The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Author: Ed

DVD Coasters: Stress Cracking

A bit less than a year ago I engraved Guilloche patterns on a stack of DVDs, stuck foam on their data sides, and defined the result to be coasters:

Laser cut CDs – Foam vs MDF-cork backing – detail

Perhaps unsurprisingly, those grooves turned out to be excellent stress raisers, to the extent that the two most-used coasters (we’re not talking heavy use) have developed cracks:

Laser-engraved DVD A – stress cracks

The parallel lines are part of the logo / pattern / design printed on the label side of the disc, which seems to have wrinkled after being glued to the foam layer. The cracks radiate outward from the laser-scarred zone around the hub.

The other one is worse:

Laser-engraved DVD B – stress cracks

None of the discs glued to rigid backing plates show anything more than minor cracks, so I think a combination of stress raising and slight flexing is really bad for cheap coaster-like objects.

No great loss, easily outweighed by knowing what not to do next time …

2024-09-09
Prusa MK4 vs. PETG-CF
Flushed with success after building a Keychain Pill Tube with orange PETG, I tried dark gray carbon-fiber PETG with the same settings:

Pill Tube – first PETG-CF

In real life, it’s a much darker gray.

It’s not only furry, it’s overstuffed: the threads didn’t engage at all.

Running a few single-thread calibration squares suggested an Extrusion Multiplier around 0.6 would produce the proper thread width. Making it so and trying again worked perfectly:

Pill tube – PETG-CF adjustments

Not only did the cap screw on easily enough, the exterior finish improved and most of the stringing went away.

However, the Mighty Dragorn of Kismet (who nerd-sniped me into getting the MK4 in the first place) observed that he’d been running PETG-CF with stock PETG settings and getting good dimensional results without further tuning.

After a few more gyrations, I did what I should have done first:

Eryone PETG-CF Temperature Tower

The label on the spool suggests a 230 °C to 250 °C extrusion temperature and 235 °C seems like the sweet spot between overly stringy and terrible bridging, although I’d never expect PETG to cross that kind of gap without some support. The 35° overhangs on the left look surprisingly good at any temperature.

With that set up, running solid calibration squares showed Dragorn was right: 1.0 EM works the way you’d expect and 0.65 EM produces under-filled surfaces:

MK4 Eryone PETG-CF 1.0 0.65 EM – top

The hand-knitted surface is more visible at a more oblique angle:

MK4 Eryone PETG-CF 1.0 0.65 EM – edge

The 0.2 mm layers look about the same on both squares.

Comparing plain PETG at 1.0 EM with those:

MK4 eSun PETG 1.0 EM – Eryone PETG-CF 1.0 0.65 EM

Set up a square with walls three threads thick:

Thinwall box – 3x 0.45 mm – slicer preview

With PrusaSlicer set to produce 0.45 mm thread widths, the walls should measure exactly 1.35 mm = 3×0.45 mm thick:
- PETG = 1.30 mm (1.29 to 1.30)
- PETG-CF = 1.40 mm (1.37 to 1.40)
While I think you could tweak the EM for both materials, it’s unlikely to make any practical difference on typical objects.

So it looks like a slightly lower temperature with 1.0 EM will produce good outside dimensions for the carbon fiber filaments, while models with precise thin sections will require careful tuning.
2024-09-08
Hiatus

The process of transferring domain ownership from Network Solutions to WordPress may take five to seven days, for reasons I do not profess to understand, and may interrupt regular service.

Until that settles out, here’s a cat picture to tide you over:

Cat on patio

I suppose if you can’t see the cat or read this post, things have really fallen off the rails.

To give you an idea of why I’m doing this:

Your request has been submitted. You will receive the authorization code in email within 3 days. If you have any questions, please chat with us

Here we go …

2024-09-03
MMU3 vs. Cart Coin Calibrators
The last time around, I used Cart Coins to verify platform alignment (a.k.a. “leveling”) on the Makergear M2. The Prusa MK4 does mesh probing to ensure accurate alignment, so these new Cart Coins exercised the MMU3 and gave me some giveaways for a recent dinner:

Cart Coin – assortment

The design, such as it is, mashes a PNG found on the InterWebs with a few go-fast stripes added in LightBurn to balance the layout inside a circle:

Cart Coin layout

The motivations for LightBurn:
- It’s convenient
- TroCraft Eco is within 0.1 mm of the proper thickness
- Laser-cut coins proceed with great speed
Normally you’d export the finished layout as an SVG, but OpenSCAD ignores “holes” within shapes, so I exported it as a PNG to serve as a binary height map:
- Import the PNG into OpenSCAD using surface()
- Resize it to 20 mm wide and 1.7 mm tall
- Knock it out of a 24 mm OD × 1.6 mm tall cylinder (which is why the extra 0.1 mm)
- Add the PNG again as a separate 1.6 mm object to refill the hole
Whereupon out pops a solid model:

Cart Coin – solid model

Export that as a 3mf file to keep the two objects aligned, import it into PrusaSlicer, then get multi-material on it:

Cart Coin – PrusaSlicer layout

There’s a fourth group with different colors in hiding. I printed 12 identical coins at a time, mostly so I could keep track of what was happening, and it ended well enough.

