Author: Ed

With the heater off for repair, I added a strip of self-adhesive stainless steel tape to the top of the plywood platform, directly under the heater. This should reduce the wood temperature and maybe, just maybe, reduce the thermal expansion that shifts the X axis location of the Z-minimum platform height switch.

It’s stainless steel because that’s what was in the Tape Lookaside Buffer; a hunk of aluminum tape, even a pair of 2 inch / 50 mm strips would work just fine.

Not shown here is the M3 screw through the front-center hole (invisible under the tape) that will eventually anchor the new heater connector.

While I was rebuilding the HPB heater wiring, I drilled / countersunk / tapped a 4-40 hole in the middle of the aluminum sub-plate for a screw to secure the middle of the heater PCB:

Remember: this plate is firmly secured to the plywood build platform with three leveling screws over springs. Another aluminum plate, with Kapton tape as the build surface, sits on top, providing an absolutely flat build platform. If you’re using a single plate, you could backfill the hole with a dab of JB Industro Weld epoxy atop a lightly greased screw, then file the top flush with the plate.

A flat-head screw harvested from a chunk of electronic junk came from the Drawer o’ Short 4-40 Screws and fit perfectly:

Mirabile dictu, the screw was short enough that it didn’t require any trimming to stay below the top surface.

Securing the center of the PCB to the aluminum plate cuts the heater’s free span in half: the PCB originally had screws only along the left and right edges. Its thermal expansion visibly bowed it away from the plate and I hope this will reduce that problem. Of course, now the PCB’s expansion has nowhere to go and those thermal stresses will probably begin chewing up the mounting holes.

While I was at it, I removed the MBI “heat spreader” tape from the PCB. I’d been reluctant to do that, for fear of peeling the traces right off the board, but the surface was in fine shape. Whew!

More on the wiring and epoxy blobbed brass tube later…

Unlike that pattern, this OpenSCAD program produces an STL file that gets sliced in the usual manner, so that the end result shows exactly how the first layer of all other objects gets laid down.

It’s two threads wide and one thread thick: customize the OpenSCAD code to match the settings in Skeinforge (or Slic3r or whatever you’re using) to make it build properly.

The two tabs mark the +X and +Y directions. The bottom surface will be wonderfully shiny from the build plate, so the symmetry along the diagonal shouldn’t pose a problem.

Should the thickness vary more-or-less linearly along any of the bars, then you know which way to level the platform. If it varies non-uniformly, then either the build plate isn’t flat or the printer has other problems.

The actual width depends on the actual thickness, of course: a too-low nozzle will create a too-wide pattern regardless of the extrusion settings. The thickness should be uniform across the entire pattern, so you can still adjust the platform leveling screws.

If you’re using a Z-minimum platform height sensor, now’s the time to adjust the switch touch-off height to make the thread thickness come out right.

When the thread thickness comes out right, then the width should match the extrusion settings: the bottom layer will be exactly like all the others. That’s the ideal situation, anyway.

A thickness snap gauge comes in handy for this sort of thing.

The OpenSCAD source code:

// Platform Level test pattern
// Ed Nisley KE4ZNU - Dec 2011

//-------
//- Extrusion parameters must match reality!

ThreadThick = 0.25;
ThreadWidth = 2.0 * ThreadThick;

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

BoxSize = 80;

//-------

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);
}

//-------

ShowPegGrid();

for (Index=[0:3])
  rotate(Index*90)
    translate([0,BoxSize/2,ThreadThick/2])
      cube([BoxSize,2*ThreadWidth,ThreadThick],center=true);

for (Index=[-1,1])
  rotate(Index*45)
    translate([0,0,ThreadThick/2])
      cube([sqrt(2)*BoxSize,2*ThreadWidth,ThreadThick],center=true);

translate([BoxSize/2,0,ThreadThick/2])
  cube([BoxSize/6,2*ThreadWidth,ThreadThick],center=true);

translate([0,BoxSize/2,ThreadThick/2])
  rotate(90)
    cube([BoxSize/6,2*ThreadWidth,ThreadThick],center=true);