Gas station without pumps

2023 January 2

Ceramics class?

Filed under: Uncategorized — gasstationwithoutpumps @ 17:17
Tags: , , , ,

I’m hoping to be able to get into the beginning ceramics course taught through adult education at Santa Cruz High.  It is one of those courses where the registration opens at a specific time, and if you aren’t on the computer obsessively refreshing, then you might not get in. (I’m not telling when the window opens—I don’t want more people competing for the slot!)

Although I have long had a fondness for pottery as objects, I’ve never learned to shape clay myself. I don’t expect to get good at it, but it should be fun to learn first-hand how much skill is involved.

I do have one specific project I’m interested in doing—replacing a broken (and badly mended) soap dish.

I had two of these soap dishes, but at various times they have been knocked off the counter in the bathroom and broken. This one was mended with superglue, but I’d really like to replace it.

I can’t remember who we bought the soap dishes from originally (I showed one to a few potters that I thought might have made it, but they did not recognize it as anything they or anyone they knew made), so now I’m thinking of making one myself. I think it should be a doable project for the beginning ceramics course.

One of the potters I showed it to suggested that the soap dish had been made with a clay extruder, which seems extremely likely to me. I sent email to the instructor of the ceramics course, asking about whether there was a clay extruder available in the studio. There is, but she did not know what model, and she suggested that I wait to see whether I get into the course before looking into it further.

I decided to start designing a clay-extruder die to roughly match the cross-section of the soap dish. I used SVG (manually edited as text, but viewed with Inkscape) to create the outline. I then printed a thin piece of plastic with half the outline cut out, to see how close I had come. It took me 3 iterations to get an SVG file that satisfied me as being close enough.

Here is a PNG rendition (from Inkscape) of the 3D shape. The image is upside down (because Inkscape has positive Y down the page).

I looked around for ways to create custom extruder dies, and found that there are various materials used for the dies: ⅛” aluminum,  ½” HDPE, marine-grade plywood, and 3D-printed PLA. I have no way to do precision cutting of aluminum, HDPE, or plywood (though I could probably get a membership at Idea Fab Labs and learn to use their CNC mill), so I decided to try designing for 3D printing, even though my 3D printer is not big enough for the approximately 112mm square dies used by the North Star extruders. I can find other 3D printers around town or on campus and use them, if needed.

I used OpenSCAD to create the model. It was very easy to do a simple linear extrusion of the SVG file, but the clay extruder dies I’ve found online all have heavily beveled edges on the top side of the die, to compress the clay as it enters the die. That was a bit harder to manage in OpenSCAD.  I ended up using the plugin for Inkscape to convert the SVG file into an OpenSCAD file that could generate points along the curve, then I used the BOSL2 library for OpenSCAD to generate an offset outline for the entrance shape on the top surface of the die and to “skin” between the exit shape and the entrance shape. I had to fix one bug in, as it had a check for strings that only worked in Python 2, not Python 3.

Here is an image of the STL file showing the beveled edge on the top of the die.

I don’t think that I’ll end up printing this STL file, as it would take 40 hours to print on an Ultimaker 3 (at 0.1mm resolution and 100% fill). I could make a cruder version in about 5 hours (at 0.2 mm resolution and 30% fill), but I don’t know whether it would be strong enough to withstand the pressure of the clay. Even if I never 3D print this die, it was fun figuring out how to create it in OpenSCAD.

I looked for other ways to convert my design to an actual die and found a few:

