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Why the Panel Gaps Look Wrong When Every Part Measures Right

July 18, 2026

A customer opens the box, looks at the front of the amplifier, and forms a judgement in about a second. Most of that judgement is the gaps: between the panel and the cover, around the knobs, at the corner where the sleeve meets the face.

That judgement is not about any single part. It is about how the parts stack up.

Gap is a stack, not a dimension

Consider a front panel that sits inside an extruded sleeve with a nominal 1 mm gap all round. The gap you see is affected by:

  • Panel width tolerance
  • Panel height tolerance
  • Sleeve internal dimensions, which are extrusion tolerances and therefore loose
  • Squareness of both parts
  • The position of the mounting holes on both parts
  • The clearance in those holes
  • Which way the panel drifts when the screws are tightened

Six or seven contributors. If each is plus or minus 0.1 mm, the worst case at one edge is well over half a millimetre. On a nominal 1 mm gap that is a gap that varies between 0.4 and 1.6 mm around the part, which reads as badly made even though every part is in tolerance.

The three design moves that fix it

1. Make the gap bigger

The most effective and least popular answer. A 1.5 mm shadow gap absorbs the same absolute variation as a 0.4 mm gap but the proportional variation is a quarter of it. The eye reads proportion.

Very tight gaps are a signature of products built with machined interfaces and controlled assembly, not of products built from a mix of extrusion and sheet.

2. Reference the visible feature, not the mounting feature

If the panel is located by its mounting holes, the gap is at the mercy of hole position tolerance on two parts plus the clearance between the screw and the hole.

If the panel is located by a machined register, a step, a rebate or a dowel, the gap is controlled by one machined feature on each part. That is two contributors instead of five.

A common and cheap version: machine a shallow rebate into the sleeve ends after cutting to length, and let the panel drop into it. The extrusion tolerance stops mattering because the register is machined.

3. Make one part adjustable

Slot the mounting holes on the hidden part. The assembler positions the panel by eye or with a shim and then tightens. This moves the problem from tolerance control to assembly technique, which for a boutique run of 30 units is often the cheapest place for it.

If you do this, say so in the assembly instruction and supply a gap gauge or a shim set. Otherwise every unit gets a different operator’s judgement.

Knob to panel clearance

The gap around a knob shaft is the closest one to the user’s eye and it has its own stack:

  • Panel cutout position and diameter
  • Potentiometer or encoder body position, which depends on the PCB
  • PCB position in the chassis
  • Shaft perpendicularity
  • Knob bore concentricity

The usual failure is a knob that sits visibly off centre in its cutout because the PCB is 0.5 mm off. Options:

  • Panel mount the control rather than PCB mounting it, and wire it. More labour, perfect alignment.
  • Use a floating PCB mount so the control self-centres in the panel cutout, then tighten.
  • Open the cutout and hide the gap with a bezel or a recess so a small offset is not readable.

Chamfers and shadow lines

A sharp corner where two parts meet shows every misalignment. A chamfer or a radius on both edges creates a shadow line that hides a surprising amount.

This is why so many well regarded enclosures have a chamfer running down every visible edge. It looks like a style decision and it is also a tolerance strategy.

The same trick works for a step: if the panel stands proud of the sleeve by 1 mm rather than being flush with it, flushness stops being a requirement.

Specifying it on the assembly drawing

Put the requirement where it belongs, on the assembly:

GAP BETWEEN FRONT PANEL AND SLEEVE:
  1.2 mm NOMINAL, 0.9 TO 1.5 ACCEPTABLE.
  VARIATION ALONG ANY ONE EDGE: 0.3 mm MAX.
  PANEL FACE TO SLEEVE FACE STEP: 1.0 mm NOMINAL,
  MUST NOT BE FLUSH OR RECESSED.
ASSEMBLE WITH 1.2 mm GAP GAUGE, TOOL 9004.

The variation line is the one that matters. It is also the one that is almost always left out.

A quick self check on your own design

  • Count the contributors to each visible gap. More than three is a warning.
  • Is the panel located by a machined register or by clearance holes?
  • Would the product still look right if every part were at its worst case limit?
  • Is any gap below 0.8 mm? If so, can it be bigger without changing the design intent?
  • Is there a chamfer or a step at every joint between parts made by different processes?
  • Does the assembly instruction say how to set the gap, or does it assume it happens by itself?

Frequently asked questions

How small can a panel gap be?

Physically you can get to 0.2 mm. Visually you should not want to. A very small gap magnifies any variation along its length, so a 0.3 mm gap that opens to 0.5 mm at one end reads as a defect. A deliberate 1 to 1.5 mm shadow gap is far more forgiving and usually looks better.

Is a uniform gap more important than a small gap?

Much more. The eye detects taper and variation instantly and absolute width hardly at all. A consistent 1.2 mm gap all round looks precise. A gap varying from 0.4 to 0.9 mm looks broken, even though it is smaller everywhere.

Should I tolerance the gap on the drawing?

You cannot tolerance a gap on a part drawing because the gap does not exist until assembly. Put it on the assembly drawing as a requirement, then work backwards to the part tolerances that deliver it.

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Keep reading

This guide is part of Audio Chassis Manufacturing: The Whole Picture, one of four sections in the guide library.