Over-constraining a part bends it
How redundant location turns a fixture into a press.
A redundant locating point, one added beyond the six a rigid part needs, forces the part to change shape slightly so that its surface reaches a point that real manufacturing variation never guaranteed it would meet, and the clamping force holding the part against its other six points is usually more than enough to supply the squeeze.
A shelf board sprung into an alcove
A shelf board cut a hair too long for a fixed alcove can often still be forced into place, sprung into a slight bow between the two end walls until both ends seat. It sits there looking installed and secure for as long as the walls hold that bow. Taken back out, it snaps straight again, showing that it was never the length the alcove required and was held under compression the whole time to fake a perfect fit. A part squeezed into contact with a redundant seventh locating point behaves the same way. It appears fully and solidly located while the fixture meant only to hold it is bending it, and the bow vanishes the instant the part is removed, taking with it any accuracy the machining achieved while that hidden stress was locked in.
The geometry explains why. A part designed to touch a fixture at seven points is asking for all seven contact surfaces to agree perfectly despite the ordinary tolerance present on both the part and the fixture. On almost every real part they do not quite agree, and once the other six points have located it, the seventh sits a small distance away from the part's actual surface. A fixture built to hold firmly does not leave that gap alone. The clamping force meant to keep the part seated against its first six points also presses the surface across the remaining gap until the seventh point makes contact. The part is pushed into touching that point, which means it is bent, however slightly, for as long as the fixture holds it.
Three hundredths of a millimetre is plenty
The gap involved can be tiny. An interference of 0.03 mm, about a third of the thickness of a sheet of paper, sits comfortably within ordinary manufacturing tolerance on either the part or the fixture. Against a thin plastic part, closing that gap generates a real, measurable bending stress, and a fixture clamp typically has far more force available than the squeeze requires. Nothing about a clamp knows to stop early once the part underneath has reached its true, unforced shape, so it keeps pushing until the redundant point is touching and the part has bent to meet it.
The same 0.03 mm matters much less in a stiff part. Thick metal can absorb that interference with no meaningfully measurable bending, since the deflection needed to close the gap in a much stiffer material is too small to register against the tolerance the job works to. That is why over-constraint counts as a serious design fault for the thin bonded plastic parts in this set, and as a minor inefficiency in a great deal of ordinary metalworking.
Extra rests hide better than extra clamps
A locating scheme with an obvious extra clamp, such as a second vice jaw added for reassurance, tends to draw scrutiny because it looks like an addition. A redundant point hidden inside an ordinary-looking support draws much less. A rest pad placed under a long overhanging section purely to stop it drooping under its own weight reads as sensible extra support, and rarely as a seventh constraint competing with the six that already located the part.
That rest pad does the same mechanical job as the fourth leg of the stool in the previous article. Whether it bends the part depends entirely on whether it happens to arrive exactly where the other six points had already, independently, put the part's surface. A reviewer who looks for extra clamps and never for extra rests has checked half the problem.
Overhangs still sag, and there is a middle path between a rigid redundant rest and no support at all. A rest that is spring-loaded or self-adjusting follows the part into whatever position its six locating points have already set, and is then locked in place without pushing. It offers real support against sag and cutting vibration without imposing a position of its own, so it adds stiffness to the setup while leaving the location to the original six points. The locking step is where care is needed, since a rest that is locked while still pressing upward becomes the rigid seventh point it was meant to replace.
Counting contacts
Checking a fixture for over-constraint means counting its actual load-bearing contacts against the six a rigid part needs, and treating any support, rest or secondary contact beyond that count as a suspect, however sensible it looks on the drawing. For each extra contact the reviewer should be able to say either that it is self-adjusting and carries no locating load, or that it is one of the six. A fixture that passes that count is one where every contact does necessary work locating the part and none of them presses it into a shape it would not hold once released.
The check can also be confirmed on the part itself. Measuring a feature with the part still in the fixture and again after release shows whether the fixture was holding it in a shape of its own; a feature that moves between the two readings is the shelf board springing straight. This single habit, counting the contacts and accounting for every one, catches the same class of mistake whether it arrives as an obvious second clamp or as an innocent-looking support pad that nobody thought to question.