The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Machine Shop

Mechanical widgetry

  • Thing-O-Matic: MBI Stepper Motor Analysis

    After pondering the stepper motor data collected there and the driver data there, plus running some experiments with different motors, I’ve concluded that the MBI stepper motors aren’t appropriate for the Thing-O-Matic. This post summarizes my doodles and provides some background and justification for what I’ve been doing…

    During the next week or two I’ll continue writing up the results of installing better stepper motors in my TOM, plus some mods required to take advantage of the improved performance. I’ll do a wrapup of the new motors when everything’s settled down and I (think I) understand what’s going on.

    The ideal situation

    This diagram from page 15 of the Allegro A3977 datasheet shows how the current varies in each winding during the course of 32 microsteps. The motors have 200 full steps/rev and 1600 microsteps/rev, so this diagram repeats 50 times during the course of one complete shaft rotation.

    Allegro A3977 microstepping current waveforms
    Allegro A3977 microstepping current waveforms

    The peak current of each waveform corresponds to the REF pot setting on the MBI driver board:

    current in amperes = (REF pot voltage) / 2

    That peak current must not exceed the motor’s rated Maximum Current, because the winding resistance dissipates that much power as heat. The maximum temperature occurs deep in the windings, far from the metal part of the armature, so blowing air on the motor helps, but does not cure, an overtemperature problem.

    The driver adjusts the current in each winding to generate an approximation of a sinusoid waveform for each microstep. Because the motor torque varies directly with the winding current, the REF pot sets the maximum torque available from the motor.

    The A3977 driver controls the current by switching the MOSFETs on and off: on = increasing current, off = decreasing current. You can fiddle with the rates of increase and decrease, but those are all in the nature of fine tuning. What’s important is that the A3977 shuts off the winding current when it exceeds the product of the REF pot setting and the sinusoidal value for the microstep, then turns it back on when it falls below a somewhat lower level.

    Therefore, the current isn’t actually constant: the whine you hear when the motors are standing still is an audible harmonic or sub-harmonic of the switching frequency. That’s not a bug, it’s a feature!

    At the microsteps corresponding to the peaks of each sinusoidal waveform, one winding carries the maximum current and the other winding carries zero current. There are 200 such positions, each corresponding to one full motor step. At those points, the armature holds the rotor in position with the much-quoted Holding Torque.

    For all other microsteps, the A3977 controls the Pythagorean sum of the two currents to equal the maximum current setting. The two currents pull the rotor toward two adjacent full-step positions, with the actual (nominal) rotor position determined by a bit of trig.

    The power dissipation in the motor at every microstep is therefore:

    (peak current)2 x (winding resistance)

    All that applies to the DC situation with the motor halted at a particular microstep. In order to turn the rotor, the drive must change the winding currents to the values for the next microstep.

    The motor windings are basically inductors with energy stored in their magnetic field, so the current cannot change instantly. The ratio of the inductance (L) and the total circuit resistance (R) is the time constant, abbreviated with a Greek tau (τ):

    τ = L/R

    The current change from one microstep to the next requires 3 time constants to settle within 5% of the final value and 5 time constants to settle within 1%. Those are characteristics of the exponential function and have nothing to do with the particular circuit; once you know the time constant, you know what’s going to happen.

    The voltage applied to the motor winding determines the final value of the current that you use with the time constant. Microstepping drivers expect to apply a voltage far higher than the winding’s rated voltage, then limit the current to the winding’s rated value: the current never reaches the “final value”, but that’s still what you use in the computation. If the supply voltage equals the winding’s rated voltage, then the final value is simply the winding’s rated current.

    The MBI Situation

    The MBI motors (all of them, XYZ and Stepstruder) in a Thing-O-Matic do not operate like that. I’ll use the XY motors as examples, but feel free to run the same analysis on the others.

    To summarize the datasheet values:

    • Inductance L = 44 mH
    • Resistance = 35 Ω
    • Rated voltage = 14 V
    • Rated current = 400 mA

    The motors operate from a +12 V supply, so the maximum winding current will be at most 12 / 35 = 340 mA. The actual power supply voltage seems to around 11.5 V with the heaters running, the A3977 MOSFETs (inside the chip) have a total on-state resistance of about 800 mΩ, and I’ll assume another ohm of wiring resistance along the way. All that reduces the actual maximum current to around 300 mA; I’ll use that, because it’s within 10% off the actual value.

