The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Author: Ed

HP 7475A Plotter: Pen Carousel Geneva Drive
A note arrived from someone who obviously couldn’t pass up an orphaned HP 7475A plotter:

The plotter I received works beautifully, except that the carousel doesn’t rotate. I found a YouTube video showing a 7475a running with the cover off, and there’s a little plastic piece – it looks like a teardrop – that advances the carousel, and is apparently part of the carousel motor assembly. Mine is missing that piece …

The keyword is Geneva drive, a wonderfully simple technique to convert one rotation of the stepper motor into 1/6 turn of the pen carousel, with no need for fancy sensors.

The (unofficial) HP Computer Museum has All The HP 7475A Documents and the Plotter Service Manual shows All The Parts. And, of course, I’ve written a bit about my adventures with an old 7475A.

Back in the day, you could get the entire Pen Carousel Housing Assembly w/ Motor (PN 07475-60175) as a unit and the Carousel Motor Only (PN 3140-0687) as a separate thing, but not the Geneva drive wheel:

HP7475A Carousel – Geneva drive cam

The cam’s drive wheel end (in inches, because early 1980s):
- 0.25 thick overall
- 0.10 thick plate under pin end
- 1.09 OD – rounded end
The pin sticking up from the cam:
- 0.154 OD (or fit to slot?)
- 0.16 tall (above base plate)
I have no good (i.e., easy + accurate) way to measure the distance from the motor shaft to the pin, but I doubt it’s critical. As long as the pin doesn’t quite whack the hub end of the slot, it’s all good:

HP7475A Carousel – cam driving

The 0.10 plate + 0.16 pin height don’t quite add up to the 0.25 overall measurement, but that’s certainly measurement error. I’d round the pin length downward and carve the drive from a 1/4 inch sheet.

A 3D printed part would probably work, apart from the accuracy required to fit the D-shaped motor shaft. Perhaps a round hole, reamed to fit the shaft, carefully aligned / positioned, with epoxy filling the D-shaped void, would suffice. A dent in the round hole would give the epoxy something to grab.

I’d be sorely tempted to use an actual metal / plastic rod for the pin, rather than depend on a stack of semi-fused plastic disks. The pin must withstand hitting the end of the “missing” slot during the power-on indexing rotation, because turning the carousel isn’t quite a non-contact sport. Normally, though, it enters the end of the slot without much fuss:

HP7475A Carousel – cam engaging

The blocked slot sits at the bottom of that picture, with a small locating pin sticking upward just above the circular feature at the end of the arm: we’re seeing the negative of a plug inserted into the original injection mold.

With a bit of luck, another HP 7475A plotter will fascinate everybody within hearing distance!

[Update: It lives! ]
2017-06-28
LM75A vs. SOIC Adapter: Mirror Imaging

After hairballing an LM75A I²C temperature sensor to verify at least one of the eBay lot worked, a bag of SOIC-to-DIP space transformers arrived, so I soldered up another LM75A:

LM75A on DIP8 adapter – top

The SOIC chip pattern sits at right angles to the DIP pins, which took some getting used to.

The slightly defocused wire connecting pin 4 (on the IC) to pins 5, 6, and 7 (on the PCB) selects address 0x48.

So I flipped it over, soldered four wires (+5 V, GND, SDA, SCL) to the numbered pins on bottom of the board, made up a little header for the other end, wired a socket strip on the crystal tester board, plugged it in, and … nothing worked.

Turns out that the other side of the board carries a TSSOP pattern, which I’d neatly masked off with a snippet of Kapton tape, surrounded by eight numbered pins. Of course, those pin numbers correspond to the TSSOP pattern facing you, so they’re mirror-imaged for the SOIC pattern on the other side.

Soooo, the proper wiring for the SOIC pattern as seen from the TSSOP side has the pin numbers exactly bass-ackwards:

LM75A on DIP8 adapter – bottom

The insulation looked a lot better the first time I soldered the wires to the PCB. Honest.

