A CNC fixture is a workholding device that positions a workpiece at a defined location relative to the machine tool and holds it there while the tool cuts. It does not create accuracy by itself: it makes the machine's programmed coordinates land in the same place on every part, and keeps the part still while cutting force pushes against it. The mechanism rests on locating and clamping; the practical limits rest on deformation.
What counts as a fixture
The defining feature is a fixed, repeatable relationship between the workpiece and the machine coordinate system. Locating elements — pins, pads, stops, nests — establish where the part sits; clamping elements press it against them. A jig also holds the part, but additionally guides the tool, so the tool path comes from drill bushings rather than from machine coordinates. A plain vise leaves position to the operator; once machined soft jaws and a hard stop are added, it is a simple fixture.
The 3-2-1 locating principle
A free rigid body has six degrees of freedom: three translations and three rotations. Drawing practice fixes the datums that remove them — ASME Y14.5 and ISO 1101 both formalise datum features and datum reference frames — and the fixture has to reproduce the datum scheme that the CNC program already assumes.
The classical arrangement is 3-2-1. Three locators on the primary datum remove one translation and two rotations; two on the secondary datum remove one translation and one rotation; one on the tertiary datum removes the last translation. Six contacts, six freedoms, no redundancy. A seventh contact in the same direction over-constrains the fixture: on a part that is not perfectly flat, the extra point either rocks it or bends it.
Clamping force and deformation
Clamps close the gap between part and locator. The force must resist the cut while staying small enough not to distort the part, and thin walls make that window narrow. Three effects stack:
- Clamping distortion — a clamp pressing on an unsupported wall bows it; the tool cuts the bowed shape, and the wall springs back on release.
- Cutting-force deflection — low stiffness lets radial tool pressure push the wall away, leaving it thicker than programmed.
- Residual stress — removing material releases stress locked into the stock, and the part moves after the final pass.
Spreading force over a larger area, through soft jaws machined to the part profile or low-pressure pneumatic and vacuum clamping, and supporting directly beneath the clamp, address the first two. Only stress relief between roughing and finishing addresses the third.
Where fixtures stop helping
A fixture repeats a position; it cannot correct machine error. Published repeatability for zero-point interfaces — commonly 3–5 µm, with some manufacturers specifying as fine as 3 µm — describes how consistently a pallet returns to the same interface, not whether the part sits correctly on it. Access is the second limit: a fixture that blocks coolant, or lets chips pack under the part, produces scrap for a different reason.
| Approach | Mechanism | Characteristic limit |
|---|---|---|
| Dedicated plate fixture | Locators machined to one part geometry | Every design change makes it scrap |
| Modular base plate | Reconfigurable grid, such as the 52 mm and 96 mm zero-point plates sold by European makers | Joints are less stiff than a one-piece body |
| Soft jaws in a standard vise | Jaws machined to the part profile | Jaw wear forces remachining |
| Vacuum or magnetic clamping | Force distributed across a face | Needs a sealed flat face, or ferromagnetic material |
Common questions
Is a vise a fixture?
Only once its jaws or stops define the part's position. Otherwise it is general-purpose workholding, and the position is reset by the operator every cycle.
Should the fixture be stiffer than the workpiece?
By a wide margin, yes. Fixture deflection adds to part deflection and is harder to predict, because it depends on the load path through bolted joints rather than through one solid body.
Can probing replace a fixture?
It can find where a part actually is and shift the program to match. It cannot stop a part moving under clamping force, or springing back once the clamps are released.