Category: Machine Shop

Mechanical widgetry

MPCNC: Belt Tensioners

The GT2 / GT3 belt specs call for 10-ish pounds of tension, but I don’t yet have a good feel for the actual MPCNC belt tension … and it’s hard to measure in-situ. So I picked up some spring tensioners and yanked one with a luggage scale:

GT2 Belt Tensioner – 4 kg

You’re looking at 4 kg = 8-ish pounds of tension. When they’re relaxed, the arms sit at roughly right angles.

I installed them on the far end of the belts, although that’s a bit snug under the roller:

GT2 Belt Tensioner – installed

An endstop switch will eventually add some clearance and it’ll be all good.

Even though they’re neither linear nor precisely calibrated, they’ll serve as a reminder to check the tension every now and again.

Install them with the same casual disregard you reserve for fish hooks and you’ll emerge unscathed.

2017-12-02
MPCNC: Makerbot-style Endstop Switch Modification

The Protoneer CNC Shield has headers for two endstops on each axis, although they’re wired to the same Arduino input pin. I installed a pair of Makerbot-style endstops on the Y axis, plugged them in, triggered one, and … the Arduino crashed. Hard. As in, a complete loss of power and reboot.

Some fiddling around produced absolutely baffling symptoms, as I replaced each endstop board and their cables to no avail.

Perusing the schematic (the “full instructions” link is dead, of course) eventually revealed the problem:

Makerbot style endstop – schematic

Got it?

Although there’s a pullup on the COM switch terminal, the switch’s NC terminal is connected to the +5 V supply, shorting across the both resistor and the LED+resistor. With two endstops in parallel, triggering one crowbars the other’s power supply to ground. I’m sure it made sense at the time, perhaps by ensuring no possible noise source could interfere with the pullup.

The solution is simple: disconnect the NC terminal from the power supply. As it turns out, the PCB layout routes +5 V on the bottom layer, up through the via around the NC switch terminal, thence to the LED and resistor, leaving only one choice:

MPCNC – dual MG endstop hack

Yup, amputate the NC terminal and be done with it.

After that, the pullup resistor lets the endstops cooperate like you’d expect: triggering either one lights up both LEDs.

2017-11-30
MPCNC: 12 V Supply vs. Stepper Current vs. Axis Speed

The default MPCNC configuration wires the two stepper motors on each axis in series, doubling the total resistance and inductance of a single motor. The stock Automation Technology motor presents 2.8 Ω and 4.8 mH in each winding to the driver, for an L/R time constant of τ = 1.7 ms. Doubling both doesn’t change the ratio, but including the harness wiring resistance gives 1.6 ms = 9.6 mH / 6 Ω.

The default DRV8825 driver configuration uses 1:32 microstepping, which I thought was excessive. I replaced the stock RAMPS setup with a Protoneer / GRBL setup using A4988 drivers in 1:16 microstepping mode, got it configured, and made a few measurements:

MPCNC CNC Shield – Current Measurement Setup

The current probe measures the winding current in the red wire. The voltage probe at the bottom isn’t doing anything, because I ran out of hands.

Here’s a 10 mm X axis move at 3600 mm/min = 60 mm/s:

MPCNC X 10mm 60mm-s 500mA-div

The top trace shows the winding current at 500 mA/div. The bottom trace shows the voltage applied to the winding at the A4988 driver pin.

Basically, the +12 V supply doesn’t provide enough headroom to let the driver force the required current into the winding at full speed, which is why the peak current decreases as the step rate increases and the sinusoid becomes a square(-ish) wave. The applied voltage switches rapidly to maintain the proper winding current when the axis is stationary or moving slowly (where the driver’s PWM current control works fine), but turns into a square (well, rectangular) wave as the pace picks up (and the driver loses control of the current).

The motor drives a 16 tooth pulley with a 2 mm belt pitch, so each revolution moves 32 mm of belt. With 1:16 microstepping, each revolution requires 3200 = 200 full step × 16 microstep/step pulses, which works out to 100 step/mm = (3200 step/rev) / (32 mm/rev). At the commanded speed near the middle of the trace, the driver must produce 6000 step/s = 60 mm/s × 100 step/mm, so each step lasts 167 μs, about τ/10.

