The motors to drive the machine are (assuming the usual cartesian layout):

X motor, Y motor. Large speed range required. Stepper motors are the preferred, from a mechanical point of view (no gearing). If I use an angular layout I still need two motors with the same requirement.

Z Motor. This will either just move a small amount (next layer) or move a large amount (to either end stop, for starting or finishing a job). An ordinary DC motor with some feedback mechanism is equally possible, especially with screw-drive to move the platform/head (it will not move if the DC motor is without power)

Extruder motor. Defines material flow. (With exchangeable heads, they should be the same). It just needs an simple speed control, basically on/off. (Should the speed of the head movement and extruder speed be carefully match and synchronized?) Stepper or DC are equal candidates.

Stepper or DC

DC Stepper
Simple, presumably cheaper More expensive?
Can be scavenged, presumably with gearing attached. Can be scavenged (old matrix printers) more challenging to get driving circuits adjusted
Positioning requires some feedback as turning the motor on and off will not move the exact same amount each time. Either measuring the actual position of the carriage, or count the number of revolutions of a shaft. Positioning is inherit in the design. The drive circuits know exactly how much the motor axel has turned.
When the motor is standing still it is also freewheeling. The gearing may make it impossible to move the head/bed by hand. You can apply power, but without defining motion, effectively braking to hold a given position. Without power it only lightly resists rotation (depends on exact model).
Speed is very variable, by using PWM (Puls Width Modulation) but not very accurate/repeatable, particular at very low speeds. Can run faster. Speed is limited by the max step rate, but it can run very accurate at any speed below that, including very slow
When stopping, the motor should be shortcircuited (or use short burst of reverse power) to avoid the motor coasting to a stop Stops immediately, when no “step” is applied.
Two wire, reverse polarity to reverse direction. Simple amplifier from controllers TTL levels to drive 4, 6 or 8 wires, needs twice the amplifiers. The waveform applied between the wires defines a “step”, this requires an extra controller or more firmware. Depending on version you need to switch polarity to keep normal motion
Gears probably needed, as it must turn with at least tens of RPM at low speed. Gears not required as it can turn arbitrarily slow. May need gears if torque is too weak.

I will need to get at least two stepper motors, and will initially try to scavenge.

For experimental purposes I bought a small used stepper motor. (Simple driver circut test done mid July).


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