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Step2 Garden Seat: Replacement Seat

A pair of Step2 rolling garden seats (they have a new version) served in Mary’s gardens long enough to give their seat panels precarious cracks:

Step2 Seat - OEM seat
Step2 Seat – OEM seat

The underside was giving way, too:

Step2 Seat - cracks
Step2 Seat – cracks

We agreed the new seat could be much simpler, although it must still hinge upward, so I conjured a pair of hinges from the vasty digital deep:

Rolling Cart Hinges - solid model - bottom
Rolling Cart Hinges – solid model – bottom

The woodpile disgorged a slab of 1/4 inch = 6 mm plywood (used in a defunct project) of just about the right size and we agreed a few holes wouldn’t be a problem for its projected ahem use case:

Step2 Seat - assembled
Step2 Seat – assembled

The screw holes on the hinge tops will let me run machine screws all the way through, should that be necessary. So far, a quartet of self-tapping sheet metal (!) screws are holding firm.

Rolling Cart Hinges - solid model - top
Rolling Cart Hinges – solid model – top

A closer look at the hinges in real life:

Step2 Seat - top view
Step2 Seat – top view

The solid model now caps the holes; I can drill them out should the need arise.

From the bottom:

Step2 Seat - bottom view
Step2 Seat – bottom view

Three coats of white exterior paint make it blindingly bright in the sun, although we expect a week or two in the garden will knock the shine right off:

Step2 Seat - painted
Step2 Seat – painted

After the first coat, I conjured a drying rack from a bamboo skewer, a cardboard flap, and some hot-melt glue:

Step2 Seat - drying fixture
Step2 Seat – drying fixture

Three small scars on the seat bottom were deemed acceptable.

The OpenSCAD source code as a GitHub Gist:

This original doodle gives the key dimensions, apart from the rounded rear edge required so the seat can pivot vertically upward:

Cart Hinge - dimension doodle
Cart Hinge – dimension doodle

The second seat looks just like this one, so life is good …

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MPCNC: Calculating Spring Rates

Calculate the spring rates for the drag knife, diamond engraver, and collet pen holders by measuring the downforce every 0.5 mm (or so):

LM12UU Collet Pen Holder - spring rate test
LM12UU Collet Pen Holder – spring rate test

Then plotting the data points and eyeballing a straight-line curve fit:

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

Doing it on hard mode definitely has a certain old-school charm. The graph highlights mis-measured data and similar problems, because, if you don’t see a pretty nearly straight line, something’s gone awry.

But we live in the future, so there’s an easier way:

Droid48 - Spring Rate - Linear Fit coefficients
Droid48 – Spring Rate – Linear Fit coefficients

Well, OK, it’s the future as of the early 1990s, when HP introduced its HP 48 calculators. I’m using the Droid48 emulator on my ancient Google Pixel: living in the past, right here in the future.

Start by firing up the STAT library (cyan arrow, then the 5 key), selecting Fit Data … from the dropdown list, then selecting the Linear Fit model:

Droid48 - Spring Rate - Linear Fit screen
Droid48 – Spring Rate – Linear Fit screen

Then tap EDIT and enter the data in a tiny spreadsheet:

Droid48 - Spring Rate - Linear Fit data
Droid48 – Spring Rate – Linear Fit data

My default “engineering mode” numeric display format doesn’t show well on the tiny screen. Tapping the WID→ key helps a bit, but shorter numbers would be better.

With the data entered, set an X value and tap the PRED key to get the corresponding Y value:

Droid48 - Spring Rate - Linear Fit prediction
Droid48 – Spring Rate – Linear Fit prediction

Tapping the OK button puts the line’s coefficients on the stack, as shown in the first picture. Write ’em on a strip of tape, stick to the top of the holder, and it’s all good:

LM12UU Collet Pen Holder - test plot - overview
LM12UU Collet Pen Holder – test plot – overview

Works for me, anyhow.

HP still has the HP 48g manuals online. The (unofficial) HP Museum has a page on the HP 48S. More than you want to know about the 48 series.

