Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
A picture from a previous repair shows the foot pedal’s innards:
Foot control – inside view
The top cover pivots on small studs that lock into the front of the case. A projection on the cover passes behind the bar near the top of the picture and presses the roller forward as the cover pivots downward under foot pressure.
The bar has an absolute maximum travel of about 15 mm, although it’s impossible to measure in situ with the cover in place:
Kenmore 158 foot pedal – actuating roller
The shaft in the middle of the carbon rheostat aligns the bar and actuates the full-speed switch contacts on the far right (not shown here). The compression spring vanishing into the ceramic body pushes the bar back against the projection on the top cover and ensures the whole affair turns off with the pedal released. The brass plate connects the two carbon buttons on the ends of the disk piles, which is what controls the motor speed from low to high, with the conical spring applying pressure to the piles as the bar moves forward:
Kenmore 158 – carbon-pile speed control – detail
The conical spring compresses about 4 mm after the brass plate contacts the buttons and has about 2 mm of overtravel after the shaft touches the full-speed contacts.
The carbon rheostat in the crash test dummy machine’s foot pedal works better than the much-repaired one from Mary’s machine, with smoother low-speed control and slower starts.
The resistance varies from about 1 kΩ with the most gentle of button touches down to about 30 Ω just before the full-speed contacts close. That’s across 4 mm of travel, so it’s rather sensitive. Most of the range seems to produce 300 to 50 Ω, more or less, kinda-sorta.
Which explains why my repairs were unavailing: the carbon piles must produce the proper resistances as the bar travels over that short distance. Changing the pile length, as happens when the disks erode and I rebuild parts, changes the resistance.
The unloaded motor draws about 300 mA regardless of the applied voltage, which suggests that the motor really wants to see a variable resistance, not a current source. More measurements are needed…
The entire Kenmore Model 158 sewing machine tilts on a pair of pivots extending from the rear of the base, just below the top surface. Mary’s slightly more recent machine has all-steel pivots:
Kenmore 158 – steel pivot pin
The older crash test dummy machine has two-part pivots, with a plastic housing molded around a steel pin:
Kenmore 158 – plastic pivot pins
Obviously, plastic was the wrong material for the cross pins that rest in the base, leading to the all-steel redesign. Sears no longer stocks replacement parts for those pins, sooo …
Both machines have a large plastic base that’s gradually disintegrating. The plan is to embed the machine frames in countertops, with those cross pins resting on plastic plugs set flush with the surface.
The frame sockets aren’t quite 1/4 inch in diameter; the rest of the hardware uses hard metric sizes, so they’re most likely 6 mm. A 15/64 inch (5.95 mm) drill bit fits snugly and a length of 0.228 inch (5.79 mm) drill rod fits loosely. The round pins are 18 mm long from the shoulder.
The square section is 8.5 mm wide, 9.5 mm tall, and 16 mm long. I have no idea what that mysterious tab on the end is supposed to do.
The cross pins are 5 mm diameter, a scant 15 mm end-to-end, stand 3 mm proud of the central block, and are centered 11 mm out from the edge of the block. I’d make them longer, to distribute the machine’s weight over more of the plugs in the countertop when it’s tilted back.
I can’t duplicate the newer forged steel pins and, for sure, they’re not good candidates for 3D printing. Perhaps:
Saw off 16 mm of 3/8 inch (9.5 mm) square stock
Blind drill 16/64 inch for the 0.228 main pin
Cross drill #12 for a 3/16 inch pin
Epoxy everything together
File off the sharp edges
For the moment, the crash test dummy sits happily on the three legs that the designers thoughtfully cast into its frame.
The handwheel on the Kenmore Model 158 sewing machine has a shiny knurled knob in the middle:
Kenmore 158 handwheel – knob
Turning the knob clockwise screws the knob inward and clamps a friction clutch that locks the handwheel to the main shaft; the motor belt drives the handwheel, the handwheel drives the shaft, and the shaft drives everything inside the sewing machine.
