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.
Making the world a better place, one piece at a time
Another two months of dots for the record:
The eyeballometric slope continues at 1 lb/month.
I started low-key upper-body strength training in June with encouraging results: my biceps no longer require exotic instrumentation for detection and my abs may soon transition from “throw pillow” to “two-pack”.
This is, however, the season of bounteous garden harvests, including delicious corn-on-the-cob and summer squash …
NYS DOT Region 8 Dutchess South recently did enough over-the-rail clearcutting to make Rt 376 bicycle-able from Red Oaks Mill to Maloney Rd!
To the best of our memories and judging from the tree stumps along the rail, it’s been a decade since DOT last clearcut that section; the Japanese Knotweed has definitely taken over since then.
Here’s what the Knotweed looked like in June, just north of Maloney Rd, after a trimming in May:
Now, it’s not nearly so snug out there:
Here’s a slide show starting with Dutchess North’s routine grass mowing in Red Oaks Mill and ending with Dutchess South’s clearcut just north of Maloney Rd:
The Wappinger Creek bridge seems to be a no man’s land between the two Residencies, but we can generally take the lane:
We hope Dutchess South’s over-the-rail maintenance will become an annual event and prevent the brush from taking over again.
Up to this point, the Sherline has been drilling 3.5 inch hard drive platters to serve as as reflecting bases for the vacuum tubes:
The CNC 3018-Pro has a work envelope large enough for CD / DVD platters, so I mashed the Sherline fixture with dimensions from the vacuum tube code, added the 3018’s T-slot spacing, and conjured a pair of fixtures for a pair of machines.
Because I expect to practice on scrap CDs and DVDs for a while:
And a 3.5 inch hard drive platter version:
The holes sit at half the 3018’s T-slot spacing (45 mm / 2), so you can nudge the fixtures to the front or rear, as you prefer.
The alignment dots & slots should help touch off the XY coordinate system on the Sherline, although it can’t reach all of a CD. Using bCNC’s video alignment on the hub hole will be much easier on the 3018.
After fiddling around with the 3018 for a while, however, the CD fixture doesn’t have many advantages over simply taping the disc to a flat platen. Obviously, you’d want a sacrificial layer for drilling, but it’s not clear the OEM motor / ER11 chuck would be up to that task.
The OpenSCAD source code as a GitHub Gist:
| // Machining fixtures for CD and hard drive platters | |
| // Ed Nisley KE4ZNU February … September 2016 | |
| // 2019-08 split from tube base models | |
| PlatterName = "CD"; // [3.5inch,CD] | |
| CNCName = "3018"; // [3018,Sherline] | |
| PlateThick = 5.0; // [5.0,10.0,15.0] | |
| RecessDepth = 4.0; // [0.0,2.0,4.0] | |
| //- Extrusion parameters must match reality! | |
| /* [Hidden] */ | |
| ThreadThick = 0.25; | |
| ThreadWidth = 0.40; | |
| HoleWindage = 0.2; | |
| Protrusion = 0.1; // make holes end cleanly | |
| inch = 25.4; | |
| function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit); | |
| module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes | |
| Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2); | |
| FixDia = Dia / cos(180/Sides); | |
| cylinder(d=(FixDia + HoleWindage),h=Height,$fn=Sides); | |
| } | |
| ID = 0; | |
| OD = 1; | |
| LENGTH = 2; | |
| //———————- | |
| // Dimensions | |
| P_NAME = 0; // platter name | |
| P_ID = 1; // … inner diameter | |
| P_OD = 2; // … outer diameter | |
| P_THICK = 3; // … thickness | |
| PlatterData = [ | |
| ["3.5inch", 25.0, 95.0, 1.75], | |
| ["CD", 15.0, 120.0, 1.20], | |
