Tuesday, October 17, 2017

More Ceramic Vessels Made with Robots

In an earlier post I discussed getting started making ceramic vases using a clay extruder and a robot. After those initial attempts I realized I needed to make a few improvements. This post documents the refinements I've made and shows some of the results.

The improvements I wanted to make were:
  • Extrude in a continuous helix rather than individual contours to remove the seam on the side of the vase. 
  • Get the air out of the clay. Air bubbles wreak havoc with the walls of the pot. 
  • Automatically generate the bottom of the vessel so I don't have to manually attach a base. 

Helix Extrusions

Previously I was taking horizontal sections through the 3D form and moving vertically between them. That robot motion creates an ugly seam as shown on the early test pot pictured below:

My new method generates the robot path in a single continuous motion. The process works like this... (in these diagrams the vertical distance of the helix is expanded greatly and the number of points along the helix is reduced greatly to make them more readable)

From the centerline of the vessel horizontal lines are generated starting at the bottom of the vase projecting outward. The intersection of lines and the vase surface are computed (shown as green Xs below). These are connected together in sequence (shown as a green helix).

That forms the path of the robot motion. However this example is far too course a spiral.

This shows an accurate representation of the helix. The vertical spacing is about 3mm per rotation. There are usually about 150 points along each rotation.

Air Bubble Free, Soften Clay

It was clear that a consistently softened and air free tube of clay was critical to getting acceptable results.

The clay has to be softened to work with the extruder. To get more moisture into the clay I cut it into quads, vertically.

The I put towels into the joints. Then add 15 ounces of water to the 25# bag. I let this soak in for at least two days, rotating the bag a few times so all the clay gets exposed. When done the clay is much softer and appropriate for the extruder.

Getting the Clay into the Tube via a Pugmill

A pugmill is used to unify the moisture content in clay and remove air from it. It uses a powerful motor to push the clay through a set of grates which divide the clay into thin rods. These are then pushed through a vacuum chamber. This pulls all the air bubbles out of the thinned clay tubes. The clay is then unified back into a solid mass which is extruded through the end of the mill.

Here's a view of the pugmill disassembled so you can see the auger which pushes, mixes, and compacts the clay, and the grates the clay is pushed through.

Milling the Fixture

I cut the fixture which holds the tube to the pug mill on my router. The fixture was designed by Taubman College FabLab Manager Asa Peller - thanks, Asa.

It goes together with glue and screws. It holds the tubes tightly. And the circular inlet to the tube tapers from the 3" output of the pugmill to the 2-7/8" inside of the tubes.

You can see the finished fixture in the video below.

Milling the Tubes

3D Potter sells the tubes ready to use for $56 each. I wanted 9 so that's $504. That's not acceptable. So I bought  (3) 6' clear PETG plastic tubes, 3" O.D., 2.75" I.D. Those were $188 from McMaster-Carr. I just needed to cut them to length and mill the holes.

I cut plywood supports to hold the tube at each end. There's a dowel through the middle to align them precisely. They are cut 0.01" smaller than the tube diameter.

It's a simple process to cut them on my 4-axis setup. There are 8 holes at each end of the tube, 45 degrees apart. So I helix bore in a few times at increasing depth to get a hole at one end. Move to the other end and do the same. Then rotate the tube 45 degrees. Rinse, repeat - 7 more times.

Using the Pugmill

My research assistants Yixen Xiao and Issam Bourai helped with the pugmill use. Here's a video of it doing a great job loading up air-free clay!

It took 2 hours to load 10 tubes of clay. That was actually fun:

It then took me 4 hours to completely clean the pugmill. Painful! I should have extruded 50 tubes!! Well, I'll enjoy it while I have all this clay ready to go.

The Results

After all the preparation it was time to extrude some new, improved vases! Here's a look at the motion of the robot. As you can see the extruder is fixed in space. The robot draws the helix beneath it.

Here are some pots having just finished extruding and waiting to dry out:

More Refinement Needed

Extrusion of the base layers is messed up! This is easy to clean up manually but can easily be improved. The basic problem is how evenly the extruder runs at startup. That wavy pattern you see in some pots is uneven initial clay output. I can start at the center of the pot and work outward. That'll put the uneven flow on the inside. There are two floor layers so the second one will cover up the first.

It may be time to mount the extruder on the robot and see how that can improve things. This has some potential for better adhesion from layer to layer when cantilevering.

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