Category: Machine Shop

Mechanical widgetry

Skeinforge Feed Settings
As part of the general reshuffling, I’ve started running the printer with different feeds for different functions:
- Travel = 250 mm/s (non-printing!)
- Basic rate = Infill = 60 mm/s (SF Speed plugin → Feed Rate)
- Perimeter = 0.33 → 20 mm/s
- First layer Infill = 0.25 → 15 mm/s
- First layer Perimeter = 0.15 → 9 mm/s
All of the corresponding Flow rates have the same values, which seems to be the right way to go. In Skeinforge 45, these are all collected in the Speed plugin.

The very slow first layer ensures good adhesion to the Kapton build surface, with the rebuilt HBP now maintaining a very stable 0.25 mm across the whole platform. I’ll try goosing the first layer infill to 20 mm/s and the perimeter to 15 mm/s at some point, but this is entirely tolerable; I’d rather have it Just Work than occasionally come unstuck.

The 20 mm/s perimeter reduces the Extruder Zittage problem, with the 9 mm/s Perimeter on the first layer coming out entirely zit-free. However, the sequential version of Amdahl’s Law applies here: a slow perimeter around a fast infill produces a fairly slow overall layer. Making the infill rather sparse doesn’t help, of course, but overall it’s a win.

This collection of speeds hopelessly confuses Pronterface’s estimated print time calculation; the most amazing prediction reported just under 24 hours for a fairly simple set of objects that took maybe half an hour. A recent gizmo had an estimated time of 4:34 and an actual time of 28:07, off by a factor of 6.2. If Pronterface divides the total filament length by the first speed it finds in the file, it’d be off by a factor of 6.7, so maybe that’s close to what happens under the covers.
2012-01-29
Reversal Zits: Speed, Acceleration, and a Bestiary
The Skeinforge Dimension plugin subsumes the obsolete Reversal plugin’s features. At the end of each thread, if the nozzle will move more than the Minimum Travel distance (1 mm by default, which is what I’m using) to the start of the next thread, the extruder yanks Retraction Distance of filament out of the hot end at the Retraction Speed.

Some experimentation at 30 mm/s showed that 2 mm of filament would eliminate all drooling, 1.5 mm left thin threads, and 1.0 mm wasn’t nearly enough.

Similar experimentation suggested 60 mm/s as the upper limit for Retraction Speed, with the SJFW acceleration limiting parameters set for 250 mm/s². The usual extrusion speed isn’t much faster than a crawl, so the distance required to reach a backwards 60 mm/s is:

dist = (60 mm/s)² / (2 * 250 mm/s²) = 7.2 mm

What that means, of course, is that the extruder doesn’t have enough torque to reach the programmed speed in the required distance. Assuming SJFW uses trapezoidal limiting, it will accelerate to some maximum speed at the halfway point and decelerate to a stop at the same rate. Pegging the midpoint at 1 mm, the extruder will reach a peak speed of:

v = √(2 * 250 mm/s² * 1 mm) = 22 mm/s

In order to hit 60 mm/s in the middle of the retraction, the extruder must accelerate at:

a = (60 mm/s)² / (2 * 1 mm) = 1800 mm/s²

Which requires way more torque than the piddly little motor I’m using can provide.

While I could swap in that larger motor, crank the current up a bit, and goose the extruder acceleration, the current Reversal Zittage is small enough for my purposes. I’d rather expend that effort on doodling up a direct-drive extruder, but that’s on the back burner until something horrible happens to the current extruder.

One easy alternative: lower the perimeter speed sufficiently far as to reduce the pressure in the hot end enough that the current speeds can suppress the zits. Notice the difference in the pix below; what you can’t see is that the first layer has no zittage whatsoever. Of course, that means the perimeter must trundle along at maybe 10 mm/s…

Herewith, a Reversal Zittage bestiary at various perimeter speeds, with Dimension set as described above and these extrusion settings:
- 0.25 mm layer height
- 0.50 mm thread width
- 60 mm/s infill
- 250 mm/s travel
A Dishwasher Rack Protector vertical tube at 30 mm/s:

Rack protector – Reversal zits

The tube’s interior had equivalent zits that cleaned out easily with a twist drill.

