Custom Fixture Quotes: What to Send, and Why It Changes the Price

Ask three suppliers to quote the same custom fixture and you will often get three prices that differ by more than the fixture is worth. That is rarely because one is greedy. It is because each of them filled the gaps in your brief differently — and the gaps, not the part, are what they were pricing.

A custom fixture quote is an estimate of how much uncertainty the supplier is absorbing. Reduce the uncertainty and the number moves, usually in your favour, and always toward being comparable between suppliers. Here is what actually reduces it.

The inputs that turn a guess into a quote

The datum scheme costs more than the part shape

Buyers tend to assume a complicated part means an expensive fixture. More often, the driver is the datum scheme: which faces locate the part, in what order, and how tightly the drawing controls them. A simple part with an awkward or over-constrained datum reference frame can cost more to fixture than a complex part with a clean one.

This is why the drawing matters more than the model. The model tells a supplier what the part looks like; the GD&T tells them what has to be true when it is clamped. Send only the model and the supplier must choose datums themselves — and if their choice differs from your inspection's, the fixture and the CMM will disagree about the same part.

Involving fixture design early is not a formality

The most productive projects start while the part design is still open. At that stage a fixture engineer can flag features that are expensive to hold — a datum face that is interrupted, a clamping area that is also a finished surface, a wall thin enough to move under clamping load — while changing them is still free. After the design is frozen, the same problems get solved in metal, at your cost, forever.

Say what must not move

Clamping applies force. On thin walls, castings and anything long and unsupported, that force becomes distortion, and distortion becomes a part that measures correctly in the fixture and wrongly on the bench. If you know a region is delicate, say so explicitly — it changes clamp placement, support strategy and sometimes the sequence in which clamps close.

• Name the surfaces that must not be marked, clamped on or supported.
• Flag thin sections and unsupported spans where clamping force will show up as movement.
• State whether the part is expected to be checked free-state or constrained — they are different results.
• Say if the same fixture must serve machining and inspection; the requirements do not automatically agree.

How to compare quotes that are not comparable

A cheaper quote is only cheaper if it covers the same scope, on the same datums, with the same documentation at handover. Two of those three usually explain the difference.

The short version: what to attach

1. 13D model in STEP or IGES, plus the dimensioned drawing with its GD&T.
2. 2Machine model and table interface, including available air or hydraulics.
3. 3The operations this fixture must support, and the tools that need access.
4. 4Target cycle time, batch size and annual volume.
5. 5Material, and any regions that must not be clamped, marked or distorted.
6. 6Whether the fixture must also serve inspection, and what documentation you need at handover.

See clamping and fixture detail up close

Frequently asked questions