Screw Threads and Gear Manufacturing Methods

(a) Machining methods

• Single-point cutting on a lathe: a tool ground to thread form is moved by the lead screw; flexible but slow, used for single parts.
• Taps and dies: hand or machine cutting of internal and external threads.
• Thread milling: a multiple- or single-form milling cutter on a thread milling machine or CNC machine; the cutter cuts the full thread depth in about one revolution of the work.
• Die heads (self-opening), thread chasing and whirling for production work.

(b) Thread rolling

The blank is pressed between hardened dies carrying the thread form and the metal is made to flow plastically (cold working) to form roots and crests, with no chips.

• Flat dies: one stationary, one reciprocating; or cylindrical rolls (two or three) fed radially.
• Very fast (hundreds per minute), used for bolts, screws and studs.

(c) Thread grinding

A grinding wheel with the thread form (or a single-rib wheel) is traversed along the rotating job. It gives high accuracy and fine finish and can cut hardened threads: lead screws, taps, gauges, ball screws. Centreless grinding of threads is also used.

Why rolled threads are stronger

• Grain fibres follow the thread contour instead of being cut.
• Cold working increases hardness and surface strength; the surface is smooth and compressive stress improves fatigue life by 10 to 20% or more.
• No material waste.

Gear manufacturing methods

• Casting (sand casting, die casting) for rough gears.
• Forming: powder metallurgy, extrusion, cold rolling of gears.
• Machining: milling with a form cutter, gear shaping, gear hobbing, gear planing, broaching.
• Finishing: shaving, grinding, lapping, honing.

Hobbing versus form milling

(a) Gear dimensions

$$\begin{aligned} D_o &= m(z+2) = 3\times 36 = 108\ \text{mm} \\ \text{Tooth depth} &= 2.25m = 6.75\ \text{mm} \\ \text{Pitch circle diameter} &= mz = 102\ \text{mm} \end{aligned}$$

A module 3 involute cutter, No. 4 of the 8-cutter set (covers 26 to 34 teeth), is used.

(b) Indexing

Simple indexing with a 40:1 worm gear:

$$\text{Index crank turns} = \frac{40}{z} = \frac{40}{34} = 1\frac{6}{34} = 1\frac{3}{17}$$

So after cutting each tooth, turn the crank by 1 full turn plus 3 holes in the 17-hole circle (set the sector arms to 3 spaces).

Answer: outside diameter = 108 mm, tooth depth = 6.75 mm; indexing = 1 full turn + 3 holes on the 17-hole circle.