Some of the half-tube ends came out slightly angled with zits here & there, but remember that they’re 4.5 mm tall:

The Zire 71 Protector had a lot more infill with very few perimeter joints. This corner shows a few zits at 30 mm/s:

Zire 71 protector – Reversal zits

One of the Dr. Who Cookie Cutters showed much more conspicuous zittage on the inside of a corner at 20 mm/s:

Dr Who cutter – Reversal zit – interior corner

Than on the outside of the same corner:

Dr Who cutter – Reversal zit – Exterior corner

The zits on the other cutter fell along one edge. The inside:

Dr Who cutter – Reversal zits – interior side

And the outside:

Dr Who cutter – Reversal zits – exterior side

The Dr. Who set included flat cookie presses with patterns. Although these islands show some zittage, they’re about 1 mm tall and perhaps 5 mm long:

Dr Who cutter – Reversal zits – islands

The rest of the perimeter extrusions look essentially perfect, so these really are very minor imperfections.
2012-01-26
Dr. Who Cookie Cutters
These Dr. Who themed cookie cutters came out nearly perfect:

Dr Who Cookie Cutters

Each consists of an outer cutter rim and an inner dough press that fit neatly together.

The STL files contain a few triangle errors that seem to be typical of objects made with Google Sketchup, but the final G-Code came out fine despite a few Skeinforge warnings.

No strings, no cleanup, no muss, no fuss: the printer is back in operation once again!

The relevant Skeinforge 45 settings, about which more later:
- 0.25 mm layer thickness + 0.50 mm thread width
- First layer: 9 mm/s perimeter + 15 mm/s infill
- Other layers: 20 mm/s perimeter + 60 mm/s infill
- 250 mm/s travel (!)
- +0 extra shells, 3 solid layers
- 0.20 infill + 45°/90° rectangular
- 200 °C extrusion + 110 °F platform
Dimension plugin settings:
- Filament dia = 2.96 mm, FPD = 0.93 (natural ABS from MBI)
- Retraction 2 mm @ 60 mm/s, min 1 mm travel
I’m not a big Dr. Who fan, but I know someone who is…
2012-01-25
Shower Basket Sucker Relocation

The outer suckers on the basket in the corner of the shower didn’t line up with the tiles; either tile dimensions have changed in the last half-century or it’s a hard-metric basket. It didn’t look right when I installed it (now that is a grandiose term if I’ve ever misused one), so (when the thing fell off and landed with a clatter a few days ago) I drilled two additional holes as far away from the corner as I could, using a step drill to prevent the plastic from shattering, and it’s all good.

Shower basket – redrilled

Sometimes, they’re easy…

You’ll note that I heroically resisted the urge to fire the Thing-O-Matic to print some kind of weird-ass safety-orange interposer plate, just because I could.

2012-01-24
HP 50g Calculator Screen Protector

A week or so after I got my HP 49GX calculator, I managed to drop a vernier caliper on it. Interior points downward, of course, putting a nice divot on the non-glare plastic over the LCD panel.

A week or so after I got my HP 50g calculator, I applied a screen protector sheet harvested from the lifetime supply I bought for my original Zire 71, back in the day.

HP 50g calculator screen protector

The fact that it’s an almost perfect fit and that the calculator sports a monochrome LCD with lower resolution is a sad commentary on the state of the calculator art.

Taking that picture in low-angle full sunlight makes the protector sheet look awful. In actual use, it’s nearly invisible. Haven’t dropped anything on it yet, either.

And, yes, I did cut it out around the HP logo button in the upper right corner.

2012-01-23
Brita Pitcher Lid Hinge Redux

Well, that fix didn’t last nearly as long as I’d hope, although I must admit whacking the pitcher lid against the refrigerator door certainly hastened its demise.

