Threading Pass Calculator

Programming G76: How Many Passes for a 1.5 mm Pitch Thread?

A 20 mm × 1.5 external thread on a steel shaft needs a total thread depth of approximately 0.92 mm (0.61 × pitch for standard 60° thread form). The G76 cycle needs to distribute this depth across multiple passes with decreasing depth per pass — the first pass takes the heaviest cut, and subsequent passes get progressively lighter.

For a 1.5 mm pitch thread in steel, use 5-6 passes. The first pass removes about 0.35 mm, the second 0.25 mm, the third 0.15 mm, and so on down to a 0.03 mm finishing pass. In aluminum, the same thread can be cut in 3-4 passes because the material is more forgiving. In stainless, increase to 7-8 passes to distribute the cutting load and prevent work-hardening at the thread root.

The infeed method — compound infeed at 29.5° for standard G76 cycles — creates a more favorable chip formation than straight infeed. The thread-cutting insert cuts on one edge only, forming the thread profile progressively with each pass. This calculator uses the standard decreasing depth formula from the Machinery's Handbook.

1. Step 1: Calculate total thread depth = 0.6134 × pitch (external) or 0.5413 × pitch (internal)
2. Step 2: Starting depth = total depth × 0.4 (reduced for material toughness)
3. Step 3: Each subsequent pass depth = previous depth × 0.65-0.75
4. Step 4: Minimum pass depth = 0.02-0.05 mm for finishing

Frequently Asked Questions

How do you calculate thread depth for CNC threading? External thread depth = 0.6134 × pitch. Internal thread depth = 0.5413 × pitch. These values come from the ISO 68-1 thread profile standard.

How many passes for a 2 mm pitch thread in steel? 7-8 passes. The first pass at approximately 0.4 mm depth, decreasing to 0.03 mm for the final finishing pass.

What is the difference between G92 and G76 threading cycles? G92 is a simple pass-repeat cycle (same depth each pass). G76 is a multi-pass cycle with automatic depth decrease and compound infeed. Use G76 for all threading except very coarse pitches where G92 gives better control.

Should I use coolant for thread cutting? Yes, for all materials except cast iron. Coolant prevents built-up edge on the threading insert, which degrades thread form accuracy. For carbide threading inserts, consistent coolant flow also prevents thermal cracking.