In round numbers, the first full cycle on the left has a 20 ms period. Each full cycle = 4 full steps = 64 microsteps, so the belt moved (60 step) / (100 step/mm) = 0.6 mm, at an (average) speed of 30 mm/s = 1800 mm/min. The current begins to fall off by the third cycle with a 12 ms period, a pace of 50 mm/s = 3000 mm/s, and pretty much falls off a cliff by 60 mm/s in the middle.

To be fair, those are aggressive speeds for milling, but lasers and 3D printers tick along pretty quickly, so they’re not unreasonable.

More study is indicated, as the saying goes …

2017-11-29
MPCNC: Endstop Mount, Now With Recess

There being nothing like a new problem to take your mind off all your old problems, now there’s a cable tie latch recess:

X min endstop – recessed cable tie latch

A sectioned view of the model shows the layout:

MPCNC MB Endstop Mount – latch recess

On the other side, a ramp helps bend the tie toward the MPCNC rail:

X min endstop – recessed strap

Which looks thusly in the realm of applied mathematics:

MPCNC MB Endstop Mount – strap recess

I’ll leave the OpenSCAD code to your imagination, because the endstop block turns out to be a bit small for the recesses. Eventually, they need a dust cover and some cleanup.

So, there!

2017-11-27

MPCNC: GRBL Configuration

This collection of GRBL settings gets the MPCNC hardware up and running:

	$$
	$0=10
	$1=255
	$2=0
	$3=2
	$4=0
	$5=0
	$6=0
	$10=1
	$11=0.010
	$12=0.002
	$13=0
	$20=0
	$21=0
	$22=1
	$23=7
	$24=500.000
	$25=1000.000
	$26=250
	$27=2.000
	$30=1000
	$31=0
	$32=0
	$100=100.000
	$101=100.000
	$102=400.000
	$110=6000.000
	$111=6000.000
	$112=3000.000
	$120=1000.000
	$121=1000.000
	$122=1000.000
	$130=650.000
	$131=475.000
	$132=100.000
	ok

view raw MPCNC-GRBL.cfg hosted with ❤ by GitHub

Conveniently, the $$ command (in the first line) produces output in exactly the format it will accept as input, so just pour the captured file into GRBL’s snout. I used ascii-xfr with a 250 ms line delay:

ascii-xfr -s -v -l 250 MPCNC-GRBL.cfg > /dev/ttyACM0

Now, to be fair, the MPCNC hasn’t yet done any useful work, but it moves.

Notes:

Setting $22=1 requires home switches to be installed and working, with $23=7 putting them on the negative end of the axes, which may not work well in practice. In particular, having the Z axis homing downward is just plain dumb.

The step/mm values in $10[012] require 1/16 microstepping with 2 mm belts on 16 tooth motor pulleys. The MPCNC’s Marlin config uses 1/32 microstepping, which doubles the step frequencies and (IMO) doesn’t provide any tangible benefit.

The speeds in $11[012]=6000 seem aggressive, although they actually work so far.

The accelerations in $12[012] may push the motors too hard with anything installed in the toolholder.

The travel limits in $13[012] depend on the rail lengths you used.

2017-11-23

Makerbot-style Endstop Power Adapter for Protoneer Arduino CNC Shield

The Protoneer Arduino CNC shield (*) has a row of 2-pin headers for bare endstop switches. Being a big fan of LED Blinkiness, I have a stock of 3-pin Makerbot-style mechanical endstops that require a +5 V connection in addition to ground and the output.

A crude-but-effective adapter consists of half a dozen header pins soldered to a length of stout copper wire, with a pigtail to a +5 V pin elsewhere on the board:

3-pin to 2-pin Endstop Power Adapter

A closer look:

3-pin to 2-pin Endstop Power Adapter – detail

The pins get trimmed on the other side of the bus wire, because they don’t go through the PCB.

Installed on the board, it doesn’t look like much:

3-pin endstop adapter on Prontoneer board

Looks like it needs either Kapton tape or epoxy, doesn’t it?

Three more endstops at the far end of the MPCNC rails (for hard limits) will fill the unused header pins.

