If you are new to UHV vacuum chambers and how to create a seal using copper gaskets when mounting optics, this blog post has some useful tips.
In many cases, installing the copper gaskets that are used to seal flanges on UHV vacuum chambers is as simple as placing the copper gasket in the knife edge recess. If the flange is facing down, then the gasket can be placed on the optics part being installed and the gasket will stay in place.
But in cases where the flange is perpendicular to the floor, the gasket will not stay in place on its own because the gravitational constant will prevent the gasket from staying in place. In other words, the gasket will fall.
There are a few ways to install a gasket onto the flange. But first, you need to make sure you’ve removed the old gasket properly.
When you remove the old gaskets, you want to be very careful not to nick the knife edge on the flange when removing the old copper gasket because a nicked knife edge often results in a vacuum leak. And, nicked knife edges are difficult and sometimes very expensive to repair.
Usually the gaskets will come off easily with a minimum amount of force. For those gaskets that are pressed in tightly and very hard to remove, I have found that using a long-nosed vice grips locking pliers works quite well.
You can adjust the gap on the pliers so that it firmly clamps down on the gasket, then simply bend the pliers so that the leverage will pop the gasket off the flange. It works every time and most importantly, it follows the number one rule of flanges – protect the knife edge. You can use a screwdriver to pry up the edge of the gasket, but if the screwdriver slips, you risk damaging the knife edge.
Two resources for removing gaskets:
Removal of metal UHV gaskets; Journal of Vacuum Science and Technology 12, 654 (1975) https://doi.org/10.1116/1.568642 https://avs.scitation.org/doi/abs/10.1116/1.568642?journalCode=jvs
A UHV-Gasket Removal Tool; Published March 1, 1983 https://www.nist.gov/publications/uhv-gasket-removal-tool
Installing New Gaskets
Method 1: Gasket Clips
The first method is to use a gasket clip. Gasket clips hold the copper gasket to the flange via a spring action. They line up with the flange’s leak check groove and hold the gasket in an area that is just past the knife edge of the flange.
Here is a link to the gasket clips that Ideal Vac provides:
Gasket clips work well most of the time. Sometimes they will not work due to geometry limitations with other nearby flanges or optics.
If you do not have gasket clips, there are some other ways to mount a copper gasket to a horizontal flange.
Method 2: Elongate the Gasket
The second method is to elongate the gasket. This works well for larger copper gaskets, such as those for 10-inch and 8-inch flanges. It works with smaller gaskets as well, but you will need to drop them from a higher distance from the floor.
For an 8-inch gasket, hold the gasket about 1 foot above the floor. Hard concrete floors work best. This technique will not work on carpet.
Drop the gasket on its edge and it will hit the floor and bounce back up. You need to catch it when it bounces back up. If you don’t catch it and it falls to the floor, that is OK. You will just need to clean the gasket off with some isopropanol or methanol.
It takes a little bit of practice to get the correct height from which you are dropping the gasket. But the general rule is that the smaller the gasket, the higher the height. I have used this technique on 10-inch to 4.5-inch gaskets with good results. 10-inch flange gaskets should be dropped from about 6 inches. 4.5-inch gaskets should be dropped from about 18 inches. 2.75-inch gaskets are more difficult as they are harder to deform than the larger gaskets.
When you insert the gasket into the flange, you need to press it into the knife edge recess. The slight elongation will act like a spring and the gasket will stay in place.
Method 3: Cellophane (Scotch) Tape
Which brings us to the third method for mounting copper gaskets – cellophane (Scotch) tape.
You can use Scotch tape to mount the gasket as long as the tape is just barely on the gasket. Since the tape is mounted outside the knife edge, then it is OK if any of the tape stays on the gasket because it will be on the air side of the knife edge.
The pictures below show the knife edge cuts on common gasket sizes. For most copper gaskets (2.75- to 10-inch flange sizes) the knife edge is approximately .100 inches from the outside edge of the gasket. For the very small 1.33” flange, it is .050 inches from the gasket’s outside edge.
The procedure is to place the tape to where it is barely on the gasket (outside the knife edge region) and then very lightly touch the flange. You can use 2 or 3 sections of tape as needed.
Once you get the optics part mounted to the flange with just a few nuts or bolts to hold it in place, pull the tape straight up and away from the flange. Usually all of the tape will come out. But if any small piece of tape is left behind, it will not matter since it is on the air side of the knife edge and will not have an effect on the vacuum.
Rotate the flange slightly to make sure that the copper gasket is properly seated before you tighten the bolts.
And don’t forget that if you are up to air for awhile and need to cover a flange or optics component with aluminum foil, you should use a UHV foil such as All Foils. UHV foil does not have an oil coating on it like foil from the super market does.
RBD Instruments provides copper gaskets for less than what most other vacuum supply companies charge. – https://rbdinstruments.com/phi/uhv-parts.html
Tightening a CF con flat copper gasket
There are 2 schools of thought on tightening copper CF gaskets. The most prevalent is to use a star pattern where you crisscross the bolts that you are tightening as shown in the drawing below.
The other method is to simply tighten in a circle pattern (which is the method that I prefer). The trick with this method is that you need to use very small increments of increased torgue as you move from one bolt to the next, otherwise you can over-tighten one section of the gasket or possibly bend the flange out of shape which would result in a leak. But, going in a circle is easier to keep track of which bolt is next.
Here are some links to other posts on the subject of how to tighten a CF copper gasket, so that you can make your own decision on which way is best. The top link has some information on recommended torgue for different size flanges.