Screw thread production and change gears

(a) Production of screw threads

Machining (cutting):

• Single-point cutting on a lathe: a tool ground to the thread form is moved along the work by the lead screw, taking several passes. It gives any pitch and size and is accurate, but slow.
• Taps and dies: internal threads are cut by a tap and external threads by a die, in one or a few passes; fast for small sizes.
• Thread milling: a rotating multi-tooth cutter is used for large threads, worms and lead screws; on CNC machines thread milling is done by helical interpolation.
• Thread chasing, whirling.

Thread rolling: the blank is pressed between hardened dies (flat reciprocating or round rolling dies), and the thread is formed by cold plastic deformation. There are no chips; the grain follows the thread contour, and the surface is work hardened and smooth. It gives higher strength, much better fatigue life and a very high production rate, but it suits only ductile materials of limited hardness, and the dies are costly.

Thread grinding: a wheel dressed to the thread form grinds the thread, either by traverse (single-rib wheel) or by plunge (multi-rib wheel). It gives the highest accuracy and finish and can cut hardened material (gauges, lead screws, ball screws), but it is costly and slow.

(b) Change gears

For thread cutting, the work (spindle) and the lead screw are linked by a gear train so that the carriage moves one pitch of the job per revolution of the work:

$$\frac{\text{Pitch of job}}{\text{Pitch of lead screw}} = \frac{\text{Driver (on spindle)}}{\text{Driven (on lead screw)}}$$ $$\frac{2.5}{6} = \frac{5}{12} = \frac{25}{60}$$

The driver on the spindle stud has 25 teeth and the driven gear on the lead screw has 60 teeth (a simple train, with gears available in steps of 5 teeth from 20 to 120). If these exact gears are not available, a compound train such as $\frac{20}{40}\times\frac{50}{60}$ gives the same ratio $5/12$.

Answer: driver 25 teeth, driven 60 teeth (ratio 5/12).