    The MBI-recommended REV voltage setting of 1.5 V sets a 750 mA output current. However, there’s no magic involved: the motors cannot draw more than 300 mA in a Thing-O-Matic, no matter what the REF trimpot may call for.

    With the REF trimpot set to 750 mA and the maximum current limited to 300 mA by the circuit, the A3977 cannot produce the correct current for most of the microsteps. Whenever the microstep current exceeds 300 mA, the A3977 cannot make that happen.

    This diagram shows the actual winding current in the MBI motors for each of the 32 microsteps in four full motor steps:

    Allegro A3977 waveform - current saturation
    Allegro A3977 waveform – current saturation

    The 100% level corresponds to the 750 mA set by the REF trimpot and, as above, the nice sinusoids show the target current for each microstep. The red line shows the actual current for each microstep, none of which can exceed the 300 mA limit. That limit corresponds to 300/750 = 40% of the maximum, just slightly over the 38.3% for the second microstep in each sequence.

    The pink zones mark the microsteps where both windings become current-limited to 300 mA. During those microsteps, the current in the windings doesn’t change and the motor cannot move. Of the 32 microsteps in each group of four full steps, the motor can move during only 16.

    The horrible sounds you hear from an MBI motor happen as the rotor encounters those pink zones: the rotor literally jams to a stop when both windings limit at 300 mA, remains immobile while the currents remain steady, then jerks across the 4 microsteps in the pink zone when the current in one winding drops below 300 mA. This is obviously not conducive to smooth motion or high torque.

    Try this: reduce REF to, say, 400 mV to limit the peak current to 200 mA. Run the motor slowly, because it won’t have much torque, and listen. Set REF back to 1.5 V, run it at the same speed, and listen.

    The power dissipation for all the microsteps in the pink rectangles is:

    2 x (300 mA)2 x 35 = 6.4 W

    The factor of 2 comes from the fact that both windings carry 300 mA in that condition.

    The motor’s rated maximum power is:

    (400 mA)2 x 35 = 5.6 W

    There’s no factor of 2 because the rating applies to one winding carrying the rated current.

    During the other microsteps the power drops slightly, with the best case when one winding carries zero current:

    (300 mA)2 x 35 = 3.2 W

    That’s why MBI motors overheat: they operate at the ragged edge of their power limit while tucked inside a thermally insulating plywood box. If the motor stops on a microstep inside those pink zones, it’ll dissipate 6.4/5.6 = 114% of its rated power.

    Changing the current between microsteps also poses a problem. The time constant for the MBI XY motors is:

    τ = 44 mH / 35 Ω = 1.3 ms

    That means the current settles within 5% in 4 ms and 1% in 6 ms.

    Stock Thing-O-Matics move at about 30 mm/s. The motor pulley has 17 teeth and the belt has teeth on a 2 mm pitch, so the motor must turn at 1 rev/s to move the stage at 34 mm/s. With 1600 microsteps/rev, each microstep takes 625 µs, which is half the time constant.

    I think you can see where this is going…

    The microsteps outside the pink zones could have active current limiting, because the A3977 has some voltage headroom. The first microstep has a current limit 20% of the 750 mA maximum (set by the trimpot = what you want) = 150 mA.

    The current starts rising toward the actual 300 mA maximum (set by the supply voltage and winding resistance = what you get) and after 625 µs it reaches:

    300 mA × (1 - e-0.5) = 120 mA

    So the current doesn’t quite reach the target and the A3977 doesn’t get a chance to do active current limiting.

    The next microstep has a 38% current limit that sets a target of 285 mA, marginally below the 300 mA limit set by the winding resistance. The A3977 continues to apply the full supply voltage, so the winding doesn’t notice anything’s changed and the current continues to rise. At the end of the second microstep the current has reached:

    300 mA × (1 - e-1) = 190 mA

    Which is about 2/3 of the target and the A3977 still doesn’t do active current limiting.

    The full analysis is messier than that, but what you see is pretty close. I won’t go into what happens when the A3977 is trying to reduce the winding current, but a similar analysis applies.

    Also, when the motor rotates slower the microsteps last longer and the current can get closer to the target value. Print at 15 mm/s to get microsteps about 1 time constant long; that’s still short, but it’s better.

