FANUC multi-start threading: Q controls starting phase, so why must F be interpreted as lead?
The 0i-F lathe manual’s G32 multi-start threading rules relate a three-start thread’s lead to its 120° phases and integer Q values. The article separates first-block rules for continuous threading from independent cuts and explains why ordinary G76 Q is not a phase angle.
Contents
In multi-start threading, adding starting angles must not be confused with changing feed per revolution to thread pitch. Phase determines where each helix starts around the circumference; lead determines how far the same helix advances axially in one revolution.
Command meanings are limited to this 0i-F lathe edition
In the FANUC 0i-F lathe manual B-64604EN-1/01, §3.5–3.6, pages 32–33 (public mirror of the original manual), G32 in G-code system A uses F for longitudinal lead and Q for the starting-angle offset relative to the spindle one-revolution signal. Q is an integer in units of 0.001°, with no decimal point permitted. Omission means 0; it is not a retained modal value. Continuous threading uses only Q from the first threading block and ignores Q in subsequent blocks.
Three starts with 2 mm pitch: why the lead is 6 mm
This original model is a cylindrical, constant-lead, uniformly spaced three-start thread with millimeter input. Pitch p=2 mm and number of starts m=3 give axial advance along one helix per full revolution L=mp=6 mm. Adjacent starts are separated around the circumference by 360°/m=120°.
| Thread start | Starting angle | Integer representation of Q | Lead of each start |
|---|---|---|---|
| Start 1 | 0° | 0 | 6 mm/rev |
| Start 2 | 120° | 120000 | 6 mm/rev |
| Start 3 | 240° | 240000 | 6 mm/rev |
Here F corresponds to 6 mm/rev, not 2. If lead 2 is used incorrectly with three starts spaced by 120°, the same geometry gives axial spacing between adjacent starts of 2/3≈0.667 mm, no longer the designed 2 mm. This reverse calculation helps identify confusion between pitch and lead fields in CAM output or process sheets.
Check independent cuts separately from continuous blocks
If start 2 takes several passes, a previous use of 120000 does not mean the next independent cut automatically retains that phase. Conversely, splitting one continuous thread into several blocks does not let Q in an intermediate block switch to another start. The former requires checking starting phase for every cut; the latter is governed by the first-block rule.
Q in ordinary G76 has a different meaning
Page 33 of the same edition distinguishes Q in the ordinary G76 / G78 format as minimum depth of cut or first depth of cut. Only the FS10/11 format has the starting-phase use described there. Values of Q in the table therefore cannot be inserted directly into an ordinary multiple repetitive threading cycle.
This article checks only geometry and field semantics. It supplies no approach, withdrawal, spindle-speed or depth-of-cut plan and does not assume that a particular machine has the required function. An actual program must also match spindle synchronization and the machine's cycle format. Further threading and feed resources are in the OPTION section.