MPCNC Vinyl Cutting: Squidwrench Logo

The Mighty Thor provided the new-ish Squidwrench logo in various digital formats, not including DXF, but dxf2gcode can process PDF files (and a few others), and the cutting / weeding / transfer ended well:

MPCNC Vinyl Cutting - Squidwrench logo on mug
MPCNC Vinyl Cutting – Squidwrench logo on mug

That’s the same 14 mil gold vinyl you saw in the Crown test.

Alas, I re-covered the pattern with the transfer film when I ran the mug through the dishwasher, in the mistaken belief the film would protect the vinyl. Come to find out the film adheres better to the vinyl than to the mug: it pulled loose during washing and peeled most of the logo off the mug.

Setting the drag knife to cut hot pink 9 mil = 0.25 mm vinyl film produced another logo:

SqWr logo - hot pink
SqWr logo – hot pink

It’s now survived several trips through the dishwasher with no protection, so I’ll call it a win.

I set dxf2gcode to use a cutting depth = 1.0 mm for about 400 g of downforce, which seems to work, although the vinyl surface showed some marks from the flat nose around the drag knife blade.

The USB camera provides a convenient way to set the “workpiece origin” before cutting:

bCNC - Video align
bCNC – Video align

Because the camera sits 130 mm beyond the blade in the +Y direction, it can’t see the swathe along the front of the MPCNC. Hard and soft limits in bCNC / GRBL keep you (well, me) from smashing the gantry into the rails, but it’s a nuisance when you forget to tape the vinyl far enough from the front.

MPCNC Vinyl Cutting: First Cuts

It somehow seemed appropriate to use the standard MPCNC Crown drawing for the first vinyl cutting test:


That’s a PNG converted from the SVG original, because WordPress regards SVG and DXF files as security risks.

Run the DXF through dxf2gcode (from the Ubuntu repository) to produce G-Code suitable for my MPCNC’s GRBL controller, tape a sheet of paper to a sacrificial acrylic sheet, fire up bCNC, set the origins, and run the G-Code:

First Paper Crown - test cut
First Paper Crown – test cut

As expected, the cut paper pulled off the acrylic, because it’s not glued down; I have some Cricut adhesive cutting mats which are definitely in the nature of fine tuning. In any event, the paper showed I could get from a DXF image to drag knife cutting action.

This being a crown, gaudy gold vinyl seemed appropriate:

First Vinyl Crown - weeding
First Vinyl Crown – weeding

The weeding process removes everything that’s not the crown; I used a razor knife to cut a square and remove the vinyl around the crown. A good needle-nose tweezer works wonders!

Apply transfer film to the weeded crown and peel it from its backing paper:

First Vinyl Crown - transfer film
First Vinyl Crown – transfer film

Stick it on something desperately in need of decoration and peel off the transfer film:

MPCNC Vinyl Cutting - crown on mug
MPCNC Vinyl Cutting – crown on mug

The tricky part is setting the drag knife cutting depth to match the vinyl sheet thickness (14 mil = 0.36 mm), so the blade cuts the vinyl without cutting through the backing paper. This seems best done with manual trial cuts on scrap vinyl, pressing the drag knife holder down firmly by hand and tweaking the depth adjustment for a clean cut.

The G-Code cuts at 400 mm/min = 6.7 mm/s, perhaps a bit on the slow side.

2N3904 and 2N3906 Transistor Assortments: Consistency Thereof

A note about building a discrete equivalent of the classic LM3909 prompted me to measure some 2N3904 and 2N3906 transistors:

ESR02 Tester - 2N3904 measurement
ESR02 Tester – 2N3904 measurement

The DC gain and VBE for each flavor look comfortingly uniform:

Transistor measurements - 2N3904 2N3906
Transistor measurements – 2N3904 2N3906

Quite unlike those Hall effect sensors, indeed.

Most of the VBE variation comes from temperature differences: re-measuring the 2N3904 transistors with VBE ≅ 672 mV put them with their compadres at 677 mV.

The 2N3906 transistors have wider gain and VBE variations, so selecting a matched pair for the LM3909 current mirror makes sense.

The sheet inside the lid collects some essential parameters for ease of reference:

            Class   Type   VCE     IC    HFE
1   2N2222    GP     NPN    40    600    100
2   2N3904    LP     NPN    40    200    100
3   2N3906    LP     PNP    40    200    100
4   2N5401    HV     PNP   150    600     60
5   2N5551    HV     NPN   160    600     80

6    A1015    OSC    PNP    50    150     70
7    C1815    OSC    NPN    50    150     70
8     C945    GP     NPN    50    150     70
9    S8050   PP AMP  NPN    40    500    120
10   S8550   PP AMP  PNP    40    500    120

11   S9012   PP AMP  PNP    40    500     64
12   S9013   PP AMP  NPN    40    500     64
13   S9014   LN LF   NPN    50    100    280
14   S9015   LN LF   PNP    50    100    200
15   S9018   VHF OSC NPN    15     50    100

You’re welcome.

