How Does an Ordinary Lathe Make Rectangular Threads?

1. First, understand the rectangular thread

Type of thread:

Type of thread

Rectangular thread, the thread profile is square, and the thread thickness is equal to 1/2 of the pitch.

It has high transmission efficiency, but low centering accuracy and weak root strength.

It is difficult to manufacture rectangular threads accurately, and the clearance of worn screw pairs is difficult to compensate or repair.

The most recent example around is the thread of a bench vice.

It needs to be tightened and loosened all day, which requires a lot of force.

Threads such as valves and water pipes, regardless of the size (the small one is the water faucet), are rectangular threads, but the size is slightly different.

Jacks and screw presses also use such threads.

Therefore, the rectangular thread is suitable for large loads, that is, the pitch thread is required to be strong, and there is no more requirement for the accuracy of the thread.

2. Thread cutting process

1. Principle of thread turning

The lathe spindle and the tool must maintain a strict movement relationship: that is, the tool moves a lead distance evenly every time the spindle (workpiece) rotates.

How Does an Ordinary Lathe Make Rectangular Threads? 1

2. Turning process flow:

How Does an Ordinary Lathe Make Rectangular Threads? 2

3. Rectangular thread has no fixed profile like other threads

Generally, when the tooth size is not specified, the cross-section of the thread is made into a square.

The turning tool is fed in a straight line to process rectangular threads, and it is OK to process them according to the depth of the groove (the height of the tooth).

3. Turning tools

Rectangular thread turning tool:

How Does an Ordinary Lathe Make Rectangular Threads? 3

Thread turning tool, considering the shape of thread teeth, grooving turning tool is suitable for machining short threads.

Only the blade width of the tool should be half of the pitch.

At the same time, because there are many threads with wide pitch, high-speed steel elastic turning tools are often used.

The biggest problem in machining rectangular thread is the grinding method of turning tool.

The blade width is half of the pitch, which is not easy to operate.

If the pitch and lead increase, the thread rise angle (lead angle) will be large, and the rear angle of the turning tool will touch the side of the thread.

In addition, note that for threads with large lead, the thread rise angle of the top and bottom of the thread will be different.

Therefore, on the outer circumference (thread crown), the width of the cutting edge is the same as that of the groove, and the lower part of the thread on the groove at the bottom of the tooth will be recessed.

In order to avoid these phenomena, the concave part uses the trapezoidal turning tool with small front edge width.

4. Turning rectangular thread

How Does an Ordinary Lathe Make Rectangular Threads? 4

When turning rectangular threads, the machining allowance should be allocated reasonably.

For details, please refer to the following figure:

Reasonably allocate thread machining allowance

(generally rough turning ap:0.10~0.50; fine turning ap: 0.01~0.10.)

Feed times

Back cutting depth

Processing steps

Machining allowance

First feed


Rough machining


Second feed


Third feed


Fourth feed


finish machining


Fifth feed


Sixth feed


5. Summary

Rectangular thread is different from triangular thread and trapezoidal thread.

Rectangular thread does not have the trouble of single-sided cutting or double-sided cutting, nor the problem of pitch diameter, because the accuracy requirement is not high.

In fact, this is relatively simple in thread machining.

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