(*) It’s significantly more expensive than the Chinese knockoffs, but in this case I cheerfully pay to support the guy: good stuff, direct from the source.

2017-11-21

Prototype Board Holder: Now With Mounting Holes and Common Board Sizes

The folks I’ve been coaching through their plotter build project showed it off at the local MiniMakerFaire this past weekend. Next time around, I’ll insist they secure their circuit boards and use good wiring techniques, so as to avoid destroying more stepper drivers.

To that end, adding mounting holes to my proto board holder seems in order:

Proto Board Holder 90x70 - Flange mounting holes - Slic3r preview — Proto Board Holder 90×70 – Flange mounting holes – Slic3r preview

The board dimensions now live in an associative array, so you just pick the board name from a Configurator drop-down list:

/* [Options] */

PCBSelect = "ArdUno"; // ["20x80","40x60","30x70","50x70","70x90","80x120","ArdDuemil","ArdMega","ArdPro","ArdUno","ProtoneerCNC"]

PCB_NAME = 0;
PCB_DIMENSION = 1;

PCBSizes = [
  ["40x60",[40,60,1.6]],
  ["30x70",[30,70,1.6]],
  ["50x70",[50,70,1.6]],
  ["20x80",[20,80,1.6]],
  ["70x90",[70,90,1.6]],
  ["80x120",[80,120,1.6]],
  ["ArdDuemil",[69,84,1.6]],
  ["ArdMega",[102,53.5,1.6]],
  ["ArdPro",[53,53.5,1.6]],
  ["ArdUno",[69,53.1,1.6]],
  ["ProtoneerCNC",[69,53.1,1.6]],
];

Which seems easier than keeping track of the dimensions in comments.

You can now put the PCB clamp screws and mounting holes on specific corners & sides, allowing oddball locations for Arduino boards with corner cutouts along the right edge:

Proto Board Holder ArdUno - Slic3r preview — Proto Board Holder ArdUno – Slic3r preview

A “selector” notation separates the hole location from the board dimensions & coordinates:

ScrewSites = [
//  [-1,1],[1,1],[1,-1],[-1,-1],        // corners
//  [-1,0],[1,0],[0,1],[0,-1]           // middles
  [-1,1],[-1,-1],[1,0]                  // Arduinos
];

Might not be most obvious way, but it works for me. Most of the time, corner clamps seem just fine, so I’m not sure adding the clamp and mounting hole locations to the dimension array makes sense.

The OpenSCAD source code as a GitHub Gist:

	// Test support frame for proto boards
	// Ed Nisley KE4ZNU – Jan 2017
	// June 2017 – Add side-mount bracket, inserts into bottom
	// 2017-11 – Selectable board sizes, chassis mounting holes

	/* [Options] */

	PCBSelect = "ArdUno"; // ["20×80","40×60","30×70","50×70","70×90","80×120","ArdDuemil","ArdMega","ArdPro","ArdUno","ProtoneerCNC"]

	Layout = "Frame"; // [Frame, Bracket]

	ClampFlange = true; // external flange

	Mounts = true; // frame to chassis screw holes

	Channel = false; // wiring channel cutout

	WasherRecess = false; // cutout around screw head

	/* [Extrusion parameters] */

	ThreadThick = 0.25; // [0.15, 0.20, 0.25]
	ThreadWidth = 0.40;

	function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);

	/* [Hidden] */

	Protrusion = 0.1;

	HoleWindage = 0.2;

	inch = 25.4;

	Tap4_40 = 0.089 * inch;
	Clear4_40 = 0.110 * inch;
	Head4_40 = 0.211 * inch;
	Head4_40Thick = 0.065 * inch;
	Nut4_40Dia = 0.228 * inch;
	Nut4_40Thick = 0.086 * inch;
	Washer4_40OD = 0.270 * inch;
	Washer4_40ID = 0.123 * inch;

	Tap6_32 = 0.106 * inch;
	Clear6_32 = 0.166 * inch;
	Head6_32 = 0.251 * inch;
	Head6_32Thick = 0.097 * inch;
	Nut6_32Dia = 0.312 * inch;
	Nut6_32Thick = 0.109 * inch;
	Washer6_32OD = 0.361 * inch;
	Washer6_32ID = 0.156 * inch;

