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Machining Vacuum Chamber Parts: What Separates a Pass From a Leak

September 21, 2026

A chamber part fails in one of three ways: it leaks, it outgasses, or the sealing face is out of flat. All three are decided by process sequence and surface condition, not by whether the shop can hold a dimension.

Sequence is the design

A chamber body that is welded from plate and then finish machined behaves. One that is finish machined and then welded almost never does. Welding pulls metal, and a sealing face that was flat before the weld is not flat after it.

A sane sequence looks like this:

  1. Rough machine the components with stock left on sealing and locating features
  2. Weld, full penetration on anything that is part of the vacuum boundary
  3. Stress relieve
  4. Finish machine sealing faces, port positions and bores
  5. Clean, inspect, leak test
  6. Finish, if any, then final clean and pack

Write the sequence on the drawing or in the purchase order. It is the difference between a part that passes and a part that argues.

O-ring grooves decide the seal

The groove, not the flange, is the seal. Three things matter and all three are cheap to specify:

  • Groove dimensions to the seal supplier standard, with the tolerance on width and depth, not just a nominal
  • Surface finish in the groove and on the mating face, commonly 16 to 32 Ra microinch on the groove and smoother on the face
  • No radial scratches across the sealing face, which means the last cut direction and deburring method matter

A groove finished by a tool that left a spiral path across the seal line will hold in a test and leak in service.

Surface finish and why smoother is not always better

Smooth surfaces trap less gas, so vacuum facing surfaces are usually specified in the 16 Ra range and gas wetted surfaces smoother still. Going tighter than the process needs costs real money, because every step down means another operation, a slower feed or an electropolish step.

Specify Ra with the area it applies to. A single note that says 16 Ra everywhere prices the whole part at the tightest number on it.

Leak testing and what to require

Helium mass spectrometer testing is the normal method for a vacuum boundary. Pressure decay finds gross leaks only. Require the method, the rate and a record and require it after the final finish, not before, because plating and polishing can open a marginal weld.

Ask for these three with the shipment: the material certificate, the weld record or welder qualification and the leak test report with the rate and the method. A part without paper is not qualified, whatever it measures.

Cleanliness is part of the part

A chamber part that is machined correctly and cleaned badly is a contaminated part. Cutting oil in a blind hole finds its way to the wafer. The cleaning and packing chain is covered in what cleanroom machining actually requires, and the material side in aluminum alloys for semiconductor parts.

If you have a chamber drawing and want quotes from shops that already weld and leak test to a written spec, send the RFQ with the sequence and the leak rate included.

Frequently asked questions

Aluminum or stainless for a chamber body?

Aluminum 6061 is common for lighter chambers and parts that need thermal conductivity. Stainless 304 or 316L is used where the chamber sees aggressive chemistry, higher bake temperatures or very low base pressure. The process decides, not the machinist.

What leak rate should I write on the drawing?

Write the rate your system needs and the test method, for example helium mass spectrometer leak test to a stated rate. Without a method, a shop can test with a method that cannot detect the leak you care about.

Why does the part have to be machined after welding?

Welding moves metal. Sealing faces, bore positions and flatness all shift. Critical features are finish machined after the weld and after stress relief, which is why the sequence belongs on the drawing.

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