Thread Cutting on Lathe – Lathe Machine Operations

On lathe, internal and external threads are cut either with the help of a threading tool or with the help of tap and die respectively.

While cutting threads with the help of a tool, the following requirements are to be fulfilled:

a. There should be a certain relation between job revolutions and the revolutions of the lead screw to control linear movement of the tool parallel to the job when the half nut is engaged with the lead screw.

b. The tool should be ground to the proper shape or profile of the thread to be cut.

In modern engine lathe, quick change gear box is provided in which different ratios of the spindle and lead screw revolutions can be easily obtained by simply shifting the change gear lever. In ordinary engine lathe, every time one has to change the gears.

For cutting a lathe an accurate screw is necessary, that the relation between the movement of the saddle, and the revolution of work be carefully controlled. This is brought about by means of a lead screw which is driven by a train of gears as shown in a set up described in Fig.

Usually the drive is first carried to the spindle called the stud which for all purposes may be assumed to be the spindle itself as it rotates at the same speed and in the same direction, unless made to reverse by the tumbler mechanism. From the stud, the drive is conveyed to the lead screw by a train of gears.

To vary the relationship between the revolutions of the lead screw and those of the stud, the train of gears may be varied. Note that the spindle gear is never changed for cutting different pitches of thread. The stud gear (driver gear) and the lead screw gear (driven gear) are changed whenever a particular ratio is needed between the spindle and lead screw.

Here is a formula which gives the required ratio between the teeth on stud and lead screw gears.

Suppose;

p1 = Pitch in mm to be cut on the job,

p2 = Pitch on lead screw, mm.

N1 = No. of revolutions stud gear is making.

N2 = No. of revolutions lead screw is making.

The second form is generally used when multi-start threads are to be cut on the job.

While cutting internal and external threads, the following points are to be considered:

i. The tool should be set exactly at the height of the centre line of job and at 90° to the job. For setting the tool, the tool setting gauge is generally used.

ii. Tool is fed either exactly at 90° to the job axis or at an angle of 27° to 30° to the job axis. The latter method is mostly adopted because complete side of the tool is used for cutting action which gives better finish on the threads.

iii. While cutting threads, the speed of job is reduced to one third or one-fourth of the job speed used in turning operations.

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