	ID = 0;
	OD = 1;
	LENGTH = 2;

	//- PCB sizes
	// the list must contain all the selection names as above

	//* [Hidden] */
	PCB_NAME = 0;
	PCB_DIMENSION = 1;

	PCBSizes = [
	["40×60",[40,60,1.6]],
	["30×70",[30,70,1.6]],
	["50×70",[50,70,1.6]],
	["20×80",[20,80,1.6]],
	["70×90",[70,90,1.6]],
	["80×120",[80,120,1.6]],
	["ArdDuemil",[69,84,1.6]],
	["ArdMega",[102,53.5,1.6]],
	["ArdPro",[53,53.5,1.6]],
	["ArdUno",[69,53.1,1.6]],
	["ProtoneerCNC",[69,53.1,1.6]],
	];

	PCBIndex = search([PCBSelect],PCBSizes)[0];
	PCBSize = PCBSizes[PCBIndex][PCB_DIMENSION];

	//echo(str("PCB Size Table: ",PCBSizes));
	//echo(str("PCB Select: ",PCBSelect));
	//echo(str("PCB Index: ",PCBIndex));
	echo(str("PCB Size: ",PCBSize));

	/* [Sizes] */

	WallThick = 4.0; // basic frame structure

	FrameHeight = 10.0;

	/* [Hidden] */

	Insert = [3.9,4.6,5.8];

	PCBShelf = 1.0; // width of support rim under PCB

	Clearance = 1*[ThreadWidth,ThreadWidth,0]; // around PCB on all sides

	ScrewOffset = ThreadWidth + Insert[OD]/2; // beyond PCB edges
	echo(str("Screw offset: ",ScrewOffset));

	/* [Screw Selectors] */

	// ij selectors for PCB clamp screw holes: -1/0/1 = left/center/right , bottom/center/top

	ScrewSites = [
	// [-1,1],[1,1],[1,-1],[-1,-1], // corners
	// [-1,0],[1,0],[0,1],[0,-1] // middles
	[-1,1],[-1,-1],[1,0] // Arduinos
	];

	// ij selectors for frame mounting holes

	MountSites = [
	[0,-1],[0,1],
	// [-1,0],[1,0]
	];

	function ScrewAngle(ij) = (ij[0]ij[1]) ? ij[0]ij[1]*15 : ((!ij[1]) ? 30 : 0); // align screw sides

	OAHeight = FrameHeight + Clearance[2] + PCBSize[2]; // total frame height
	echo(str("OAH: ",OAHeight));

	BossOD = 2*Washer4_40OD; // make bosses oversized for washers

	FlangeExtension = 4.0 + Washer6_32OD/2 – WallThick; // beyond frame structure

	FlangeThick = IntegerMultiple(2.0,ThreadThick); // plate under frame

	Flange = PCBSize
	+ 2*[ScrewOffset,ScrewOffset,0]
	+ [BossOD,BossOD,0]
	+ [2FlangeExtension,2FlangeExtension,(FlangeThick – PCBSize[2])]
	;

	FlangeRadius = BossOD/4;

	echo(str("Flange: ",Flange));
	NumSides = 4*5;

	WireChannel = [Flange[0],15.0,3.0 + PCBSize[2]]; // ad-hoc wiring cutout
	WireChannelOffset = [
	Flange[0]/2,0,(FrameHeight + PCBSize[2] – WireChannel[2]/2)
	];

	//- Adjust hole diameter to make the size come out right

	module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes

	Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);

