Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Although small power diodes make fine flyback diodes for relays, the motor can draw several amps during the startup pulse, which will be a bit out of spec for the usual 1N4007-class diodes. Pressing an old 5 A / 200 V stud diode into service produces the ungainly black-and-blue lump eating the end of the green wire:
Motor flyback diode – installed
For completeness, here’s the entire AC line interface part of the schematic:
AC Power Interface
The diode’s 200 V limit should suffice, even for cold starts at high line peaks, but, when you build this with new parts, get something rated a bit higher, OK?
A Circuit Cellar reader sent me a lengthy note describing his approach to slow-motion AC motor drives, designed for an already ancient truck mounted radar antenna back in 1972-ish, that prompted me to try it his way.
The general idea is to pulse the motor at full current for half a power line cycle with an SCR (rather than a triac) at a variable pulse repetition rate: the high current pulse ensures that the motor will start turning and the variable repetition frequency determines the average speed. As he puts it, the motor will give off a distinct tick at very low speeds and the maximum speed will depend on how the motor reacts to half-wave drive.
Note that this is not the chopped-current approach to speed control: the SCR always begins conducting at the first positive-going 0 V crossing after the command and continues until the motor current drops to zero. There are no sharp edges generating high-pitched acoustic noise and EMI: silence is golden.
The existing speed control circuitry limits the peak current and assumes that the motor trundles along more-or-less steadily. That won’t be the case when it’s coasting between discontinuous current pulses.
When I first looked at running the motor on DC, these measurements showed the expected relationship:
Kenmore Model 158 AC Motor on DC – Loaded and Unloaded RPM vs Voltage
Eyeballometrically, the slowest useful speed will be 2 stitch/s = 120 shaft RPM = 1300 motor RPM. At that speed, under minimal load, the motor runs on about 20 V and draws 550 mA. At that current, the 40 Ω winding drops 22 V, which we’ll define as “about 20 V” for this discussion, so the back EMF amounts to pretty nearly zilch.
That’s what you’d expect for the fraction of a second while the motor comes up to full speed, but in this case it never reaches full speed, so the motor current during the pulses will be limited only by the winding resistance. At the 200 V peak I’ve been using for the high-line condition, that’s about 5 A peak, although I’d expect 4 A to be more typical.
So, in order to make this work:
the optocoupler driving the base needs more current
the differential amp from the Hall effect sensor needs less gain
Given the ease with which I’ve pushed the hulking ET227 transistor out of its SOA, the motor definitely needs a flyback diode to direct the winding current away from the collector as the transistor shut off at the end of the pulse. Because it’s running from full-wave rectified AC, the winding current never drops to zero: there will definitely be enough current to wreck the transistor.
The firmware needs reworking to produce discrete pulses at a regular pace, rather than slowly adjusting the current over time, but that’s a simple matter of software…
Unfortunately, the smooth interior of the temple spring pocket and the smooth exterior of the hinge plate didn’t provide enough mechanical lock for the epoxy; the pieces pulled apart after a week.
So I put a stake in its heart:
Eyeglass temple – tapered pin
That’s a tapered brass pin from the Box o’ Clock Parts, buttered up with a dab of epoxy, then shoved firmly into a 41 mil (#59) hole drilled through the pocket and the edge of the hinge plate.
Fast-forward overnight, apply a Dremel grinding bit, and it looks passable:
The replacement probe has a woven metal jacket that’s allegedly more rugged than the original plastic, but I think the main difference comes from the additional strain relief at the end of the probe:
Kitchen thermometer – new probe
That still looks abrupt to me, so I wrapped a silicone tape snippet around the joint:
Kitchen thermometer – new strain relief
Probably not food-safe, definitely butt-ugly, but I don’t want to replace the probe again for a long time.
FWIW, although the probe description says it’s compatible with Taylor 1970N thermometers and doesn’t mention the 1478 we have, the 2.5 mm plug fits (no suprise there) and the display shows appropriate temperatures; it seems no less accurate than the original probe.
A surplus haul of 24 V / 150 mA white LED panels arrived:
LED Panel – 24 V 150 mA
I wired a pair to a 24 V wall wart and stuck them under the M2’s bridge supporting the X stage:
LED Panel – on M2 Gantry
I thought about epoxying them in place to get better heatsinking to the metal bridge. The ever-trustworthy description said the big copper baseplate meant the panels didn’t need any heatsinking, so I used tapeless sticky and will hope for the best. Should the sticky give out, then I’ll use epoxy.
They’re much better than the previous white LED strip, although it’s tough to tell in the pictures. The chain mail armor appears under the new lights; some older pictures will creep in from time to time.
Just for completeness, here’s how the MAKE Magazine 2015 Test Objects came out on my somewhat modified MakerGear M2. I ignored the instructions, lumped all the models together, sliced ’em with my ordinary Slic3r settings, and printed the entire lot in one go:
MAKE Magazone 2015 Test Objects – on platform
Some details…
There’s no point in showing the Dimension Accuracy Tower-of-Hanoi (hiding behind the smokestack), as it looks exactly like it should. The 20 mm diameter platter came out at 19.7 ± 0.05 mm in both X and Y, so that’s a score of 2 or 3. It’s exactly the same along both axes, both diagonals, and, in fact, all the way around, within ±0.07 mm tolerance. In fact, all the layers worked out about that way; it’s consistently a bit too small. That’s what I’d expect for an uncalibrated model.