Anyhow, when correctly wired, the LM75A worked as it should:

LM75A Temperature Sensor – installed

It’s snuggled chip-down against the top of the 125 MHz oscillator can, with a dab of heatsink compound improving their thermal bond and a yellow cable tie around the foam holding them together. The socket header is wired pin-for-pin to the DAC I²C socket directly above it.

The OLED temperature display shows 28.250 °C, because the oscillator just started up in a cool basement. It’ll eventually settle around 39-ish °C, where its output should be pretty close to the 125 MHz – 344 Hz value hardcoded into the source.

Oh, that’s a 3 mm amber LED next to the relay can: much less glaring than the white LED, no matter what it looks like here.

2017-06-27

Mailing Tube End Caps: Screw-in Version

The mailing tube arrived with contents intact, although the USPS inlet scanning didn’t work and the tube pretty much teleported across several states without leaving any tracking data behind. The recipient suggested several modifications to the caps:

Review of user experience of tube end:
The ribs on the endcap are very good at holding the cap on, so much so that I had to use a prying implement to remove it, which cracked the flange.
Would consider less depth on the cap, and possibly another layer on the flange.

Some continuous process improvement (a.k.a OpenSCAD hackage) produced a swoopy threaded cap with thumb-and-finger grips:

Mailing Tube Screw Cap - top - Slic3r — Mailing Tube Screw Cap – top – Slic3r

The finger grips are what’s left after stepping a sphere out of the cap while rotating it around the middle:

Mailing Tube Cap - finger grip construction — Mailing Tube Cap – finger grip construction

That worked out surprisingly well, with the deep end providing enough of a vertical-ish surface to push against.

The two hex holes fit a pin wrench, because the grips twist only one way: outward. The wrench eliminates the need for a flange, as you can now adjust the cap insertion before slathering packing tape over the ends. Man, I loves me some good late binding action!

A three-start thread seemed like overkill, but was quick & easy. The “thread form” consists of square rods sunk into the cap perimeter, with one edge sticking out:

Mailing Tube Cap - thread detail — Mailing Tube Cap – thread detail

They’re 1.05 times longer than the cap perimeter facets to make their ends overlap, although they’re not tapered like the ones in the broom handle dingus, because it didn’t (seem to) make any difference to the model’s manifoldhood.

Not needing any endcaps right now, I built one for show-n-tell:

Threaded mailing tube end cap - installed — Threaded mailing tube end cap – installed

The OpenSCAD source code as a GitHub Gist:

	// Mailing tube end cap
	// Ed Nisley KE4ZNU – June 2017

	Layout = "Build";

	Model = "Screw";

	//- Extrusion parameters – must match reality!

	ThreadThick = 0.25;
	ThreadWidth = 0.40;

	function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);

	Protrusion = 0.1;

	HoleWindage = 0.2;

	//- Screw sizes

	inch = 25.4;

	TubeID = 2 * inch;
	TubeWall = 0.1 * inch;

	CapInsert = 15.0;
	CapRim = 6*ThreadThick;
	CapWall = 3*ThreadWidth;

	NumFlanges = 3;
	FlangeHeight = 3*ThreadThick;
	FlangeWidth = ThreadWidth/2;
	FlangeSpace = CapInsert / (NumFlanges + 1);

	ThumbHoleOD = 20.0;
	ThumbHoleAngle = 100;
	ThumbHoleSteps = 10;

	SpannerPinOD = 5.0;
	HelixOD = 4*ThreadThick;
	HelixHeight = 0.75*CapInsert;
	HelixAngle = atan(HelixHeight/(PI*TubeID));
	HelixStarts = 3;

	OAHeight = CapInsert + CapRim;

	NumRibs = 3*4;
	NumSides = 3*NumRibs;

	//- Adjust hole diameter to make the size come out right

	module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes

	Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);