  1. I could use a commercial 3D printing service, but ones like Shapeways would charge around $140.
  2. I could get North Star (who make extruders) to use their CNC mill to make one for about $40 out of HDPE. I’d have to get them the SVG file somehow, as their ordering procedure for custom dies involves making a hand sketch on a paper form! Update 2023 Jan 3: North Star no long offers this service. “Unfortunately, we have discontinued our custom die service until further notice. North Star is focusing primarily on production of our main product line. Equipment, manpower, and materials are dedicated solely to this effort. We will review our ability to offer custom die services sometime in 2023.”
  3. I could order a 3D-printed custom die from a shop on Etsy ( for about $12—the procedure for submitting the SVG file there is pretty clear.  I need to find out how solid the dies are and how well they hold up—I’ll need to decide between their cheap dies and the more expensive ones from North Star (which are almost certainly stronger and smoother). Update: I contacted the seller, and the dies are printed with 20% fill and 0.2 mm height, which Cura estimates would take about 4:25 to print. The seller claims that they work as well as the North Star dies, but hasn’t used them long enough to know how durable they are. Prices are likely to go up as they get more experience with selling them.
  4. There is another Etsy shop selling custom dies for 4″ clay extruders, but they are a bit more expensive, and their images don’t show the beveling—I would certainly be better off with a North-Star-made die.
  5. I could join Idea Fab Labs and learn to use their CNC mill, which would cost me at least $150 (for a one-month “pro” membership).
  6. I could join Idea Fab Labs and 3D print there (I suspect that they’d allow a long print, especially if it was on one of their Ender 5 printers, rather than the more popular Prusas). But that would still cost me $110 for a one-month membership, and the 3D-printed version would probably not be as good as one milled by North Star.

Right now, I’m leaning towards getting a 3D printed die from the Etsy shop, since the price is low and I can live with wasting the money if the print fails. The North Star die is a safe backup.

But first, I have to get into the class and then find out what model extruder the classroom has.

Update 2023 Jan 6: I registered for the class successfully.


  1. First – this is awesome and I hope you enjoy it! I think you’ll like it if you make it in (I’ll keep my fingers crossed for you)

    I took a ceramics class a couple years ago and one of the things I learned is that clay shrinks during the process. When it’s wet it’s one size, by the time it’s bone-dry enough to fire it’s already shrunk some, and then it shrinks some more in the kiln. I could never get my cups to look right in part because they shrank a bunch during the process.
    (Thinking about it now – they have have actually shrunk differently on different axes. Like, it got more shorter than it got less wide. I don’t know how that would even be possible, but it might be what happened).

    Anyways, I’m really curious to know how you’ll size the die in order to produce the correct size final shape.

    Comment by mikethetall — 2023 January 2 @ 19:26 | Reply

    • I made the die outline based on the final glazed shape, so I was worried that it would be a bit too big. Stoneware clay shrinks between 11% and 13%, so maybe I’ll scale the die up a bit. []

      Thanks for the info—I forgot to look up clay shrinkage. Of course, for this particular application, it does not matter exactly what size the soap dish ends up, so if I get the size “wrong”, it will just be different, not bad. Differential shrinkage, causing warping, would be more of a problem.

      Comment by gasstationwithoutpumps — 2023 January 2 @ 21:24 | Reply

  2. Not as “you should have done it this way” but as additional information for others, or if you run into problems with this die.

    I initially moved from OpenSCAD to FreeCAD for basically all of my modeling because I frequently want to use fillets, and they are hard in OpenSCAD. Now I’m used to FreeCAD and rarely use OpenSCAD. FreeCAD’s fillets can be at different angles, such as for a die like this.

    FreeCAD has an OpenSCAD workbench, so I would expect it to be possible to use it to apply fillets to solids defined in OpenSCAD. I haven’t used this in practice because I just switched to using FreeCAD’s Part Design workbench from the ground up for most of my modeling, and haven’t been doing much in OpenSCAD in the past couple of years.

    Comment by Michael K Johnson — 2023 January 5 @ 05:43 | Reply

    • I agree that OpenSCAD makes fillets and chamfers much harder than they need to be (what is needed here is a chamfer, rather than a fillet). I wish that the capabilities of the BOSL2 library were fully integrated into OpenSCAD, which would help a lot. I’ve considered using other CAD tools, but I prefer a programming interface to a drawing interface (I can program, but I can’t draw well). It looks like FreeCAD does include a Python interface, so I’ll have to check out whether it would meet my needs better than OpenSCAD.

      Comment by gasstationwithoutpumps — 2023 January 5 @ 08:32 | Reply

  3. […] Mini: a die for a clay extruder, to fit a 4″ round Scott Creek extruder. Unfortunately, the extruder die design I had to match my broken soap dish will not fit—the biggest dimension is more than the 4″ […]

    Pingback by UCSC’s soon to be fab lab (and an existing one) | Gas station without pumps — 2023 January 19 @ 22:22 | Reply

RSS feed for comments on this post. TrackBack URI

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: