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.

Author: Ed

  • Thing-O-Matic Stepper Extruder: First Steps

    Over the past few weeks I’ve printed the gears and plate from TheRuttmeister’s Coloso-Gear MK5 extruder Thing and flatted the shaft on a moderately husky (but not hyperthyroid) NEMA 17 stepper motor. While tearing the Thing-O-Matic down to add thermal switches to the Extruder Head, I converted the MK5 Filament Drive into a stepper extruder. Much to my astonishment, when I plugged the cable in and fired up ReplicatorG … It Just Worked!

    Even more amazing: the first pinout arrangement turned the motor in the correct direction!

    Coloso-Gear Stepper Extruder
    Coloso-Gear Stepper Extruder

    Some nasty pincushion distortion makes the larger gear look misaligned, but it’s parallel to the mounting plate and correctly engaged with the drive gear.

    The motors arrived with short stubs of thin yellow wire on the IDC motor connectors, which I soldered directly to a much longer cable. The Parts Heap disgorged a chubby 8-conductor signal cable; I used pairs of wires for each motor connection, although one conductor would have entirely enough copper. The two cable ties around the motor prevent flexing those delicate wires as the Z stage moves.

    Two tweaks to the MK6 Stepstruder profile in thingomatic.xml produced the right answers:

    • Set motor_steps = 1456
    • Set stepspermm = 48.2

    Running the motor at 2.0 rpm for 30 sec should produce exactly 1 revolution of the big gear. I marked and counted the teeth on the larger gear as it rotated, and came up with 56 teeth. It’s a 51 tooth gear, so reducing the default 1600 steps/rev by 51/56 produces 1457. A defunct MBI stepper driver board that now only does full steps provides power; I resoldered all the chip pins and the fault isn’t due to external causes like no-lead solder.

    Then run it for 60 seconds at 2.0 rpm and it’s under by maybe 1/10 of the tooth-to-tooth spacing. Adjust 1457 x 101.9/102 = 1456. Run it for another minute and it’s spot on.

    I measured 60.45 mm for two revolutions of the big gear, so it’s 30.23 for one rev, which requires the aforementioned 1456 steps. Averaging more revolutions would yield more digits, but given the rubbery nature of molten filament, three significant figures seems entirely sufficient. I suspect this depends greatly on how deeply the extruder drive embosses the filament, so it’ll require some fine tuning.

    Back of the envelope for the DC extruder at 255 PWM: feed = 45 mm/s, 0.35 mm thickness, w/t = 1.7 = 0.56 mm width gives 6.9 mm3/s. The filament is about 2.9 mm dia = 6.6 mm3, so it passed through the extruder at a bit over 1 mm/sec. There’s some windage involved in all those numbers and the extruding rate obviously depends on the temperature.

    The stepper (from the usual eBay seller) is a Minebea 17PM-K150, which doesn’t appear in their catalog listing, so it’s likely one of their many custom motors. The stack length resembles the 17PM-K3xx series, which means roughly 1 A rated current. Setting the driver current to 500 mA (VREF = 1 V) produces enough torque that I cannot pull the filament back hard enough to stop it.

    The step rate at 2 rpm is:

    48.6 step/s = (2 rev/min) x (51/7) x (1 min/60 s) x (200 step/rev)

    At that lethargic pace, the K3xx motors have something like 0.250-0.300 N·m of torque at rated current. At half current, call it 0.100 N·m and multiply by 51/7 to get 0.700 N·m = 100 oz·in.

    The effective drive diameter is 30.23/π = 9.6 mm, so the available force on the filament is 0.7 N·m / 0.01 m = 70 N ≈ 7 kgf = 15 lb. Yeah, but that little 7-tooth gear will snap right off …

    The reversal plugin cranks the big gear backwards at 35 rpm, which works out to 850.5 step/s. That ought to work, particularly seeing as how it’s not actually pushing anything.

  • Thing-O-Matic: Flatting Motor Shafts

    The NEMA 17 steppers I picked up from eBay as part of the stepper extruder upgrade project have round shafts; that’s not surprising, as they came with pressed-on timing gear pulleys. In their new application they’ll sport plastic herringbone gears and those have setscrews.

    Herringbone gears with nut inserts
    Herringbone gears with nut inserts

    Both nuts have epoxy potting to prevent moving / rotating under duress. Remember to load the screw threads with beeswax and run it all the way through before you pot the nuts, lest the screw become one with the nut. Yes, the left gear fits a NEMA 23 stepper.

    (Those are 14-tooth gears. I’ll actually use a 7-tooth gear, but I printed a bunch of gears to get the hang of it.)

    Any time you tighten a setscrew on a motor shaft, it’ll raise a burr on the shaft. You can pull a plastic / printed gear off a ruined shaft because the burr will simply carve a gash through the plastic. A metal-hub gear or pulley will jam solid on the burr; you definitely don’t want that to happen.

    The solution, which comes standard on many motor shafts, is a flatted section where the screw can raise a burr without causing a problem. In addition, the flat prevents the screw from sliding around the shaft and producing a circular scar that makes the gear impossible to remove.

    Adding a flat requires a few minutes of Quality Shop Time, but will save you considerable hassle later on. Just Do It!

    Mummify the motor in masking tape to keep grinding grit and metallic dust out of the shaft bearings, then grab the shaft in a smooth- or soft-jaw vise. I grabbed a machinist’s vise in the bench vise, but use what you have.

    Masked motor in vise
    Masked motor in vise

    Apply a Dremel grinding stone / cutoff wheel along the shaft to produce a flat about the same width as the tip of the screw. The object of the game is to make the flat wide enough to keep the burr on the flat, but not grind half the shaft away.

    Don’t grind the shaft without clamping it, because the vibration will destroy the bearings. Clamp the shaft to stabilize it and isolate the motor, then do the grinding.

    Flatted shaft with screw
    Flatted shaft with screw

    Here’s the shaft after installing & removing the gear. Notice the burr:

    Flatted shaft with screw scar
    Flatted shaft with screw scar

    And a detail of the burr:

    Flatted shaft scar - detail
    Flatted shaft scar – detail

    It’s not like I’m over-tightening the screw, either: that’s what a hardened screw does to a soft motor shaft.

  • Building an LED Floodlight Into a Task Lamp

    Building an LED Floodlight Into a Task Lamp

    LED Gooseneck Floodlight
    LED Gooseneck Floodlight

    Eks forced me to take a pile of crap useful make-froms, including a gooseneck task lamp that was probably bolted onto a machine tool in its former life. It sported a 20 W halogen bulb, but looked to be just about exactly the right size for those LED floodlights, which is why I didn’t put up much of a fuss about taking it off his hands.

    The LED lamps are much bigger than the halogen bulb, but they fit neatly into the housing diameter. All they needed was a bit more front-to-back room, which looked a lot like a chunk of PVC pipe. The housing screws together with a 1.5 mm thread that I can’t produce on my inch lathe; I’m still not set up for thread milling. This being a low-stress application with a lamp that ought to outlast me, I figured I’d just make the belly band slip-fit the two threads, glue it in place, and move on.

    I sawed off a length of PVC pipe, faced off the ends in the lathe, then CNC milled a recess to clear the male threads on the gooseneck part (I hate precision boring in the lathe). Given the rather tenuous grasp of that 3-jaw chuck, I made two passes around the perimeter: pipe ID 52.1, thread OD 54.5, remove 1.2 mm all around, about 9 mm down.

    Milling top recess
    Milling top recess

    On the other end, the female thread ID = 52.2 and the pipe ID = 52.1, so I glued another ring of PVC pipe inside to provide enough meat to turn it down. Once again, saw off a ring, face the ends, then cut out a segment so that the OD circumference of the inner ring is just slightly smaller than the ID circumference of the outer pipe. The result looked like this:

    PVC insert sizing
    PVC insert sizing

    Apply a heat gun to the inner ring until it’s soft enough to stuff into the pipe, clamp it until it hardens, apply PVC cement, and clamp overnight. Contrary to appearances, the ends of the two pipes are flush at the surface. Once again, you cannot have too many clamps:

    Clamped PVC insert
    Clamped PVC insert

    Turning down the outside to fit the threads shows just how little meat was left on that pipe:

    Skinning down to the insert
    Skinning down to the insert

    While it was chucked up (and despite my dislike of boring) I bored a bevel to accept the LED lamp and adjusted the OD so the lamp fit snugly between the end of the belly band and the lens holder on the front of the housing:

    Floodlight in holder
    Floodlight in holder

    The switch comes from the Parts Heap. A D drill puts a slightly undersized hole that’s just right for the threaded switch; I simply turned it in by hand. A length of zip cord carries the power up the gooseneck, where various ends get soldered to the switch and lamp.

    I applied some hot-melt glue to the threads and pushed everything together:

    Finished LED Floodlight
    Finished LED Floodlight

    The glass lens on the front fits in a molded holder with an annular air gap. The LED lamp housing has all those fancy cooling fins against the inner pipe, so there’s a bit of cooling air flow around the lamp and out through the rear black section. A thermocouple reports the lamp temperature gets up around 75 °C in a 14 °C shop; a 50 °C rise might be a tad warm in the summer, but we’ll see what happens.

    The power supply came from the Parts Heap: a 12 V 1 A wall switching power supply in the shape of a wall wart. For now, the zip cord from the lamp terminates in a coaxial power jack that (amazingly enough) fits the wart’s connector, but I’ll eventually put a box in there somewhere.

    Clamped the butt end of the gooseneck to the backsplash on the countertop under the mill and It Just Works!

  • LED Floodlight Rehabilitation

    I picked up a pair of 12 V 4 W 3-LED floodlights (datasheet, newer datasheet, and catalog) with 34 degree and 24 degree beams from All Electronics, with the intent of making some task lighting fixtures for the shop. Somebody decommissioned the lamps by snipping off a pin, so they’re not immediately useful.

    The back pulls off with a bit of difficulty, after removing the two obvious screws and holding the connector body in place while pulling. I didn’t try to remove the circuit board, which would require unsoldering the clearly marked Anode and Cathode LED wires that enter from the bottom of the board.

    LED Floodlight - interior
    LED Floodlight – interior

    I plan to build these lamps right into the fixtures, so soldering a wire directly onto the pin makes sense; I expect they’ll outlast my usage and a socket won’t add any value. As an intermediate step, I soldered a short brass tube onto the pin stump:

    LED Floodlight - repaired pin
    LED Floodlight – repaired pin

    In new condition, these retail somewhere beyond $60, so cutting 6 mm from one pin shaved about fifty bucks off the price. I suspect they were extracted from somebody’s shiny new, recently abandoned, and probably foreclosed, office complex and were ruined to prevent resale-as-new. The fact that the reflectors got a bit scuffed up along the way wouldn’t help their value any, either.

    They draw 330-odd mA from a 12 V supply, run from AC or DC (either polarity), and seem to have a constant-current driver inside. I wouldn’t buy ’em new, but for eight bucks a pop they’re a pretty good deal.

  • Target Unit Pricing Amusement

    I’m not sure whether all unit prices must be converted to “per 100 count” or not, but it really doesn’t work out well for CFL bulbs (clicky for more dots):

    CFL 2-bulb pack
    CFL 2-bulb pack

    That’s $549.50 per 100 bulbs, if you can’t make it out.

    On the shelf below, however, a single-bulb pack priced out to $979 per 100 count, so the 2-pack is definitely a better deal!

  • Thing-O-Matic: Oozebane Turds

    Printing those fairing mounting plates gave me an opportunity to explore the Oozebane parameter space. I wasn’t quite sure how it would work and now I’m certain that it can’t.

    Here’s the joint at the start/end of the perimeter extrusion around one of the plates, with Oozebane set for a 4 mm early shutdown:

    Perimeter joint - Oozebane
    Perimeter joint – Oozebane

    You’re looking straight down at three edges (bottom = 2 layers, middle & top = 3 layers), but the shadow obscures the vertical faces; they’re firmly joined. The nozzle enters the picture from the left, slows and stops at the joint, then departs for another location.

