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How To Choose The Right Tap
Jul 17, 2017

Thread is the most common method of connecting mechanical parts, and taps are the most commonly used tools for machining internal threads. The correct selection of tap to process internal thread ensures the quality of thread connection and improves the service life of tap.

Tap tolerance zone

Domestic machine taps are marked with the middle diameter tolerance zone code: H1, H2 and H3, respectively, indicating the same location of tolerance bands, but the tolerances are different. The tolerance band code of hand tap is H4, and the tolerance value, pitch error and angle error ratio are larger than that of machine tap. The material, heat treatment and production process are not better than those of machine tap. H4 can not be marked as required. The tolerance level of the inner thread tolerance that can be machined by the middle diameter tolerance band of tap is as follows:

Taps, tolerances, codes, internal threads, tolerances, grades

H1, 4H, 5H

H2, 5G, 6H

H3, 6G, 7H, 7G

H4, 6H, 7H

Some companies use the imported tap, the German manufacturer 4H, ISO2 often labeled as ISO1 6H, ISO3 6G (ISO1-3 international standard and national standard H1-3 is equivalent), so the internal thread tolerance code and tap the tolerance zone are marked with.

Select the thread system

The common thread has three types: metric, imperial, unified system (also called us). The metric system is in millimeters, with a tooth angle of 60 degrees. For example: "M8X1-6H" stands for metric fine thread with a diameter of 8 mm, with a pitch of 1 mm and an internal thread tolerance zone of 6H.

The inch is in inches and has a tooth angle of 55 degrees. For example: BSW 1/4-20 means diameter 1/4 inches, coarse tooth pitch is 20 teeth per inch, this kind of thread has been seldom used. The unified system is in inches, with a tooth angle of 60 degrees.

The diameter is less than 1/4 inches and is indicated by the usual numbers. Sizes from 0 to 12 indicate the diameter of 0.06 inches to 1/4 inches respectively. The main use of the United States is still unified thread.

Select types of taps

We often use is: straight fluted tap, spiral tap, screw tap, squeeze tap, the performance of each one has his good points.

Straight groove tap is the most versatile, through hole or through hole, non-ferrous metal or ferrous metal can be processed, the price is also the cheapest. But targeted is also poor, what can be done, nothing is the best. Spiral groove tap is more suitable for machining hole thread, cutting chip back to the rear. Because of the screw angle, the actual cutting angle of the tap increases with the increase of the helix angle.

Experience tells us: the processing of ferrous metal, spiral angle election of a smaller, generally around 30 degrees, to ensure the strength of spiral teeth. Processing non-ferrous metal, spiral angle election of a larger, can be about 45 degrees, cutting a bit sharp. When the threaded tap processes the thread, the chip is discharged forward. Extrusion tap is more suitable for non-ferrous metal processing.

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