    If the motor stops on a microstep outside the pink zones, then the two winding currents will eventually exceed the values for that microstep and the A3977 will begin active current limiting: that’s when you hear the chopper whine. However, if the motor stops on a microstep inside the zones, then it’s dead silent: the currents never reach the level where the A3977 can apply active current limiting.

    Because torque is proportional to current, the motor never delivers its rated torque in any microstep while it’s turning. The motor datasheet includes this torque-speed curve:

    Cupcake TOM Stepper Torque Curve
    Cupcake TOM Stepper Torque Curve

    The much-quoted Holding Torque is irrelevant. That measures the motor’s ability to hold its position with an external torque applied to the shaft. Unlike CNC milling machines, 3D printers do not impose torques on the XY motors due to forces from a cutting tool.

    What’s important is the bottom curve showing the pull-in torque: the torque available to accelerate the load from a dead stop to the speed shown along the bottom, given in full-step pulses per second.

    At 1 rev/sec the motor sees 200 full steps/sec, at which speed the pull-in torque is about 12 mN·m. However, that’s at the 400 mA full rated current applied from a 24 V source through a current-limiting driver. Because the maximum torque depends on the current and the resistance limits the maximum current to 300 mA, the maximum pull-in torque scales to 9 mN·n.

    I’ll grant the possibility that there’s a misprint and Kysan simply dropped a zero. Pull-in torque around 150 mM·m seems more common with short NEMA 17 motors, but the data sheet is what the data sheet is. The motors behave as though they have no mojo, which leads me to believe the printed word.

    Anyhow, that torque assumes the driver applies the proper winding current in each microstep, which, as you’ve just seen, doesn’t happen. Oddly, the MBI motors provide the highest torque in the pink zones, where the Pythagorean sum of the resistance-limited winding currents is:

    √(2 x 300 mA2) = 420 mA

    So the motors run in a crippled full-step mode that produces more-or-less the rated torque only when the motor isn’t moving, while dissipating too much power. When the motor is moving, the current never reaches the proper level.

    The measurements I made when I had the printer apart indicate that the X and Y stages require far more torque than the MBI motors can provide, even if they were driven correctly. The fact that they work at all has more to do with good luck and spec tolerances than anything else.

    The inadequate torque also answers the question of whether a higher power supply voltage will improve things: no, not much. A 24 V supply (as specified in the datasheet!) will permit operation at the rated current with correct microstepping, but that torque is still far too low.

    I don’t have any inside knowledge, but I think what happened is that these motors date back to the Cupcake printer, which used a simple H-bridge without active current limiting. For that type of driver, the rated voltage of the winding must equal the supply voltage, because the winding resistance provides the current limiting.

    Using A3977 drivers seemed like a simple upgrade to produce the Thing-O-Matic, but a microstepping driver must apply a voltage much higher than the winding’s rated voltage in order to get fast current changes and apply active current limiting. The old motors simply aren’t suited for the new drivers.

    Tomorrow: what better motors can do for a Thing-O-Matic.

    I’m certain I’ve made at least one error in what you’ve just read; comments, criticisms, and corrections are welcome. However, before you comment, RTFM for the A3977 driver, the MBI stepper motors, and any other hardware you’re proposing. Run the numbers first, OK?

    Update:  A reader suggests a rule of thumb relating voltage to inductance …

    Marris Friemannis of Gecko drive fame quotes this rule of thumb (http://www.mechmate.com/forums/showthread.php?t=1618)

    Drive Supply Voltage = 32 * √mH Inductance of the motor

    So in case of 44mH motor, correct voltage would be in excess of 200V, which I choose read as “the motor is junk” :)

    In the stuff 3D printers use, single digit mH values at 24V seem to work fine.

  • Thing-O-Matic: New X Axis Stepper Motor

    New X axis 34 mm NEMA 17 motor in place
    New X axis 34 mm NEMA 17 motor in place

    A group of 34 mm NEMA 17 steppers arrived from the usual eBay seller, I wired one up, and popped it in place under the original cork sheet. The bolts sit on steel washers riding atop compliant bushings from the batch of 43 mm NEMA 17 steppers that drive the extruder and Y axis.