| ]; | |
| PlatterSides = 3*4*5; // polygon approximation | |
| B_NAME = 0; // machine name | |
| B_OC = 1; // … platform screw OC, use small integer for slot | |
| B_STUD = 2; // … screw OD clearance | |
| BaseData = [ | |
| ["3018", [5.0, 45.0], 6.0], // slots along X axis | |
| ["Sherline", [1.16*inch,1.16*inch], 5.0], // tooling plate | |
| ]; | |
| //———————- | |
| // Drilling fixture for disk platters | |
| module PlatterFixture(Disk,Machine) { | |
| PI = search([Disk],PlatterData,1,0)[P_NAME]; // get platter index | |
| echo(str("Platter: ",Disk)); | |
| Platter = [PlatterData[PI][P_ID], | |
| PlatterData[PI][P_OD], | |
| PlatterData[PI][P_THICK]]; | |
| BI = search([Machine],BaseData,1,0)[B_NAME]; // get base index | |
| echo(str("Machine: ",Machine)); | |
| AlignOC = IntegerMultiple(Platter[OD],10); | |
| echo(str("Align OC: ",AlignOC)); | |
| AlignSlot = [3*ThreadWidth,10.0,3*ThreadThick]; | |
| StudClear = BaseData[BI][B_STUD]; // … clearance | |
| StudOC = [IntegerMultiple(AlignOC + 2*StudClear,BaseData[BI][B_OC].x), // … screw spacing | |
| BaseData[BI][B_OC].y]; | |
| echo(str("Stud spacing: ",StudOC)); | |
| NumStuds = [2,1 + 2*floor(Platter[OD] / StudOC.y)]; // holes only along ±X edges | |
| echo(str("Stud holes: ",NumStuds)); | |
| BasePlate = [(20 + StudOC.x*ceil(Platter[OD] / StudOC.x)), | |
| (10 + AlignOC), | |
| PlateThick]; | |
| echo(str("Plate: ",BasePlate)); | |
| PlateRound = 10.0; // corner radius | |
| difference() { | |
| hull() // basic plate shape | |
| for (i=[-1,1], j=[-1,1]) | |
| translate([i*(BasePlate.x/2 – PlateRound),j*(BasePlate.y/2 – PlateRound),0]) | |
| cylinder(r=PlateRound,h=BasePlate.z,$fn=4*4); | |
| for (i=[-1,0,1], j=[-1,0,1]) // origin pips | |
| translate([i*AlignOC/2,j*AlignOC/2,BasePlate.z – 2*ThreadThick]) | |
| cylinder(d=4*ThreadWidth,h=1,$fn=6); | |
| for (i=[-1,1], j=[-1,1]) { // alignment slots | |
| translate([i*(AlignOC + AlignSlot.x)/2, | |
| j*Platter[OD]/4, | |
| (BasePlate.z – AlignSlot.z/2 + Protrusion/2)]) | |
| cube(AlignSlot + [0,0,Protrusion],center=true); | |
| translate([i*Platter[OD]/4, | |
| j*(AlignOC + AlignSlot.x)/2, | |
| (BasePlate.z – AlignSlot.z/2 + Protrusion/2)]) | |
| rotate(90) | |
| cube(AlignSlot + [0,0,Protrusion],center=true); | |
| } | |
| for (i=[-1,1], j=[-floor(NumStuds.y/2):floor(NumStuds.y/2)]) // mounting stud holes | |
| translate([i*StudOC.x/2,j*StudOC.y/2,-Protrusion]) | |
| rotate(180/6) | |
| PolyCyl(StudClear,BasePlate.z + 2*Protrusion,6); | |
| translate([0,0,-Protrusion]) // center clamp hole | |
| rotate(180/6) | |
| PolyCyl(StudClear,BasePlate.z + 2*Protrusion,6); | |
| translate([0,0,BasePlate.z – Platter[LENGTH]]) // disk locating recess | |
| rotate(180/PlatterSides) | |
| linear_extrude(height=(Platter[LENGTH] + Protrusion),convexity=2) | |
| difference() { | |
| circle(d=(Platter[OD] + HoleWindage),$fn=PlatterSides); | |
| circle(d=Platter[ID] – HoleWindage,$fn=PlatterSides); | |
| } | |
| translate([0,0,BasePlate.z – RecessDepth]) // drilling recess | |
| rotate(180/PlatterSides) | |
| linear_extrude(height=(RecessDepth + Protrusion),convexity=2) | |
| difference() { | |
| circle(d=(Platter[OD] – 10),$fn=PlatterSides); | |
| circle(d=(Platter[ID] + 10),$fn=PlatterSides); | |
| } | |
| } | |
| } | |
| //———————- | |
| // Build it | |
| PlatterFixture(PlatterName,CNCName); | |
The CAMTool V3.3 board on the CNC 3018-Pro hardwires the three DRV8825 stepper driver chips in 1:32 microstep mode by pulling all three Mode pins high. Unlike most CNC boards, it does not include jumpers to let you select different microstep modes; the designers know you want as many microsteps as you can possibly get.