Remove the small screw, turn the knob counterclockwise to remove it, and you see the clutch:
Kenmore 158 – handwheel clutch – detail
Yes, the black stamped metal part is the clutch.
Those three projections around the exterior limit the knob’s travel to a bit under 1/3 turn, with the little screw you just removed traveling between two of the projections. When you reinstall the knob:
Turn it until it’s snug
Insert and tighten the screw
Done!
The two dogs in the middle project outward from the shaft notches: the bases engage the notches, the tips bears on the knob’s inner surface. Tightening the knob compresses the dogs, presses the clutch against the handwheel, and locks everything together.
It’s entirely possible to install the clutch backwards and, while it’ll come pretty close to working, it’s not quite right.
Having a NEMA 23 stepper fit almost exactly into the spot vacated by the sewing machine’s AC motor was too good to pass up:
Kenmore 158 – NEMA 23 stepper – on adapter
So I wired a power supply to an M542 stepper driver brick, connected the pulse output of a function generator to the brick’s STEP inputs, swapped motor leads until it turned the proper direction (CCW as seen from the shaft end), and turned the function generator knob:
Kenmore 158 – NEMA 23 stepper test
The object was to find the step frequency where the motor stalls, for various winding currents and supply voltages. The motor won’t have enough torque to actually stitch anything near the dropout speed, but this will give an indication of what’s possible.
With a 24 V DC supply and 1/8 microstepping (40 k step/s = 1470 RPM):
1.00 A = 11 k step/s
1.91 A = 44 k/s
2.37 A = 66 k/s
3.31 A = 15 k/s
With a 36 V DC supply and 1/8 microstepping:
1.91 A = 70 k/s
3.31 A = 90 k/s
With a 36 V DC supply and 1/4 microstepping (40 k step/s = 2900 RPM):
1.91 A = 34 k/s
2.37 A = 47 k/s
2.84 A = 47 k/s
3.31 A = 48 k/s
The motor runs faster with a higher voltage supply, which is no surprise: V = L di/dt. A higher voltage across the winding drives a faster current change, so each step can be faster.
The top speed is about 3500 RPM; just under that speed, the motor stalls at the slightest touch. That’s less than half the AC motor’s top speed under a similarly light load and the AC motor still has plenty of torque to spare.
90 k step/s at 1/8 microstepping = 11 k full step/s = crazy fast. Crosscheck: 48 k step/s at 1/4 microstepping = 12 k full step/s. The usual dropout speed for NEMA 23 steppers seems to be well under 10 k full step/s, but I don’t have a datasheet for these motors and, in any event, the sewing machine shaft provides enough momentum to keep the motor cruising along.
One thing I didn’t expect: the stepper excites howling mechanical resonances throughout its entire speed range, because the adapter plate mounts firmly to the cast aluminum frame with absolutely no damping anywhere. Mary ventured into the Basement Laboratory to find out what I was doing, having heard the howls upstairs across the house.
She can also hear near-ultrasonic stepper current chopper subharmonics that lie far above my audible range, so even if the stepper could handle the speed and I could damp the mechanics, it’s a non-starter for this task.
Given that the AC motor runs on DC, perhaps a brute-force MOSFET “resistive” control would suffice as a replacement for the carbon disk rheostat in the foot pedal. It’d take some serious heatsinking, but 100 V (or less?) at something under 1 A and intermittent duty doesn’t pose much of a problem for even cheap surplus MOSFETs these days.
That would avoid all the electrical and acoustic noise associated with PWM speed control, which counts as a major win in this situation. Wrapping a speed control feedback loop around the motor should stiffen up its low end torque.
For completeness, the matching socket (not shown) joins two cords:
AC line cord (two wire, not polarized, no ground)
Foot pedal
Extract the motor wiring from that block and connect it to a 50 V / 3 A bench supply, with the positive lead to the marked wire conductor:
Kenmore 158 AC motor – DC power
Cranking the voltage upward from zero:
Kenmore Model 158 AC Motor on DC – RPM vs V
So that’s about 200 RPM/V, offset by 2800 RPM. Totally unloaded, of course.