    [Update: Not that you’ll ever find another one, but here’s the straight dope directly from the motor label…

    • Astrosyn P/N FH5-1043 02
    • Minebea 17PM-J034-P2VS
    • No. T6Z01-03

    ]

    Putting a cork sheet under each motor was a nice idea, but  it didn’t work as intended: the bolts quite effectively couple the vibration to those resonant acrylic and plywood sheets. I had to cut the two front bushings in half to ease the bolt heads under the X axis stage at that end of its travel, but the motor is now isolated from the Y stage. I’m sure the bolts touch the slots, but even I am unwilling to fit compliant bushings around the bolts.

    Lashing the cable to the side of the box should suffice for strain relief, as the jacket forces it to flex in a large upward loop with no sharp bends.

    X axis motor wire strain relief
    X axis motor wire strain relief

    That picture’s inverted so the flash lights up the stuff sticking out of the bottom of the box. The thin silvery arch is a cable tie around the motor connector holding it securely against the motor frame, but the Y axis follower bearing just to its right actually sticks out slightly more.

    This motor has six active terminals and could run in either unipolar or bipolar mode. The wiring harness has four leads and that’s why I bought it: the MBI driver board expects a bipolar motor.

    The winding resistance is a mere 2 Ω, compared to the MBI motor’s 35 Ω. Of course, I don’t have any specs for this motor, but similar Minebea 17PM-J0xx motors run around an amp with 130-180 mN·m of pull-in torque, compared to the MBI motor’s 14-ish mN·m. I expect to run it around 500-750 mA at half the rated torque, where it’ll dissipate maybe a watt, tops, with no overheating.

  • Thing-O-Matic: Stepper Motor Torture Test

    The G-Code in BenJackson’s Smooth Motion test does a good job of identifying mechanical constraints and stepper motor problems. Even after doing all the rod alignments and sundry tweaks, my TOM wasn’t reliable around 4000 mm/min = 65 mm/s and had terrible mechanical resonances around 5000 mm/min. While normal extrusion feeds run around 30 to 40 mm/s (1800 to 2400 mm/min), I didn’t have a warm fuzzy feeling that everything was operating correctly and, in fact, the TOM suffered the occasional missed step.

    The per-axis speeds in thingomatic.xml limit the maximum speed attainable in the G-Code; you can set the F value in the G-Code as high as you like, but the axis won’t move any faster than it’s allowed. I think ReplicatorG applies those limits when it converts human-readable G-Code into binary Sanguino3G code.

    After installing the new Y axis stepper motor and X rod follower, I ran through another series of tests to see what was new & different. With all the mechanical binding eliminated and a decent Y motor at 800 mA, the TOM now traverses reliably up to about 5500 mm/min = 90 mm/s. The X axis loses steps dependably by 6000 mm/min = 100 mm/s with the current set to 300 mA.

    Note that this is not printing, just moving. Printing requires attention to a whole bunch of details, but first you gotta have reliable motor control.

    So I set the upper limit at 5000 mm/min and we’re both perfectly happy. Indeed, with the new Y axis motor, the mechanical resonances have vanished and it’s a much quieter machine at the normal speeds. As a confidence builder, that one will suffice for now.

    I have a 34 mm NEMA 17 in hand to replace the X axis motor; it’ll be all good.

  • What Would Barbie Pack …

    Nerf pistol on build plate
    Nerf pistol on build plate

    … if Barbie would pack heat?

    It’s actually a snub-nosed version of that, cut down by 15 mm to fit the TOM’s vertical space; the nozzle homed 3 mm above the last of the 345 layers.

    I wanted to discover three things:

    • Are there any axis skips in a 4 hour print?
    • Can ABS film + aluminum plate anchor a tall object?
    • Can I use up all the pink filament?

    Answers: no, just fine, not quite.

    I did not re-check the platform alignment after installing the new Y axis motor and fiddling a bit with the Y axis rods. Quite to my dismay, the platform was about 0.5 mm too high (crunch!), so I gave the Z axis leadscrew a mighty twist and salvaged the first layer during the Outline extrusion. Despite that, the first layer seemed to be flat within the usual 0.2 mm (eyeballometrically measuring the first infill, as the Outline was trashed) and adhesion was fine.

    The grip delaminated a bit and the butt pulled the film up, which isn’t entirely unexpected for huge objects.

    Nerf pistol grip - lifting and delamination
    Nerf pistol grip – lifting and delamination
    Nerf pistol - grip detail
    Nerf pistol – grip detail

    A better view of the grip showing the cracks:

    I enclosed the build chamber before starting this print, but the temperature still isn’t all that high in the Basement Laboratory and the plastic was barely warm when I took it out. I’m not convinced any reasonable chamber temperature will solve the problem; it may work out better to assemble large objects from thinner parts.