As it turns out, 1:32 microstep mode requires 1600 steps for each millimeter of travel and, because GRBL tops out around 30 k step/s, the maximum speed is about 18.75 mm/s = 1125 mm/min. Which isn’t at bad, but, because I intend to use the thing for engraving, rather than the light-duty machining it’s (allegedly) capable of performing, running at somewhat higher speeds will be desirable.
For sure, a 3018-Pro does not have a physical resolution of 625 nm.
If you’re willing to settle for a mere 400 step/mm = 2.6 µm, then you can just ground the Mode 2 pin to get 1:8 microstep mode:
Rewiring the CAMTool board isn’t feasible, but hacking the DRV8825 carrier PCB doesn’t require much effort.
So, we begin.
Clamp the PCB in a vise, grab the Mode 2 pin with a needle-nose pliers, apply enough heat to melt the solder completely through the board, and yank that pin right out:
I do wonder how the layout folks managed to reverse the “N” for the Enable pin. Perhaps it’s a Cyrillic И in a dead-simple font?
With that done, add a snippet of wire from M2 to the GND pin in the opposite corner to complete the job:
Despite that picture, remember to plug the DRV8825 boards into the CAMTool V3.3 board with the heatsink downward and the twiddlepot on the top, as shown in the little instruction book you got with the hardware:
Recompute the step/mm value in 1:8 microstep mode:
400 step/mm = (200 full step/rev) × (8 microstep/full step) / (4 mm/rev)Then set the corresponding GRBL parameters:
$100=400
$101=400
$102=400The 3018-Pro should work exactly like it did before, maybe a little noisier if your ears are up to the task.
Moah Speed comes later …
The CAMTool V3.3 board dispenses with fancy USB power switching circuitry:
The NUP2201 is an ESD clamp diode / suppressor IC, which is a nice touch, but FU1, a simple 300 mA polyfuse, is the only thing standing between the USB cable and the on-board +5 V regulator. In real life, it looks like this:
It’s the little black rectangle between the USB jack and the CH340 USB-to-serial chip. The
The far end of the USB cable plugs into a Raspberry Pi, a device known for unseemly fussiness about USB power, so I unsoldered the fuse and installed a diode:
It’s a BAT54 Schottky diode, pointed toward the right to prevent current from the board getting to the Pi. Pin 2 (toward the bottom) isn’t connected to anything inside the package, either, so it’s all good.
I suppose if one were a stickler for detail, one could gimmick the diode in series with the fuse, but I figured that’s a solution for a problem well down on the probability list …
Herewith, a look at CNC 3018-Pro stepper motor current waveforms as a function of supply voltage, PWM decay mode, and motor speed.
The scope displays X and Y axis motor current at 1 A/div, with sensing through a pair of Tektronix Hall effect current probes:
The X axis driver is an unmodified DRV8825 PCB operating in default mixed-decay mode. The Y axis DRV8825 has its DECAY pin pulled high, thereby putting it in fast decay mode.
