Guides / Design and Fit

Putting Real Tolerances on a Keyboard Case Drawing

September 21, 2026

The fastest way to make a keyboard case expensive is to put plus or minus 0.05 mm in the title block and walk away. It doubles the inspection, slows every finishing pass, raises the scrap rate and buys you nothing on the twenty features where a tenth of a millimetre was irrelevant.

Good drawings are selective. Here is where to be selective.

The dimensions worth tightening

Gasket shelf depth, on a gasket mount case. This controls compression and compression controls the sound. It is the single most influential dimension on the part. Give it its own callout.

Plate pocket, wherever the plate has to sit within the case. Too tight and the plate will not drop in after coating. Too loose and it moves. This is a fit, so dimension it as a fit rather than as two independent sizes.

Screw and boss pattern, because it has to agree with the PCB you already have. This is a position tolerance, not a size tolerance, and stating it that way tells the shop what you actually need.

Port cutouts. A USB-C socket has a housing, and the cable that plugs into it has a moulded boot. Both need clearance, and both are visible from outside. Get the position right and be generous with the clearance, because a cutout that is too tight for a thick cable is a defect your buyers will find within an hour.

Mating faces between the halves. The seam around the board is visible from every angle. Flatness and the step between the two parts matter more here than any diameter on the part.

Plate flatness, over the full span rather than at a point. A plate is thin, it has a lot of material removed from it, and cutting relieves internal stress. Ask the shop what it can hold and write that number down.

What can run at block tolerance

Almost everything else. Wall thickness away from a mating face. Internal pockets that only hold air, foam or a hidden weight. Cosmetic radii. Overall length and width of the case, within reason. Decorative chamfers. The depth of an internal cavity that nothing sits in.

None of these get better by being tighter. Some get worse, because a tight tolerance on a large outside dimension can force a slower and riskier finishing pass on the most visible surface of the part.

State position as position

A common drawing habit is to dimension a hole pattern as a chain of sizes from one edge. It works, but it accumulates tolerance across the chain and it tells the shop nothing about what you actually care about, which is that the holes line up with the PCB.

Use a datum and give the pattern a position tolerance relative to it. The shop then knows which features must agree with each other and which are free to float.

Before or after finishing

This is the note most first drawings are missing.

Type II anodising adds a small amount of thickness per surface. Hardcoat adds considerably more. E-coating, cerakote and powder coat add more again. In a pocket, the coating goes on both walls, so the pocket gets smaller by twice the build.

If a plate pocket is drawn as a precise fit and the case is then e-coated, the plate does not go in. This is one of the most common causes of a prototype that “measures perfectly” and still fails assembly.

Put a line on the drawing: dimensions apply after finishing, or dimensions apply before finishing with a stated allowance. Either is fine. Silence is not.

Add the stack up once

Take the one assembly that matters most, usually plate to case, and add every tolerance in the chain: case feature, coating, gasket, plate thickness, plate flatness, boss height, coating again.

You are not trying to design a perfect stack. You are finding out which single term dominates. Tighten that one, leave the rest alone, and let compliant components absorb what remains. On a gasket board the gasket is doing that job for you, which is a real manufacturing advantage of the mount.

What to write on the drawing

A workable title block for a keyboard case looks roughly like this:

• General tolerance: plus or minus 0.1 mm unless stated.
• Specific callouts on the five or six dimensions above.
• Surface finish requirement on cosmetic faces, with the blast grit named.
• A note on whether dimensions are before or after finishing.
• A note naming which faces are cosmetic and which are hidden.
• Alloy and temper, with no substitution without written approval.

That is a drawing a shop can quote accurately and hold reliably, and it will come back cheaper than the one with a tight number in the title block. The rest of what belongs in the package is in what to send a shop before it can quote.

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