    This was the first full-up test of the X Rod Follower and the new Y axis stepper motor. Prior to printing this thing, I did a quick torture test (about which, more later) and dialed the motor currents back:

    • X REF = 0.63 V → 315 mA
    • Y REF = 1.76 V → 880 mA (in a 2 Ω winding)
    • Z REF = 0.54 V → 270 mA
    • A REF = 0.99 V → 450 mA (in a 2 Ω winding)

    After four hours the Y, Z, and A steppers were barely warm to the touch and a thermocouple stuck into one of the X stepper’s bolt holes reported it was 38 °C, just above barely warm. I’m adducing evidence that the MBI steppers aren’t appropriate for the TOM’s requirements and that the default current settings are much too high.

    Now, for some Nerf darts…

  • NEMA 17 Shaft Adapter

    NEMA 17 5mm to 0.1875 inch shaft adapter
    NEMA 17 5mm to 0.1875 inch shaft adapter

    The NEMA standards for stepper motors don’t specify the shaft dimensions, alas. While most NEMA 17 steppers have 5 mm shafts, the X and Y axis motors in a Thing-O-Matic have 3/16 inch shafts: MBI belt pulleys with 4.76-ish mm ID won’t fit on 5 mm OD shafts.

    (Note: the “17” in NEMA 17 means the mounting holes are on a more-or-less 1.7 inch circle. The side of the motor frame will be close, but that’s not the controlled dimension. Some relevant diagrams live there.)

    I plan to replace the Y axis stepper with a better motor (I got a set of three, one of which is now driving the stepper extruder), which means either buying a new pulley or having some Quality Shop Time. Plus, a bit more length on the Y axis shaft than what comes standard would be a Good Thing, too.

    [Update: From the motor label, not that you’ll ever find one like it…

    • 38 mm case
    • Minebea-Matsushita 17PM-K150-P1V
    • No T6824-02

    ]

    So I built an adapter from 5/16 and 3/16 rod with a setscrew to grab a flat on the stepper shaft and a pin for the torque. The larger rod turned out to be La Salle Fatigue-Proof steel, not that it matters, and the smaller rod is plain old W-1 Water Hardening Drill Rod, both from Brownell’s, a long time ago in a universe far away. You could turn and drill the adapter from a single length of 5/16 rod if you prefer, but take some care to maintain the alignment.

    A bit of lathe & Sherline CNC work:

    • Face one end of the 5/16 rod
    • Drill half an inch with a #9 drill (0.196 + runout = 5 mm)
    • Drill another quarter inch with a #12 drill (0.189 = 4.8 mm)
    • Saw off 3/4 inch, face the raw end
    • Saw & face an inch of 3/16 rod
    • Epoxy little rod in big rod, set upright, wait overnight
    • Cross drill #43 and tap 4-40 near big end
    • Cross drill #56 for 0.045 music wire pin
    • Chamfer pin hole, clean, epoxy pin in place, wait overnight
    • File two flats on 3/16 shaft for MBI pulley setscrews
    Tapping shaft adapter
    Tapping shaft adapter

    I grabbed the small rod in the vise with the large rod resting on the top of the jaws while the epoxy cured, figuring that it’d be pretty much self-aligning. Not that a few mils one way or the other will matter, as it’s driving a timing belt in a flexy machine anyway.

    Cross-drilling the pin hole required eyeballing the center of the length of 3/16 rod within the 5/16 rod. It’s not critical, but avoid missing the poor thing entirely. You want to minimize the nested length, so as to keep the adapter as short as possible, but keep at least one diameter (3/16 inch) so as to maintain alignment.

    Tapping should involve a bottoming tap, but I used what I had and it worked out OK.

    Now, one reason I was willing to do this is that the stock Y axis motor shaft was already too short. As nearly as I can tell, the TOM dimensions were set before MBI started shipping those cork sound-deadening plates, because the shaft is recessed into the pulley by about the thickness of that plate.

    The MBI pulleys are an extremely tight fit on a 3/16 inch rod, so, rather than forcing the pulley, I enlarged the hub with a #12 drill (same as in the adapter) to get another 1.5 mil of clearance; it’s now an easy slip fit on the rod.

