Electron Gun Progress

29 05 2012

The work on the electron gun armature is progressing nicely. Shapeways is currently printing this version:

Similar to the last one, but this time with a phosphor screen holder in the right shape for our phosphor shard.

This project demands a high level level of precision for its components. Everything needs to be exactly the right size and shape, and so in order to  use 3D printing effectively, we need to understand its limitations and work around them.

In all likelihood, there will be a problem with the armature above. Possible problems include:

1) The holes in the first column which are supposed to accept the hot cathode are smaller than they should be.

2) the columns are warped by the kiln-firing process.

3) Some unforeseen problem.

To address problem 1, We have also printed a hole-gauge:

The holes have a range of diameters, all clustered around 1.28 mm, the right size for the holes in the hot cathode holder.

The range of diameters in the gauge will give us a good idea of the relationship between the hole diameters in the OpenSCAD files, and the hole diameters in the printed part. If the holes in the printed armature are smaller(or bigger) than they should be, then this will tell us how to compensate in the next printing. Source code

Here’s the hole gauge in plastic

The hole into which the cathode is inserted was designed to have a diameter of 3mm, but as you can see, it tightly holds the cathode foot, which has a diameter of about 1.3 mm, which means that the diameter of the plastic version of the hole is 1.7mm bigger than that of the OpenSCAD version.

with further measurements, it might be possible to find a formula which converts OpenSCAD dimensions to real dimensions for this material and MakerBot setting, but that’s not important right now.

Another concern (problem 2), is keeping the columns straight. I’ve been in correspondence with the Shapeways, and according to them,  long thin pieces like the armature columns sometimes warp unpredictably when fired. This is problematic because a straight line of sight from the cathode, to the center of the accelerator, to the phosphor screen is integral to the electron gun’s operation.

If they don’t warp, great.
If they do, then we must alter the design so that this doesn’t happen, and reprint. Here’s a candidate:

In this version, the columns are buttressed in the x and y dimensions, so they shouldn’t warp. If they do anyway, or if something else goes wrong, then it’s back to the drawing board for another armature design, but that’s OK, because OpenSCAD and 3D printing make the prototyping process fast and inexpensive. Source code

Domenick Bauer




3 responses

30 05 2012

The holes will tend to maintain their roundness better if their openings are oriented perpindicular to the build direction than if they are parallel. You should print a hole gauge on a right angle bracket. That way you can get some idea of the effect of part orientation with respect to build direction.

31 05 2012

I’ll try that, thanks.

28 12 2012
Dr. Lawlor

Domenick, regarding undersized 3D printed holes, nophead’s “Polyholes” are the standard OpenSCAD solution:

I usually just drill holes to size, which results in a much cleaner cylindrical profile than I can print. But this would be problematic in ceramic…

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