So I found a suitable screw in the Tiny Box o’ Teeny Screws (in a sub-container of eyeglass repair screws), drilled a snug hole where the plastic pin used to be (entirely by hand on the drill press, feeding the lid into the drill), and snapped everything together again:

Brita pitcher lid hinge – screw

The remaining plastic pin had a fracture at its base, but I just glued it and will defer installing a screw until it finishes disintegrating. At some point we’re going to be forced to buy a new pitcher…

2012-01-21

Zire 71 Printed Button Shield

After fixing that old plate, I just had to do this:

Zire 71 protector - solid model — Zire 71 protector – solid model

Which pretty much fills up the build platform:

Zire 71 protector - on build platform — Zire 71 protector – on build platform

And fits perfectly:

It’s printed with 100% infill to produce a solid plastic plate.

In retrospect, I think it’d work better if I put the notch on the bottom side with a bit of support, so that the glass-smooth surface faced the Zire. Maybe next time?

The OpenSCAD source code:

// Protector plate for Zire 71 PDA
// Ed Nisley KE4ZNU - Jan 2012

//-------
//- Extrusion parameters must match reality!
//  Print with +0 shells, 3 solid layers, 0.2 infill

ThreadThick = 0.25;
ThreadWidth = 2.0 * ThreadThick;

Protrusion = 0.1;           // make holes end cleanly

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

//-------
// Dimensions

$fn=8*4;

Length = 110;
Width = 73;
Thickness = IntegerMultiple(2.0,ThreadThick);
CornerRadius = 5.0;

NotchLength = 20;

PocketWidth = 22;
PocketLength = 12;
PocketRadius = 3.0;
PocketOffsetX = -1;
PocketOffsetY = 10;

SlotLength = 20;
SlotWidth = IntegerMultiple(8.0,ThreadWidth);
SlotDepth = IntegerMultiple(0.75,ThreadThick);

//-------

module ShowPegGrid(Space = 10.0,Size = 1.0) {

  Range = floor(50 / Space);

    for (x=[-Range:Range])
      for (y=[-Range:Range])
        translate([x*Space,y*Space,Size/2])
          %cube(Size,center=true);
}

//-------
// Outer section of plate
//  with nice rounded edges

module PlateHalf() {

  translate([0,0,Thickness/2])
    difference() {
      minkowski(convexity=3) {
        cube([(Width - SlotWidth)/2 - 2*CornerRadius,(Length - 2*CornerRadius),(Thickness - 2*Protrusion)],center=true);
        cylinder(r=CornerRadius,h=Protrusion);
      }
      translate([PocketOffsetX,PocketOffsetY - Length/2,0])
        minkowski() {
          cube([PocketWidth - 2*PocketRadius,PocketLength - 2*PocketRadius,Thickness],center=true);
          cylinder(r=PocketRadius,h=Protrusion);
        }
    }
}

//-------

ShowPegGrid();

translate([(Width - SlotWidth)/4 + SlotWidth/2,0,0])
  PlateHalf();

translate([-(Width - SlotWidth)/4 - SlotWidth/2,0,0])
  mirror([1,0,0])
    PlateHalf();

difference() {
  translate([0,0,(Thickness - SlotDepth)/2])
    cube([SlotWidth + 2*Protrusion,Length - 2*SlotLength + SlotWidth,(Thickness - SlotDepth)],center=true);
  for (Index=[-1,1])
    translate([0,(Index*(Length/2 - SlotLength + SlotWidth/2)),-Protrusion])
      cylinder(r=SlotWidth/2,h=Thickness + 2*Protrusion);
}

2012-01-20

Category: Machine Shop

Skeinforge Feed Settings

Reversal Zits: Speed, Acceleration, and a Bestiary

Dr. Who Cookie Cutters

Shower Basket Sucker Relocation

HP 50g Calculator Screen Protector

Brita Pitcher Lid Hinge Redux

Zire 71 Printed Button Shield