    Drilling MBI motor pulley
    Drilling MBI motor pulley

    Anyhow, the bottom flange of the pulley is 17 mm above the ridge on the motor and this one worked out to a bit over 20. No problem, I can just lower the motor a little bit, flip the pulley over to get the setscrew end of the hub on the top, and it’ll have plenty of room. A bit more shaft is much better than not enough, sez I.

    Y axis motor shaft extension
    Y axis motor shaft extension

    The motors came from the usual eBay seller complete with a squishy silicone sound deadening panel that turned out to be exactly the right thickness, when stacked atop a cork sheet, to put the pulley where it needed to be. I cut a second cork sheet, so as to isolate the bolt heads from that acrylic body panel, and it’s all good.

    Y axis motor with shaft adapter
    Y axis motor with shaft adapter

    Now, to print a suitable test object…

  • Digital Caliper Thumbwheel Holder: Another Repair

    The digital caliper on my desk has been getting a lot of use lately and, as expected, that delicate glued repair failed.

    Well, I can fix that…

    Thumbwheel holder - installed
    Thumbwheel holder – installed

    That’s a somewhat chopped-up Version 1; as always, I must build one prototype to see how everything fits, then make a real part incorporating all the changes. The models  and code below have those changes and should print fine.

    This picture from the previous repair shows what broke and why:

    Broken Roller Mount
    Broken caliper thumb roller mount

    I removed the remainder of the arch, filed the stub square, made a bunch of tedious measurements, and wrote a chunk of OpenSCAD code to create a repair part that looks like this:

    Thumbwheel holder - build model
    Thumbwheel holder – build model

    There’s also a layout arrangement to confirm that it’ll fit the stub:

    Thumbwheel holder - fit model
    Thumbwheel holder – fit model

    And then I printed four so I could pick the best one. The horizontal hole and notch come out surprisingly well, although this thing is right down around the minimum size you’d want to print:

    Thumbwheel holders - as built
    Thumbwheel holders – as built

    The 1-72 screw threads itself into the hole without a nut; I simply match-drilled a hole in the stub under the hole in the part. Of course, that means I must fit the next part to that hole…

    I really wish I was printing with, say, black filament. Even dark green would be better. Heck, I’d go with yellow, but if I don’t get rid of this pink stuff I’ll have it forever.

    The OpenSCAD source code:

    // Digital Caliper thumbwheel holder
// Ed Nisley - KE4ZNU - Apr 2011

Build = true;						// set true to generate buildable layout

$fn = 8;							// default for holes

// Extrusion values
// Use 0 extra shells behind the perimeter
//     2 solid shells on the top & bottom

ThreadThickness = 0.33;
ThreadWT = 1.75;
ThreadWidth = ThreadThickness * ThreadWT;

HoleWindage = ThreadWidth;			// enlarge hole dia by extrusion width

Protrusion = 0.1;					// extend holes beyond surfaces for visibility

// Caliper dimensions

WheelDia = 10.0;					// thumbwheel OD
WheelRadius = WheelDia/2;
WheelMargin = 1.0;					// space around wheel
WheelRimThick = 2.5;				// subtract from repair block

ShaftDia = 2.90;					// axle between knurled wheels
ShaftRadius = ShaftDia/2;
ShaftLength = 2.7;
ShaftRetainer = 3.0;				// thickness around shaft

StubThick = 2.45;					// stub of holder on caliper head
StubLength = 5.0;					// toward caliper head
StubHeight = 6.0;					// perpendicular to caliper head
StubClearance = 0.5;				// distance to caliper frame

FrameLength = 50;					// for display only
FrameHeight = 16.0;
FrameThick = 3.0;

// Repart part dimensions

ForkLength = StubLength - StubClearance;	// toward caliper head around stub
ForkHeight = StubHeight;			// perpendicular to caliper head
ForkGap = 0.2;						// clearance to stub on all sides
ForkBladeThick = 2.0;				// on each side of stub

ShaftClearance = 0.0;				// Additional clearance around shaft
ShaftOffset = 8.5;					// Shaft center to stub

BoltHoleDia = 1.8;					// 1-72 machine screw, more or less
BoltHoleRadius = BoltHoleDia/2;

// Convenient sizes and shapes

FrameBlock = [FrameLength,FrameThick,FrameHeight];

StubBlock = [StubLength,StubThick,StubHeight];
StubMargin = [ForkGap,2*ForkGap,ForkGap];

