SEM Buildblog

Turbomolecular porn by hslphotosync on Flickr.

The turbomolecular pump has been attached! The largest flange on the chamber is a KF-50 for various reasons including cost of fitting hardware. Above this size, ISO fittings must be used and these are much more expensive than the KF style.

The turbopump that was donated to the project uses a 4.5” CFF conflat style metal knife gasket mount. 4.5”CFF to KF-50 adapters are several hundred dollars so it was decided instead to machine an adapter out of aluminum and use a square lathe-cut viton gasket instead of the soft metal gasket. The gasket was purchased from ANCorp for $14.50. The 3x5” aluminum stock was purchased from Industrial Metal Supply for $22. But of course nothing is free, the 10 hours or so it took to machine the piece is worth something right?! Luckily time IS free!

The profile of the piece was turned first, with the KF-50 end on one side and a square shoulder on the other.

Though I suppose its not completely necessary when using viton rubber instead of metal as a gasket material, I attempted to machine the knife edge into the face of the CFF half. The circular sawtooth would normally bite into a soft copper gasket but of course that’s not possible when using aluminum for the flange material as the aluminum is too easily deformed. Stainless is too expensive and I’m not sure I have the skill required to pull it off just yet… The knife edge came out OK, though slightly over sized. I must’ve been tired or something.

Finally the hole pattern was added using a mill and a (horribly undersized) rotary table. A very long day at the lathe, and a somewhat tired yet nonetheless glorious victory!

Edit: If I made this piece again, I would probably have attempted to make it tapered from the CFF bore to the KF-50 bore. If I ever come across enough free time and another hunk of metal I might just try to machine another one and compare the pumping speed between the two.

Compression Fitting Assembled with BA Gauge

The BA gauge which I want to use to measure the ultimate vacuum level is a glass tube type and lacks a flange. The solution is either to torch a metal fitting out of kovar (expensive) or to use a simple viton O-ring compression fitting. Since the latter isn’t destructive and I don’t yet trust my glassblowing skills, I went with the compression fitting.

Instead of buying a compression fitting from a vacuum supplier, I decided to go ahead and turn one on the lathe for practice. The combination of threading, tight tolerances, dissimilar materials and knurling made this the most advanced lathe project I have yet taken on.

An exploded view of the pieces is shown below

In order to prevent any burrs in the threads of the steel nut, a thread form was turned and is shown below. Made from the same steel, the threads were worked against the nut threads under force for multiple mate demate cycles until the motion was smooth.

And finally a photo of the gauge connected to the chamber. Just one step closer :)

Edit: If I made this piece again I would have added a small internal shoulder for the neck of the gauge to sit on so that it couldn’t be pushed all the way into the flange and potentially hit things inside the chamber.

High Voltage Feedthrough Experiment on Flickr.

So it turns out that spark plugs make terrible high vacuum devices, and this became quite apparent before even introducing one into the chamber. While attempting to “alter” one on the lathe, the metal sheath became very loose and it was evident that there was neither an o-ring nor any sort of sealant nor braze connecting the ceramic core to the metal jacket which contains the threads.

A couple of lathe-hours later…

“Yo dawg, I heard you liked holding work-pieces so I put a chuck in your chuck so you could use inner jaws while you use outer jaws”

Upon discovering this I simply removed the entire jacket and turned the profile of the Al2O3 core into a custom KF-25 flange. The fit was so snug it required a special jig just to remove once I had “test fit” the part.

Add in some hysol 0151 low out-gassing epoxy and viola! A high voltage feedthrough :)

Vacuum Leak Plot annotated on Flickr.

The HSL SEM Chamber has several tiny pinhole leaks in the welds. The pinholes were detected by spraying the welds with isopropanol from a spray bottle and watching the response as the alcohol leaked through and then vaporized (and spoiled the vacuum momentarily).

Here I stand, mighty spray bottle in one hand, and determination in the other :) (Photo by Erica Weems)

As you can tell from the plot above, painting the holes with clear nail polish (nitrocellulose laquer) while under vacuum made a nice seal as the liquid was pulled through the leaks and solidified. The DV-6M gauge was brought to 10mV which indicates ~10^-3 Torr.

The next step involves getting cheap surplus valves to seal off the chamber and estimate leak rate in prep for attaching the turbomolecular pump to the system for an ever deeper vacuum. WE NEED TO GO DEEPER!

Just another day lathing up vacuum fittings!

Heatsync Labs has aquired a 12x36 lathe and I have been learning to use it for turning KF/NW vacuum flanges. This beauty is an 18” NPT to KF16 flange turned out of aluminum. These flanges mate to our DV-6M and DV-4D vacuum gauge tubes which are used to measure the vacuum level between ~10 Torr and ~10mTorr. The flange on the right was purchased from Ted Pella and is shown for comparison. They both hold a nice seal, though the NPT threads are not high vacuum and will need epoxied with a (low cost!) low outgassing vacuum epoxy (Hysol 0151)

SEM Vaccum Chamber by hslphotosync on Flickr.

The vacuum chamber has been welded!! HUGE thanks to Karl Kammerzell and his welder David Iannaccone for making this project possible. With this HUGE obstacle out of the way I am excited to proceed onwards!

Removing the paint from the chamber. This takes FOREVER!!

Lid welds have been verified!

The SEM welds have been leak tested and verified to be good to at least 10^-3 Torr! Click below to read on and see some vacuum porn!