Max feedrate checker

How it is worked out

A step/direction controller moves an axis one pulse per step. The fastest feed is therefore the controller’s pulse rate shared out over the pulses each millimetre needs, and the frequency a feed needs is the same sum turned round.

pulse frequency (Hz)   = steps per mm × feed (mm/min) ÷ 60
maximum feed (mm/min)  = step-rate limit (Hz) × 60 ÷ steps per mm
steps per mm           = full steps per rev × microsteps ÷ screw lead (mm)
headroom (%)           = (limit − pulse frequency) ÷ limit × 100

The limit used is the controller’s, or the drive’s maximum input frequency if you enter one and it is lower. m/min is mm/min ÷ 1000, and in/min is mm/min ÷ 25.4.

Worked example

A 200-step (1.8°) motor on a drive set to 40 microsteps, turning a 5 mm lead ball screw directly, on a TNC-M34:

steps per mm   = 200 × 40 ÷ 5            = 1 600 steps/mm
maximum feed   = 100 000 × 60 ÷ 1 600    = 3 750 mm/min  (3.75 m/min, 147.6 in/min)
3 000 mm/min   : 1 600 × 3 000 ÷ 60      = 80 000 Hz = 80 kHz, 20 % headroom
5 000 mm/min   : 1 600 × 5 000 ÷ 60      = 133 kHz, past the limit

The 3 000 mm/min feed fits. A 5 000 mm/min rapid cannot be delivered at this resolution, whatever max_rate is set to.

Assumptions

• Steps per mm already includes any belt or gear ratio between the motor and the screw.
• The figures are for one axis at a steady feed. On a TNC-M34 the ceiling applies to the fastest-moving axis of a move; the other axes get fewer pulses from the same interrupt.
• For a rotary axis, enter steps per degree and a feed in degrees per minute; the arithmetic is the same.
• Headroom under 10 % is flagged as a warning. That margin is this tool’s choice, not a controller figure.

Why the step rate, not the motor, often limits rapids

Every microstep is another pulse the controller has to send for the same distance. At 1 600 steps/mm a 100 kHz ceiling runs out at 3 750 mm/min, which on a 5 mm lead is the motor turning only 750 rpm. Many stepper systems still have usable torque there, so the rapid stops at the pulse ceiling long before it stops at the motor. Typing a bigger number does not help: the TNC-M34 accepts a max_rate of up to 200 000 mm/min, but its pulse engine delivers no more than 100 kHz, and at this resolution that is 3 750 mm/min.

The trade-off of fewer microsteps

Halving the microsteps halves steps per mm and doubles the top feed at the same step rate — the table beside the result shows that ladder for your numbers. What it costs:

• Coarser steps: 1 600 steps/mm is 0.625 µm a step, 400 steps/mm is 2.5 µm. Both are usually finer than a stepper holds position under load, so the loss in real accuracy is smaller than those numbers suggest.
• Rougher motion at low speed, with more noise and vibration, because each step is a bigger jump in the motor’s current.
• More chance of exciting resonance at particular speeds.

What it can buy besides speed: on a TNC-M34 a lower step-rate demand lets a wider STEP pulse survive. step_pulse_us is held to half the shortest step period the axes can demand — 5 µs at 100 kHz, 10 µs at 50 kHz — so a drive that needs a 10 µs pulse needs the demand at or below 50 kHz.

The limits in this tool

On the Tstep-087 and the TIO-0808 each pulse lasts about one 10 µs tick and steps land on that tick grid, so rates between the grid points are averages. Up to 50 000 steps/s there is at least a full tick with the line low between pulses; above that, some pulses fall on consecutive ticks with the line low for only a moment.

Why 100 kHz is flagged

100 kHz is the ceiling of the TNC-M34, the Tstep-087 RS-485 drive and the TIO-0808, so none of them can meet a demand past it. The TNC-L32 can go higher, but only with DIR SETUP + PULSE WIDTH under 5 µs. Its manual gives microsecond settle times as what optocoupler inputs need, which leaves a drive with a differential line-receiver input as the realistic case. Past 100 kHz, check the drive’s maximum input frequency as well.

The drive’s limit

The drive has its own maximum input pulse frequency. If you know it, enter it and the tool uses whichever limit is lower. Take the figure from the datasheet of the drive you have; there is no preset for it here.

What this does not check

A pulse rate the controller can send is not a speed the axis can reach. A stepper’s torque falls as speed rises, because winding inductance limits how fast current builds (a higher supply voltage helps). Mid-band resonance can stall a motor at particular speeds, acceleration needs torque margin on top of that, and a long screw has a critical speed of its own. A stepper torque-speed estimator for that side is planned at /tools/torque-speed/.

Tiny Controls products

• TNC-M34 four-axis CNC controller: 100 kHz per axis.
• TNC-L32 two-axis lathe controller.
• Tstep-087-485 stepper drive with an RS-485 interface.
• Tstep-087X stepper drive: 4, 5, 8, 10, 16, 20, 32 or 40 microsteps.
• TIO-0808 RS-485 I/O card.