A Weekly Check On Cleaning Fixtures

A fixture that has drifted 0.2 mm will still clean the part perfectly. It will also put every feature slightly out of position, and the cleaning result will look fine while the measurement fails. Nobody looks at the fixture, because the fixture is not what the machine reports on.

That is the reason a weekly check pays for itself. Everything below takes about ten minutes, and none of it needs instruments beyond a feeler gauge and a torch.

Before Every Shift

These are the checks an operator can do in under a minute, and they catch the failures that stop the line rather than the ones that spoil the result.

• Locating pins present and seated — a pin that has worked loose lets the part sit low by 0.1 mm or more
• Clamps closing fully by hand before the cycle starts, not after
• No part debris left on a locating face from the previous cycle
• Drain holes in the fixture base clear, so liquid does not pool under the part

Once A Week

The weekly list is where the slow failures get caught. Each item degrades over months, which is exactly why nobody notices it happening.

• Wear on locating faces — more than 0.05 mm of dishing means the face has lost its datum, and a polished face has lost its datum and will repeat less accurately than the tolerance assumes
• Play in quick-change pins and bushes, checked by hand rather than by gauge
• Seals and plugs on plug-sealing heads: hardness, cuts, and whether the sealing face is still flat
• Debris packed into the shadowed corners of the fixture, where the spray pattern never reaches
• Fixture frame straightness, particularly on long parts where a slight bow changes the stand-off at one end
• Variant identification still correct — a mislabelled pad sends the next batch through the wrong program

After A Variant Change

Changeovers are when a fixture is most likely to be put back wrongly, and the error is often invisible until parts fail downstream.

• Every pad and stop in the position the program expects, confirmed against the setup sheet rather than from memory
• First part measured, not only washed — the first part after a change is the only one that tells you the fixture is right
• Nozzle positions not disturbed by handling during the change
• Fasteners torqued rather than hand-tight, since a loose clamp moves under spray pressure

Once A Quarter

The quarterly items are the ones that need the machine stopped and a part measured rather than looked at. They are also the items that explain a slow drift nobody could attribute.

• Measure a known part in the fixture and compare its position with the value recorded at commissioning — a change beyond 0.1 mm means the fixture has moved, not the part
• Check fixture height at both ends of a long bed with a straight edge; a 0.5 mm bow changes the stand-off and therefore the impact force at one end
• Inspect seal grooves and O-ring seats for compression set, which shows as a seal that seals at 10 MPa but weeps at 30 MPa
• Confirm the spray pattern still hits the fixture in the same place, using water-sensitive paper at the working distance
• Review whether the fixture is still the right one for the current variant mix — fixtures outlive part designs more often than anyone expects

A quarter is also a sensible interval for replacing rather than adjusting: locating pins, spring plungers and seal sets are consumables that cost little and are usually adjusted three or four times before anyone replaces them.

When A Part Fails

The fixture is usually the last thing checked and often the actual cause. Three questions settle it quickly.

• Does the failure repeat at the same location on the part? A fixed location points at the fixture or the nozzle pattern, not at the bath
• Does it follow the fixture rather than the machine? Run one part in a second fixture if one exists
• Did it start after a changeover, a crash or a maintenance stop? Those are the three events that move a fixture

What To Keep On The Sheet

A checklist only works if the result is recorded, and what matters is not a tick but a value. Record the date, who checked, and one number for anything that can drift: pin play, wear depth, or the measured position of the first part after the check.

If you only keep three of these, keep the first-part measurement after every changeover, the weekly look at locating faces, and the drain holes. Those three account for most of the failures that look like machine problems and are not.