Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
By now, I have half a dozen baggies each containing half a dozen plotter pens, plus a demo program that can produce good-looking Superformula plots, so I can do this without any hassle:
HP 7475A 2541A 68465 – Random pens
And this:
HP 7475A 2641V 26599 – Random pens
I must confess to not being good at withstanding temptation; the second plot comes from another HP 7475A plotter that I won on eBay:
Stacked HP 7475A Plotters
Apparently, nobody else wanted a plotter advertised as “non-working”, leaving me as the sole bidder. The photos showed that it powered up properly, sported a serial (not HPIB) interface, had an (empty) carousel with rubber pen boots (that were, oddly enough, not fossilized), and came with a complete set of manuals. Turns out any one of those items sells for more than the entire package, so I can part it out, flip the pieces, and Profit! if I were so inclined.
Load the carousel with a handful of restored pens, insert a sheet of paper, hold down the P1 + P2 buttons, flip the power switch, and out comes a perfectly drawn demo plot:
During this plot, an Inmac purple pen (in the Pen 5 slot) pretty much ran out of ink:
HP 7475A – Pen 5 before refill
It printed the legend perfectly and started the trace solidly enough, proceeding upward from the far right, but after ten circuits around the center it returned dragging a very faint line behind it.
Just for a change, I punched a new-old-stock Inmac pen from its sealed blister pack, only to find that it left a spotty trail. These being easy to refill, I popped the top, flipped the fiber reservoir, added ten drops of HP2000C black (i.e., not the crappiest ink I’ve ever used), and scribbled a few feet to get it started again. It left a good-enough trace, so I ran an A-size plot with the ball pen, two liquid ink pens, and a refilled ceramic pen:
HP 7475A – Inmac ball – liquid – ceramic – pens
It’s a bit pallid compared with the black ceramic pen. The line is continuous and, in comparison with all the other plotter pens in the collection, very very very fine.
Turns out that it’s a miniature ball-point pen:
Inmac ball pen tip
I’m certain that water-based inkjet juice isn’t the right stuff for a ball-point pen, which may account for the lack of color. It’d be most appropriate for a document with fine text details, not that I must plot any of those at the moment.
There’s a bag with a dozen more of ’em in various cheerful colors, so, if I could just think of something that needed stubby ballpoint pens, I’d be all set.
It might be intended for a different HP plotter that applies more downforce to give the ball more encouragement. We’ll never know…
Being that type of guy, I measure the single-layer skirt threads to keep track of the platform alignment. Most of the time, nothing happens, because the M2 has a remarkably stable platform, but some of the objects I’d done in early August showed more than the usual variation and, worryingly, no discernible trend.
Adjusting the platform alignment between each of those sets produced no consistent effect, which is most unusual. The X in the bottom set shows where that thinwall box came unstuck from the platform, indicating that the clearance was considerably more than the nominal 0.25 mm layer height.
Peering under platform revealed something else that was quite unusual:
M3 washer – bad seating
That washer should be flat against the spider mounting plate. My first thought was a burr on the plate, but that didn’t make any sense, as the plate was clean & smooth when I installed the platform; I’d enlarged those holes with a fine file and would have checked for burrs as part of that operation.
Removing the screw nut and extracting the washer revealed the true problem:
M3 washer with burrs
It’s a bad washer!
Tossing that one in the trash and installing a good washer put everything in order:
M3 washer – proper seating
Well, that’s after re-doing the alignment to un-do the previous flailing around, of course.
As nearly as I can tell, that washer sat there without causing any trouble since I installed the hotrod platform. or, more likely, when I repaired a failed screw. In late July I poked the platform to measure how much it moved under pressure, which apparently dislodged the washer and put the burr in play.
That’s how sensitive a 3D printer is to mechanical problems…
Victoreen 710-104 Ionization Chamber Fittings – Show V2
There’s not much difference from the first iteration, apart from a few code cleanups. The engraved text is kinda-sorta gratuitous, but I figured having the circuit board dimensions on all the key parts would avoid heartache & confusion; the code now autosizes the board to the holder OD. Skeletonizing the board template didn’t save nearly as much printing time as I expected, though.
Now I can build a second electrometer amp without dismantling the two-transistor version.
The OpenSCAD source code:
// Victoreen 710-104 Ionization Chamber Fittings
// Ed Nisley KE4ZNU August 2015
Layout = "Show";
// Show - assembled parts
// Build - print can parts + shield
// BuildShield - print just the shield
// BuildHolder - print just the can cap & PCB base
// CanCap - PCB insulator for 6-32 mounting studs
// CanBase - surrounding foot for ionization chamber
// CanRim - generic surround for either end of chamber
// PCB - template for cutting PCB sheet
// PCBBase - holder for PCB atop CanCap
// Shield - electrostatic shield shell
//- Extrusion parameters must match reality!
