Posts Tagged MPCNC

MPCNC: Guilloche Engraving First Light

A diamond point drag engraving bit in the MPCNC scratched a suitable Guilloché pattern into a scrap hard drive platter much much better than I had any reason to expect:

MPCNC - Guilloche 835242896 - HD plattter - 0.1mm

MPCNC – Guilloche 835242896 – HD plattter – 0.1mm

That’s with a 0.1 mm cut depth, sidelit with an LED flashlight.

Feeding those nine digits into the Guilloché pattern generator script should get you the same pattern; set the paper size to 109 mm and use Pen=0 to suppress the legend.

The same pattern at 0.3 mm cut depth looks about the same:

MPCNC - Guilloche 835242896 - HD plattter - 0.3mm

MPCNC – Guilloche 835242896 – HD plattter – 0.3mm

It’s slightly more prominent in real life, but not by enough to make a big difference. I should try a graduated series of tests, of course, which will require harvesting a few more platters from dead drives.

Either side will look great under a 21HB5A tube, although the disks are fingerprint and dust magnets beyond compare.



MPCNC: Diamond Drag Engraver vs. Acrylic

Drawn at Z=-0.1 mm on scrap acrylic with the diamond engraver in the modified collet holder:

MPCNC - Diamond point - acrylic 0.1mm

MPCNC – Diamond point – acrylic 0.1mm

The badly rounded corner comes from a Z touch off in facepalm mode; the poor diamond must have been trying to dig a 2 mm trench through the acrylic.

Then again at Z=-0.5 mm:

MPCNC - Diamond point - acrylic 0.5mm

MPCNC – Diamond point – acrylic 0.5mm

At half a millimeter, the holder applies well over 100 g of downforce. There’s no way to know how much lateral force the tip applies to the holder, but it’s obvious the parallel beams on the MPCNC drag knife adapter lack lateral stability:

MPCNC knife adapter mods - OpenSCAD model

MPCNC knife adapter mods – OpenSCAD model

Bending beams still seem much better than a linear bearing, though.

Leave a comment

Modifying a 2.5 mm Collet Pen Holder for a 3 mm Diamond Engraver

Of course, the diamond engraving points have a 3 mm shaft that doesn’t fit in the 2.5 mm Collet Pen Holder, but making a hole bigger isn’t much of a problem …

Commence by drilling out the collet closer nut:

Collet Holder - closer nut drilling

Collet Holder – closer nut drilling

The hole didn’t start out on center and I didn’t improve it in the least. A touch of the lathe bit and a little file work eased off the razor edge around the snout.

The knurled ridges at the top are larger than the threaded body, which requires a shim around the threads to fit them into the lathe chuck. Start by cutting a slightly larger ID brass tube to the length of the threaded section:

Collet Holder - brass shim cutoff

Collet Holder – brass shim cutoff

I finally got a Round Tuit and ground opposing angles on the cutoff tool ends, so I can choose which side of the cut goes through first. In this case, the left side cuts cleanly and the scrap end carries the thinned slot into the chip tray.

Grab the tube in a pair of machinist vises and hacksaw a slot:

Collet Holder - brass shim slitting

Collet Holder – brass shim slitting

Apply a nibbler to embiggen the slot enough to leave an opening when it’s squashed around the threads:

Collet Holder - brass shim around threads

Collet Holder – brass shim around threads

Put a nut on the collet threads in an attempt to keep them neatly lined up while drilling:

Collet Holder - collet drilling

Collet Holder – collet drilling

Drill the hole to a bit over 3 mm in small steps, because it’s not the most stable setup you’ve ever used. Eventually, the diamond point just slips right in:

Collet Holder - 3 mm scribe test fit

Collet Holder – 3 mm scribe test fit

Reassemble in reverse order and It Just Works:

Collet Holder - finished

Collet Holder – finished

Now, to scratch up some acrylic!

, ,


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.

Leave a comment

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.


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.


1 Comment

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.