Posts Tagged MPCNC

MPCNC: Jogging Keypad for bCNC

The bCNC G-Code sender sends jogging commands to GRBL from an ordinary numeric keypad:

MPCNC - Jogging keypad

MPCNC – Jogging keypad

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.

Jogging to align the spindle (well, a pen or drag knife) with a target using the video camera works really well:

bCNC - Video align

bCNC – Video align

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.


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Raspberry Pi vs. MicroSD-as-Disk Memory

The MPCNC has bCNC running on a Raspberry Pi, with a Samsung EVO MicroSD card serving as the “hard drive”:

Sandisk Extreme Plus vs. Samsung EVO MicroSD cards

Sandisk Extreme Plus vs. Samsung EVO MicroSD cards

The picture also shows a defunct Sandisk Extreme Plus killed by continuous video recording in my Fly6 bike camera. I later replaced the EVO with a video-rated Samsung card which has been running fine ever since, albeit with the occasional crash-and-reformat expected with “action” cameras.

With that as background, a different Samsung EVO card from the same batch has been running the MPCNC’s Raspberry Pi for about a year. Over the course of a few days last week, the RPi went from an occasional stall to a complete lockup, although waiting for minutes to hours would sometimes resolve the problem. As I’ve learned by now, it’s not a software crash, it’s the controller inside the card suffering from write amplification while trying to move data from failing sectors.

Applying f3write to the card shows the problem:

MPCNC MicroSD - f3write slowdown

MPCNC MicroSD – f3write slowdown

The write speed started out absurdly high as the card’s write cache fills, then slowed to to the flash memory’s ability to absorb data, and eventually ran out of steam during the last few files.

But, as you might not expect, f3read reported the data was fine:

sudo f3read /mnt/part
F3 read 7.0
Copyright (C) 2010 Digirati Internet LTDA.
This is free software; see the source for copying conditions.

                  SECTORS      ok/corrupted/changed/overwritten
Validating file 1.h2w ... 2097152/        0/      0/      0
Validating file 2.h2w ... 2097152/        0/      0/      0
Validating file 3.h2w ... 2097152/        0/      0/      0
Validating file 4.h2w ... 2097152/        0/      0/      0
Validating file 5.h2w ... 2097152/        0/      0/      0
Validating file 6.h2w ... 2097152/        0/      0/      0
Validating file 7.h2w ... 2097152/        0/      0/      0
Validating file 8.h2w ... 2097152/        0/      0/      0
Validating file 9.h2w ... 2097152/        0/      0/      0
Validating file 10.h2w ... 2097152/        0/      0/      0
Validating file 11.h2w ... 2097152/        0/      0/      0
Validating file 12.h2w ... 2097152/        0/      0/      0
Validating file 13.h2w ... 2097152/        0/      0/      0
Validating file 14.h2w ... 2097152/        0/      0/      0
Validating file 15.h2w ... 2097152/        0/      0/      0
Validating file 16.h2w ... 2097152/        0/      0/      0
Validating file 17.h2w ... 2097152/        0/      0/      0
Validating file 18.h2w ... 2097152/        0/      0/      0
Validating file 19.h2w ... 2097152/        0/      0/      0
Validating file 20.h2w ... 2097152/        0/      0/      0
Validating file 21.h2w ... 1322894/        0/      0/      0

  Data OK: 20.63 GB (43265934 sectors)
Data LOST: 0.00 Byte (0 sectors)
	       Corrupted: 0.00 Byte (0 sectors)
	Slightly changed: 0.00 Byte (0 sectors)
	     Overwritten: 0.00 Byte (0 sectors)
Average reading speed: 43.04 MB/s

Obviously, the card’s read speed isn’t affected by the write problems.

Assuming the actual data & programs on the card were still good, I slurped the partitions:

sudo partimage save /dev/sdf1 mpcnc_boot.gz
sudo partimage save /dev/sdf2 mpcnc_partition.gz

And wrote them back:

sudo partimage restmbr  mpcnc_boot.gz.000 
sudo partimage restore /dev/sdf1 mpcnc_boot.gz.000 
sudo partimage restore /dev/sdf2 mpcnc_partition.gz.000

Unshown: a finger fumble requiring MBR restoration.

Having forced the card controller to reallocate all the failed sectors, the card works now fine and runs at full speed again. This won’t last long, but it’ll be interesting to see how it plays out.

While I was at it, I wrote the partitions to a new-ish / unused Samsung EVO Plus card, now tucked under the MPCNC’s monitor in case of emergency.

An old SFF Optiplex with an SSD may be a better fallback.

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MPCNC: Acrylic Engraving First Light

Just to see what happened, I chucked a drag knife in the collet pen holder:

MPCNC Collet Pen Holder - drag knife

MPCNC Collet Pen Holder – drag knife

The blade, being fixed in the collet and lashed to the adapter, doesn’t rotate: it’s not behaving as a true drag knife.

Twiddling the cut depth to about 0.2 mm produced a credible Guilloché pattern in some scrap acrylic:

MPCNC Collet Pen Holder - drag engraved acrylic - 0.2 mm depth

MPCNC Collet Pen Holder – drag engraved acrylic – 0.2 mm depth

The sidelight comes from an LED flashlight off to the side, with a bit of contrast tweaking to suppress the workbench clutter.

