap you need you should pay attention to the type of hole you are tapping. When tapping a through hole you can use a standard cut tap which has a lead on it. The lead is the tapered portion on the end of the tap that essentially centers the tap in the hole when engaging.

When tapping a through hole you need to make sure you go deep enough to cut threads all the way through the hole – the length of the lead will depend on the size of the thread. The larger the thread the longer the lead.

Threading a Blind Hole with a Bottoming Tap

This bottoming tap has little or no lead and allows you to thread deeper into a blind hole.

However, if you are tapping a blind hole you would be wise to consider a bottoming tap. A bottoming tap has the lead almost completely ground off. This allows you to engage the tap deeper into a blind hole. These are used when there is a tighter tolerance on the depth of the hole, in situations where a hole too deep will break through into a feature. This is due to the main problem with cut taps … CHIPS. When using cut taps in a blind hole, regardless of standard tap or bottoming tap, you need to make room for chips. As the tap engages the hole it is cutting the thread geometry out of the material, therefore creating chips. Since you are engaging from above the chips are forced down in the hole along with the tap. If you do not provide enough room at the bottom of your hole, then you will break your tap. Simple as that. That is why bottoming taps are so helpful in blind holes – with a very short lead you do not have to drill as deep to form full threads to a certain depth. Keep in mind, there are taps available with helical geometry with the goal of lifting chips up out of the hole. In my experience, I have gotten mixed results. The complex geometry ultimately weakens the tap, so if you are tapping a tough material be careful. Just make sure you do your homework.

The Strength of a Roll Form Tap

Roll form taps like this are stronger than cut taps and forms threads rather than cutting them.

Next, we have roll form taps. When I first discovered roll form taps, I was in heaven.  It was after a particularly frustrating week of broken taps and bad parts. Chances are if you are reading this then you have had some of those weeks.  We all have.  Roll form taps are much stronger than cut taps, and the geometry is completely different. The one drawback to roll form taps, and a major reason most shops I have worked for never adopted them completely is because the standard tap drill size no longer applies.  Most standard drill charts (the large ones you put on the wall in the shop) now have standard tap, roll form tap, metric tap and STI tap drill sizes all listed separately. However, after years of using standard taps many of us don’t reference the chart as much as we should, and since the hole for a roll form tap is significantly larger than that for a cut tap, bad things can happen. A roll form tap does exactly as the name hints – it forms the threads rather than cutting them. When the tap engages the hole rather than cutting material away it changes the form of it, and shapes it into the thread geometry. If you have ever done any work for a defense contractor, this is why most prints will have a note that all threads must be formed by a cut tap. The military generally frowns upon operations that change the structure of the material, at least in my experience. They also frown upon castings as opposed to solid plates for complex parts – too much unknown in what you can’t see. Anyway, roll form taps are great.  They are difficult to break (no, that’s not a challenge) but they also require a bit more torque. I have really only used roll form taps in aluminum and other soft materials, rarely in cold rolled steel. I am not sure how they perform in harder materials, but most shops don’t like the idea of two sets of drills for the same size thread, which is why they are not more widespread.

The Versatility of Making Threads with a Thread Mill

Helical boring rather than drilling combined with thread milling allows you to produce a multitude of different sized threaded holes with just two tools.

The final type of thread cutting is thread milling. Thread milling is a great operation that seems scary at first, but once you get it down it is truly amazing. There are many different types of thread mills, which I will get into in a different blog post. For now, I will discuss a single point thread mill. With a single point thread mill, you have great versatility, with most thread mills cutting a wide range of threads. You can create custom pitch threads, right hand or left hand, inside threads or outside threads all with one tool. Since I have recently started using helical boring for my holes rather than drilling, you can accomplish many different holes, with many different threads all with two tools. You bore a hole to the minor diameter of the thread, send the thread mill in to cut the threads, and you can use the thread mill itself to chamfer the top of the hole, as long as it doesn’t have to be a ninety-degree countersink, since most thread mills will be somewhere between thirty and sixty degrees. Due to the geometry of the tool, the only distance you need to make up for is from the outside edge (cutting edge, or point) of the tool to the flat tip, which is generally less than .02” on the smaller thread mills. The real benefit here is that once you mill the hole, you can send the thread mill to the bottom of the hole and mill from the bottom up, rather than top down. By doing this you are avoiding any concern of the tool running into chips at the bottom of the hole since you are moving away from the bottom of the hole. Looking at the thread mills you may not believe at first that it is going to do what it is supposed to. I know when I used one for the first time I was sure it was going to break – but it didn’t.  Thread milling is the most versatile and efficient of thread forming strategies, and it is going to be my go-to from here on out unless there is a good reason I can’t do it.

Mill a Hole and Thread it with the Same Tool

More Info. on Thread Milling vs. Tapping

Do your homework, know your tools and your materials and approach anything you do in the machine shop with a good understanding and clear head.  This doesn’t change when you are threading holes.  Trust the numbers, and in this case, don’t be afraid of trying new things. I always scoffed at thread milling – I only wish I had found it sooner.  Stay safe folks.