HP 7475A Plotter: Pen Holder Height Map Cap

The “pen holder” in an HP 7475A plotter carries the pen across the width of the paper:

HP 7475A - Pen Holder - overview

HP 7475A – Pen Holder – overview

Given that it was designed to carry pens, not knives, I wasn’t surprised that the spring-loaded finger clamping the knife adapter didn’t apply enough force to hold the adapter in place against the cutting forces. I figured a quick test of a gizmo to stabilize the adapter would be in order, even though I knew:

  • The pen holder doesn’t apply enough downward force
  • The knife adapter doesn’t have a depth-of-cut shroud around the blade

In order to build the gizmo, I need the carrier’s dimensions…

An overhead photo of the pen holder shows the layout in the XY plane:

HP7475A - pen holder - top view

HP7475A – pen holder – top view

I shouldn’t have used graph paper as a background, because the next step was to remove the background and isolate the carrier:

HP7475A - pen holder - top view - isolated

HP7475A – pen holder – top view – isolated

The carrier measures 26.8 mm front-to-back, so scaling a grid to match that dimension provides a coordinate system overlay:

HP7475A - pen holder - top view - 1 mm grid

HP7475A – pen holder – top view – 1 mm grid

The (0,0) origin sits at the lower left, so you can read off all the relevant coordinates as needed.

However, rather than go full-frontal digital, I resized the isolated image to 20 pixel/mm, turned it into a height map, and treated it like a chocolate mold or cookie cutter with gray values scaled to the desired height:

  • Black = background to be removed
  • Dark gray = 2.5 mm thick
  • Medium gray = 3.5 mm
  • Light gray = 7 mm
  • White = 10 mm

Drawing the walls with a 40 pixel diameter pen makes them 2 mm wide at 20 pixel/mm:

HP7475A - knife stabilizer

HP7475A – knife stabilizer

It’s painfully obvious why I don’t do much freehand drawing, although the knife adapter hole is supposed to be oval.

As with cookie cutters and chocolate molds, there’s no need for that much resolution, so I rescaled it to 4 pixel/mm, saved that tiny image as a PNG file, and handed it to OpenSCAD’s surface() function to get a solid model. This being a one-off, I typed this OpenSCAD source code directly into the OpenSCAD editor on the fly, then remembered to save it (!) before shutting down:

difference() {
  translate([0,0,-2.0]) render(convexity=10)
    surface("/long-and-tedious-path/HP7475A - knife stabilizer - scaled.png",center=true);

The mirror() transformation inverts the model top-to-bottom along the Z axis, compensating for the flip from drawing the height map as though the walls rise upward from the pen carrier, after which the flip() transformation puts the flat side down to make it buildable.

The height map image conversion produces a bazillion irrelevant faces, but it’s quick and easy:

HP7475A - Roland knife stabilizer - height map model

HP7475A – Roland knife stabilizer – height map model

I’ve been using Slic3r’s Hilbert Curve  pattern for top & bottom infill to get a nice textured result:

Roland knife stabilizer - height map - Slic3r preview

Roland knife stabilizer – height map – Slic3r preview

Which printed just about like you’d expect:

HP 7475A - Roland knife adapter and stabilizer - height map - bottom view

HP 7475A – Roland knife adapter and stabilizer – height map – bottom view

I reamed out the hole with a step drill (the HP pens are close enough to 7/16 as to make no difference here) to get the knife adapter to fit, but the walls and suchlike came out close enough.

Then it just snapped into place:

HP 7475A - Roland knife adapter and stabilizer - height map

HP 7475A – Roland knife adapter and stabilizer – height map

Actually, no, it didn’t just snap into place: some (dis)assembly was required.

First, remove the brass knife bearing from the adapter, push the knife adapter shell into the pen holder, slide the stabilizer cap down over the adapter, press it firmly around the pen holder, reinstall the brass knife bearing, then it’s ready.

The cuts in the green vinyl just to the left of the knife blade (in a window decoration sheet I spotted in a trash can) show that the blade can cut, albeit with some finger pressure, but the fancy red stabilizer didn’t stay stuck on the pen carrier nearly as well as I expected. A screw attachment will help with that, which calls for going all digital on those coordinates.

But it was quick & easy…



  1. #1 by solaandjin on 2015-04-27 - 14:54

    Forgive me for the heresy, but have you ever considered trying a “real” CAD system? You’re a whiz with the OpenSCAD, but when modeling more complex objects, sometimes it’s a lot easier to do some things visually. In the past, the options for a CAD program with power, free (to use)-ness, and that ran on Linux were practically non-existent, not counting some programs of ludicrous difficulty (Blender) or of vaguely experimental open-source quality (FreeCAD). But now there’s OnShape, which is very good, free for five “active” models (you can toggle “activeness”), and pretty easy to learn. Claims to run on Linux. Invites to their beta come pretty quick.

    • #2 by Ed on 2015-04-27 - 20:25

      have you ever considered trying a “real” CAD system?


      I’m continually frustrated by finding a Thingiverse widget that’s almost what I need, but I don’t have $10k lying around for the CAD program required to tinker with the “source code”.

      OpenSCAD puts a definite upper bound on model complexity (although nophead’s Mendel90 shows what can be done by someone with high-end coding chops and a mission), but I can build fully parametric models using Free Software: anybody can make use of those models, at any time, anywhere, for any reason. Even better, the model isn’t locked into a program that will become obsolete or, worse, abandoned after a few years; even if the OpenSCAD project dies, I’ll still be able to run OpenSCAD and (with a bit of a tailwind) compile it as needed.

      Unleashing find ! -path \*MCAD\* -iname \*scad | wc -l on the directory containing (most of) my models reports 267 files. That’s an overestimate, because some came from Thingiverse and others represent dead-end trials, but let’s say I’ve conjured 200 original models. A limit of five “active” models makes no sense, but I don’t have $100/month lying around for the “professional” version, either.

      So, yeah, sometimes I’d really like to just doodle up a design, but … most of my stuff looks like a bracket, so the models aren’t really all that difficult.

      • #3 by solaandjin on 2015-04-27 - 23:09

        The five active documents is for active private documents. Public documents are unlimited. You can toggle the “active” status easily, so only people with a lot of private models that need to be worked on simultaneously will need to fork up. I agree with you about the lock-in downside; even with CAD interchange formats like STEP and IGES, you still lose the history and parametric features, and there is no interchange format that retains all those things.

        I like OpenSCAD, too, but hand-entering polygon coordinates and trying to remember which direction things are supposed to run is no fun. I think Fusion 360 (Windows and Mac only, so I didn’t mention it earlier) and OnShape are very exciting developments, and the first really powerful options available for cheap people like me.

        • #4 by Ed on 2015-04-28 - 08:16

          hand-entering polygon coordinates

          My rule of thumb: never, ever enter anything that’s not a direct measurement of a physical property.

          Everything else gets computed from those measurements, which is what you’d have to do anyway in a parametric CAD “drawing program”, so OpenSCAD gets me almost all the way to the goal. All the rest is just, mmm, window dressing… [grin]

      • #5 by solaandjin on 2015-04-27 - 23:12

        Oh, I might as well add, even they are not a really suitable option for “modify a thingiverse object”. STL is basically a write-only format, even with things like Meshmixer available. I end up redrawing things from scratch just so I can have a nice parameterized history very often.

        • #6 by Ed on 2015-04-28 - 08:17

          redrawing things from scratch

          Or rebuilding the parametric model they should have started with. [mutter]

          The real problem is not having the “source code” for the objects, which is a problem for CAD designs: everything’s in a silo. Right now, I don’t know any way around that, other than modeling my brackets the hard way…