A part machined from the wrong datum can measure within every tolerance on the drawing and still fail to assemble. The fixture's datum scheme should follow the part's functional datums, and the clamps should push the part onto those locators rather than away from them.
7 min read · Last reviewed 28 September 2026 · All resources
A free part can move along three axes and rotate about three. A location scheme removes all six, deliberately, using a known set of contact points. The classic arrangement uses three points on the primary face, two on the secondary and one on the tertiary.
The value of doing this explicitly is that you know which surface controls what. When a part comes out wrong, you can reason about which locator was responsible instead of guessing.
The drawing's datums represent how the part functions in its assembly. A fixture that locates from a different surface introduces a stack between what was machined and what actually matters, and that stack consumes tolerance you thought you had.
Where the functional datum is unusable for machining — unreachable, unfinished, or the surface being cut — say so early. There are ways to handle it, but they need to be a deliberate decision rather than a silent substitution.
Adding a fourth pad to a three-point face feels safer and is not. The part now rocks between contacts, and which combination it settles on depends on clamp order, swarf and how the operator loaded it. Results vary run to run for no visible reason.
If more support is genuinely needed — a large or flexible part — the extra supports should be adjustable or compliant, set after the part is located rather than defining the location themselves.
For thin, cast or slender parts, clamping changes the shape of the part before the cut starts, and it springs back afterwards. The part is then machined accurately to the wrong shape.
The cheapest way to find this is to measure a first-off both clamped and released. If the readings differ meaningfully, the fixture needs lower force, more spread, or support in different places.
Locators work only if they are clean. A design where swarf can settle on a locating pad will drift through a shift, and the operator will usually adapt without reporting it.
Designing for escape — clearance under pads, no upward-facing pockets, room for a blast of air — is a small cost at design stage and difficult to retrofit.
FAQ
The questions engineers and buyers ask before sending a drawing. If yours is not here, ask it directly — you will get an engineer's answer, not a brochure.
Still have a question?
Talk to Anita M.N, Managing Partner, or send the drawing and let the review answer it.
Locate from the best available surface and account for the resulting stack, or add a temporary machined feature to work from. Either is workable; what causes trouble is substituting silently.
Load the same part several times and measure without machining it. If the readings move, the part is not locating repeatably before anything is cut.
Where the fixture will see long service or abrasive conditions, yes — a worn locator moves position slowly and invisibly. It is a cost-versus-life decision, worth making explicitly.
Sometimes, with adjustable or interchangeable locating elements. The risk is that adjustability reintroduces variation, so it works best where the differences are clear and positively set rather than dialled in.
Start a project
Share what you are trying to make or fix. Our engineering team reviews every enquiry and responds with the questions that matter, a feasibility view and next steps.