The original data:
DC V
DC A
RPM
Notes
15
0.29
690
Barely turning
20
0.28
1380
Finger-stoppable
25
0.29
2350
30
0.29
3450
35
0.30
4450
40
0.29
5740
45
0.29
6780
Still finger-holdable at start
50
0.29
8000
I can hold the shaft stopped between my fingers up through 45 V, with 0.54 A locked-rotor current at 25 V. The motor doesn’t have a lot of torque, although it’s operating at less than half the normal RMS voltage.
I should take those numbers with the motor driving the sewing machine to get an idea of the actual current under a more-or-less normal load.
Reversing the power supply leads shows that the motor rotates only counterclockwise, which is exactly what you’d expect: both polarities of the normal AC sine wave must turn the motor in the same direction.
Fancy new sewing machines can stop with the needle either up (so you can remove the fabric) or down (to nail it in place while you rotate it). This requires sensing the needle position, which prompted me to spend far too long contemplating all the mechanical gadgetry driven by the motor.
As nearly as I can tell, the crank counterweight behind the handwheel produces the most unambiguous position reports. Here’s what it looks like with the needle down:
Kenmore 158 – main shaft counterweight
As you’d expect, with the shaft rotated exactly 180° from that point, the needle is up.
The inviting space just above the shaft provides room for the bobbin winder that engages a knurled ring on the back of the handwheel, but the lower space seems to be available. The counterweight sits about halfway into the back of the handwheel, so the sensors must look at the frame side of the counterweight.
Two adjacent sensors could detect the edge of the counterweight, which would be enough to uniquely identify both positions. If they were spaced across the lower-left edge in that picture:
01 = trailing edge = bottom dead center = needle down (as shown)
00 = open air = needle rising
10 = leading edge = top dead center = needle up
11 = solid steel = needle falling
Either sensor gives you one pulse per handwheel revolution and the combination gives you a quadrature output of both position and direction. The top speed of 1000 RPM produces 17 Hz square waves.
An additional pulse/rev sensor on the motor shaft would give better control over the motor speed, as the handwheel runs at 1/10 the motor speed with belt slip built right in. Figure 10 kRPM → 170 Hz pulses.
From a cold start, you know the shaft angle to within a bit under 180°. If the motor can turn in both directions (as would a stepper or DC motor), you can always move the needle upward. If it turns only forward (as does the AC motor) and the needle is falling, then you probably don’t want to move the motor until you get a button push indicating that all fingers are clear.
A pair of Hall effect sensors might suffice to detect that big hunk of steel, perhaps with a pair of teeny magnets glued to the face or a magnetic circuit closed by the counterweight.
Huh. Who’d’a thunk it? That’s just too good to pass up…
Although you wouldn’t use PLA for the real motor mount, this was easy:
Drive Motor Mount – solid model
And the whole affair fits pretty much like you’d expect:
Kenmore 158 – NEMA 23 stepper – on adapter
The NEMA 23 motor doesn’t have the same end profile as the AC motor and the adapter plate gets in the way of the pulley, but flipping the pulley end-for-end perfectly aligned the belt.
For whatever it’s worth, here’s how I removed the pressed-on gear from the shaft:
NEMA 23 Stepper – removing gear
I’m pretty sure I have a little gear puller somewhere, but it’s not where I expected to find it, which means it could be anywhere.
Much to my astonishment, the shafts on both motors are exactly 1/4″ inch. I filed a flat on the shaft to avoid having the setscrew goober the poor thing.
A stepper isn’t the right hammer for this job, because it can’t possibly reach 8000 rpm, but it’ll be good enough to explore the parameter space and weed out the truly stupid mistakes. A brushless DC motor from halfway around the planet would fit in the same spot.
The OpenSCAD source code:
// NEMA 23 Stepper Mounting Plate
// Ed Nisley - KE4ZNU - June 2014
Layout = "Build"; // Build Show
//- Extrusion parameters must match reality!