Sharing the Road on Raymond Avenue: Squeeze Play

We’re riding home with groceries along Raymond Avenue, approaching the Vassar Main Gate roundabout, and, as is my custom, I’ve been pointing to the middle of the lane for maybe five seconds as I move leftward to take the lane:

Raymond Passing - Approach - 2018-10-04
Raymond Passing – Approach – 2018-10-04

The driver of HCX-1297 is having none of it:

Raymond Passing - Near Miss - 2018-10-04
Raymond Passing – Near Miss – 2018-10-04

The mirror passed maybe a foot away from my shoulder; I’d reeled my arm in as the front fender passed by.

All three traffic circles / roundabouts on Raymond neck the lane down and angle it rightward into the circle, which is supposed to “calm” traffic:

Raymond Passing - Roundabout - 2018-10-04
Raymond Passing – Roundabout – 2018-10-04

The design doesn’t allow much flinch room for cyclists and certainly isn’t calming for us.

The NYS engineer who designed the Raymond roundabouts said the whole thing was “standards compliant”, refused to go on a check ride with me to experience what it was like, and told me to detour through the Vassar campus if I felt endangered while sharing the road.

Obviously, NYS DOT personnel do not dogfood their “share the road” bicycle standards by riding bicycles.



This just in (clicky for more dots, but not clearer dots):

Spam image - xxx
Spam image – xxx

Yes, the attachment was named xxx.jpg, presumably so I wouldn’t suspect it of containing anything untoward.

The name-dropping definitely adds verisimilitude: not just Microsoft (or Micro Soft) Windows and Google, but Yahoo, too. Be still, my heart!

It’s unclear how I would contact their “fiduciary agent in LIMA PERU” by dialing a 909 area code in California or sending an email to, um,, but, hey, why not? Perhaps another version of me in a parallel universe used the Peruvian Internet?

This must be one of those scams where, if you’re bright enough to notice the problems, they won’t need to waste any time on you.

You’re welcome to my identification numbers. When you get the check, slip me maybe 100 large, preferably under the table, and we’ll call it square.


MPCNC: Drag Knife Holder Spring Constant vs. Stiction

Sliding a drag knife body in a PETG holder, even after boring the plastic to fit, shows plenty of stiction along 2 mm of travel:

MPCNC - Drag Knife Holder - spring constant
MPCNC – Drag Knife Holder – spring constant

Punching the Z axis downward in 0.5 or 1.0 mm steps produced the lower line at 210 g/mm. Dividing by three springs, each one has a 70 g/mm spring constant, which may come in handy later.

The wavy upper line shows the stiction as the Z axis drops in 0.1 mm steps. The line is eyeballometrically fit to be parallel to the “good” line, but it’s obvious you can’t depend on the Z axis value to put a repeatable force on the knife.

I cranked about a turn onto the three screws to preload the springs and ensure the disk with the knife body settles onto the bottom of the holder:

MPCNC - DW660 adapter drag knife holder - spring loaded
MPCNC – DW660 adapter drag knife holder – spring loaded

The screws are M4×0.7, so one turn should apply about 140 g of preload force to the pen holder. Re-taking a few data points with a 0.5 mm step and more attention to an accurate zero position puts the intercept at 200 g, so the screws may have been slightly tighter than I expected. Close enough, anyway.

The stiction is exquisitely sensitive to the tightness of the two DW660 mount clamp screws (on the black ring), so the orange plastic disk isn’t a rigid body. No surprise there, either.

Loosening the bored slip fit would allow more lateral motion at the tip. Perhaps top-and-bottom Delrin bushings (in a taller mount) would improve the situation? A full-on linear bearing seems excessive, even to me, particularly because I don’t want to bore out a 16 mm shaft for the blade holder.

It’s certainly Good Enough™ as-is for the purpose, as I can set the cut depth to, say, 0.5 mm to apply around 250-ish g of downforce or 1.0 mm for 350-ish g. The key point is having enough Z axis compliance to soak up small  table height variations without needing to scan and apply compensation.

Cheap Scale Calibration Check

Before doing another spring constant test with the old Harbor Freight scale, I found deployed my cheap calibration weight sets to verify it displayed the right numbers:

US-Magnum Scale - calibration check
US-Magnum Scale – calibration check

It’s spot on for all weights above 1 g, although I must tap the pan to settle on the reading from above for it get the last 0.1 g right.

Below 1 g, it’s the wrong hammer for the job; I expected no better from it.