Axis drive motors for CNC router gantries: a NEMA 34 stepper or a 750 W AC servo on a belt-reduced rack-and-pinion drive, compared by travel per revolution, resolution, thrust, speed and inertia

01

From motor turn to gantry travel: a 1 in pitch-circle pinion behind 2:1 to 3.2:1 belt reductions gives 0.98 to 1.57 in per revolution

Avid CNC, which sells router kits and drive components, bases its rack-and-pinion drives on a pinion with a 1 in pitch circle, so one pinion turn moves the gantry 3.14159 in. A belt reduction between motor and pinion divides that distance: [1] [2]

Avid's racks are imperial 20 pitch (20 teeth on a 1 in pitch-circle pinion, a linear pitch of 0.1571 in) with a 20° pressure angle; it warns that module 1, 1.5 or 2 racks, or racks with another pressure angle, are incompatible and cause excessive wear and noise. Its NEMA 34 standard drive uses 2:1 rather than the NEMA 23 drive's 3:1 because the larger motor's torque allows higher speed, and uses larger belts and pulleys for four times the torque capacity. [1] [2] A spring-loaded pivot keeps the pinion in mesh for low backlash, and a cam on top sets belt tension. [2]

Editorial calculation: 0.00049087 in per microstep is 12.5 µm at the gantry, and 1.5708 in per motor turn on the standard NEMA 34 drive is 39.9 mm. [1] [2] On a ball-screw axis, like those in the image above, the screw lead takes the place of the pinion circumference divided by the reduction.

The image shows a gantry carriage with three motors: black motors with orange cables in drag chains, coupled through coupling housings to ball screws on the vertical and horizontal axes.

State the drive type on each axis (rack and pinion, belt, ball screw), the pinion pitch and rack standard or screw lead, the reduction ratio, and the resolution needed at the tool.

02

Same drive, two motors: an 8 N·m NEMA 34 stepper and a 750 W servo rated 2.39 N·m, 7.16 N·m peak, 6000 r/min, with 8,388,608 counts per turn

Leadshine's 86CM80 is a two-phase NEMA 34 stepper 98 mm long with 8.0 N·m holding torque at 6.0 A per phase, 0.5 Ω, 4.2 mH, rotor inertia listed as 2.8 kg·cm², IP40 and 3.2 kg; holding torque is a standstill figure, and Leadshine publishes a separate speed-torque curve. [3] Panasonic's MINAS A6 MSMF082L1U2M is a 750 W, 200 V low-inertia AC servo motor: [4]

Editorial calculation, both motors on Avid's 3.2:1 NEMA 34 drive (0.9817 in = 24.9 mm per motor turn, pinion radius 0.5 in = 12.7 mm), ignoring belt and mesh losses: the stepper's 8 N·m holding torque gives at most 8 × 3.2 / 0.0127 = 2016 N of thrust at standstill, falling with speed along its curve; the servo gives 602 N continuously and 1804 N at its 7.16 N·m peak. [3] [4] [2] The servo's 3000 r/min rated speed is 74.8 m/min of axis travel and its 6000 r/min maximum 149.6 m/min, and one encoder count is 24.9 mm / 8,388,608 = 0.003 µm, so the rack, pinion and belt, not the encoder, set the axis accuracy. [4] [2]

Editorial calculation, with an example gantry mass of 100 kg (an assumed value, not from a source): the inertia reflected to the motor is 100 × (0.0249 / 2π)² = 1.57 × 10⁻³ kg·m², 16.4 times the servo's 0.96 × 10⁻⁴ kg·m² rotor inertia and so inside Panasonic's recommended ratio of 20 times or less; at the 2:1 standard drive's 39.9 mm per turn the same example gantry would reflect 4.0 × 10⁻³ kg·m², 42 times. [4] [1]

Give each axis's moving mass, required rapid speed and acceleration, cutting thrust, drive ratio and pinion or screw data, and ask the supplier for the motor's speed-torque curve and rotor inertia, not only holding or rated torque.

03

The belt pulls on the motor shaft: 392 N radial load in operation on a 750 W servo

A belt reduction puts the belt tension onto the motor shaft as a side load. Panasonic lists the permissible shaft loads for the MSMF082L1U2M: during operation 392 N radial and 147 N thrust in either direction; during assembly 686 N radial, 294 N thrust in the A direction and 392 N in the B direction. [4]

Avid's drives carry the motor on a pivot plate with a tensioning cam, so belt tension is set by adjustment rather than fixed by the mounting. [2] Editorial note: when a router's axis motor is replaced, the new motor's shaft diameter (Avid's PRO drive takes 1/4, 3/8 or 1/2 in shafts with matching pulleys), its permissible radial load and the belt tension the drive needs have to be checked together. [2] [4]

State the motor shaft diameter and length, the pulley and belt fitted, the belt tension, and the radial load the motor is rated for in operation.

04

Axis drive details to send

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Axes: [X, Y, Z travel; moving mass per axis]
Drive: [rack and pinion, belt, ball screw; pinion pitch or screw lead; reduction ratio]
Motion: [rapid speed; acceleration; cutting thrust]
Resolution: [at the tool; microstepping or encoder counts]
Motor: [stepper or servo; frame; shaft diameter; brake on Z]
Controller: [step/direction or fieldbus; supply voltage]

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