The scope timebase varies to match the programmed feed rate. Because the X and Y axes move simultaneously, each axis moves at 1/√2 the programmed speed:
G1 X10 Y10 F100 → 71 mm/min on X and Y
The motor generates minimal back EMF at slow speeds, so the winding sees nearly the full supply voltage. As described in the previous post, the basic problem arises when the current rises too fast during each PWM cycle:
V = L di/dt
di/dt = 24 V / 3 mH = 8 kA/sThe first 1:32 microstep away from 0 calls for 5% of max current = 50 mA at a 1 A peak. The DRV8825 datasheet says the PWM typically runs at 30 kHz = 33 µs/cycle, during which the current will change by 270 mA:
267 mA = 8 kA/s × 33.3 µs
Notice how the current slams to a nearly constant, much-too-high value just after the first microstep. The incorrect current level decreases with lower supply voltage, because the rate-of-change decreases and the commanded current level reaches the actual (incorrect) current sooner.
Varying the motor voltage at a constant 10 mm/min:
Note that reducing the supply voltage doesn’t change the motor winding current, because the DRV8825 controls the current during each microstep, at least to the best of its ability.
Also note that the current overshoots the target for those microsteps, even when the motor is stopped, because there’s no back EMF, so the power dissipation is too high even at rest.
Enough back EMF appears at 100 mm/min to begin tamping down the current overshoot at 24 V:
The current waveform looks good at 12 V:
The back EMF at 1000 mm/min nearly eliminates the overshoot at 24 V, with fast decay in the Y axis causing some PWM ripple:
Both decay modes look good at 12 V:
At 1500 mm/min, the highest reasonable speed for the thing, and a 24 V supply, both waveforms still look good:
However, the back EMF is now high enough to buck the 12 V supply, preventing the current from decreasing fast enough in mixed decay mode (top trace):
Tweaking the GRBL config to allow 2000 mm/min feeds shows the waveforms starting to become triangular, even at 24 V:
And a 12 V supply opposed by the back EMF simply can’t change the current fast enough to keep up with the DRV8825 microstep current levels:
Bottom line: a +12 V motor supply and DRV8825 drivers modified to run in fast decay mode look like the best setup for the 3018-Pro: good current control at low speeds with enough moxie to handle higher speeds.
I should hack the DRV8825 boards into 1:8 microstep mode to reduce the IRQ rate by a factor of four, then see what happens to the back EMF at absurd speeds.
After nigh onto 18 years, the pipe straps holding the Zzipper fairing struts to the handlebars of our Tour Easy recumbents finally shrugged off their plastic wraps:
Although they still worked, riding over broken pavement produced distinct rattles; alas, the roads around here feature plenty of broken pavement.
The solution is a rugged plastic block capped with aluminum plates to spread the clamping load:
The solid model is straightforward:
A slight bit of tinkering made the stack exactly the right height for 45 mm screws secured with nyloc nuts. No washers on either end, although that’s definitely in the nature of fine tuning.
The three sections print without support:
I reamed the smaller hole with a 3/8 inch drill to match the fairing strut rod. The as-printed larger hole fit the handlebar perfectly, although the first picture shows the tubing isn’t exactly round on the near side of the block, where it starts the outward bend toward the grips.
The cap plates cried out for CNC, but I simply traced two outlines of the block on 1/8 inch aluminum sheet, bandsawed near the line, introduced them to Mr Disk Sander for finishing & corner rounding, transfer-punched the holes from the plastic blocks, and drilled to suit:
Making two pairs of plates by hand counts as Quality Shop Time around here.
The first few rides confirm the fix: no rattles!
