Step2 Garden Seat: Replacement Seat
Posted by Ed in Home Ec, Machine Shop, Software on 2019-07-19
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:
The underside was giving way, too:
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:
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:
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.
A closer look at the hinges in real life:
The solid model now caps the holes; I can drill them out should the need arise.
From the bottom:
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:
After the first coat, I conjured a drying rack from a bamboo skewer, a cardboard flap, and some hot-melt glue:
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:
The second seat looks just like this one, so life is good …
MPCNC: Calculating Spring Rates
Posted by Ed in Machine Shop, Oddities, Software on 2019-07-18
Calculate the spring rates for the drag knife, diamond engraver, and collet pen holders by measuring the downforce every 0.5 mm (or so):
Then plotting the data points and eyeballing a straight-line curve fit:
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:
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:
Then tap EDIT and enter the data in a tiny spreadsheet:
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:
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:
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.
MPCNC: Diamond Drag Engraving Speed Tests
Posted by Ed in Machine Shop, Science on 2019-07-17
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:
Diamond drag engraving produces a thinner line and makes the wobbulations more obvious:
Another test showed similar results:
Slowing down definitely reduces the shakes:
Producing the best results takes quite a while:
Similar results on another test:
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:
A closer look at the text:
And some digits:
When I want to brand an engraved CD, this will suffice:
All in all, the MPCNC engraves much better than I expected!
Drag Knife Blade Ejector Handle
Posted by Ed in Machine Shop on 2019-07-16
The LM12UU drag knife holder buries the blade ejector pin deep inside the machinery:
So a handle with a pin makes sense:
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.
Monthly Image: And Then There Were Two
Posted by Ed in Oddities, Photography & Images, Science on 2019-07-15
The turkey hen who once had nine chicks, then seven, now has only two:
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.
Power Lift Chair Upholstery Protection
Posted by Ed in Home Ec, Machine Shop on 2019-07-14
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:
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:
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?
Chipmunk Shelter
This chipmunk has been hanging out near the collection of yard & garden tools on the patio:
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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