// Print with 2 shells and 3 solid layers
ThreadThick = 0.25;
ThreadWidth = 0.40;
HoleWindage = 0.2;
Protrusion = 0.1; // make holes end cleanly
AlignPinOD = 1.75; // assembly alignment pins = filament dia
inch = 25.4;
function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);
//- Screw sizes
Tap4_40 = 0.089 * inch;
Clear4_40 = 0.110 * inch;
Head4_40 = 0.211 * inch;
Head4_40Thick = 0.065 * inch;
Nut4_40Dia = 0.228 * inch;
Nut4_40Thick = 0.086 * inch;
Washer4_40OD = 0.270 * inch;
Washer4_40ID = 0.123 * inch;
//----------------------
// Dimensions
OD = 0; // name the subscripts
LENGTH = 1;
Chamber = [91.0,38]; // Victoreen ionization chamber dimensions
Stud = [ // stud welded to ionization chamber lid
[6.5,IntegerMultiple(0.8,ThreadThick)], // flat head -- generous clearance
[4.0,9.5], // 6-32 screw -- ditto
];
NumStuds = 3; // this really isn't much of a variable...
StudAngle = 360/NumStuds;
StudSides = 6; // for hole around stud
BCD = 2.75 * inch; // mounting stud bolt circle diameter
PlateThick = 2.0; // minimum layer atop and below chamber ends
RimHeight = 4.0; // extending along chamber perimeter
WallHeight = RimHeight + PlateThick;
WallThick = 3.0; // thick enough to be sturdy & printable
CapSides = 8*6; // must be multiple of 4 & 3 to make symmetries work out right
RimOD = Chamber[OD] + 2*WallThick;
echo(str("Rim OD: ",RimOD));
//PCBFlatsOD = 82.0; // desired hex dia flat-to-flat
PCBFlatsOD = floor(RimOD*cos(30)) - 2.0; // .. maximum possible
//PCBFlatsOD = floor(Chamber[OD]*cos(30)) - 2.0; // .. chamber fitting
PCBClearance = ThreadWidth; // clearance beyond each flat for mounting
PCBThick = 1.1;
PCBActual = [PCBFlatsOD/cos(30),PCBThick]; // OD = tip-to-tip
PCBCutter = [(PCBFlatsOD + 2*PCBClearance)/cos(30),PCBThick - ThreadThick]; // OD = tip-to-tip dia + clearance
PCBSize = str(PCBFlatsOD, " mm");
echo(str("Actual PCB across flats: ",PCBFlatsOD));
echo(str(" ... tip-to-tip dia: ",PCBActual[OD]));
echo(str(" ... thickness: ",PCBActual[LENGTH]));
HolderHeight = 13.0 + PCBCutter[LENGTH]; // thick enough for PCB to clear studs + batteries
HolderShelf = 2.0; // shelf under PCB edge
HolderTrim = 5.0; // remove end of holder to clear PCB edge solder blobs
echo(str("Holder trim distance: ",HolderTrim));
HolderTrimAngle = StudAngle/2 - 2*atan(HolderTrim*cos(StudAngle/2)/(PCBActual[OD]/2)); // atan is close for small angles
echo(str(" ... angle: ",HolderTrimAngle));
PinAngle = 15; // alignment pin angle on either side of holder screw
echo(str("PCB holder across flats: ",PCBCutter[OD]*cos(30)));
echo(str(" ... height: ",HolderHeight));
ShieldInset = 0.5; // shield inset from actual PCB flat
ShieldWall = 2.0; // wall thickness
ShieldLid = 6*ThreadThick; // top thickness (avoid one infill layer)
Shield = [(PCBFlatsOD - 2*ShieldInset)/ cos(30),40.0]; // electrostatic shield shell dimensions
TextSize = 4;
TextCharSpace = 1.05;
TextLineSpace = TextSize + 2;
TextDepth = 1*ThreadThick;
//----------------------
// Useful routines
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,
$fn=Sides);
}
//- Locating pin hole with glue recess
// Default length is two pin diameters on each side of the split
module LocatingPin(Dia=AlignPinOD,Len=0.0) {
PinLen = (Len != 0.0) ? Len : (4*Dia);
translate([0,0,-ThreadThick])
PolyCyl((Dia + 2*ThreadWidth),2*ThreadThick,4);
translate([0,0,-2*ThreadThick])
PolyCyl((Dia + 1*ThreadWidth),4*ThreadThick,4);
translate([0,0,-Len/2])
PolyCyl(Dia,Len,4);
}
module ShowPegGrid(Space = 10.0,Size = 1.0) {
RangeX = floor(100 / Space);
RangeY = floor(125 / Space);
for (x=[-RangeX:RangeX])
