Although the bCNC GUI has conspicuous Run / Hold buttons, it’s easier to poke a physical switch when you really really need a pause in the action or have finished a (manual) tool change. Rather than the separate button box I built for the frameless MPCNC, I designed a chunky switch holder for the CNC 3018XL’s gantry plate:
The original 15 mm screws were just slightly too short, so those are 20 mm stainless SHCS with washers.
The switches come from a long-ago surplus deal and have internal green and red LEDs. Their transparent cap shows what might be white plastic underneath:
I think you could pry the cap off and tuck a printed legend inside, but appropriate coloration should suffice:
Making yellow from red and green LEDs always seems like magic; in these buttons, red + green produces a creamy white. Separately, the light looks like what you get from red & green LEDs.
The solid model shows off the recesses around the LED caps, making their tops flush with the surface to prevent inadvertent pokery:
The smaller square holes through the block may require a bit of filing, particularly in the slightly rounded corners common to 3D printing, to get a firm press fit on the switch body. The model now has slightly larger holes which may require a dab of epoxy.
A multi-pack of RepRap-style printer wiring produced the cable, intended for a stepper motor and complete with a 4-pin Dupont socket housing installed on one end. I chopped the housing down to three pins, tucked the fourth wire into a single-pin housing, and plugged them into the CAMtool V3.3 board:
The CAMtool schematic matches the default GRBL pinout, which comes as no surprise:
The color code, such as it is:
Black = common
Red = +5 V
Green = Run / Start (to match the LED)
Blue = Hold (because it’s the only color left)
The cable goes into 4 mm spiral wrap for protection & neatness, with the end hot-melt glued into the block:
The model now includes the wiring channel between the two switches, which is so obviously necessary I can’t imagine why I didn’t include it. The recess on the top edge clears the leadscrew sticking slightly out of the gantry plate.
The LEDs require ballast resistors: 120 Ω for red and 100 Ω for green, producing about 15 mA in each LED. Those are 1/8 W film resistors; I briefly considered SMD resistors, but came to my senses just in time.
A layer of black duct tape finishes the bottom sufficiently for my simple needs.
Yeah, a Portrait mode plot kinda squinches the annotations into the corners.
Rotating the coordinates to put the X axis along the length of the new platform is, of course, a simple matter of mathematics, but it’s just a whole lot easier to rearrange the hardware to make the answer come out right without fancy reprogramming.
Shut down the Raspberry Pi and turn off the power!
At the CAMtool V3.3 board:
Swap the X and Y motor cables
Move the former Y endstop switch to the X axis input
Plug the new endstop switch into the Y axis input, routing its cable across the top of the gantry
Abandon the former X axis switch and its cable in place
Modify the GRBL configuration:
$3=4 – +Y home @ gantry left, +X home @ frame front
$130=338 – X axis travel along new frame
$131=299 – Y axis travel across gantry
Tweak the bCNC config similarly, if that’s what you’re into.
Verify the new home position!
I reset the G54 coordinate system to put XY = 0 at the (new!) center of the platform, redefined G28 as the “park” position at the (new!) home pulloff position, and set G30 as the “tool change” position at the -X -Y (front right) corner of the platform, with bCNC icons to simplify moving to those points.
And then It Just Worked™:
The Spirograph patterns definitely look better in landscape mode:
I eventually turned the whole machine 90° clockwise to align the axes with the monitor, because I couldn’t handle having the X axis move front-to-back on the table and left-to-right on the screen.
The cartridge is a nice 6 mm cylinder, eminently transformable into a plotter pen:
A few minutes with a caliper provides key measurements for a snout surrounding the business end:
The green letters & numbers give the nearest drill sizes. The “T” values along the bottom are the tailstock turns (at 1.5 mm/turn) required to poke the drills to the indicated depths, eyeballed when the body just enters the hole.
Having recently decomissioned the Thing-O-Matic and harvested its organs parts, I have a vast collection of 3/8 inch = 9.52 mm shafts and matching bronze bushings:
Bronze bushings have low stiction, at least when they’re co-axial, and are much shorter than linear ball bearings.
I chopped off a 70 mm length of shaft and faced the raw end:
The other end had a maker’s logo, but I don’t recognize it:
I really wanted an 8 mm bore around the snout, but it just didn’t work out. The ring around the 7.5 mm counterbore shows where the larger drill just … stopped:
A trial fit with the pen cartridge:
The top of the shaft gets a somewhat longer knurled ring for the 3 mm SHCS holding the cartridge in place:
The screw bears on a split collar turned and drilled from a Delrin rod:
The “split” came from a simple saw cut across one side and I milled a flat spot in the knurling to seat the screw. As usual, the knurled ring got epoxied to the shaft.
