Cut a thread correctly

Even home improvement, it happens again and again that a thread either no longer good or even not available. Then you have to cut a thread first. Although this is actually part of the training of many metalworking professions due to the complexity, you can also easily cut or re-cut a thread.

The distinction of threads

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

  • metric thread
  • inch thread
  • normal thread
  • fine thread

Simplification of thread variety

But globalization has not stopped at the threads. So threads were further simplified. The new DIN ISO threads have not only replaced the old metric threads, but also the areas where formerly predominantly Zöllische threads were used (gas / water installation, aviation), have the new ISO metric thread introduced. Of course, it can still happen, especially with old threads (for example, classic cars) to find such threads.

DIN-ISO thread

The DIN-ISO threads include the standard threads (also standard threads) and the fine threads in sizes M1 to M68. They also largely replace the inch threads, of which the following threads are still found in older applications and in certain areas.

inch thread

  • UTS thread (Unified Thread Standard)
  • Witworth threads, also known as BSW (British Standard Witworth), BSF (British Standard Fine) or BSP (British Standard Pipe Thread)
German industrial standards (DIN)

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

DIN 11, DIN 12: Witworth threaded
DIN 13: old metric units

Today, however, the new DIN ISO classifications apply.

DIN ISO 13-1: metric ISO standard thread
DIN ISO 13-2: metric ISO fine thread

Right and left hand thread

In addition, you still have to distinguish between the common right-hand and the less frequently encountered left-hand threads. Left-hand threads are used above all where the mechanical movement of a part would give rise to the danger that a screw or nut with right-hand thread could thereby loosen. For this reason, for example, a distinction was made between the driver's and front passenger's side for vehicle rims for a long time. One side had studs as left-hand, the other side as a right-hand thread.

The cutting of threads

To cut a thread you need different tools. Manual manual threading tools often use three different internal drills or external dies.

  • rough precut
  • fine pre-cutter
  • Fertigschneider

Manual and machine tapping

These drills or dies are successively clamped in the wind or turning iron ("wind" of "winds") and used. When machine tapping usually a tap or thread cutting dies is used.

The peculiarities of the core and core hole diameter

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

Calculation of the core diameter metric

For example, it is 1mm for M6, so the size of the drill is exceptionally consistent with conventional drill sizes. In this case 5. For M4, the diameter of the core hole drill would be 3.3 because the internal thread pitch is 2.46 mm. Of course, the (gross) core diameter must also be considered for external threads.

Calculation of the core diameter zöllisch

In the case of inch threads, however, 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.

Now you can cut threads

With this knowledge, you can now perform the correct core hole drilling and then cut the thread correctly.

Instructions for cutting a thread (inside and outside)

  • Component into which a thread is to be cut
  • manual or mechanical thread cutters (inside or outside, drills or dies)
  • suitable wind or turner
  • alternatively drill press
  • cutting oil

1. Preparation

First of all, you have to determine the correct core hole drill for the internal thread if you want to produce a completely new thread. For external threads, you usually use metal rods or spigots that are turned as a core. Again, you must take into account the appropriate core diameter for the thread size.

2. Cut the thread

For a manual threading kit, you will use the three different drills or dies in order. The most difficult task is to set the drill or the cutting iron absolutely straight.

Then pour some oil on the cutting surface and start screwing in the die or the drill. Never turn further than you can turn with both hands without settling. Then turn back a bit to cut the next piece of thread. How to remove the chips and ensure an even distribution of the cutting oil.

For triple cutting sets, perform this work in the same way with all three drills or dies. Then the thread is cut.

Tips & Tricks

The core hole and core diameter data can be found in metalworking handbooks. In the corresponding tables, all drill dimensions for the different thread types are listed there. If you cut threads with a drill press, 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 rotate the chuck by hand, so use such a drill in this way, even for manual tap. However, the drill must not be switched on! Machine thread cutters can also be used for manual thread cutting. Manual taps must never be driven by a machine. As already mentioned, you can turn the chuck by hand.

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