    The turd appears on the far side of this picture, just above the left hole:

    Fairing mount - outside
    Fairing mount – outside

    Here’s the same joint, but with Oozebane turned off:

    Perimeter joint - normal
    Perimeter joint – normal

    Any questions?

    Ah: layer thickness 0.3 mm, w/t=1.7 → width =0.56 mm, 45 mm/s feed, 255 PWM flow.

    As nearly as I can tell, Oozebane can’t possibly work the way it’s currently defined, at least for the DC extruder on my Thing-O-Matic. The problem is that Oozebane simultaneously shuts off the extruder and slows the feed rate, but the pressure on the molten plastic inside the extruder continues to force it out at about the same rate for quite some time.

    Thus, with the feed rate reduced to some unknown (and unprogrammable) value and the flow continuing at the original rate, each thread endpoint accumulates an oversized turd.

    Maybe Oozebane works for somebody else, but a stepper extruder is the right solution…

  • Tour Easy: Zzipper Fairing Upper Mount Plates

    The stock Zzipper fairing handlebar mount consists of an aluminum bar with a plate welded to each end at more-or-less the correct angle to match the fairing curve. The plate has a 1/4 inch hole in one end, wherein a 1/4-20 nylon machine screw clamps the fairing to the plate, with a nylon washer distributing the stress. That doesn’t cope well with the vibrations caused by riding around here, let alone our summer vacation trips on crushed-stone rail trails, and the fairings tend to stress-crack at the holes.

    These 3D printed plates are just the latest in a long series of attempts to distribute the stress over a larger area. The outside view:

    Fairing mount - outside
    Fairing mount – outside

    The open hole gets another screw to hold the plates in position. The bump on the far side is an Oozebane turd, about which more later.

    The view from inside the fairing:

    Fairing mount - inside
    Fairing mount – inside

    You can’t see the layer of black foam rubber salvaged from a mouse pad between each plate and the fairing. That should prevent any local stress concentration at the screw and ease the transition to the tapered plate edges.

    The solid model looks about like you’d expect:

    Fairing Mount Plates - Upper
    Fairing Mount Plates – Upper

    The hole position depends on the fairing position, as the fairings have three holes. The pictures show the fairing on my bike; it’s in the lowest position, with the screw in the topmost hole. The OpenSCAD file has an option to put the holes where you need them.

    The plates are only 8 layers thick, printed with 4 solid layers top and bottom to eliminate any fill. You could do the same by setting the fill to 100%, I suppose. Using 4 outer shells (3 additional) makes the flanged edge nice and flat and uniform.

    The layer height is 0.33 mm, with w/t=1.7 for a width of 0.56 mm. Feed rate = 43 mm/s and flow rate = 255. DC Extruder, alas.

    Running the first layer at feed = 0.5  and flow = 0.75 produces some fluffing in the fill, but there’s no way to get a lower flow from the DC extruder motor. Flow = 0.75 corresponds to PWM=191; anything lower sometimes fails to start the motor. If it starts, it’ll run, but … that’s not dependable.

    I printed them on an aluminum plate for a nice flat bottom surface.

    The OpenSCAD source code:

    // Clamp plates for Zzipper fairing on Tour Easy recumbents
// Ed Nisley - KE4ZNU - Mar 2011

// Build with...
//	extrusion parameters matching the values below
//	4 outer shells
//	4 solid surfaces at top + bottom
//  slow feeds to ensure hole perimeters stick to fill

include </home/ed/Thing-O-Matic/lib/MCAD/boxes.scad>
include </home/ed/Thing-O-Matic/lib/MCAD/units.scad>

// Select hole layout
// The if statement seems to work only for CSG object trees
// Fortunately, I need only two different layouts...

