Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Tag: Improvements
Making the world a better place, one piece at a time
Then plotting the data points and eyeballing a straight-line curve fit:
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
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
Then tap EDIT and enter the data in a tiny spreadsheet:
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
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:
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:
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
A closer look at the text:
MPCNC Engraving – hard drive platter – detail A
And some digits:
MPCNC Engraving – hard drive platter – detail B
When I want to brand an engraved CD, this will suffice:
MPCNC Engraving – CD attribution text
All in all, the MPCNC engraves much better than I expected!
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.
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
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
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?
The retina-burn orange ring is printed in PETG with my usual slicer settings: three perimeter threads, three top and bottom layers, and 15% 3D honeycomb infill. That combination is strong enough and stiff enough for essentially everything I do around here.
The insert on the left came out of its hole carrying its layer of epoxy: the epoxy-to-hole bond failed first. Despite that, punching it out required enough force to convince me it wasn’t going anywhere on its own.
The column of plastic around the insert standing up from the top fits into the central hole (hidden in the picture) in the bench block. Basically, the edge of the hole applied enough shear force to the plastic to break the infill before the epoxy tore free, with me applying enough grunt to the drill press quill handle to suggest I should get a real arbor press if I’m going to keep doing this.
The third insert maintained a similar grip, as seen from the left:
Brass Insert Retention test – C left
And the right:
Brass Insert Retention test – C right
The perimeter threads around the hole tore away from the infill, with the surface shearing as the plastic column punched through.
Bottom line: a dab of epoxy anchors an insert far better than the 3D printed structure around it can support!
Encouraged by the smooth running of the LM12UU drag knife mount, I chopped off another length of 12 mm shaft:
LM12UU Collet Pen Holder – sawing shaft
The MicroMark Cut-off saw was barely up to the task; I must do something about its craptastic “vise”. In any event, the wet rags kept the shaft plenty cool and the ShopVac hose directly behind the motor sucked away all of the flying grit.
The reason I used an abrasive wheel: the shaft is case-hardened and the outer millimeter or two is hard enough to repel a carbide cutter:
LM12UU Collet Pen Holder – drilling shaft
Fortunately, the middle remains soft enough to drill a hole for the collet pen holder, which I turned down to a uniform 8 mm (-ish) diameter:
LM12UU Collet Pen Holder – turning collet body
Slather JB Kwik epoxy along the threads, insert into the shaft, wipe off the excess, and it almost looks like a Real Product:
LM12UU Collet Pen Holder – finished body
The far end of the shaft recesses the collet a few millimeters to retain the spring around the pen body, which will also require a knurled ring around the outside so you (well, I) can tighten the collet around the pen tip.
Start the ring by center-drilling an absurdly long aluminum rod in the steady rest:
M12UU Collet Pen Holder – center drilling
Although it’s not obvious, I cleaned up the OD before applying the knurling tool:
LM12UU Collet Pen Holder – knurling
For some unknown reason, it seemed like a Good Idea to knurl without the steady rest, perhaps to avoid deepening the ring where the jaws slide, but Tiny Lathe™ definitely wasn’t up to the challenge. The knurling wheels aren’t quite concentric on their bores and their shafts have plenty of play, so I got to watch the big live center and tailstock wobbulate as the rod turned.
With the steady rest back in place, drill out the rod to match the shaft’s 12 mm OD:
LM12UU Collet Pen Holder – drilling shaft
All my “metric” drilling uses hard-inch drills approximating the metric dimensions, of course, because USA.
Clean up the ring face, file a chamfer on the edge, and part it off:
LM12UU Collet Pen Holder – parting ring
Turn some PVC pipe to a suitable length, slit one side so it can collapse to match the ring OD, wrap shimstock to protect those lovely knurls, and face off all the ugly:
LM12UU Collet Pen Holder – knurled ring facing
Tweak the drag knife’s solid model for a different spring from the collection and up the hole OD in the plate to clear the largest pen cartridge in the current collection:
Collet Holder – LM12UU – solid model
Convince all the parts to fly in formation, then measure the spring rate:
LM12UU Collet Pen Holder – spring rate test
Which works out to be 128 g + 54 g/mm:
LM12UU Collet Pen Holder – test plot – overview
I forgot the knurled ring must clear the screws and, ideally, the nyloc nuts. Which it does, after I carefully aligned each nut with a flat exactly tangent to the ring. Whew!
A closer look at the business end:
LM12UU Collet Pen Holder – test plot – detail
The shaft has 5 mm of travel, far more than enough for the MPCNC’s platform. Plotting at -1 mm applies 180 g of downforce; the test pattern shown above varies the depth from 0.0 mm in steps of -0.1 mm; anything beyond -0.2 mm gets plenty of ink.
Now I have a pen holder, a diamond scribe, and a drag knife with (almost) exactly the same “tool offset” from the alignment camera, thereby eliminating an opportunity to screw up.
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The yard camera now resides outdoors and plugs into one of three outlets on the patio, all of which have weatherproof covers attached by a bead chain to the trim plate:
Patio Outlet – new chain installed
That’s the after-repair condition, as two of the three chains were broken when we bought the house.
Stipulated: the covers needed scrubbing, but sometimes ya gotta stay focused on the Main Goal.
Two feet of 3.4 mm brass bead chain (because spares: ya gotta have stuff) arrived from eBay, I dismounted all three covers, and discovered the bell-shaped brass caps on the old chains were perfectly serviceable after six decades:
Patio Outlet – chain retainers
The outlets are wired to circuit breaker 28, of course.
Having enough chain to go around, each cover now sports a slightly longer leash than before:
Patio Outlet – chain assembly
Reinstall in reverse order, the camera rebooted as it should, and it’s all good out there:
The Smell of Molten Projects in the Morning 