Archive for category Science
Drawn at Z=-0.1 mm on scrap acrylic with the diamond engraver in the modified collet holder:
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:
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:
Bending beams still seem much better than a linear bearing, though.
The corrosion growing on our long-suffering cheese slicer finally ruptured its epoxy coating:
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!
The DC gain and VBE for each flavor look comfortingly uniform:
Quite unlike those Hall effect sensors, indeed.
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:
Class Type VCE IC HFE 1 2N2222 GP NPN 40 600 100 2 2N3904 LP NPN 40 200 100 3 2N3906 LP PNP 40 200 100 4 2N5401 HV PNP 150 600 60 5 2N5551 HV NPN 160 600 80 6 A1015 OSC PNP 50 150 70 7 C1815 OSC NPN 50 150 70 8 C945 GP NPN 50 150 70 9 S8050 PP AMP NPN 40 500 120 10 S8550 PP AMP PNP 40 500 120 11 S9012 PP AMP PNP 40 500 64 12 S9013 PP AMP NPN 40 500 64 13 S9014 LN LF NPN 50 100 280 14 S9015 LN LF PNP 50 100 200 15 S9018 VHF OSC NPN 15 50 100
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:
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:
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.
Whiteboards from the SqWr Electronics Session 5, covering transistors as switches …
Reviewing I vs V plots, starting with a resistor and then a transistor as a current amplifier:
Reminder of why you can’t run a transistor at its maximum voltage and current at the same time:
A resistor load line, with power calculation at the switch on and off coordinates:
Detail of the power calculations, along with a diagram of the current and voltage when you actually switch the poor thing:
Oversimplification: most of the power happens in the middle, but as long as the switching frequency isn’t too high, it’s all good.
Schematic of the simplest possible switched LED circuit, along with a familiar mechanical switch equivalent:
We started with the “mechanical switch” to verify the connections:
Building the circuitry wasn’t too difficult, but covering the function generator and oscilloscope hookup took far more time than I expected.
My old analog Tek 2215 scope was a crowd-pleaser; there’s something visceral about watching a live CRT display you just don’t get from the annotated display on an LCD panel.
I’d planned to introduce capacitors, but just the cap show-n-tell went well into overtime. We’ll get into those in Session 6, plus exploring RC circuitry with function generators and oscilloscopes.
The bars on the original MPCNC drag knife / plotter pen adapter had a 100 g/mm spring constant:
Making the bars slightly thicker improved their print-ability:
The reddish tint marks the new bars, with their location carefully tweaked to be coincident with the stock STL.
Shoving the pen into the scale with 0.1 mm steps produces another unnervingly linear plot:
Real plotter pens want about 20 g of force, so this isn’t the holder you’re looking for.
A bunch of plots at Z=-1.0 mm turned out well with the ballpoint pen insert, though:
The globs apparently come from plotting too fast for conditions; reducing the speed to 1500 mm/min works better.
This compact fluorescent lamp seems to have survived nearly two decades of use in a desk lamp:
It had plenty of starts, although maybe not so many total hours, as the other CFLs you’ll find mentioned around here.
I swapped in a similar CFL and we’ll see what happens.