How to cut internal threads with a tap

How to cut a thread for a bolt or screw? To perform threading, you need to understand the question and have a minimum set of tools, the main of which is the tap.

Tap scheme

Tap scheme.

General information about the tool

Internal threads in the holes are cut by hand or machine special tools - taps, which are found in two versions: machine-manual and machine. Types and versions of the tool for cutting internal threads:

Types of taps

Types of taps.

  1. Machine products are designed to be fixed in the chuck or collet clamp of a metalworking machine on which this type of work can be performed. The machine-manual version provides for a square section shank, so that the tool can be installed in the hand-wheel.
  2. By the method of cutting taps can be universal (continuous) or complete. The latter are a set of two or three tools of the same diameter, which should be used in the same hole alternately: first, draft No. 1, then No. 2 and 3, if there is one. In the case of some alloys, sets of five products are used. The universal tap performs all threads at once, but requires great physical effort and often breaks.
  3. According to the thread configuration, the articles for cutting are divided into pipe, metric and cylindrical.
  4. The type of holes distinguish tools for through and blind holes. The first part of the entry part is long. Most often, they are universal, and the second cut is cut. Such works are performed with a set of 2-3 products.
  5. By the performance of the working part of the product are straight, screw or shortened grooves. They are used to work with conventional, carbon or low alloyed steel grades. For stainless or heat-resistant viscous steels, an instrument with a staggered arrangement of teeth is used.
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Preparing for threading

Before you cut the thread, you need to drill a hole of the required diameter. It is clear that it will be smaller than the diameter of the thread, but how much? For standard cases, the hole sizes are shown in Table 1.

Table 1

Thread designation Diameter of hole, mm Thread designation Diameter of hole, mm
M 1 0.75 M 10 8.5
M 1.1 0.85 M 11 9.5
M 1.2 0.95 M 12 10.2
M 1.4 1.1 M 14 12.0
M 1.6 1.25 M 16 14.0
M 1.8 1.45 M 18 15.4
M 2 1.6 M 20 17.4
M 2.2 1.75 M 22 19.4
M 2.5 2.05 M 24 20.9
M 3 2.5 M 27 23.9
M 3.5 2.9 M 30 26.4
M 4 3.3 M 33 29.4
M 4.5 3.8 M 36 31.9
M 5 4.2 M 39 34.9
M 6 5.0 M 42 37.4
M 7 6.0 M 45 40.4
M 8 6.7 M 48 42.8
M 9 7.7 M 52 46.8


What to do if the thread is non-standard? There is a universal formula for determining the diameter of the hole. On each tick there is a marking denoting the type, diameter and thread pitch in millimeters, for example, one of the non-standard - M 6 x 0.75. The size of the hole is determined as follows: its pitch is taken away from the size of the thread (6-0.75 = 5.25 mm). The pitch is the distance between two turns.

Pipe female cylindrical thread is measured in inches. The diameters of the holes for this type of connection are shown in Table 2.

table 2

Designation in inches 18 14 3/8 12 5/8 3/4
Diameter of hole, mm 8.8 11.7 15.2 18.9 20.7 24.3
Designation in inches 7/8 one 1 1/8 1 1/4 1 3/8 1 1/2
Diameter of hole, mm 28.1 30.5 35.2 39.2 41.6 45.2

The drill needs to be picked up correctly sharpened, without beating. Otherwise, the hole will come out of a larger diameter, and subsequently there will be a backlash at the joint due to defective turns.

The angle of sharpening the drill should match the hardness of the material.

The table of the choice of taps for cutting thread

The table of the choice of taps for cutting threads.

The harder the treated metal, the greater the angle should be (but not more than 140 °).

Before performing work, you should prepare such tools or fixtures as:

  • electric low-speed drill or drilling machine;
  • drill size required;
  • drill is 10 mm larger than the feedthrough or countersink for chamfering;
  • set of taps;
  • hand crank;
  • manual vice (if necessary);
  • hammer;
  • core;
  • grease;
  • rags.
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The order of work

When the preparatory stage is completed, you can drill a hole and cut the thread in the following order:

Female threading scheme

Female threading scheme.

  1. Decide on the location of the hole, marking the part and putting the mark in the right place. Attach the core and with one strong blow of the hammer on it make a recess in the place of entry of the drill. Clamp an electric drill or a drill in a chuck, set a low rotational speed, lubricate the cutting part of the tool. It is best to use for these purposes the old lard, if not, you can lubricate the grease. When working with viscous grades of steel (stainless or heat resistant), lubrication is produced with engine oil.
  2. If necessary, clamp the item in a manual vice. Attach the drill bit to the groove on the part, visually align the electric drill so that the cutting tool is at a 90 ° angle to the surface. Drill a hole at low revs, periodically lubricating the drill with lard or pouring with engine oil if the part is thick. Do not allow the tool to skew or overheat. At the end, insert the countersink into the cartridge and make a high-quality chamfer, the beginning of the cutting process depends on it. Distortion when countersinking is also unacceptable. The depth of chamfer is up to 1 mm, with very small hole diameters up to 0.5 mm.
  3. Insert the shank of the tap No. 1 into the sliding prism of the driver, fix it with the rotation of the handles, lubricate the working part well. After removing the drill chips from the part, insert the tap with the key into the hole and cut the thread. Do not try to forcibly change the position of the incoming tool, it will go as smooth as you have drilled a hole and made a chamfer. Adjust the position of the tap relative to the detail at the very beginning so that there is no obvious skew. Cut according to the algorithm: make 2 turns forward, then - 1 turn back. This is necessary to reset the chips and reduce the load on the tool.
  4. Repeat the operation with taps 2 and 3 (if there is one). You can not count the momentum, you need to cut the thread until you feel an increase in resistance. Then make a turn back to reset the chips. After the operation, you can verify the quality of the work done by screwing the bolt into the hole. It can go tight and require rotation with a key, then you can try to screw another bolt. If he goes with the tension, it is necessary to cut the threads with a tap first, drive the first and second numbers.

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Precautionary measures

You should not try to perform internal threading with the methods of small-scale mechanization, that is, insert the tap into the chuck of the drilling machine, especially into the chuck of an electric drill. You cannot correctly adjust the force and angle of attack. Everything will end with a broken tool, and in the worst case, with spoiled threads.

Worst of all, when the broken off working part of the tap number 1 gets stuck and remains in the hole. Then you have to find a special extractor for extracting debris and a diamond drill in the trade network or on the market. Without these accessories, it is difficult to get a piece of debris, but you can sacrifice aesthetics and make a hole in the metal around the fragment to capture it. This is done with a small angle grinder. If a part of the product protrudes above the plane of the part, you can try to grasp it with pliers or jaws and unscrew.

The best way is to learn how to cut the thread properly and to do the work, as recommended, calmly and without haste.

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