RepairBlockLength = ForkLength + ShaftOffset;
RepairBlockThick = 2*ForkBladeThick + StubThick;
RepairBlockHeight = WheelRadius + ShaftRadius + ShaftRetainer;

RepairBlock = [RepairBlockLength,RepairBlockThick,RepairBlockHeight];

// Caliper parts to show how repair fits in

module CaliperParts() {
  union() {
	translate([0,0,-(StubClearance + FrameHeight/2)])
	  cube(FrameBlock,center=true);
	translate([-(StubLength/2 + ShaftOffset),0,(StubHeight/2)])
	  cube(StubBlock,center=true);
  }
}

// Repair block with origin below wheel shaft

module RepairPart() {

  difference() {

// Body of repair part
	union() {
	  translate([-RepairBlockLength/2,0,RepairBlockHeight/2])
		cube(RepairBlock,center=true);
	  translate([0,0,WheelRadius])
		rotate([90,0,0])
		  cylinder(r=ShaftRadius+ShaftRetainer,h=ShaftLength,center=true,$fn=12);
	}

// wheels
	translate([0,(ShaftLength + WheelRimThick)/2,WheelRadius])
	  rotate([90,0,0])
		cylinder(r=(WheelRadius + WheelMargin),h=WheelRimThick,center=true,$fn=16);
	translate([-(WheelRadius + WheelMargin)/2,
			  (ShaftLength + WheelRimThick)/2,
			  (WheelRadius - Protrusion)/2])
	  cube([(WheelRadius + WheelMargin),WheelRimThick,(WheelRadius + Protrusion)],
			center=true);
	translate([0,-(ShaftLength + WheelRimThick)/2,WheelRadius])
	  rotate([90,0,0])
		cylinder(r=(WheelRadius + WheelMargin),h=WheelRimThick,center=true,$fn=16);
	translate([-(WheelRadius + WheelMargin)/2,
			  -(ShaftLength + WheelRimThick)/2,
			  (WheelRadius - Protrusion)/2])
	  cube([(WheelRadius + WheelMargin),WheelRimThick,(WheelRadius + Protrusion)],
			center=true);

// axle clearance
	translate([0,0,WheelRadius])
	  rotate([90,0,0])
		cylinder(r=ShaftRadius,h=(ShaftLength + 2*Protrusion),center=true);
	translate([0,0,(WheelRadius - Protrusion)/2])
	  cube([ShaftDia,(ShaftLength + 2*Protrusion),(WheelRadius + Protrusion)],
		   center=true);

// stub of previous wheel holder
	translate([-(ShaftOffset + (ForkLength - ForkGap)/2 + Protrusion),
			  0,
			  (StubHeight + ForkGap - Protrusion)/2])
	  cube([(ForkLength + ForkGap + Protrusion),
		   (StubThick + 2*ForkGap),
		   (StubHeight + ForkGap + Protrusion)],
		   center=true);

// mounting screw hole
	translate([-(ShaftOffset + ForkLength/2),0,StubHeight/2])
	  rotate([90,0,0])
		cylinder(r=(BoltHoleDia + HoleWindage)/2,
				 h=(RepairBlockThick + 2*Protrusion),
				 center=true,$fn=6);
  }
}

// Build it!

if (!Build) {
  CaliperParts();
  RepairPart();
}

if (Build) {
  translate([-RepairBlockLength/2,0,RepairBlockHeight])
	rotate([0,180,0])
	  RepairPart();
}

  • ABS Plate Film: Bottom View

    I built a quartet of very small knots to see how they’d stick to the ABS film and whether Reversal could cope with tiny things.

    Multiple knots on platform
    Multiple knots on platform

    The left rear knot lost its footings and I removed the rubble while the nozzle was busy with another knot. The top grew surprisingly well out of a tangle deposited in mid-air.

    The ABS film peeled neatly off the aluminum plate and shows what the adhesion looks like from below. The tangle is now in the right front.

    Bottom view of ABS coating with knots
    Bottom view of ABS coating with knots

    Reversal at 20 rpm, 75 ms, no early start. Still some blobbing during travel, so the reverse parameter isn’t quite large enough.

    My Shop Assistant prettied one up by cleaning off the snots, dunking it in the jar of pink goo, and applying several layers of bronze acrylic paint. Next time, we’ll use thinner goo and scuff the knot a bit before painting.

    Painted knot
    Painted knot