The OpenSCAD source code as a GitHub Gist:
| // Fairing strut mount for Tour Easy handlebars | |
| // Ed Nisley – KE4ZNU – 2019-08 | |
| Layout = "Show"; // [Show,Build,Block] | |
| Support = false; | |
| /* [Hidden] */ | |
| ThreadThick = 0.20; | |
| ThreadWidth = 0.40; | |
| HoleWindage = 0.2; | |
| Protrusion = 0.1; // make holes end cleanly | |
| function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit); | |
| ID = 0; | |
| OD = 1; | |
| LENGTH = 2; | |
| inch = 25.4; | |
| //———————- | |
| // Dimensions | |
| // Handlebar along X axis, strut along Y, Z=0 at handlebar centerline | |
| HandlebarOD = 0.875 * inch + HoleWindage; | |
| StrutOD = 0.375 * inch + HoleWindage; | |
| PlateThick = 1.0 / 16.0 * inch; | |
| WallThick = 2.0; | |
| Screw = [3.0,6.8,4.0]; // M3 OD=washer, length=nut + washers | |
| RoundRadius = IntegerMultiple(Screw[OD]/2,0.5); // corner rounding | |
| ScrewOC = [IntegerMultiple(StrutOD + 2*WallThick + Screw[ID],0.5), | |
| IntegerMultiple(HandlebarOD + 2*WallThick + Screw[ID],0.5)]; | |
| echo(str("Screw OC: ",ScrewOC)); | |
| BlockSize = [ScrewOC.x + 2*RoundRadius,ScrewOC.y + 2*RoundRadius,HandlebarOD + StrutOD + 3*WallThick]; | |
| echo(str("Block: ",BlockSize)); | |
| HandleBarOffset = WallThick + HandlebarOD/2; // block bottom to centerline | |
| StrutOffset = HandlebarOD/2 + WallThick + StrutOD/2; // handlebar centerline to strut centerline | |
| echo(str("Screw length: ",BlockSize.z + 2*PlateThick + Screw[LENGTH])); | |
| NumSides = 2*3*4; | |
| //———————- | |
| // 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, | |
| $fn=Sides); | |
| } | |
| // Basic shapes | |
| // Block with handlebar along X axis | |
| module Block() { | |
| difference() { | |
| hull() | |
| for (i=[-1,1], j=[-1,1]) | |
| translate([i*ScrewOC.x/2,j*ScrewOC.y/2,-HandleBarOffset]) | |
| cylinder(r=RoundRadius,h=BlockSize.z,$fn=NumSides); | |
| for (i=[-1,1], j=[-1,1]) | |
| translate([i*ScrewOC.x/2,j*ScrewOC.y/2,-(HandleBarOffset + Protrusion)]) | |
| PolyCyl(Screw[ID],BlockSize.z + 2*Protrusion,8); | |
| translate([-BlockSize.x,0,0]) | |
| rotate([0,90,0]) | |
| cylinder(d=HandlebarOD,h=2*BlockSize.x,$fn=NumSides); | |
| translate([0,BlockSize.y,StrutOffset]) | |
| rotate([90,0,0]) | |
| cylinder(d=StrutOD,h=2*BlockSize.y,$fn=NumSides); | |
| } | |
| if (Support) { // totally ad-hoc | |
| color("Yellow") | |
| cube(1,center=true); | |
| } | |
| } | |
| //- Build it | |
| if (Layout == "Block") | |
| Block(); | |
| if (Layout == "Show") { | |
| Block(); | |
| color("Green",0.25) | |
| translate([-BlockSize.x,0,0]) | |
| rotate([0,90,0]) | |
| cylinder(d=HandlebarOD,h=2*BlockSize.x,$fn=NumSides); | |
| color("Green",0.25) | |
| translate([0,BlockSize.y,StrutOffset]) | |
| rotate([90,0,0]) | |
| cylinder(d=StrutOD,h=2*BlockSize.y,$fn=NumSides); | |
| } | |
| if (Layout == "Build") { | |
| translate([-1.2*BlockSize.x,0,HandleBarOffset]) | |
| difference() { | |
| Block(); | |
| translate([0,0,BlockSize.z]) | |
| cube(2*BlockSize,center=true); | |
| } | |
| translate([1.2*BlockSize.x,0,StrutOD/2 + WallThick]) | |
| difference() { | |
| rotate([180,0,0]) | |
| translate([0,0,-StrutOffset]) | |
| Block(); | |
| translate([0,0,BlockSize.z]) | |
| cube(2*BlockSize,center=true); | |
| } | |
| translate([0,0,StrutOffset]) | |
| rotate([180,0,0]) | |
| intersection() { | |
| Block(); | |
| translate([0,0,StrutOffset/2]) | |
| cube([2*BlockSize.x,2*BlockSize.y,StrutOffset],center=true); | |
| } | |
| } |
The Smell of Molten Projects in the Morning 