Clamping Principles & Clamps

Clamping is the act of holding work while it is machined, and it is the half of workholding that this wiki’s workholding group treats alongside locating. The two jobs are different, and the difference is the whole of the principle: locating puts the part in the right place, and clamping keeps it there. The locators — the fixed stops, the faces of a vice, the pins of a 3-2-1 setup — establish where the part sits in the machine; the clamps then press it against those locators and hold it firm against the forces of the cut, so that every feature is machined in the same place on every part. Get locating right and clamping wrong, and the part moves, lifts or distorts under the cutter; get both right, and the cutting forces of the job pass harmlessly through a workholding that holds the part rigidly in the position the datum setup defined. This entry sets out the principles of clamping and the clamps that apply them.

A clamp holds, it does not locate

The first principle is that clamping and locating must not be confused, and the mistake is one of the commonest in the shop. The locators define the position; the clamp’s only job is to hold the part against them. So a clamp is always arranged to press the work into its locators — down on to the solid support of the table or the fixture, and against the fixed stops — never to slide it out of place. A part that is clamped before it is pushed home against its stops, or a clamp that bears on an unsupported corner and rocks the part off its seat, is a part whose position is lost: the clamping has overridden the locating, and the machined features will sit in the wrong place relative to the datums. The rule that shops repeat is that the clamp presses the part down and in — down on to the surface that supports it, and in against the stops that locate it — and a well-arranged clamp is one that cannot lift the work at all.

Clamping over solid support

A clamp is only as good as what is beneath the part, and the second principle is that clamping force must always bear over solid support. When a clamp presses down on a part, the part must have something solid underneath to take the load: the table of the machine, a parallels or a solid stop under the clamped area, the floor of a fixture pocket. A part clamped over a hollow — over an open hole, a recess, or an overhang that is not supported — will flex, spring and distort under the clamp before the cutter ever arrives, and the distortion is stored in the part, to spring back when the clamp is released and ruin the machined surface or the size. The shop therefore packs the work: solid parallels beneath the part, a support under every clamp point, and where a thin or delicate region must be held, the support comes first and the clamp bears on it. This is also where clamping meets rigidity: the whole chain — clamp, part and support — must be stiff enough that the cutting force cannot deflect it, because every deflection is a machining error.

Clamping force and its limits

The third principle is that the clamp must be tight enough to hold, and no tighter, and the two failures bracket the correct setting. Too little clamping force lets the cutting forces move the work: the part shifts, chatters or lifts, and the cut is spoiled. Too much force distorts the part: the springy workpiece bows under the clamp, and when it is unclamped it springs back, leaving a face that is not flat or a size that is not true. Between the two lies the right clamping force, and the shop finds it by judgment: enough force that the part cannot move under the heaviest cut it will take, spread over a large enough area that the pressure on the part stays modest, and applied consistently — the same tightening on every part, so that every part is held the same way. That consistency is why production work turns to power clamping, where a pneumatic or hydraulic cylinder applies the same force every time and can be released and re-applied in seconds, and why even hand-clamped work is often tightened with a torque wrench when the holding is critical.

The common clamps

The clamps that apply these principles are a small family of standard forms, and a shop keeps them in a box by the machine. The strap clamp — the flat bar that spans the part, held by a stud at one end over a heel block and pressing down on the work at the other — is the everyday clamp of the machine table and the fixture, cheap, simple and infinitely adjustable with packing pieces. The step clamp is its cousin, with stepped blocks that let the clamp sit level on work of different heights. The toe clamp — the low clamp that hooks over the edge of the part and presses down and in — holds work that must be clamped from the side, and the edge clamp presses horizontally against the part’s side. The toggle clamp locks open and shut with an over-centre action, giving instant, repeatable manual clamping for quick setups. And for production, the swing clamp and the link clamp are the power clamps that swing or slide into place and clamp down, all of them air- or oil-driven, all of them often mounted on the tombstone or the fixture plate of a machining centre.

Clamps in the setup

In the setup itself the clamps must clear the work and the tool, and their placing is part of the machining plan. A clamp that lies in the path of the cutter, or that overhangs the edge of the part into the region being machined, must be moved or the cut must stop short, so clamps are arranged clear of the tool’s travel, holding the part at its edges and its ends rather than across its face. The clamps are set to press over the solid regions of the part, clear of the pockets and the features to be cut, and their order matters too: the part is seated against its locators first, then clamped progressively, a little at a time, so that no single clamp drags the part off its seat before the others hold it. And when the job is done, the clamps come off in a deliberate order, releasing the part without the stored spring of a distorted workpiece flicking it from the fixture. The clamps are the most adaptable of workholding, and the shop that understands them holds almost any part: the principle — press the work into its locators, over solid support, with a force that holds but does not distort — is the same on the machining centre table, the tombstone and the one-off fixture.