// Print with 4 shells and 3 solid layers
ThreadThick = 0.20;
ThreadWidth = 0.40;
HoleWindage = 0.2; // extra clearance
Protrusion = 0.1; // make holes end cleanly
AlignPinOD = 1.70; // assembly alignment pins: filament dia
inch = 25.4;
function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);
//----------------------
// Dimensions
// Origin at bottom front corner of plate as mounted on machine
// motor mounted on rear surface, so recess is on that side
PlateThick = 4.0; // overall plate thickness
SlotOffset = [10.0,13.0,0]; // center nearest origin, motor in X+,Y+ direction
SlotSize = [8.0,25.0]; // diameter of mounting screw , overall end-to-end length
CutoutOffset = [0.0,40.0,0]; // cutout around machine casting
CutoutSize = [18.0,18.0];
MotorBase = 58.0; // square base plate side
MotorHoleOC = 47.2; // hole center-to-center spacing
MotorHoleOffset = MotorHoleOC/2;
MotorHoleDia = 5.0;
MotorBaseCornerRadius = (MotorBase - MotorHoleOC)/2;
FlangeWidth = 20.0; // mounting flange
MotorCenter = [(FlangeWidth + MotorBase/2),(MotorBase/2),0]; // XY of shaft centerline
MotorShaftDia = 7.0; // allow some clearance
HubDia = 38.5; // allow some clearance
HubHeight = 1.8;
//----------------------
// Useful routines
module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes
Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);
FixDia = Dia / cos(180/Sides);
cylinder(r=(FixDia + HoleWindage)/2,
h=Height,
$fn=Sides);
}
module ShowPegGrid(Space = 10.0,Size = 1.0) {
RangeX = floor(100 / Space);
RangeY = floor(125 / Space);
for (x=[-RangeX:RangeX])
for (y=[-RangeY:RangeY])
translate([x*Space,y*Space,Size/2])
%cube(Size,center=true);
}
//----------------------
// Build it!
module BasePlate() {
difference() {
// cube([(MotorCenter[0] + MotorBase/2),MotorBase,PlateThick],center=false);
linear_extrude(height = PlateThick) {
hull() {
translate([MotorBaseCornerRadius,MotorBaseCornerRadius])
circle(r=MotorBaseCornerRadius);
translate([MotorBaseCornerRadius,MotorBase - MotorBaseCornerRadius])
circle(r=MotorBaseCornerRadius);
translate([FlangeWidth + MotorBase - MotorBaseCornerRadius,MotorBase - MotorBaseCornerRadius])
circle(r=MotorBaseCornerRadius);
translate([FlangeWidth + MotorBase - MotorBaseCornerRadius,MotorBaseCornerRadius])
circle(r=MotorBaseCornerRadius);
}
}
translate(MotorCenter - [0,0,Protrusion]) {
rotate(180/8)
PolyCyl(MotorShaftDia,(PlateThick + 2*Protrusion),8); // shaft hole
PolyCyl(HubDia,(HubHeight + Protrusion)); // hub recess
for (x=[-1,1] , y=[-1,1]) {
translate([x*MotorHoleOffset,y*MotorHoleOffset,0])
rotate(180/8)
PolyCyl(MotorHoleDia,(PlateThick + 2*Protrusion),8);
}
}
translate(SlotOffset - [0,0,Protrusion]) { // adjustment slot
linear_extrude(height = (PlateThick + 2*Protrusion))
hull() {
circle(d=SlotSize[0]);
translate([0,(SlotSize[1] - SlotSize[0])])
circle(d=SlotSize[0]);
}
}
translate(CutoutOffset - [Protrusion,0,Protrusion])
linear_extrude(height = (PlateThick + 2*Protrusion))
square(CutoutSize + [Protrusion,Protrusion]);
}
}
ShowPegGrid();
if (Layout == "Show") {
BasePlate();
}
if (Layout == "Build") {
translate([-(SlotOffset[0] + MotorBase/2),MotorBase/2,PlateThick])
rotate([180,0,0])
BasePlate();
}
The Smell of Molten Projects in the Morning 