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Glue on the alignment pins


During the placement of the (small) modules on the first disk, we noticed that some of the modules wobbled a bit. The modules + feet were checked on a granite flat, and found to be flat. The support disk also was flat, and the wobble was not due to excessive thermal grease. The problem was traced to a 'shoulder' of epoxy where the tiny alignment pins were glued into the disk. This cannot be reliably seen under the microscope, since you're looking down on the pin, and cannot get a good profile view of the pins (0.8mm diameter, sticking up ~0.8mm). We dismounted the modules until a solution was found.

There were two possible solutions:

  1. Cut a countersink into the bottom of the graphite feet, just enough to accomodate the glue shoulder.
  2. Try to remove the glue shoulder.
Both methods were tried using an unused graphite pedestal. I made a 90° countersink by filing the tip of a finishing nail (visible in the photograph below). This works well, and does not affect the hole diameter along most of it length. However, this would have to be done on fully assembled modules, and it generates (a small amount of conducting) carbon dust.

The second method requires a sharp eye (or the use of a microscope), and takes a little longer. but can deliver guaranteed results, and it is safer. Below is how to do this operation:

Take a small, short pedestal, and place it over a pin. Don't push it down. Under a bright light, with the tip of a scalpel, touch the spot indicated by the red arrow. If there is a glue shoulder, the pedestal will tip up a bit, and you can see it move in the reflected light. The second test is to hold the plate up to the light and see if you can see light under the pedestal.

If there is a glue shoulder, use the tip of the scalpel to clean all around the base of the pin. Repeat the test until the pedestal no longer tips when touched.

When you have checked all 24 pins on a side, take a fresh small pedestal and repeat the procedure for all pins. Graphite is very soft, and the first pedestal may have worn during the first round. I marked the used pedesals with an 'X'. Repeat on the other side of the disk.

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Hubert van Hecke
Last modified: Sat Jan 15 13:45:22 EST 2011