for (y=[-RangeY:RangeY])
translate([x*Space,y*Space,Size/2])
%cube(Size,center=true);
}
//-----
module CanRim(BaseThick) {
difference() {
cylinder(d=Chamber[OD] + 2*WallThick,h=(WallHeight + BaseThick),$fn=CapSides);
translate([0,0,BaseThick])
PolyCyl(Chamber[OD],Chamber[LENGTH],CapSides);
}
}
module CanCap() {
difference() {
CanRim(PlateThick + Stud[0][LENGTH]);
translate([0,0,-Protrusion]) // central cutout
rotate(180/6)
cylinder(d=BCD,h=Chamber[LENGTH],$fn=6); // ... reasonable size
for (i=[0:(NumStuds - 1)]) // stud clearance holes
rotate(i*StudAngle)
translate([BCD/2,0,0])
rotate(180/StudSides) {
translate([0,0,PlateThick])
PolyCyl(Stud[0][OD],Chamber[LENGTH],StudSides);
translate([0,0,-Protrusion])
PolyCyl(Stud[1][OD],Chamber[LENGTH],StudSides);
}
for (i=[0:(NumStuds - 1)], j=[-1,1]) // PCB holder alignment pins
rotate(i*StudAngle + j*PinAngle + 60)
translate([Chamber[OD]/2,0,0])
rotate(180/4 - j*PinAngle)
LocatingPin(Len=2*(PlateThick + Stud[0][LENGTH]) - 4*ThreadThick);
translate([-(BCD/2),0,-Protrusion])
rotate(90) mirror()
linear_extrude(height=(ThreadThick + Protrusion))
text(PCBSize,size=6,font="Liberation Mono:style=bold",halign="center",valign="center");
}
}
module CanBase() {
difference() {
CanRim(PlateThick);
translate([0,0,-Protrusion])
PolyCyl(Chamber[OD] - 2*RimHeight,Chamber[LENGTH],CapSides);
}
}
module PCBTemplate() {
CutLen = 10*PCBActual[LENGTH];
difference() {
cylinder(d=PCBActual[OD],h=PCBActual[LENGTH],$fn=6); // actual PCB size
translate([0,0,-Protrusion])
cylinder(d=8,h=CutLen,$fn=12);
if (true)
for (i=[0:5]) // empirical cutouts
rotate(i*60 + 30)
translate([PCBFlatsOD/3,0,-Protrusion])
rotate(60)
cylinder(d=0.43*PCBActual[OD],h=CutLen,$fn=3);
translate([PCBActual[OD]/4,0,(PCBActual[LENGTH] - ThreadThick)])
linear_extrude(height=(ThreadThick + Protrusion),convexity=1)
text(PCBSize,size=4,font="Liberation Mono:style=bold",halign="center",valign="center");
}
}
module PCBBase() {
intersection() {
difference() {
cylinder(d=Chamber[OD] + 2*WallThick,h=HolderHeight,$fn=CapSides); // outer rim
rotate(30) {
translate([0,0,-Protrusion]) // central hex
cylinder(d=(PCBActual[OD] - HolderShelf/cos(30) - HolderShelf/cos(30)),h=2*HolderHeight,$fn=6);
translate([0,0,HolderHeight - PCBCutter[LENGTH]]) // hex PCB recess
cylinder(d=PCBCutter[OD],h=HolderHeight,$fn=6);
for (i=[0:NumStuds - 1]) // PCB retaining screws
rotate(i*StudAngle + 180/(2*NumStuds))
translate([(PCBCutter[OD]*cos(30)/2 + Clear4_40/2 + ThreadWidth),0,-Protrusion])
rotate(180/6)
PolyCyl(Tap4_40,2*HolderHeight,6);
for (i=[0:(NumStuds - 1)], j=[-1,1]) // PCB holder alignment pins
rotate(i*StudAngle + j*PinAngle + 180/(2*NumStuds))
translate([Chamber[OD]/2,0,0])
rotate(180/4 - j*PinAngle)
LocatingPin(Len=2*(HolderHeight - 4*ThreadThick));
}
if (false)
for (i=[0:NumStuds - 1])
rotate(i*StudAngle - StudAngle/2) // segment isolation - hex sides
translate([0,0,-Protrusion]) {
linear_extrude(height=2*HolderHeight)
polygon([[0,0],[Chamber[OD],0],[Chamber[OD]*cos(180/NumStuds),Chamber[OD]*sin(180/NumStuds)]]);
}
translate([-(PCBFlatsOD/2 + PCBClearance - HolderShelf),0,HolderHeight/2])
rotate([0,90,0]) rotate(90)
linear_extrude(height=(ThreadWidth + Protrusion))
text(PCBSize,size=6,font="Liberation Mono:style=bold",halign="center",valign="center");
}
for (i=[0:NumStuds - 1])
rotate(i*StudAngle + StudAngle/2 - HolderTrimAngle/2) // trim holder ends
translate([0,0,-Protrusion]) {
linear_extrude(height=2*HolderHeight)
polygon([[0,0],[Chamber[OD],0],[Chamber[OD]*cos(HolderTrimAngle),Chamber[OD]*sin(HolderTrimAngle)]]);
