Cut a thread correctly

Even with DIY enthusiasts, it happens again and again that a thread is either no longer good or not present at all. Then a thread has to be cut first. Although this is actually part of the training course for numerous metal trades due to the complexity, you can also cut or recut a thread without much effort.

The distinction of threads

Some do-it-yourselfers have a respect for thread cutting because there seem to be so many different thread types. In fact, the threads have been simplified more and more. Basically, you should only distinguish a few types of thread.

  • metric thread
  • imperial thread
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  • Normal thread
  • Fine thread

Simplification of the variety of threads

But globalization has not stopped at the threads either. In this way, threads have been further simplified. The new DIN-ISO threads have not only replaced the old metric threads, the new metric ISO threads have also found their way into areas where previously imperial threads were used (gas / water installation, aviation). Of course, it can still happen with old threads (for example on vintage cars) to come across such threads.

DIN-ISO thread

The DIN-ISO threads include normal threads (including regular threads) and fine threads in sizes M1 to M68. They also substantially replace the tariff threads, of which the following threads can be found in older applications as well as in certain areas.

Customs thread

  • UTS Customs Thread (Unified Thread Standard)
  • Witworth Customs Thread, also known as BSW (British Standard Witworth), BSF (British Standard Fine) or as BSP (British Standard Pipe Thread)

German industrial standards (DIN)

Depending on the tool used or table book, however, the old DIN standards may still be important for you.

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DIN 11, DIN 12: Custom Witworth Thread
DIN 13: Old metric units

Today, however, the new DIN ISO classifications have validity.

DIN ISO 13-1: Metric ISO rule thread
DIN ISO 13-2: Metric ISO fine thread

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Right and left hand threads

In addition, you have to differentiate between the common right-hand threads and the less common left-hand threads. Left-hand threads are mainly used where the mechanical movement of a part would pose the risk of a screw or nut with a right-hand thread loosening. This is why, for a long time, a distinction was made between the driver and passenger side of vehicle rims, for example. One side had stud bolts as a left-hand thread, the other side as a right-hand thread.

Cutting threads

You need different tools to cut a thread. With tools for manual thread cutting by hand, three different cutting drills (internal thread) or. Cutting dies (external thread) used.

  • coarse pre-cutter
  • fine pre-cutter
  • Finishing cutter

Manual and machine thread cutting

These drills or. Cutting dies are clamped and used one after the other in the wind or turning iron ("wind " from "winches "). In the case of machine thread cutting, a tap or tap is usually used. Thread cutting dies used.

The special features of the core and core hole diameters

In addition to the taps, you will need a core hole drill, especially for new internal threads that have to be drilled first. Depending on the metric size of the later thread, this core hole drill does not necessarily correspond to conventional drill sizes - as is often assumed. With metric ISO threads, the core hole diameter is calculated based on the thread pitch.

Calculation of the core diameter metric

In the case of M6, for example, it is 1 mm, which is why the drill size is the same as conventional drill sizes. In this case 5. With M4 the diameter of the core hole drill would be 3.3, since the internal thread pitch is 2.46 mm. Of course, the (gross) core diameter must also be taken into account for external threads.

Calculation of the core diameter imperial

In the case of imperial threads, the calculation of the core hole drill is somewhat different. The thread distance of a thread rotation is not taken into account, but the thread length to one inch.

You can now cut threads

With this knowledge, you can now carry out the correct core lock drillings and then cut the thread correctly.

Instructions for cutting a thread (inside and outside)

  • Component in which a thread is to be cut
  • manual or machine taps (inside or outside, drill or. Die)
  • Matching wind or turning iron
  • alternatively standing drill
  • Cutting oil

1. preparation

First of all, you have to determine the correct core hole drill for the internal thread if you want to make a completely new thread. For external threads, you usually use metal rods or tenons that are turned as a core. Here, too, you have to consider the core diameter that matches the thread size.

2. Cut the thread

With a manual thread cutting set you will one after the other the three different drills or. Use a die. The most difficult endeavor is to position the drill or die absolutely straight.

Then drizzle some oil on the cutting surface and start screwing in the die or drill. Never turn any further than you can turn with both hands without putting your hands down. Then turn back a bit to cut the next piece of thread. This is how you remove the chips and ensure that the cutting oil is evenly distributed.

In the case of three-way cutting sets, carry out this work in the same way with all three drills or. Die through. Then the thread is finished.

Tips & Tricks The data on the core hole or. Core diameters can be found in books of tables for metal professions. All drill dimensions for the various thread types are listed there in the corresponding tables.If you cut threads with a pillar drill, you have the best prerequisites that a thread is actually cut completely straight and not offset at an angle. You can also clamp the workpiece and turn the drill chuck (€ 14.79 on Amazon *) by hand, i.e. you can also use such a drill (€ 48.00 on Amazon *) in this way for manual thread cutters. However, the drill must not be switched on for this!You can also use machine taps for manual thread cutting. Manual thread cutters, however, must never be driven by a machine. As already mentioned, you can turn the drill chuck by hand.