	FixDia = Dia / cos(180/Sides);

	cylinder(r=(FixDia + HoleWindage)/2,h=Height,$fn=Sides);
	}

	module ScrewCap() {

	union() {

	difference() {
	cylinder(d=TubeID,h=OAHeight,$fn=NumSides);

	for (a=[0,180])
	for (i=[0:ThumbHoleSteps-1])
	rotate(a + i*ThumbHoleAngle/ThumbHoleSteps)
	translate([TubeID/4,0,-iThumbHoleOD/(2ThumbHoleSteps)])
	sphere(d=ThumbHoleOD);

	for (a=[0,180])
	rotate(a – 60)
	translate([0.75*TubeID/2,0,-Protrusion])
	rotate(0*180/6)
	PolyCyl(SpannerPinOD,0.75*CapInsert,6);
	}

	for (s=[0:HelixStarts-1])
	for (i=[0:NumSides-1])
	rotate(i360/NumSides + 180/NumSides + s360/HelixStarts)
	translate([TubeID/2 – 0.25HelixOD,0,iHelixHeight/NumSides + HelixOD])
	rotate([90 + HelixAngle,0,0])
	cylinder(d=HelixOD,h=1.05PITubeID/NumSides,center=true,$fn=4);
	}

	}

	module PushCap() {

	difference() {
	cylinder(d=TubeID,h=OAHeight,$fn=NumSides);
	translate([0,0,CapWall])
	cylinder(d=TubeID – 2*CapWall,h=OAHeight,$fn=NumSides);
	}

	for (i=[1:NumFlanges])
	translate([0,0,i*FlangeSpace])
	difference() {
	cylinder(d=TubeID + 2*FlangeWidth,h=FlangeHeight,$fn=NumSides);
	translate([0,0,-Protrusion])
	cylinder(d=TubeID – 2CapWall,h=FlangeHeight + 2Protrusion,$fn=NumSides);
	}

	for (i=[0:NumRibs-1])
	rotate(i*360/NumRibs)
	translate([0,-ThreadWidth,CapWall + ThreadThick])
	cube([TubeID/2 – CapWall/2,2*ThreadWidth,CapInsert + CapRim – CapWall – ThreadThick],center=false);

	translate([0,0,CapInsert]) {
	difference() {
	cylinder(d=TubeID + 2*TubeWall,h=CapRim,$fn=NumSides);
	translate([0,0,-Protrusion])
	cylinder(d=TubeID – 32CapWall,h=2*CapRim,$fn=NumSides);
	}
	}
	}

	//- Build things

	if (Model == "Push")
	if (Layout == "Show")
	PushCap();
	else if (Layout == "Build")
	translate([0,0,OAHeight])
	rotate([180,0,0])
	PushCap();

	if (Model == "Screw")
	if (Layout == "Show")
	ScrewCap();
	else if (Layout == "Build")
	translate([0,0,OAHeight])
	rotate([180,0,0])
	ScrewCap();

view raw Mailing Tube Cap.scad hosted with ❤ by GitHub

2017-06-26

Handbag Strap Rivet Repair

One of the leather strap anchors on Mary’s giant haul-everything-to-a-concert(*) handbag pulled its rivet through the canvas fabric:

Handbag – pulled-through rivet

We knotted the strap around the zippered opening and completed the mission.

Of course, it wouldn’t have pulled through if they’d splurged on washers, but noooo too expensive:

Handbag – intact rivet – inside

Some rummaging produced a pan-head M3 screw of suitable length:

Handbag – repaired – outside

A slightly battered acorn nut was a special treat for the inside, with another washer to keep me happy:

Handbag – repaired – inside

That was easy!

(*) At Tanglewood, where they don’t strip-search you on the way in, tow-behind coolers seemed de rigueur, and a good time was had by all.

2017-06-25
Why Electricity Won

Spotted this impressive array at an apartment building:

Gas meter array

That’s just for one wing; the other end of the building has a similar installation. Each apartment has an electric stove and gas heat / AC.