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MPCNC: Diamond Drag Engraving Speed Tests

The MPCNC isn’t the most stable of CNC machine tools, given its large masses and 3D printed structure. My early plotting pen tests suggested speeds around 250 mm/min were appropriate:

MPCNC - GCMC Text - 250 mm-min
MPCNC – GCMC Text – 250 mm-min

Diamond drag engraving produces a thinner line and makes the wobbulations more obvious:

MPCNC Engraving Speed Test A - 600-900 mm-min
MPCNC Engraving Speed Test A – 600-900 mm-min

Another test showed similar results:

MPCNC Engraving Speed Test B - 700-900 mm-min
MPCNC Engraving Speed Test B – 700-900 mm-min

Slowing down definitely reduces the shakes:

MPCNC Engraving Speed Test B - 100-300 mm-min
MPCNC Engraving Speed Test B – 100-300 mm-min

Producing the best results takes quite a while:

MPCNC Engraving Speed Test A - 50-200 mm-min
MPCNC Engraving Speed Test A – 50-200 mm-min

Similar results on another test:

MPCNC Engraving Speed Test C - 50-150 mm-min
MPCNC Engraving Speed Test C – 50-150 mm-min

Those “mm/s” labels are typos; they should read “mm/min”. Plotting at -1.0 mm on scrap CDs and DVDs produces a downforce around 200 g.

Eyeballometrically, 100 mm/min seems fine, but 50 mm/min (I’d likely use 60 for a nice round 1 mm/s) eliminates all the shakes.

Smooth curves, like Guillloché patterns, can run much faster, because they don’t have abrupt direction changes. This 3-½ inch hard drive platter has text engraved at 100 mm/min and the pattern at 600 mm/min, both at -3.0 mm for 300 g of downforce:

MPCNC Engraving - Guilloche drive platter test
MPCNC Engraving – Guilloche drive platter test

A closer look at the text:

MPCNC Engraving - hard drive platter - detail A
MPCNC Engraving – hard drive platter – detail A

And some digits:

MPCNC Engraving - hard drive platter - detail B
MPCNC Engraving – hard drive platter – detail B

When I want to brand an engraved CD, this will suffice:

MPCNC Engraving - CD attribution text
MPCNC Engraving – CD attribution text

All in all, the MPCNC engraves much better than I expected!

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Drag Knife Blade Ejector Handle

The LM12UU drag knife holder buries the blade ejector pin deep inside the machinery:

Drag Knife - LM12UU ground shaft - assembled
Drag Knife – LM12UU ground shaft – assembled

So a handle with a pin makes sense:

LM12UU Drag Knife Ejector Pin Pusher
LM12UU Drag Knife Ejector Pin Pusher

It’s a variant Sherline tommy bar handle, so there’s not much to say about it.

The dark butt end comes from the traces of the black filament I used for the previous part. Even after flushing half a meter of orange through the hot end, you’ll still see some contamination, even with the same type of plastic. Doesn’t make much difference here, though.

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Monthly Image: And Then There Were Two

The turkey hen who once had nine chicks, then seven, now has only two:

Turkey Hen with two chicks
Turkey Hen with two chicks

We haven’t seen the fox since it nailed the previous chick, but it may be responsible for taking a chick a day, every day, for a week.

We wonder if she misses the rest of her brood as much as we do …

Taken through two layers of 1950s window glass, zoomed all the way in, with a phone camera.

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Power Lift Chair Upholstery Protection

For reasons not relevant here, we have a power lift chair which has been shedding upholstery tufts since the day we got it. After realizing this wasn’t going to stop on its own, I spent a while poking around underneath and discovered the steel struts supporting the leg rest rub along the upholstery during their entire travel:

Lift chair - strut vs upholstery
Lift chair – strut vs upholstery

Apparently, the padding behind the upholstery pushes it a bit further out than the original design could accommodate, letting the raw edges on the steel struts shave off the fuzz.

I put relatively smooth stainless steel tape on all the protrusions and bent it around the rough edges:

Lift chair - strut smoothing
Lift chair – strut smoothing

Those steel folds are smoother than they appear.

It’s not obvious this will solve the problem, but the struts seems to be scraping off much less fuzz than before, so it’s a step in the right direction.

Why is it all of today’s consumer products require 10% more engineering to work in the real world?

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Chipmunk Shelter

This chipmunk has been hanging out near the collection of yard & garden tools on the patio:

Chipmunk near patio shelter
Chipmunk near patio shelter

When threats appear, the critter vanishes into the clutter and waits until we go elsewhere. It’s almost as good as the roof gutter pipe!

Those stripes remain surprisingly visible in the shadows between stacks of clay pots, though, if you know where to look.

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