Covers look easy. That is the trap.
A cover is a flat disc with a sealing face, a groove, and four holes. What could go wrong? Plenty — I have seen covers scrapped at final inspection because the sealing face was flat on the machine and concave on the surface plate. Thin covers flex under clamping, so the machining results look great in the cut and lie after unclamping. Add an O-ring groove and you have three tolerance systems fighting each other: face flatness, groove depth, and groove position relative to the bore.
The sealing face comes first
On any cover with a gasket or O-ring, the sealing face flatness is the number that decides whether the part works. Typical production numbers: 0.01-0.02 mm flatness on a 100 mm cover face, with a surface finish of Ra 0.8 or better. That face is machined last, in a finishing pass with a sharp insert and a light depth — 0.1-0.2 mm — so there is no spring-back from heavy cuts.
The O-ring groove is where drawings go wrong most often. Groove depth tolerance drives squeeze, and squeeze drives sealing. A standard O-ring groove (per the Parker or AS 568 handbook) calls depth tolerance around +-0.05 mm, and the groove floor roughness should be Ra 1.6 or better. If the drawing skips the roughness callout on the groove floor, add it — it is the difference between a seal that lasts and one that leaks in six months.
Fixturing thin covers without flex
Thin covers are the classic vacuum part. A 3 mm cover held on a vacuum fixture machines flat and stays flat, because the holding force is distributed and there are no clamp point loads. On thicker covers — 6 mm and up — the options open up:
- Vacuum plate with a gasket matching the part contour — first choice for covers under 5 mm
- Soft-jaw chuck gripping the OD with full arc contact, then face the front
- Bolted through the mounting holes to a fixture plate — works when the holes are on the drawing anyway
- Peripheral clamps with support under the cover center, for larger covers that would drum under vibration
Whatever the method, never clamp a thin cover at three points and expect the sealing face to stay flat. The flex comes back the moment the clamp comes off.
Chatter control on thin-wall covers
Thin covers ring like a bell under interrupted cuts. The practical fixes, in order of what works in the shop:
- Sharp, positive-rake inserts — a worn tool is the number one chatter source
- Light depth of cut on the final pass so the wall does not deflect
- Higher spindle speed with lighter feed to stay in a stable cutting window
- Damping: fill the cover with a block of material or support the center from behind
If chatter marks show on a sealing face, that part fails the Ra callout regardless of flatness. Treat the finish pass on sealing faces as a separate, deliberate operation — not the cleanup of a rough pass.
Materials and what they change
| Cover material | Typical wall | Sealing face finish | Watch out for |
|---|---|---|---|
| Aluminum 6061-T6 | 2-6 mm | Ra 0.8 | Stress relief, gummy finish on soft grades |
| Aluminum A380 (die cast) | 2-4 mm | Ra 0.8-1.6 | Porosity under the face, hard skin |
| Stainless 304 | 3-6 mm | Ra 0.8 | Work hardening, chatter, distortion |
| Brass C360 | 2-5 mm | Ra 0.4-0.8 | Thin wall flex, burr on grooves |
One production tip for gummy aluminum: use a wiper insert or a final pass with a larger corner radius to leave a clean Ra 0.8 face without a separate polishing step. And check the bore-to-groove concentricity — a groove that runs out relative to the bore will leak even with perfect flatness.
What to put on the drawing
- Sealing face flatness: state it explicitly, plus which datum it references
- Groove depth tolerance and floor roughness — the two most-missed callouts
- Concentricity between the bore and the O-ring groove
- A note on measurement: flat covers must be measured on a flat surface plate, not in the machining fixture
Frequently Asked Questions
Why is my cover sealing face concave after machining?
The cover flexes under clamping while machined, then springs back when released. Hold it flat during machining — vacuum or full-arc soft jaws — and machine the face last with a light finishing cut.
What tolerance should an O-ring groove have?
Groove depth around +-0.05 mm, floor roughness Ra 1.6 or better, and the groove concentric to the bore. Follow the AS 568 / Parker handbook values for the specific O-ring size.
Can you machine a 2 mm cover without chatter?
Yes, with a sharp positive-rake tool, light depth of cut, and stable fixturing with center support. Thin covers need the holding and cutting strategy designed together.
Should covers be machined from plate or die cast?
For low volume, machined 6061-T6 plate wins on flatness and repeatability. Die cast A380 makes sense only at volume, and porosity under the sealing face is a real risk.