Why Is There a Scrap of Metal Under That Machine? Nothing Is Ever the Right Size
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A thin piece of material inserted to fill a gap or adjust an alignment is among the humblest components in engineering and among the most necessary. Real parts are never quite the size the drawing says.
Why gaps are unavoidable
Every manufactured part differs from its drawing by some amount, and drawings state a tolerance saying how much variation is acceptable rather than a single exact figure, because achieving an exact figure is impossible and approaching it is expensive. When several parts are assembled those variations add up, so the cumulative difference between the assembly and the design can be considerably larger than any single part's error. Foundations settle, castings distort as they cool, and machines move as they heat. Something must absorb that difference, and adding a thin piece of chosen thickness at assembly is far cheaper than making every part to an accuracy that would remove the need.
What the pieces look like
The forms are various and each suits a situation:
- •Flat sheet cut to shape, in graded thicknesses from a few hundredths of a millimetre
- •Slotted horseshoe shapes that slide in without removing the bolt
- •Laminated stacks whose layers peel off one at a time to adjust thickness
- •Tapered wedges used in pairs, sliding against each other to set a height
- •Ring forms sized to a shaft, adjusting the position of bearings
- •Plastic packers used in building work under frames and paving
Where precision depends on them
Several exacting jobs consist mostly of measuring a gap and choosing the right thickness to fill it. Aligning a pump to its motor means measuring how far the two shafts are out of line, both sideways and in angle, and raising the feet of one machine by measured amounts until they match, since a misalignment of a fraction of a millimetre destroys couplings and bearings. Valve clearances in an engine are set by fitting a metal disc of a precisely chosen thickness. Machine tools are levelled and squared the same way. Aircraft assembly uses enormous numbers of them, and liquid versions that cure in place are used where the gap is an awkward shape.
The measuring that goes with them
Choosing a thickness requires measuring a gap accurately, and the tools are as simple as the component. A feeler gauge is a fan of thin steel leaves of known thickness, and the right one is the leaf that slides into the gap with slight drag, with leaves combined where necessary. A dial indicator on a magnetic base measures how far something moves or how far it is out of position, which is how machine alignment is done. Taper gauges slide into a gap and read the thickness where they stop. Laser systems project a line across two machines and compute the correction for each foot directly, which is now standard on large installations and reports the required thickness rather than the raw measurement.
How they go wrong
The component is simple enough to be treated carelessly, and the failures are consistent enough to appear in every maintenance manual. Too many thin pieces stacked together behave like a spring, since each interface can compress slightly, so a stack of a dozen thin ones is much less rigid than one of the right thickness. Dirt or burrs trapped between them changes the effective thickness and ruins the adjustment being made. Rusted or mismatched material creates corrosion between dissimilar metals. Pieces cut smaller than the foot they sit under concentrate the load and dent. And undocumented ones are removed during maintenance and not replaced, which misaligns a machine that was fine before the work started.
The takeaway
Parts are made to a tolerance rather than an exact size and the variations accumulate through an assembly, so filling the difference with a thin piece of chosen thickness is far cheaper than making everything precisely. Aligning a pump to its motor, setting valve clearances and levelling machine tools are mostly this work. Stacking many thin pieces makes the joint springy, and undocumented ones get lost during maintenance.