HoleSelect = 1;						// 0 = his, 1 = hers

HolesTop 	= (0 == HoleSelect) ? [0,1,1] : [1,0,1];
HolesBottom = (0 == HoleSelect) ? [0,1,1] : [1,0,1];

// Set these to match the extrusion parameters for successful building

ThreadZ = 0.33;						// extrusion thickness
ThreadWidth = 0.57;					// extrusion width = ThreadZ x w/t

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

// Plate dimensions

HoleDia = 0.25 * inch;				// these are 1/4-20 bolt holes
HoleSpace = (1) * inch;				// center-to-center spacing
									//  usually 1 inch, but 15/16 on one bike

CornerR = 5.0;						// corner rounding

Layer1X = 90;						// against fairing surface
Layer1Y = 32;
Layer1Z = 2*ThreadZ;

Layer2Margin = 1.5;					// uncovered edge
Layer2X = Layer1X - 2*Layer2Margin;
Layer2Y = Layer1Y - 2*Layer2Margin;
Layer2Z = 3*ThreadZ;

MountX = 46.3 + HoleWindage;		// handlebar mounting bracket end plate
MountHoleSpace = 13.0;				//  end to hole center
MountY = 16.3 + HoleWindage;
MountZ = 4*ThreadZ;					// recess depth
MountCap = 3.0;						// endcap arc height
MountR = (pow(MountCap,2) + 0.25*pow(MountY,2)) / (2*MountCap);	// ... radius

Layer3Margin = 1.5;
Layer3X = Layer2X - 2*Layer3Margin;
Layer3Y = max((Layer2Y - 2*Layer3Margin),(MountY + 8*ThreadWidth));
Layer3Z = 3*ThreadZ;

PlateZ = Layer1Z + Layer2Z + Layer3Z;

// Convenience settings

BuildOffset = 3.0 + Layer1Y/2;		// build Y spacing between top & bottom plates

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

//---------------
// Create plate with selectable holes

module Plate(hs) {

  difference() {

	union() {
		translate([0,0,Layer1Z/2])
		  roundedBox([Layer1X,Layer1Y,Layer1Z],CornerR,true);
		translate([0,0,Layer1Z + Layer2Z/2])
			roundedBox([Layer2X,Layer2Y,Layer2Z],CornerR,true);
		translate([0,0,Layer1Z + Layer2Z + Layer3Z/2])
			roundedBox([Layer3X,Layer3Y,Layer3Z],CornerR,true);
	}

	if (0 != hs[0]) {
	  translate([-HoleSpace,0,PlateZ/2])
		  cylinder(r=(HoleDia + HoleWindage)/2,
					h=(PlateZ + 2*Protrusion),
					center=true,$fn=10);
	}

	if (0 != hs[1]) {
	  translate([0,0,PlateZ/2])
		  cylinder(r=(HoleDia + HoleWindage)/2,
					h=(PlateZ + 2*Protrusion),
					center=true,$fn=10);
	}

	if (0 != hs[2]) {
	  translate([HoleSpace,0,PlateZ/2])
		  cylinder(r=(HoleDia + HoleWindage)/2,
					h=(PlateZ + 2*Protrusion),
					center=true,$fn=10);
	}

  }

}

//---------------
//-- Build the things...

translate([0,BuildOffset,0]) Plate(HolesTop);

translate([0,-BuildOffset,0])
  difference() {
	Plate(HolesBottom);

	translate([-(HoleSpace + MountHoleSpace - MountX/2),0,PlateZ - MountZ/2 + Protrusion/2])
	  intersection() {
		cube([MountX,MountY,(MountZ + Protrusion)],center=true);
		union() {
		  cube([(MountX - 2*MountCap),MountY,(MountZ + Protrusion)],center=true);
		  translate([ (MountX/2 - MountR),0,0])
			cylinder(r=MountR,h=(MountZ + Protrusion),center=true);
		  translate([-(MountX/2 - MountR),0,0])
			cylinder(r=MountR,h=(MountZ + Protrusion),center=true);
		}
	  }
  }

    I loves me my Thing-O-Matic, despite its annoyances…

    [Update: Stepper extruder parameters and a tweak to make the mount plate track the hole position correctly.]