Posts Tagged CNC
[Update: It seems I interchanged “em” and “de” throughout this post. ]
Up to this point, I’ve been labeling printed parts with
emdebossed legends that look OK on the solid model:
Alas, the recessed letters become lost in their perimeter threads:
Raising the legend above the surface (“
deembossing”) works reasonably well, but raised letters would interfere with sliding the battery into the holder and tend to get lost amid the surface infill pattern.
The blindingly obvious solution, after far too long, raises the letters above a frame embossed into the surface:
Which looks OK in the real world, too:
The frame is one thread deep and the legend is one thread tall, putting the letters flush with the surrounding surface and allowing the battery to slide smoothly.
The legend on the bottom surface shows even more improvement:
An OpenSCAD program can’t get the size of a rendered text string, so the fixed-size frame must surround the largest possible text, which isn’t much of a problem for my simple needs.
Adapting the NP-BX1 battery holder to use SMT pogo pins worked well:
The next step is to add sockets for those 14 AWG wires:
Start by reaming / hand-drilling all the holes to their nominal size and cleaning out the pogo pin pocket.
Solder wires to the pogo pins and thread them through the holder and lid:
That’s nice, floppy silicone-insulated 24 AWG wire, which may be a bit too thick for this purpose.
The pogo pins will, ideally, seat with the end of the body flush at the holder wall. Make it so:
Dress the wires neatly into their pocket:
Butter the bottom of the lid with epoxy, clamp in place, set it up for curing, then fill the recess:
While it’s curing, make a soldering fixture for the 14 AWG wires:
The holes are on 5 mm centers, in the expectation other battery holders will need different spacing.
Solder it up and stick the wires into the base:
Jam a battery in and It Just Works™:
- Green = supply current at 20 mA/div
- Yellow = LED driver transistor base voltage
- Purple = other transistor collector voltage
- White = base – collector voltage = capacitor voltage
The measurement setup was a bit of a hairball:
For completeness, here’s the schematic-and-layout diagram behind the circuitry:
I love it when a plan comes together!
The OpenSCAD source code as a GitHub Gist:
The pips are 100 mm apart at (-50,-50) and (+50,50). Astonishingly, the laser aligner batteries are in fine shape.
I should have protected the platter before drilling all those holes:
All’s well that ends well:
It looks even better in the dark, although you’d never know it from this picture:
I wish I could engrave those patterns on already-drilled platters, but dragging a diamond point into a hole can’t possibly end well. I could deploy the Tiny Sandblaster with a vinyl mask, if I had enough artistic eptitude to lay out a good-looking mask.
The old Atom running LinuxCNC for the Sherline finally stopped booting, so I popped the Optiplex 760 off the stack and did a live-USB trial run. The latency / jitter worked out around 25 µs, slightly worse than before, but still Good Enough, and the StepConf utility coerced the motors into working OK.
What didn’t work was the old Eagle-to-HAL code defining the Logitch Gamepad as a Joggy Thing to allow smooth joystick jog control. Well, stuff changes over the course of eight years, but, in this case, the fix turned out to be a one-liner: the
probe_parport module isn’t needed nowadays.
With that out of the way, it runs fine:
The INI and HAL files defining the Sherline configuration as a GitHub Gist:
Unlike the keypads on my streaming radio players, this one requires no configuration at all, because bCNC regards it as just another keyboard input. The catch: you must select any screen element other than a text entry field to have bCNC recognize the keystrokes as “not text”.
You would get the same results from the numeric keys on the right side of a full-size / 104-key plank. I’m using a small “tenkeyless” keyboard, which means I can put the keypad wherever it’s easiest to reach while tweaking the MPCNC.
The ÷10 and ×10 keys along the top row alter the step size by factors of ten, which is pretty much what you need: jog to within a big step of the target, drop to the next lower decade, jog a few more times, maybe drop another decade, jog once, and you’re as close as you need to be with an MPCNC. The -1 and +1 keys aren’t as useful, at least to me: changing from 5 mm to 4 mm or 6 mm doesn’t make much difference.
GRBL and bCNC don’t do smooth jogging and the discrete steps aren’t as nifty as the Joggy Thing with LinuxCNC, but it gets the job done.
Drilling a pair of holes into a length of ground steel shaft turned it into a holder for a Sakura Micron pen:
The aluminum ring epoxied to the top keeps it from falling completely through the linear bearing.
The hole sizes are the nearest inch drills matching the pen’s hard metric sizes:
While I was at the lathe, I turned another layer of epoxy on the printed holder down to a consistent 11.95+ OD. It fits the bearing nearly as well as the steel shaft, although it’s not quite as smooth.
The steel version weighs about 20 g with the pen, so it applies about the same downforce on the pen nib as the HP 7475A plotter. The force varies from about 19 g as the Z axis moves upward to 23 g as it move downward, so the stiction amounts to less than 10% of the weight:
However, the more I ponder this setup, the less I like it.
When the Z-axis moves downward and the nib hits the paper, it must decelerate the weight of the pen + holder + ballast within a fraction of a millimeter, without crushing the nib. If the pen moves downward at 3000 mm/min = 50 mm/s, stopping in 0.3 mm requires an acceleration of 4.2 m/s² and a 20 g = 2/3 oz mass will apply 0.08 N = 0.3 oz to the nib. Seems survivable, but smashing the tip a few hundred times while drawing the legends can’t possibly be good for it.
Also, the tool length probe switch trips at 60 (-ish) g, which means the pen can’t activate the switch. Adding a manual latch seems absurd, but you can get used to anything if you do it enough.
Adding a lock screw to the camera mount stabilized the camera-to-spindle offset enough to make calibration meaningful. Mark the spot directly under the camera:
Then mark the spot directly under the spindle, perhaps by poking a small cutter into the tape, measure the XY distances between the two center points, and use bCNC’s camera registration process to set the camera offset.
With those numbers in place, switching to the tool view (the green button with the end mill to the right in the ribbon bar) puts the camera at the spindle location:
The view from outside shows the relation between those two pieces of tape:
Now I can align the camera view to a fixture position and be (reasonably) sure the spindle will automagically align to the same XY coordinate when I switch to the “tool” view. Seems to work well in preliminary tests, anyhow.