Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Category: Science
If you measure something often enough, it becomes science
Some handwaving discussion of construction, electrolytic capacitor innards, and The Plague:
Session 6 – Whiteboard 1 – cap construction
A 1 F cap charged through a 1.8 kΩ resistor during most of the session to show what an 1800 s time constant looked like. Nope, it never quite got to the 3.5 V from the power supply, even when we all decided it was time to shut down!
Around 1960, somebody my father knew at the Harrisburg AMP factory gave me a chunk of plugboard bandsawed from a scrapped computer or industrial controller, because he knew I’d enjoy it:
AMP Plug Board
He was right.
I spent months rearranging those little cubes (some with cryptic legends!) into meaningful (to me) patterns, plugging cables between vital spots, and imagining how the whole thing worked:
AMP Plug Board – detail
Long springs ran through the notches under the top of the blocks to connect the plug shells to circuit ground. The ends of the steel rails (still!) have raw bandsaw cuts, some of the blocks were sliced in two, the tip contact array behind the panel wasn’t included, and none of that mattered in the least.
Only a fraction of the original treasure trove survives. It was absolutely my favorite “toy” ever.
Quite some years ago, our Larval Engineer assembled the pattern you see; the hardware still had some attraction.
I’ve asked Mary to toss it in the hole with whatever’s left of me, when that day arrives …
At some point in its history, the left rail holding the wood perch on our industrial-strength “squirrel proof” seed feeder took a hit, most likely from being dropped:
Squirrel on bird feeder
I finally got a Round Tuit and un-bent the poor thing:
Bird feeder – rail un-bending
Because the bend happened at the base of the vertical strut holding the shutter, I clamped a Genuine Vise-Grip sheet metal pliers along the straight section. The Craftsman knock-off Vise-Grip then applied torque at the bend, rather than just making things worse, and some two-axis tweakage lined up the rail pretty well.
With the bend taken care of, I clamped the rail in the bench vise with some scrap wood around the strut:
Bird feeder – warped rail
A percussive adjustment jam session flattened the top flange, leaving both sections as flat as they’re gonna get.
While I was at it, I turned a pair of stepped aluminum washers for the new wood rod:
Bird feeder – parting off washer
Which looked about like you’d expect, including a little chatter from the cut off tool:
Bird feeder – perch hardware
Yeah, I drilled the wood rod on the lathe, too; I loves me some simple lathe action.
Reassemble in reverse order and it’s all good:
Bird feeder – perch installed
We’re supposed to bleach the feeder every week to kill off the bacteria causing House Finch Eye Disease and, while I can’t promise a weekly schedule, we’ll (try to) reduce the amount of crud on the feeder this year.
If you’ve got a feeder, sign up for Project Feederwatch and do some citizen science!
Drawn at Z=-0.1 mm on scrap acrylic with the diamond engraver in the modified collet holder:
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
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
Bending beams still seem much better than a linear bearing, though.
Most of the epoxy remains in good shape, but it’s obviously not the right hammer for this job.
Having recently spotted my tiny sandblaster, I think I can clear off the corrosion and epoxy well enough to try again with good old JB Weld epoxy. It’s not rated for underwater use, so I don’t expect long-term goodness, but it’ll be an interesting comparison.
Bonus: the slicer will start with a uniform gray surface!
Most of the VBE variation comes from temperature differences: re-measuring the 2N3904 transistors with VBE ≅ 672 mV put them with their compadres at 677 mV.
The 2N3906 transistors have wider gain and VBE variations, so selecting a matched pair for the LM3909 current mirror makes sense.
The sheet inside the lid collects some essential parameters for ease of reference:
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
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
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.