The Bridging Performance lattice gets a 5, with all the bars having dead-flat perimeters and no dropped infill. That would be a 1 if “dropped” should be “drooped”; I have no idea which is correct or exactly what they mean, but I have seen bridge threads drop off the sides, so I’ll assume it means what it says.
The front view shows the first bridging layer getting droopy under the longer bars, as you’d expect:
Bridging – front
All those drooping threads remain above the 2 mm tolerance, assuming that’s what they intended.
The bottom view shows the loose strands below the bars:
Bridging – bottom
The Overhang Performance arch gets a 5, because the top surface finish remains pretty much the same from 30° through 70° overhang:
Overhang – upper
Underneath, things look weirder:
Overhang – lower
I think the oddness on the left (the underside of the 30° section) is due to interference from the Fine Positive Space Features spire array; the nozzle came directly from there. The 70° overhang looks ugly, but I wouldn’t have imagined that would work at all, let alone as well as it did.
The Negative Space Tolerance block weighs in at 2, as the pins with 0.6 and 0.5 mm clearance pushed out with finger pressure. The 0.3 and 0.4 mm clearance pins have air nearly all the way around, but would require a sharp rap from a mallet. The 0.2 mm pin remains firmly stuck:
Negative Space Tolerance
I don’t know how to judge the Fine Positive Space Features bed-o’-nails:
Fine Positive
I think it’s either a 2 or a 3, but opinions will certainly differ. Hot off the platform, five of the nine spires completed successfully. Three other got almost done, but broke off in handling. The collection of drool on the left-middle spire seems to be from the uncompleted spires in the foreground; I think there just wasn’t enough adhesion to hold them together. The perimeters ran at 50 mm/s and the infill at 150 mm/s, because it’s printed with everything else, so it wasn’t done with the delicacy it would get in isolation.
Both Mechanical Resonance in XY boxes look fine to me:
XY Resonance – notch
The ripples are visible, but barely perceptible to the thumbnail. The Rules call for 0 or 2, I’d give it a 1: if those ripples pose a problem, then sheesh you’re using the wrong process.
Also, the perimeters ran at 50 mm/s perimeter and the thick walls got 150 mm/s infill.
A corner of the single-wall box looks about the same as the corresponding point on the 1 mm box (which isn’t shown):
XY Resonance – corner
I think the Mechanical Resonance in Z smokestack gets a 1 (the Rules allow either 0 or 2); I stopped it after 100 mm, because bedtime. The bottom section shows the influence of all the other stuff going on around it:
Z Resonance – lower
That’s not a missed step over there on the far left: it lines up with the bottom bar of the adjacent Bridging Performance lattice. The next glitch lines up with the top of the Negative Space block. And so forth and so on.
The top, done all by itself at 11 mm/s, shows some misalignment:
Z Resonance – upper
Each layer took 15 seconds, so I suspect it’d look better with more cooling.
So, using ordinary default settings for everything and with all the handwaving in mind, I’ll call the total score 19-ish of a possible 29. The M2 would definitely do better on individual objects sliced with carefully hand-tuned parameters after considerable iteration; this is its ordinary, day-in-and-day-out performance on crazy models that I’d never attempt without tweaking.
The score would be much much much higher if I judged it with criteria similar to what I see applied to some of the Thingiverse groupings.
The M2 works well for me, anyhow.
