Why are there four sets in 4056~4059 when I only use two ranges?

The four sets are reserved for the four combinations of CTH1A/CTH2A (00/01/10/11); the CNC uses this pair to pick the active ratio set. Even with only two physical ranges, fill all four correctly and set the unused two equal to an in-use range - otherwise after a shift the CNC applies the wrong ratio and the screen speed diverges from the real speed by a large factor.

Do I use 3751/3752, and how do they relate to 3741/3742?

Only Type-B shifting (3705#2=1) uses 3751/3752. 3741/3742 are the per-range spindle maximum speeds; 3751/3752 are the switch-point motor speeds (digital), computed as 3751 = 4095 x switch-point spindle speed / 10V-motor-speed. Type-A (3705#2=0) does not need them - the CNC selects directly by 3741~3744.

The spindle will not oscillate during shift and the fork will not enter - what now?

First check 3705#1 = 1 (enable SOR displacement) and that *SSTP (G29.6) is held at 0 during the shift; then check 3732 oscillation speed is enough to move the fork. For a stubborn hydraulic detent, change the SOR-period spindle into an alternating forward/reverse square wave so the detent hunts for position.

FANUC Spindle Gear Shifting: 3705/3732/3735/3741-3744/3751-3753/4056-4059 and PMC Signals

A geared spindle (high/low range) exists for one reason: high torque at low speed, high power at high speed. The gearbox splits the spindle into two to four ranges; the electrical side must tell the CNC each range's speed window, how to encode the gear ratio, and how the PMC drives the shift mechanism. It is parameter-heavy and signal-coupled, which is why it is one of the most frequently mis-configured areas on site.

Written for the FANUC 0i series. Parameter numbers follow the common 0i convention; always confirm initial values against your machine’s parameter manual.

1. First, decide which shift type the machine uses

There are two families, with completely different parameters and PMC handling. Identify this first:

M-type shifting (most common on M-series, 0i-MF / 0i-Mate-M): the CNC auto-selects the range from the S command and the per-range speed limits, then announces it to the PMC through the GR30/GR20/GR10 signals. It splits further into:

Type A: 3705#2 = 0. The CNC picks the range directly by comparing against 3741~3744.
Type B: 3705#2 = 1. Adds 3751/3752 as the motor-speed switch points between ranges, which resolves the auto-detection trouble when each range has a different upper limit.

T-type shifting (also available on T-series lathes): the current range is set by the GR1/GR2 signals (four ranges), chosen by an M code in the program (e.g. M41~M44) or manually; the CNC simply outputs the speed for the active range.

The two shift types: M-type lets the CNC auto-select the range and emit GR signals to the PMC, while T-type uses an M code / GR1/GR2 to pick the range

2. The core parameter list

Whether M-type or T-type, set this group first:

Parameter	Purpose	Note
`3705#2`	Gear switching method	0 = Type A, 1 = Type B
`3705#1`	Use SOR for gear displacement	=1 lets the spindle oscillate at `3732` during shift
`3705#4`	Output S code / SF signal	Must =1 on M-series to use S/SF in PMC
`3732`	Spindle motor speed during gear shift	Oscillation speed for the fork to engage
`3735`	Spindle motor minimum clamp speed	Lower motor-speed limit (digital)
`3736`	Spindle motor maximum clamp speed	=4095 means unlimited
`3741~3744`	Spindle maximum speed per range	Range 1-4 upper limit, scaled by ratio
`3751`	Motor speed at the 1->2 switch point	Type B only, computed
`3752`	Motor speed at the 2->3 switch point	Type B only, computed
`3753`	Delay after the shift M code outputs	Wait for the mechanism before FIN
`4056~4059`	Spindle-to-motor gear ratio (by CTH1A/CTH2A)	One set per range, x1/1000 unit
`4171~4174`	Motor-sensor to spindle gear ratio	Same four ranges

Correcting a common misprint: briefs often write the clamp speeds as 3734/3735, but there is no 3734 in the standard set. The minimum clamp is 3735 and the maximum clamp is 3736 (4095 = unlimited). 3741~3744 are the per-range spindle maximum speeds. Treat any “3734” as 3735/3736.

Core parameter groups: shift method (3705 three bits) + clamp speeds (3735/3736) + per-range limits (3741-3744) + Type-B switch points (3751/3752) + gear ratios (4056-4059/4171-4174)

3. How the gear ratio is actually computed

This is where people invert things. 4056~4059 hold the spindle-to-motor gear ratio (in 1/1000 units), one set per CTH1A/CTH2A combination. It is a different quantity from 3741/3742, but the two are coupled:

3741~3744 set the highest spindle speed each range can reach (already ratio-scaled).
4056~4059 / 4171~4174 tell the CNC how to convert motor speed into spindle speed for display and control; after a shift, when CTH1A/CTH2A change, the CNC switches to the matching ratio set.

Complete example (ZF two-range, high 1:1, low 4:1, motor max 7000, shift speed 800):

3705#2 = 1 (Type B)
3735 = 5, 3736 = 4095, 3732 = 3
3741 = 7000/4 = 1750 (low-range spindle limit)
3742 = 7000 (high-range spindle limit)
3751 = 800 x 4095 / 1750 = 1872 (1->2 switch-point motor speed)
3752 = 4095
4056~4059 = 100, 100, 100, 400 (ratio x1000 for the four CTH combinations)
4171~4174 = 1, 1, 4, 1 (motor-sensor to spindle ratio)

The hard check: at low range the actual spindle maximum = motor maximum / low-range ratio. Above, low = 7000 / 4 = 1750, matching 3741 – so it is not inverted.

Gear-ratio math: 3741 = motor max / low ratio, 4056-4059 filled per CTH combination, with the ZF 1:1/4:1 worked example

4. PMC signals that must be handled

Filling parameters is only half the job; the shift action is driven by the PMC. Key signals:

GR30/GR20/GR10 (G28.0/28.1/28.2): the gear-range select signals the CNC emits after choosing a range; the PMC uses them to energise the shift solenoid/valve.
CTH1A (G70.3) / CTH2A (G70.2): the serial spindle actual gear stage feedback. The CNC uses this pair to pick which of 4056~4059 to apply. The range and this pair must correspond, or the displayed speed diverges from reality by a large factor.
SOR (G29.5): spindle orientation / gear-displacement signal. With 3705#1=1, during a shift the spindle oscillates at the 3732 speed to help the fork engage; a stubborn hydraulic detent can be given an alternating forward/reverse square wave.
*SSTP (G29.6): spindle stop. Keep it 0 during the shift so the spindle is allowed to rotate at 3732; set it to 1 and the spindle locks, and the fork cannot enter.
SF (G70.5 class) / SFIN (G5.2): spindle function strobe and completion. On M-series 3705#4=1 is required before SF is emitted; the PMC issues SFIN only after the mechanism is truly in place, otherwise the CNC hangs on the M line.
M code and the 3753 delay: T-type or manual shifting uses M41~M44; after the CNC emits the M code it waits the 3753 delay before judging completion. In the ladder, issue FIN only after the mechanism is actually seated.