}
}
}
//-- Electrostatic shield
// the cutouts are completely ad-hoc
module ShieldShell() {
CutHeight = 7.0;
difference() {
cylinder(d=Shield[OD],h=Shield[LENGTH],$fn=6); // exterior shape
translate([0,0,-ShieldLid]) // interior
cylinder(d=(Shield[OD] - 2*ShieldWall/cos(30)),h=Shield[LENGTH],$fn=6);
translate([0,0,Shield[LENGTH] - TextDepth])
rotate(180) {
translate([0,0.3*Shield[OD] - 0*TextLineSpace,0])
linear_extrude(height=(TextDepth + Protrusion))
text("Gamma",size=TextSize,spacing=TextCharSpace,font="Liberation:style=bold",halign="center",valign="center");
translate([0,0.3*Shield[OD] - 1*TextLineSpace,0])
linear_extrude(height=(TextDepth + Protrusion))
text("Ionization",size=TextSize,spacing=TextCharSpace,font="Liberation:style=bold",halign="center",valign="center");
translate([0,0.3*Shield[OD] - 2*TextLineSpace,0])
linear_extrude(height=(TextDepth + Protrusion))
text("Amplifier",size=TextSize,spacing=TextCharSpace,font="Liberation:style=bold",halign="center",valign="center");
translate([0,-0.3*Shield[OD] + 1*TextLineSpace,0])
linear_extrude(height=(TextDepth + Protrusion))
text("KE4ZNU",size=TextSize,spacing=TextCharSpace,font="Liberation:style=bold",halign="center",valign="center");
translate([0,-0.3*Shield[OD] + 0*TextLineSpace,0])
linear_extrude(height=(TextDepth + Protrusion))
text("2015-08",size=TextSize,spacing=TextCharSpace,font="Liberation:style=bold",halign="center",valign="center");
}
translate([Shield[OD]/4 - 20/2,Shield[OD]/2,(CutHeight - Protrusion)/2]) // switch
rotate(90)
cube([Shield[OD],20,CutHeight + Protrusion],center=true);
if (false)
translate([-Shield[OD]/4 + 5/2,Shield[OD]/2,(CutHeight - Protrusion)/2]) // front
rotate(90)
cube([Shield[OD],5,CutHeight + Protrusion],center=true);
translate([-Shield[OD]/2,0,(CutHeight - Protrusion)/2]) // right side
cube([Shield[OD],7,CutHeight + Protrusion],center=true);
translate([0,(Shield[OD]*cos(30)/2 - ThreadWidth),0.75*Shield[LENGTH]])
rotate([90,0,180]) rotate(00)
linear_extrude(height=(ThreadWidth + Protrusion))
text(PCBSize,size=5,font="Liberation Mono:style=bold",halign="center",valign="center");
}
}
//----------------------
// Build it
ShowPegGrid();
if (Layout == "CanRim") {
CanRim();
}
if (Layout == "CanCap") {
CanCap();
}
if (Layout == "CanBase") {
CanBase();
}
if (Layout == "PCBBase") {
PCBBase();
}
if (Layout == "PCB") {
PCBTemplate();
}
if (Layout == "Shield") {
ShieldShell();
}
if (Layout == "Show") {
CanBase();
color("Orange",0.5)
translate([0,0,PlateThick + Protrusion])
cylinder(d=Chamber[OD],h=Chamber[LENGTH],$fn=CapSides);
translate([0,0,(2*PlateThick + Chamber[LENGTH] + 2*Protrusion)])
rotate([180,0,0])
CanCap();
translate([0,0,(2*PlateThick + Chamber[LENGTH] + 5.0)])
PCBBase();
color("Green",0.5)
translate([0,0,(2*PlateThick + Chamber[LENGTH] + 7.0 + HolderHeight)])
rotate(30)
PCBTemplate();
translate([0,0,(2*PlateThick + Chamber[LENGTH] + 15.0 + HolderHeight)])
rotate(-30)
ShieldShell();}
if (Layout == "Build") {
translate([-0.50*Chamber[OD],-0.60*Chamber[OD],0])
CanCap();
if (false)
translate([0.55*Chamber[OD],-0.60*Chamber[OD],0])
rotate(30)
translate([0,0,Shield[LENGTH]])
rotate([0,180,0])
ShieldShell();
if (true)
translate([0.55*Chamber[OD],-0.60*Chamber[OD],0])
rotate(30)
PCBTemplate();
if (true)
translate([-0.25*Chamber[OD],0.60*Chamber[OD],0])
CanBase();
translate([0.25*Chamber[OD],0.60*Chamber[OD],0])
PCBBase();
}
if (Layout == "BuildHolder") {
translate([-0.25*Chamber[OD],0,0])
CanCap();
translate([0.25*Chamber[OD],0,0])
PCBBase();
}
if (Layout == "BuildShield") {
translate([0,0,Shield[LENGTH]])