The snout started as a 3/8 inch aluminum rod, drilled as shown in the sketch, with a (scant) 7.5 mm section to fit the shaft. The carbide insert left a nicely rounded shoulder that required trimming to fit snugly into the shaft:
A trial fit showed the snout was a bit too long for comfort:
Making something shorter doesn’t pose much of a challenge:
Another trial fit shows it’s spot on:
The critical part is having the snout support the plastic around the pen tip to prevent wobbulation.
Epoxy the whole thing together, add a suitable spring, tighten the screws & nuts for the reaction plate, and it’s all good. I write with about 50 g of force for these pens, so a light preload seemed in order:
If I’d weighed the full-up shaft + snout + collar + cartridge, I’d know if the Y intercept matches that weight. It seems a little lighter, but I’m not taking the thing apart to find out.
The first version of the 3D printed holder (shown above) is a straightforward modification of the LM12UU diamond drag bit holder, but, after building enough of these things, I realized the circular reaction plate should be triangular to get more clearance in front of the Z-axis stepper motor when installing & removing the holder:
It also has a recess for the serrated top of the bearing, to prevent the knurled collar from clicking annoyingly as the Z-axis rises at the end of each stroke.
The CNC 3018-Pro uses cheap & readily available parts, so extending the Y axis went smoothly:
The 2040 side rails are now 450 mm long, as is the 8 mm leadscrew. I ordered 500 mm guide rods to forestall small length mismatches, then marked them to match the rails:
Cut them off slightly beyond the mark, face the raw ends to length, drill-and-tap for M5 screws, then put a pair of just-under-50-mm stubs in the bar stockpile. They ought to come in handy for something, right?
The original side rails & guide rods were 290 (not 300!) mm long, so the table gained another 160 mm of travel for a total of 340 mm; I suppose it’s now a CNC 3034-Pro. Seeing as how it’s the only one and I don’t want to kill my snicker SEO, let’s call it a CNC 3018-ProXL or a maybe 3018-Pro34. Whatever.
The embiggened 300×340 mm platform dates back to the original 1955 kitchen: genuine Formica over plywood. It sits atop the previous 300×180 mm table, now demoted to being a riser, and a sheet of closed-cell foam, with the same 50 mm long M6 screws holding everything to T-nuts in the 3018’s original aluminum platform.
And, yes, the identical Formica underneath the machine originally covered a freestanding kitchen cabinet; I knew I kept it around for some good reason. Kinda disorienting to see a piece of the pattern moving against the same background, though.
The GRBL setup now extends the Y-axis length ($131=338) and puts the G54 coordinate at the new middle, with the Z-axis origin kissing the ball-point pen on the new surface:
G10 L2 P1 X-145 Y-169 Z-24.6
While I was at it, I set the G28 position at the far left side of the gantry, with the table sticking out to the front, and the Z axis at the top:
G28.1 X-298 Y-1 Z-1
Those are absolute machine coordinates, with Y and Z pulled off home by 1 mm. I set one of bCNC’s buttons to emit G28 and park the tool carrier over there, out of the way.
With all that prepared, a full-size Tek Circuit Computer disk plots the way it should on a sheet of Letter-size paper:
I suspect the longer rods wouldn’t work quite so well for actual milling / machining any material tougher than, say, rigid foam blocks. For engraving and pen plotting, they’re all good.
Some measurements show this countertop isn’t quite as flat as the previous one, but a pair of tweaks got it within -0.15 / +0.1 mm:
Which I defined to be Good Enough™ for use with spring-loaded implements of cutting & drawing.
The fixtures are centered at X±70.0 mm / Y=0.0 from the G54 workspace coordinate origin dead-center in the middle of the platform, with G55 centered on the HD fixture to the left and G56 on the CD fixture to the right.
So the engraving workflow amounts to homing the CNC 3018 when I turn it on, taping a platter in a fixture, selecting the corresponding WCS, loading a suitable G-Code file, and firing it off. It seems bCNC returns to G54 after completing the file, so verifying the WCS selection every time is Very Good Practice.
The friable lacquer coating on some CDs fills my world with glitter whenever I engrave a pattern on their label side. I didn’t plan on a dust shoe for this thing!
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