Page 118 - Threading_Handbook

2.12 Depth per Pass and Number of Passes
THREAD TURNING In order to produce threads, the cutting tool needs to make several numbers of cuts along

the workpiece surface. The parameters of depth per pass and number of passes have a
very important role in threading production. These parameters have a direct effect on cutting
edge wear, tool life, threading surface quality, and threading production stability.
The two methods most common in determining the depth per pass and the number of
passes are constant chip area by decreasing depth per pass or constant depth per pass.
The choice of method does not depend on the selected infeed methods (radial infeed, flank
infeed, modified flank infeed, alternating flank infeed), which are described in chapter 2.12.
The depth per pass and number of passes parameters depend on the type of equipment,
tool overhang, machine stability, workpiece material, cutter geometry and the threading
depth required.

Constant Chip Area by Decreasing Depth per Pass
(Recommended)

This is the most common method and is generally recommended, as in most cases it
ensures high productivity. The principle of this method is that the initial cutting depth at
the first pass is the largest, and then gradually decreases at each pass to ensure material
removal within a constant chip area. The calculation of passes is designed so that the last
pass, which is destined to be a finish pass, will be 0.05 - 0.1 mm (0.0019 - 0.0039 inches).
Using this method ensures constant loads on cutting edge and uniform wear,
which increases tool life.

Formula for Calculation of Depth per Pass

∆ap(i) = ap × √C
√n -1
a
When:
∆ap(i) — Depth of cut i pass (i = 1...na)
i — Pass

ap — Total depth of cut
na — Number of passes

C — Constant value:
nd
rd
st
118 ISCAR For 1 pass: C=0.3; For 2 pass: C=1; For 3 pass and higher: C = i − 1