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Last updated: 22 Sep 2000
Send any comments to: pbarber@mailer.fsu.edu
Remove the Penning Gauge from the system and locate a suitable, clean work surface. Loosen and remove the knurled ring just below the electrical connector - set aside. Carefully extract the anode assembly. This is the part attached to the electrical connector. Using the proper hex key wrench, remove the small hex head set screw that holds the anode to the electrical feedthrough, and set aside. Do not use a ball end hex key wrench for this, or you will gack up the screw. (and yes, gack is a technical term) The next step is to remove the gasket from the cathode assembly (the tube with the magnet). The gasket may either be of rectangular cross-section (original design) or it may simply be an o-ring (my design). Either is ok. Use great caution when extracting this gasket. Do not use a hard, sharp instrument such as a pair of forceps or a machinists scribe. If you do, you will most likely scribe a line in the sealing surface, creating a leak. If you can't remove the gasket or o-ring with your finger or finger nail, try something else soft, such as a flattened piece of 1/8" copper tubing or the wooden shaft of a q-tip, whittled to a blade shape. This elastomer should ideally be replaced at each cleaning, but sometimes new o-rings of this size can be scarce. Retain the old one in case you have to use it again. The Parker Number for this o-ring is 2-018. Viton is the preferred material. Now take the anode assembly ( the wire loop doo-dad ) and the cathode assembly ( stainless steel tube with the magnet on it ) to the student shop. ( yes, doo-dad is a technical term ) Using the large arbor press ( ask a machinist ) in the student shop, carefully press the stainless steel tube out of the magnet. This is done by selecting the slot in the press base which is closest to the cathode tube diameter and pressing on the non-gasket side of the tube while supporting the magnet on the base. Use something soft like a small piece of wood to protect the tube. Pay attention, or the tube will fall on the floor. If the tube's diameter gets deformed, it will no longer seal in a compression fitting, rendering it useless. Remember also, that the outer surface of the tube at the end opposite the gasket is an o-ring sealing surface and must not be scratched. With the magnet removed, all the little bits of magnetic crap that cling to the pole pieces can fall off. If you don't remove the magnet, it is almost impossible to get them all out. Next, the cathode tube should be glass beaded clean. The objective is to blast the inside while not blasting the outside. If you don't think that you can do this simply by being cautious, then use tape to mask those surfaces that must not be blasted, namely the gasket sealing surface and the non-gasket end of the tube that seals in the compression fitting. Blast until all carbon deposits are removed. Cleaning the anode is tricky. Be carefull and do the following: Reduce the glass bead machine's air pressure regulator to about 30 psi! You may have to step on the foot valve to release the downstream pressure before your changes are registered on the pressure gauge. Make several test sprays with nothing in the box to make sure that the pressure is reduced. Now put the anode in the box. Using extreme caution, blast the anode assembly from all angles. Make sure that the loop is not getting deformed. Visually inspect every area of the loop to make sure that you didn't miss any spots (you will, but that's ok - just put it back in and go again). Oh, and please, DON'T BLAST THE DAD-BLAMED GLOVES! It makes them go all gooey ( and yes, gooey is a technical term ). Blast the base of the anode as well, but try to avoid the threaded holes. If you can't, install sacrificial screws, and discard them when you are done. When done, run the blaster's air pressure back up to where it was, or about 60 psi if you can't remember. That way, the machinists don't have to listen to eighteen complaints about how the bead blaster isn't working... Take all the pieces-parts back to your work area. Using a small stainless steel scratch brush ( Grainger, stock no: 3A336 ) clean the vacuum side of the electrical feedthrough. Brush it as clean as you can get it. You really don't want to blast this part, because it can be difficult ( if not impossible ) to remove all of the glass beads from the insulator recess. Now for the final cleaning: Clean the cathode tube and anode in a warm Alconox solution and rinse well with warm tap water and then deionized water. Be diligent in removing all of the glass beads. Follow the di-water rinse with a solvent rinse - ethanol, methanol or acetone are all fine. Clean the vacuum side of the electrical feed through and the set screw with a solvent rinse. Clean the new (or old) gasket with a solvent wipe - do not use acetone; methanol or ethanol are ok. Do not lubricate the o-ring or gasket with vacuum grease. It isn't needed here. Take some clean paper towells, the cathode tube assembly and the magnet back to the student shop. This time, use the arbor press to re-insert the cathode tube into the magnet. Center the magnet on the pole pieces. This is best done by pressing on the gasket end of the tube while the magnet is held in place by the base unit. Again, the tube should pass through the base unit. Return to your work area with the cathode assembly. Use clean kim-wipes and solvent to clean up anything that got dirty in the shop. With gloved hands ( unpowdered latex or cotton is fine ), attach the anode to the electrical feedthrough. Note the small bend in the base of the anode assembly. When installed, this small bend forms one side of a spark gap with the grounded portion of the feedthrough. Install the anode assembly so that the spark gap distance is between 0.010" and 0.016" to prevent damage to the loop should arcing occur due to excess system gas pressure. Still wearing the gloves, install the new (or cleaned ) gasket. Install the anode assembly into the cathode assembly, noting the position of the guide pin. Re-install the knurled ring and tighten well. Sight down the axis of the tube from the open end and make sure that the anode is both Centered between the two pole pieces, and Perpendicular to an imaginary line through the two pole pieces.
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Last updated: 22 Sep 2000
Send any comments to: pbarber@mailer.fsu.edu