How to choose the right tap

Thread is the most common way to connect mechanical parts, and tap is the most common tool to process internal thread. Proper selection of tap to process internal thread can ensure the quality of thread connection and prolong the service life of tap.

Select the tap tolerance zone

Domestic machine taps are marked with the middle diameter tolerance zone code: H1, H2, H3 respectively represent different positions of the tolerance zone, but the tolerance values are the same. The tolerance zone of hand tap is code H4, the tolerance value, pitch and Angle error are larger than that of machine tap, and the material, heat treatment and production process are also inferior to that of machine tap. H4 may not be marked as required. The grade of the inner thread tolerance zone that can be processed by the middle diameter tolerance zone of the tap is as follows:

Tap tolerance zone code applies to internal thread tolerance zone class

H1 4 h, 5 h

The H2 5 g, 6 h

H3 6G, 7H, 7G

H4 6 h, 7 h

Select the thread system

At present, there are three common threads: metric, British, unified (also known as the United States). The metric system is a thread with a tooth Angle of 60 degrees in millimeters. For example, m8x1-6h represents 8 mm diameter metric fine thread, 1 mm pitch, 6H internal thread tolerance zone.

The Inch system is a thread with a 55 degree tooth Angle. For example, BSW 1/4-20 indicates a 1/4 inch diameter and 20 coarse threads per inch, which are rarely used today.

Unified system is inch by inch, tooth Angle 60 degrees of thread. Diameter less than 1/4 inch, commonly used numbers, 0 through 12 are 0.06 inch to 1/4 inch diameter specifications, respectively. At present, the main use of the United States is still unified thread.

Select the type of tap

We often use: straight groove tap, spiral groove tap, spiral tip tap, squeeze tap, its performance has its advantages.

Straight groove tap, it has the strongest versatility, through hole or through hole, non-ferrous metal or black metal can be processed, the price is also the cheapest. But the pertinence is also poor, what can be done, what is not the best. The cutting cone part can have 2, 4, 6 teeth, the short cone is used for the through hole, the long cone is used for the through hole. As long as the bottom hole is deep enough, the cutting cone should be selected as long as possible, so as to share the cutting load of the teeth more, the service life is also longer.

Spiral groove tap, more suitable for processing impassable thread, chip back discharge during processing. Because of the screw Angle, the actual cutting Angle of tap will increase with the increase of the screw Angle. Experience tells us: when processing black metal, the spiral Angle should be smaller, generally about 30 degrees, to ensure the strength of the spiral teeth. Processing of non-ferrous metals, spiral Angle selected a little bigger, can be about 45 degrees, some sharp cutting.

Screw tip tap, the chips are discharged forward when machining threads. Its core size design is relatively large, the strength is better, can withstand large cutting force. Processing non-ferrous metal, stainless steel, ferrous metal effect is very good, through hole thread should be preferred to use screw tip tap.

Special tap

There are some special taps, such as the jumping tooth taps. It interval remove a tooth, and the pressure and friction between the workpiece is small, processing thin-walled parts internal thread, will not cause deformation of thin-walled parts. There are other content chip tap, the middle open chip hole, specialized processing dozens to several hundred millimeters of large diameter internal thread.

New internal thread cutter

It is worth mentioning that: the current CNC machining center used a kind of internal thread cutting tool, it set chamfering, drilling, thread processing in one, the use of machine tool spiral interpolation, the front edge of the tool can be drilled, chamfering, side teeth can be milling internal thread. A tool can process the same pitch, different diameter of the screw hole. Expand the scope of the tool, reduce the number of tools, but also greatly reduce the number of tool change machining center.

Extrusion tap

It is more suitable for processing non-ferrous metals. Different from the working principle of the above-mentioned cutting taps, it is extruded to the metal to make it shape and deform and form internal threads. Extrusion forming of the internal thread metal fiber is continuous, tensile, shear strength, processing of the surface roughness is also taught, but the requirements of the bottom hole of the extrusion tap is higher: too large, the amount of base metal is less, resulting in the internal thread small diameter is too large, strength is not enough. Too small, sealing extrusion metal has nowhere to go, causing the tap to break. Calculation formula: diameter of bottom hole = nominal diameter of internal thread -0.5 pitch.