The MPCNC’s lack of rigidity produces visible jitters in the Guilloché pattern and backlash makes the characters somewhat wobbly, but it’s OK for a large and inexpensive CNC gantry machine.

A brace of diamond-point engraving bits are making their way around the planet even as I type. A symmetric 60° point may reduce the swarf thrown out by the drag knife, although it surely won’t improve the overall jitter and backlash.



GCMC Guilloche Plot Generator

It turns out the Spirograph patterns I’d been using to wring out the MPCNC are also known as Guilloché, perhaps after the guy who invented a lathe-turning machine to engrave them. Sounds pretentious, but they still look nice:

Guilloche 591991062 - scanned

Guilloche 591991062 – scanned

With the ballpoint pen / knife collet holder in mind, I stripped the tool changes out of the Spirograph generator GCMC source code, set the “paper size” to a convenient 100 mm square, and tidied up the code a bit.

As with Spirograph patterns, changing the random number seed produces entirely different results. A collection differing in the last digit, previewed online:

Seed = 213478836:

Guilloche 213478836

Guilloche 213478836

Seed = 213478837:

Guilloche 213478837

Guilloche 213478837

Seed = 213478838:

Guilloche 213478838

Guilloche 213478838

Seed = 213478839:

Guilloche 213478839

Guilloche 213478839

They’re such unique snowflakes …

The Bash script now accepts a single parameter to force the PRNG seed to a value you presumably want to plot again, rather than just accept whatever the Gods of Cosmic Jest will pick for you.

The GCMC source code and Bash (or whatever) script feeding it as a GitHub Gist:

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MPCNC: Modified Drag Knife Adapter Spring Constant

The bars on the original MPCNC drag knife / plotter pen adapter had a 100 g/mm spring constant:

MPCNC - Plotter pen force test

MPCNC – Plotter pen force test

Making the bars slightly thicker improved their print-ability:

MPCNC knife adapter mods - OpenSCAD model

MPCNC knife adapter mods – OpenSCAD model

The reddish tint marks the new bars, with their location carefully tweaked to be coincident with the stock STL.

Shoving the pen into the scale with 0.1 mm steps produces another unnervingly linear plot:

Modified MPCNC pen adapter - Spring Constant data

Modified MPCNC pen adapter – Spring Constant data

Real plotter pens want about 20 g of force, so this isn’t the holder you’re looking for.

A bunch of plots at Z=-1.0 mm turned out well with the ballpoint pen insert, though:

MPCNC Modifed pen adapter - first plots

MPCNC Modifed pen adapter – first plots

The globs apparently come from plotting too fast for conditions; reducing the speed to 1500 mm/min works better.



MPCNC: Modified Drag Knife Adapter

A trio of Cutter Cutting Plotter Blade Holders arrived:

Collet pen holder

Collet pen holder

Despite the name, they’re not well-suited for drag knife blades, because they’re collets gripping a 2 mm shaft. The blade doesn’t rotate unless the plotter / cutter rotates the entire holder, which is actually a thing.

I got ’em because the snout of a common ball-point pen refill measures about 2 mm:

Collet pen holder - detail

Collet pen holder – detail

The glob around the tip comes from plotting too fast for conditions; about 1500 mm/min works better for continuous lines and 250 mm/min improves text.

The stock MPCNC adapter has a single recess suited for Genuine Plotter Pens, but the knurled lock ring on these cheapies sticks out far enough to make them wobbly. This being an inconvenience up with which I need not put, a few lines of OpenSCAD tweak the stock STL:

MPCNC knife adapter mods - OpenSCAD model

MPCNC knife adapter mods – OpenSCAD model

The original STL is ivory, new cuts are cyan, and additions are reddish.

The two support beams are now 1.6 mm = four thread widths, for improved slicing with a 0.35 mm nozzle and a higher spring constant.

It’s by-and-large indistinguishable from the old adapter:

MPCNC - Pen Holder Detail

MPCNC – Pen Holder Detail

Which I was using upside-down, because the flange fit better.

The MPCNC works reasonably well as a pen plotter with a genuine ballpoint pen:

MPCNC Ballpoint pen plots

MPCNC Ballpoint pen plots

The OpenSCAD source code as a GitHub Gist:



MPCNC: Re-Relocated Probe Camera

Although the camera doesn’t hit anything, it seemed entirely too exposed out in front:

MPCNC - relocated camera - front view

MPCNC – relocated camera – front view

So I moved it to the back, where I can’t see it and maybe won’t clobber it:

MPCNC Re-Relocated USB Camera

MPCNC Re-Relocated USB Camera

The camera sensor is now almost exactly aligned with the XY axes, so the goofy rotation is gone and the offsets look better:

bCNC - Rear-mount Camera Probe Config

bCNC – Rear-mount Camera Probe Config

The size of the “10 mm” inner circle at the crosshair depends on the target distance, so it’ll be smaller for surfaces clamped onto and thus rising above the table. Depending on how much that matters, I can tweak the camera focus and scale factor to make the answer come out right.

The setup at the home position looked like this from a different perspective:

MPCNC - Rear-mounted USB Camera

MPCNC – Rear-mounted USB Camera

No operational change, just a cleanup.


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