	FiiDia = Dia / cos(180/Sides);

	cylinder(r=(FiiDia + HoleWindage)/2,h=Height,$fn=Sides);
	}

	//- Build things

	if (Layout == "Frame")
	difference() {
	union() { // body block
	translate([0,0,OAHeight/2])
	cube(PCBSize + Clearance + [2WallThick,2WallThick,FrameHeight],center=true);

	for (ij = ScrewSites) // screw bosses
	if (ij[0] != 0 \|\| ij[1] != 0)
	translate([ij[0]*(PCBSize[0]/2 + ScrewOffset),
	ij[1]*(PCBSize[1]/2 + ScrewOffset),
	0])
	cylinder(d=BossOD,h=OAHeight,$fn=NumSides);

	if (ClampFlange) // flange for work holder & mounting screw holes
	linear_extrude(height=Flange[2])
	hull()
	for (i=[-1,1], j=[-1,1]) {
	translate([i(Flange[0]/2 – FlangeRadius),j(Flange[1]/2 – FlangeRadius)])
	circle(r=FlangeRadius,$fn=NumSides); // convenient rounding size
	}
	}

	for (ij = ScrewSites) { // screw position indeies
	if (ij[0] != 0 \|\| ij[1] != 0) {
	translate([ij[0]*(PCBSize[0]/2 + ScrewOffset),
	ij[1]*(PCBSize[1]/2 + ScrewOffset),
	-Protrusion])
	rotate(ScrewAngle(ij))
	PolyCyl(Clear4_40,(OAHeight + 2*Protrusion),6); // screw clearance holes

	translate([ij[0]*(PCBSize[0]/2 + ScrewOffset),
	ij[1]*(PCBSize[1]/2 + ScrewOffset),
	-Protrusion])
	rotate(ScrewAngle(ij))
	PolyCyl(Insert[OD],OAHeight – PCBSize[2] – 3*ThreadThick + Protrusion,6); // inserts

	if (WasherRecess)
	translate([ij[0]*(PCBSize[0]/2 + ScrewOffset),
	ij[1]*(PCBSize[1]/2 + ScrewOffset),
	OAHeight – PCBSize[2]])
	PolyCyl(1.2*Washer4_40OD,(PCBSize[2] + Protrusion),NumSides); // optional washer recess
	}
	}

	if (Mounts)
	for (ij = MountSites)
	translate([ij[0](Flange[0]/2 – Washer6_32OD/2),ij[1](Flange[1]/2 – Washer6_32OD/2),-Protrusion])
	rotate(ScrewAngle(ij))
	PolyCyl(Clear6_32,(Flange[2] + 2*Protrusion),6);

	translate([0,0,OAHeight/2]) // through hole below PCB
	cube(PCBSize – 2[PCBShelf,PCBShelf,0] + [0,0,2OAHeight],center=true);

	translate([0,0,(OAHeight – (PCBSize[2] + Clearance[2])/2 + Protrusion/2)]) // PCB pocket on top
	cube(PCBSize + Clearance + [0,0,Protrusion],center=true);

	if (Channel)
	translate(WireChannelOffset) // opening for wires from bottom side
	cube(WireChannel + [0,0,Protrusion],center=true);
	}

	// Add-on bracket to hold smaller PCB upright at edge

	PCB2Insert = [3.0,4.9,4.1];
	PCB2OC = 45.0;

	if (Layout == "Bracket")
	difference() {
	hull() // frame body block
	for (i=[-1,1]) // bosses around screws
	translate([i*(PCBSize[0]/2 + ScrewOffset),0,0])
	cylinder(r=Washer4_40OD,h=OAHeight,$fn=NumSides);

	for (i=[-1,1]) // frame screw holes
	translate([i*(PCBSize[0]/2 + ScrewOffset),0,-Protrusion])
	rotate(i180/(26))
	PolyCyl(Clear4_40,(OAHeight + 2*Protrusion),6);

	for (i=[-1,1]) // PCB insert holes
	translate([i*PCB2OC/2,(Washer4_40OD + Protrusion),OAHeight/2])
	rotate([90,0,0])
	cylinder(d=PCB2Insert[OD],h=2*(Washer4_40OD + Protrusion),$fn=6);

	}

view raw Proto Board Holder.scad hosted with ❤ by GitHub

2017-11-20

Category: Machine Shop

MPCNC: Belt Tensioners

MPCNC: Makerbot-style Endstop Switch Modification

MPCNC: 12 V Supply vs. Stepper Current vs. Axis Speed

MPCNC: Endstop Mount, Now With Recess

MPCNC: GRBL Configuration

Makerbot-style Endstop Power Adapter for Protoneer Arduino CNC Shield

Prototype Board Holder: Now With Mounting Holes and Common Board Sizes