For reference, here’s the current Slic3r configuration:
# generated by Slic3r 1.2.1 on Sun Dec 7 12:19:19 2014
avoid_crossing_perimeters = 0
bed_shape = -100x-125,100x-125,100x125,-100x125
bed_temperature = 70
bottom_solid_layers = 3
bridge_acceleration = 0
bridge_fan_speed = 100
bridge_flow_ratio = 1
bridge_speed = 150
brim_width = 0
complete_objects = 0
cooling = 1
default_acceleration = 0
disable_fan_first_layers = 1
dont_support_bridges = 1
duplicate_distance = 6
end_gcode = ;-- Slic3r End G-Code for M2 starts --\n; Ed Nisley KE4NZU - 15 November 2013\nM104 S0 ; drop extruder temperature\nM140 S0 ; drop bed temperature\nM106 S0 ; bed fan off\nG1 Z160 F2000 ; lower bed\nG1 X130 Y125 F30000 ; nozzle to right, bed front\nM84 ; disable motors\n;-- Slic3r End G-Code ends --
external_perimeter_extrusion_width = 0
external_perimeter_speed = 50
external_perimeters_first = 0
extra_perimeters = 1
extruder_clearance_height = 25
extruder_clearance_radius = 15
extruder_offset = 0x0
extrusion_axis = E
extrusion_multiplier = 1.07
extrusion_width = 0.4
fan_always_on = 0
fan_below_layer_time = 30
filament_diameter = 1.72
fill_angle = 45
fill_density = 20%
fill_pattern = 3dhoneycomb
first_layer_acceleration = 0
first_layer_bed_temperature = 70
first_layer_extrusion_width = 0.4
first_layer_height = 100%
first_layer_speed = 25
first_layer_temperature = 175
gap_fill_speed = 50
gcode_arcs = 0
gcode_comments = 0
gcode_flavor = reprap
infill_acceleration = 0
infill_every_layers = 2
infill_extruder = 1
infill_extrusion_width = 0
infill_first = 1
infill_only_where_needed = 0
infill_speed = 150
interface_shells = 0
layer_gcode =
layer_height = 0.2
max_fan_speed = 100
min_fan_speed = 75
min_print_speed = 4
min_skirt_length = 15
notes =
nozzle_diameter = 0.35
only_retract_when_crossing_perimeters = 1
ooze_prevention = 0
output_filename_format = [input_filename_base].gcode
overhangs = 1
perimeter_acceleration = 0
perimeter_extruder = 1
perimeter_extrusion_width = 0.4
perimeter_speed = 150
perimeters = 2
post_process =
raft_layers = 0
resolution = 0.01
retract_before_travel = 1
retract_layer_change = 0
retract_length = 1
retract_length_toolchange = 5
retract_lift = 0
retract_restart_extra = 0
retract_restart_extra_toolchange = 0
retract_speed = 60
seam_position = nearest
skirt_distance = 3
skirt_height = 1
skirts = 3
slowdown_below_layer_time = 10
small_perimeter_speed = 50
solid_fill_pattern = rectilinear
solid_infill_below_area = 5
solid_infill_every_layers = 0
solid_infill_extrusion_width = 0
solid_infill_speed = 150
spiral_vase = 0
standby_temperature_delta = -5
start_gcode = ;-- Slic3r Start G-Code for M2 starts --\n; Ed Nisley KE4NZU - 15 Nov 2013\n; 28 Feb 2014 - 6 Mar 2014 - tweak Z offset June July 2014\n; Z-min switch at platform, must move nozzle to X=130 to clear\nM140 S[first_layer_bed_temperature] ; start bed heating\nG90 ; absolute coordinates\nG21 ; millimeters\nM83 ; relative extrusion distance\nG92 Z0 ; set Z to zero, wherever it might be now\nG1 Z10 F1000 ; move platform downward to clear nozzle; may crash at bottom\nG28 Y0 ; home Y to be sure of clearing probe point\nG92 Y-127 ; set origin so 0 = center of plate\nG28 X0 ; home X\nG92 X-95 ; set origin so 0 = center of plate\nG1 X130 Y0 F30000 ; move off platform to right side, center Y\nG28 Z0 ; home Z with switch near center of platform\nG92 Z-4.65 ; set origin to measured z offset\nG0 Z2.0 ; get air under switch\nG0 Y-127 F10000 ; set up for priming, zig around corner\nG0 X0 ; center X\nM109 S[first_layer_temperature] ; set extruder temperature and wait\nM190 S[first_layer_bed_temperature] ; wait for bed to finish heating\nG1 Z0.0 F500 ; put extruder at plate \nG1 E30 F300 ; prime to get pressure, generate blob\nG1 Z5 F2000 ; rise above blob\nG1 X15 Y-125 F20000 ; jerk away from blob, move over surface\nG1 Z0.0 F1000 ; dab nozzle to attach outer snot to platform\nG4 P0.5 ; pause to attach\nG1 X35 F500 ; slowly smear snot to clear nozzle\nG1 Z1.0 F2000 ; clear bed for travel\n;-- Slic3r Start G-Code ends --
support_material = 0
support_material_angle = 0
support_material_enforce_layers = 0
support_material_extruder = 1
support_material_extrusion_width = 0
support_material_interface_extruder = 1
support_material_interface_layers = 3
support_material_interface_spacing = 0
support_material_interface_speed = 100%
support_material_pattern = pillars
support_material_spacing = 2.5
support_material_speed = 150
support_material_threshold = 0
temperature = 175
thin_walls = 1
threads = 2
toolchange_gcode =
top_infill_extrusion_width = 0
top_solid_infill_speed = 50
top_solid_layers = 3
travel_speed = 250
use_firmware_retraction = 0
use_relative_e_distances = 0
vibration_limit = 0
wipe = 0
xy_size_compensation = 0
z_offset = 0
With about two meters of black PLA left on the spool, a pair of spare joiner links and a few tchotckes seemed in order:
Chain Mail Armor – spares and samples
The underside of the samples shows the bridges between the pillars and the cap layer between the sides:
Chain Mail Armor – link bottom
The bridge strands start out droopy, then pull into a more-or-less straight thread as the plastic cools and shrinks. The next layer up looks much, much better.
I can spend a long time watching the nozzle stretch threads across the chasms, putting me in a definite Channel Zero state of mind…
The Smell of Molten Projects in the Morning 