G76 Threading Infeed Calculator

Per-pass depths for single-point threading at constant chip area: cumulative depth = total × √(n/N), the G76/G92 ladder with every pass listed in both units.

Machining 3 inputs 3 results

Inputs

View results

Total thread depth (d)

Number of cutting passes (N)

passes

Minimum cut depth (q_min)

Results

Default result

Edit inputs

First pass depth(Δd₁)

0.0148in

d ÷ √N — the heaviest cut of the cycle.

Also computed

Last pass depth(Δd_N)0.00316in

Above the minimum cut depth — every pass still cuts.

First : last ratio4.69

Equal-area infeed concentrates depth early; the ratio grows with pass count.

Method notes 10 notes

Pass 1: cut 0.376 mm → 0.376 mm total (0.0148 in → 0.0148 in)
Pass 2: cut 0.156 mm → 0.531 mm total (0.0061 in → 0.0209 in)
Pass 3: cut 0.119 mm → 0.651 mm total (0.0047 in → 0.0256 in)
Pass 4: cut 0.101 mm → 0.751 mm total (0.0040 in → 0.0296 in)
Pass 5: cut 0.089 mm → 0.840 mm total (0.0035 in → 0.0331 in)
Pass 6: cut 0.080 mm → 0.920 mm total (0.0032 in → 0.0362 in)
Each pass removes the same chip area (∝ d²/N) — that is the whole point of the √(n/N) ladder, and why the last passes are so shallow.
Depths are radial (X per side). Flank or modified-flank infeed (the 29-30° compound angle on 60° threads) changes the path, not these depths.
Add 1-2 spring passes at zero infeed after the ladder; they relieve deflection and clean the flanks.
Thread depth for external 60° forms ≈ 0.6134 × pitch; verify against the thread spec and measure over wires for class fits.

Single-point threading holds chip area, not depth, constant: cumulative infeed after pass n is d_total × √(n/N), so each pass removes area ∝ d²/N — the G76 ladder. An M10 × 1.5 external thread (depth ≈ 0.6134 × pitch = 0.92 mm) in 6 passes runs 0.376, 0.156, 0.119, 0.101, 0.089, 0.080 mm: a 4.7:1 first-to-last ratio with identical edge load every pass. This calculator lists the whole ladder in both units and flags passes that fall below the minimum (rubbing) cut depth.

How to use this calculator

Get the total depth. ≈ 0.6134 × pitch for external 60° forms; the thread spec governs for class fits.
Pick the pass count. 5-8 for fine pitches, 8-12 for coarse or tough materials; more passes = lighter finish cuts.
Check the last pass. It must clear the minimum cut depth or it rubs — the screen flags this.
Program it. G76 takes first-pass depth and minimum; G92 or a manual compound takes the cumulative ladder directly.

How it works

A single-point threading tool cuts a V: the deeper the pass, the wider the cut. Holding chip area constant — what the insert edge actually feels — means cumulative depth must grow with the square root of the pass number:

d_n = d_total × √(n / N) · each pass removes area ∝ d²/N

That is the ladder G76 generates from its first-pass and minimum parameters, and what a manual G92 or compound-rest job programs explicitly. Spindle speed and feed come first — feed equals the pitch — via the SFM to RPM converter and the tap-drill/thread screen; pitch geometry context lives in the thread pitch chart.

Worked example

Verified against the live calculator

External M10 × 1.5: total depth ≈ 0.6134 × 1.5 = 0.92 mm, cut in 6 passes:

0.376 → 0.156 → 0.119 → 0.101 → 0.089 → 0.080 mm (per pass)

The first pass is 0.376 mm (0.0148 in) and the last only 0.080 mm (0.0032 in) — a 4.7:1 ratio — yet every pass removes the same chip area. The last increment sits above the 0.025 mm rubbing floor, so the plan stands; add a spring pass at the finish diameter and check the fit over wires.

Frequently asked questions

How does G76 decide the depth of each threading pass?

Constant chip area: cumulative depth after pass n is total × √(n/N). The cross-section a V-thread tool removes grows with depth squared, so equal areas mean the cumulative depths follow square roots — first pass deepest, each one after shallower.

What depth per pass for an M10 × 1.5 thread?

Total external depth ≈ 0.6134 × 1.5 = 0.92 mm. In six constant-area passes that ladder runs 0.376, 0.156, 0.119, 0.101, 0.089, 0.080 mm — a 4.7:1 first-to-last ratio, plus a spring pass or two at zero infeed.

Why not use equal depths for every threading pass?

Because chip area would grow with every pass: the last equal-depth pass sweeps the widest V and takes several times the first pass’s load, right when the part is weakest and finish matters most. Equal area flips that — heavy cuts early, finishing cuts light.

What is the minimum depth of cut in threading?

The floor below which the insert rubs instead of cutting — commonly around 0.025 mm (0.001 in), entered as the minimum (Q) value on the control. If the calculated last passes fall below it, use fewer passes or let the cycle clamp.

Method & assumptions

Constant-chip-area infeed (the standard single-line G76 behavior on Fanuc-family and Haas controls); depths are radial X values per side.
Flank vs radial vs modified-flank infeed changes the approach path and which flank carries the cut — not these depths.
Total depth ≈ 0.6134 × pitch applies to external 60° metric/UN forms; Acme, buttress and class-fit work take their depths from the thread spec.
Controls clamp at their minimum-DOC parameter; this screen flags when the ladder would fall below yours instead of silently re-planning passes.