rotate([0,180,0])
ShieldShell();
}
Following madbodger’s recommendation, but finding no local sources, a bottle of Koh-I-NoorRapido-EZE Pen Cleaner solvent arrived. It’s billed as a solvent & cleaner for drafting ink, not plotter ink, which seems like an unnatural restriction.
I’d previously tried refilling some fossilized pens, only to find that the ink simply won’t flow through a nib filled with dried ink. So the pens you’ll see here have refilled reservoirs atop nibs that don’t write.
Dismantling the Koh-I-Noor black pen produced this unsightly mess:
HP 7475A – Koh-i-Noor black pen parts
I pushed the nib out of the shell using a pin punch, pretty thoroughly crushing the tip in the process. The ink reservoir looks like some sort of fluff inside a plastic sleeve, with a hole left by the butt end of the nib and a crust left by the evaporating ink. I scraped off the crust, put the nib in a cylinder filled with solvent, and let it sit for a few days, after which most of the black ink had vanished.
I reassembled the pen with the blunted end of the nib inside the body and the reservoir flipped end-for-end, in the hope that would work better.
Trying a different tactic with a Staedtlergreen pen, I removed the reservoir, filled the body with solvent, and dunked the tip in a solvent bath:
HP 7475A – Staedtler pen – nib soaking
After a few days, the body was still mostly full of solvent, so it’s not flowing freely through the nib. Perhaps leaving the nib in air would encourage the fluid to move outward, at the risk of drying the nib even more.
Pen 2, the gray K-I-N trace, seems a bit pallid, likely due to using cheap black inkjet ink. Apart from that, it’s continuous and presentable.
Pen 3, the green Staedtler, remains in the land of the undead; its ink flows better than before, but not enough to be worthwhile. The demo routine writes the annotation first and those characters came out well enouigh.
The other two pens also carry refilled ink: Pen 1 = ceramic tip, Pen 4 = Staedtler fiber (which, judging from the cap color, started out as gray and has become much darker after the inkjet ink refill).
All in all, a modest success and I’ll try again later. Better, however, to refill each pen before it dries out, as with the two “good” pens.
For reasons that aren’t relevant here, I had to reinforce some old basement stairs. Rather than drilling holes, sinking anchors, and installing screws, I just nailed painted 2×4 strips to the foundation using this Craftsman 1231.3817 Power Hammer, which is not available in a Sears / Kmart near you:
Sears Craftsman Power Hammer
It’s a handheld gun that drives two inches of hardened steel nail into solid concrete by firing what looks like an overstuffed 0.22 Short blank cartridge: load a nail, fit a cartridge, press the muzzle firmly against the target, and whack the butt end with a hammer.
Worked like a champ. Scary as you’d imagine.
If the nail stands proud of the surface, you can hit it again with a low(er) power load to drive it the rest of the way. Sometimes that sinks it below the surface, leaving a cylindrical pit. In the situations where I use this thing, nobody will ever notice.
It’s similar to the Remington Model 476 Powder Actuated Fastening Tool (manual), which you can get from Amazon and surely other vendors; fancier versions also exist. Equally surely, they’re illegal in some jurisdictions.
I have reason to use it every few decades, which is entirely enough for me…
Wear goggles, earplugs, gloves, and don’t get stupid.
The Smell of Molten Projects in the Morning 