The black coins with the translucent retina-burn orange cart look surprisingly good.

But this is way faster:

They’re the size of a US quarter, because that’s what unlocks shopping carts around here. Feel free to tweak the parameters for your locale.

The OpenSCAD source code is almost a one-liner:
```
difference() {
    cylinder(d=24.0,h=1.6);

    resize([20.0,0,1.7],auto=true)
        linear_extrude(height=1,convexity=10)
           projection(cut=true)
            surface("/mnt/bulkdata/Project Files/Prusa Mk4/Models/Cart Coin/Cart Coin layout.png",
                    center=true,invert=true);
}

    color("Black")
        resize([20.0,0,1.6],auto=true)
            linear_extrude(height=1,convexity=10)
               projection(cut=true)
                surface("/mnt/bulkdata/Project Files/Prusa Mk4/Models/Cart Coin/Cart Coin layout.png",
                        center=true,invert=true);
```
Use them responsibly, OK?
2024-09-03
Sting-Kill: Everyday Carry Tube

One of Mary’s gardening cronies suggested Sting-Kill might reduce her dramatic swelling [^1] after a bee / wasp / insect sting. Because it must be applied immediately after the sting, the swab must be on hand in the garden or on a bike ride, but the glass vial inside seem entirely too fragile to survive amid the usual clutter of a purse / pocketbook / belt pack / bike pack.

Well, I can fix that:

Pill tube – PETG default

It’s a KeyChain Pill Tube from Printables, enlarged 20% in the XY plane to fit the Sting-Kill swab, with the white applicator end fitting neatly into the domed screw-on lid for a bit of cushioning.

The solid model looks about like you’d expect:

Pill Tube – slicer preview

Despite that preview, I printed it with a brim. PETG sticks tenaciously to the Textured PEI steel sheet and a brim wasn’t really needed; just pop the parts off the platform when cool.

Somewhat to my astonishment, the threads screwed together easily, smoothed out after a few on-and-off cycles, and it’s ready for a moment we both hope will never occur.

[^1] Mary did tote an EpiPen back in the day, but a few near misses indicated she’s no longer quite as sensitized. She does swell up something powerful and we’re hoping immediately applying a Sting-Kill will help knock it down.

2024-09-02
Vole Traps: Rebaited

Although the drilled sunflower seeds worked reasonably well, various critters gnawed through the threads and escaped unharmed with the seeds. We tried gluing seeds to the trigger with good old Elmer’s Non-Toxic School Glue, only to find garden ants absolutely love the stuff.

Well, if voles like seeds, they’ll surely like nuts:

Rat traps – walnut halves

Those are rat traps (much bigger than mouse traps) with walnut halves secured to the top and bottom of the trigger with hot melt glue.

Yes, the plywood plates under the traps hold them together. There’s no reason to put fancy new traps outdoors where they succumb to weather in short order; these are veterans from previous episodes.

Having taken out two voles with sunflower seeds over the course of a week, the walnuts accounted for two more voles in three days. Mary thinks a neighbor vole needs a day to notice its buddy has gone missing, so the average pace may be a vole every other day.

Bonus: Gnawing on the nuts or trying to pull them away triggers the trap, so those walnuts are still out there.

The community gardens have enough voles to attract Red Tailed Hawks, which have started perching on fence posts and stooping on voles foolish enough to run along the paths or into grassy areas. Some gardeners seem disconcerted by the presence of such large birds in close proximity, but Mary assures them they’re helpers.

2024-08-30
Prusa MK4 Platform Protector

While cleaning the bird’s nest off the extruder, the need for a protector over the steel build plate became obvious:

Prusa MK4 – platform cover sheet

Yes, it’s laser cut, which trivialized the two little holes for the locating screws in the back.

Given the dimensions, it should take you five minutes, tops, to cut your own from some chipboard:

Prusa MK4 Platform protector sheet doodles

The blobs along the front edge mark the magnets where you could add a bit of steel to snap the protector in place, if you’re inclined to remove the plate. I just let it sit there, which seems entirely sufficient.

Long ago I made a similar corrugated cardboard sheet for the M2 that is still covering its glass platform today.

2024-08-29