Guides / Design and Fit

Gasket Mount Case Design for Manufacture

September 21, 2026

Gasket mount is the most popular mount in custom keyboards and the one where a small machining error does the most damage. The reason is simple: the feature that decides the sound is a shelf depth, and shelf depth is exactly the kind of dimension that quietly runs at a default tolerance unless you stop it.

Start from the compressed gasket, not the gasket

The most common first time error is drawing the shelf gap equal to the gasket thickness.

A gasket only works when it is compressed. Poron and silicone strips are usually specified with a working compression range, often somewhere around 30 to 50 percent for keyboard use. If you use a 2 mm strip and leave a 2 mm gap, the gasket is doing nothing at all and the plate rattles.

So the sequence is: choose the gasket material and thickness, get its compression range from the supplier, calculate the gap that puts it in the middle of that range, then draw the shelf to produce that gap once the top and bottom case are screwed together.

Write the resulting gap on the drawing as a dimension with a tolerance. It is the single most important number on the part.

Put a real tolerance on it

A blanket block tolerance of plus or minus 0.1 mm on a case is normal and sensible for most features. On the gasket shelf it is too loose, because 0.2 mm of range across the tolerance band is a meaningful fraction of your compression target.

Call out the shelf depth at a tighter tolerance and leave the rest of the case at the block tolerance. This is much cheaper than tightening everything, and it directs the shop’s attention exactly where it belongs. The general principle is in which tolerances actually matter.

Groove, shelf or tabs

Three common arrangements, and the choice is about assembly rather than sound.

A flat shelf with the gasket strip laid on it is the cheapest to machine and the easiest to revise later. The gasket can shift during assembly, which is fine on a board that gets built once.

A groove locates the strip positively. It costs a little more machining and it commits you to a gasket size, but the board is far easier to reassemble. Design the groove around the compressed condition, and pay attention to the corner radius: a cutter leaves a radius in the corner and a square gasket does not fit into it.

Tabs on the plate that sit between gasket strips move the compliance onto the plate rather than the case. The tab positions and their tolerance then become critical dimensions on the plate drawing, and they need calling out just as the shelf does.

Remember the finish

Anodising adds thickness. Type II adds little, but e-coating and hardcoat add enough to matter on a pocket that was drawn as a precise fit.

The shelf is on the inside and usually gets coated along with everything else. A shelf that is exactly right raw is slightly shallow once coated, and the gasket compresses more than you designed. It is a small effect and it stacks with everything else.

The fix is a note on the drawing saying whether the dimension applies before or after finishing. Shops ask this question when they are experienced and assume when they are not.

The tolerance stack nobody draws

Sit down and add it up once. Between the plate and the top case you have: the case shelf depth, the gasket thickness variation, the plate thickness variation, the plate flatness, the screw boss height and the coating on both surfaces.

Every one of those has a tolerance. If each is independently reasonable and they all land at one end of their range, the result can be outside anything you intended. This is the arithmetic that explains why one unit in a run of two hundred feels wrong while every part measures in specification.

You do not need to solve it perfectly. You need to know which single dimension has the most influence, tighten that one, and let the gasket’s compliance absorb the rest.

Both halves, one anodising load

The top case and the bottom case meet in a visible seam along the whole perimeter of the board. Two anodising loads of the same dye will not match closely enough for that seam.

Put it in writing on the purchase order: matching halves run in the same load. This costs nothing and it prevents the most expensive failure mode this design has.

A short drawing checklist

• Shelf gap dimensioned to the compressed gasket, with its own tolerance.
• Gasket material, thickness and durometer named, not just “poron”.
• Corner radii on any groove compatible with the gasket’s shape.
• Plate tab positions dimensioned if the design uses them.
• A note saying whether dimensions are before or after finishing.
• A note that matching parts share an anodising load.

Six lines. They are the difference between a board that sounds like your prototype and a board that sounds like a lottery. When the drawing is ready, send it out for quotes.

Frequently asked questions

Keep reading

• Machining a Knob for a Keyboard Encoder, and the Case Hole It Sits In
• Sizing a USB-C Cutout for the Cable Boot and the Daughterboard Behind It