The plumbing!

2017-06-24
LF Crystal Tester: Bring the Noise!

The OLED display refresh contributes 100 Hz noise pulses to the low-level sine wave from the crystal test fixture:

OLED Enabled – 100 Hz display refresh

Disabling the display by activating its powersave option reveals 60 Hz pulses from the USB port on the Arduino Nano:

OLED Powersave – 60 Hz USB Ground Loop

Unplugging the USB cable, leaving just the +5 VDC power supply and coax cable to the oscilloscope, solves most of the problem:

OLED Powersave – USB unplugged

A closer look shows some (relatively) low frequency noise remains in full effect:

OLED Powersave – USB unplugged – detail

Disabling the display while measuring the crystal seems sensible, although, to avoid surprises, a pushbutton should start the process. Unplugging the USB port puts a real crimp in the data collection, although that’s probably survivable with a USB isolator, one of which is on the way around the planet.

The remaining low-level chop requires more thought. Somewhat to my surprise, holding the Arduino Reset button down doesn’t change much of anything, so it’s not a firmware thing.

Those 10 µF coupling caps gotta go.

With the OLED dark and the USB carrying data:

Spectrum – OLED Powersave – USB in

Compare that to the first pass:

Spectrum-60

Tamping down the noise seems to reduce the overall amplitude variation, but it also makes the capacitor-in and capacitor-out curves more consistent. There may be other things going on that I haven’t accounted for.

The peak frequencies differ by 0.2 Hz, which is probably due to a few degrees of temperature difference. Obviously, it’s badly in need of a temperature calibration & correction.

2017-06-23
LF Crystal Tester: LM75 Temperature Sensor
A strip of NXP (née Philips plus Freescale, including the part of Motorola that didn’t become ON) LM75A I²C temperature sensors arrived from beyond the horizon. To see if they worked, I soldered thin wires directly to the SO-8 pins, entombed it in Kapton tape to prevent spitzensparken, and jammed it under the foam insulation atop the AD9850 DDS module:

AD9850 DDS module with LM75A Temperature Sensor

This turns out to be easier than screwing around with thermistors, because the chip reports the temperature directly in Celcius with ⅛ °C resolution. Classic LM75 chips from National (now absorbed by TI) had ½ °C resolution, but the datasheet shows the bits have an easily extensible format:

LM75A Temperature Data Format

Huh. Fixed-point data, split neatly on a byte boundary. Who’d’a thunk it?

There’s a standard Arduino library using, naturally enough, floating point numbers, but I already have big fixed point numbers lying around and, with the I²C hardware up & running from the X axis DAC and OLED display, this was straightforward:
```
Wire.requestFrom(LM75_ADDR,2);
Temp.fx_32.high = Wire.read();
Temp.fx_32.low = (uint32_t)Wire.read() << 24;
PrintFixedPtRounded(Buffer,Temp,3);
u8x8.drawString(0,ln,"DDS C          ");
u8x8.drawString(16-strlen(Buffer),ln,Buffer);
printf(",%s",Buffer);
ln += 1;
```
The next-to-last line squirts the temperature through the serial port to make those nice plots.

Casually ignoring all I²C bus error conditions will eventually lead to heartache and confusion. In particular, the Basement Laboratory temperature must never fall below 0 °C, because I just plunk the two’s-complement temperature data into an unsigned fixed point number.

Which produces the next-to-bottom line:

DDS OLED with LM75 temperature

Alas, the u8x8 font doesn’t include a degree symbol.

Given sufficient motivation, I can now calibrate the DDS output against the GPS-locked 10 MHz standard to get a (most likely) linear equation for the oscillator frequency offset as a function of temperature. The DDS module includes a comparator to square up its sine wave, so an XOR phase detector or something based on filtering the output of an analog switch might be feasible.
2017-06-22