Stepper Motor Sync Wheel

A need for pix of the current waveforms in a stepper motor produced a need to synchronize to the shaft rotation. Rather than cobble something up using random spare parts, I printed a wheel with a tab:

The model looks about like you’d expect:

Those stretched pentagonal holes give it a vaguely religious aspect, don’t they?

The tab is 2/50 of the circumference, so that the resulting pulse neatly brackets two consecutive groups of four full-step pulses. There’s no way to align the tab with the rotor position, so producing a good scope sync pulse becomes a simple matter of software.

The tab’s length and radial position corresponds to this carefully engineered bit of mayhem:

The shaft hole will be just slightly too small for the motor shaft, which is perfectly fine. Drill the hole to 5 mm using a #9 drill, working your way up from about #12 to keep the hole concentric.

Actually, that was the second version. The first was a quick-and-dirty disk with a tab, but it came out too floppy at only 1 mm thick and utterly boring:

But it served as the prototype to settle the tab dimensions and location:

The OpenSCAD source:

// Optical Interrupter
// Suited for low speed demonstrations!
// Ed Nisley KE4ZNU June 2011

//- Extrusion parameters - must match reality!
//  Print with +2 shells and 3 solid layers

ThreadThick = 0.33;
ThreadWidth = 2.0 * ThreadThick;

//- Plate dimensions

MotorShaftDia = 5.0;
MotorShaftDiaSides = 8;
MotorShaftPolyRadius = (MotorShaftDia/2)/cos(180/MotorShaftDiaSides);

HubDia = MotorShaftDia + 16*ThreadWidth;
HubThick = ceil(10.0/ThreadThick)*ThreadThick;		// total, not added to plate
HubSides = 8;

BladeRadius = 31.5;				// to center of optical switch gap
BladeThick = 2*ThreadWidth;		// measured radially
BladeAngle = (2/50)*360;		// 50 repeats of 4 full step sequences per rev
BladeHeight = 7.0;				// beyond ribs

PlateRadius = BladeRadius + 5.0;
PlateThick = ceil(3.0/ThreadThick) * ThreadThick;

HoleCenterRad = (BladeRadius + HubDia/2)/2;
HoleDia = 0.75 * (3.14159 * 2 * HoleCenterRad)/HubSides;
HoleSides = 5;

//- Convenience items

Protrusion = 0.1;
$fn = 128;						// make large circles very smooth

//- Build it!

difference() {
  union() {
	cylinder(r=PlateRadius,h=PlateThick);			// base plate

	cylinder(r=HubDia/2,h=HubThick,$fn=HubSides);	// hub

	translate([0,0,PlateThick])						// blade
	  difference() {
		cylinder(r=BladeRadius+BladeThick/2,h=BladeHeight);
		cylinder(r=BladeRadius-BladeThick/2,h=BladeHeight + Protrusion);
		rotate([0,0,(180 - BladeAngle/2)])
		  translate([PlateRadius,0,(BladeHeight + Protrusion)/2])
			cube([PlateRadius*2,PlateRadius*2,BladeHeight+Protrusion],center=true);
		rotate([0,0,(BladeAngle/2)])
		  translate([PlateRadius,0,(BladeHeight + Protrusion)/2])
			cube([PlateRadius*2,PlateRadius*2,BladeHeight+Protrusion],center=true);
	  }

  }

  translate([0,0,-Protrusion])						// shaft hole
	cylinder(r=MotorShaftPolyRadius,
			 h=HubThick+2*Protrusion,
			 $fn=MotorShaftDiaSides);

  for (Angle = [0:(HubSides-1)])					// beautification holes
	rotate([0,0,Angle*(360/HubSides)])
	  translate([HoleCenterRad,0,-Protrusion])
		rotate([0,0,180])
		  scale([1.33,1.0,1.0])
			cylinder(r=HoleDia/2,
					 h=(PlateThick + 2*Protrusion),
					 $fn=HoleSides);

}

Yeah, that optical switch really is older than you are…