Where fixture error comes from
Almost all fixture error is one of three things: locating on a surface that varies, wearing the locating feature until it no longer repeats, or clamping in a way that deflects the part before the cut starts.
The first is a design problem, the second is a maintenance problem and the third is usually both. Knowing which one you have is what makes the fix obvious.
| Fault | Symptom | Fix |
|---|---|---|
| Locating on a rough or variable surface | Size scatter that changes between batches | Locate on a machined datum or use a compensating pin |
| Worn locating feature | Gradual drift in one direction over weeks | Replace the pin or pad; harden the wear surface |
| Clamping deflection | Part measures correctly off the machine but not on it | Clamp over a support, or reduce clamp force and use a stop |
| Insufficient support under the cut | Chatter that varies with the part, not the tool | Add support or reduce the overhang of the part |
Soft jaws, hard jaws and marking
Hard jaws locate accurately and last, but they mark a finished surface. Soft jaws are machined in place to the part and distribute the clamping force, which suits finished or thin-walled parts and batch work where the jaw can be dedicated to one job.
The choice is usually decided by whether the clamped surface is a finished face. If it is, a jaw that marks it becomes a scrap source regardless of how accurate it is.
Top pins and repeatable location
A top pin locates a bore or a counterbore and clamps in one action, which removes the separate locating and clamping steps that introduce error. Because the pin is ground to the bore it locates, the position repeatability is set by the pin and the bore together.
That also means the pin is a wear item in the same way a collet is. A pin that has worn or picked up a burr will locate the part slightly differently on each cycle, and the resulting scatter is often misread as a machine or tool problem.
Wear surfaces decide how long a fixture stays accurate
A fixture is a wear item. Locating pins, rest pads and jaw faces all see sliding contact on every cycle, and how long they stay accurate is decided by the hardness of the wear surface rather than by the accuracy of the original grind.
Case-hardening or nitriding the contact surfaces is what turns a fixture that drifts after a few thousand cycles into one that holds for a production campaign. Where a surface must not mark the part, the wear resistance has to come from the geometry instead, which is why soft jaws are machined to fit rather than hardened.
Frequently asked questions
What is a top pin used for?
A top pin locates a bore or counterbore and clamps the part in one action, which removes the separate locating and clamping steps. It is common in production where a part must be positioned the same way on every cycle.
Should I use hard jaws or soft jaws?
Hard jaws where the clamped surface is raw or unimportant, because they last longer and locate accurately. Soft jaws where the surface is finished or the wall is thin, because they distribute the clamping force and avoid marking.
How often should fixture components be replaced?
In production, treat locating pins, pads and jaws as wear items with an inspection interval rather than replacing them on failure. A worn locating feature produces scatter long before it fails outright, and the scatter is usually blamed on the tooling.
Can you make fixture components from our drawing?
Yes, that is the normal way we supply workholding. Send the drawing, the part it locates and the required repeatability, and we will confirm the locating method as well as the dimensions.
Do you supply complete fixtures or only components?
We supply components and sub-assemblies to drawing. Complete fixture design is normally done by the machine builder or the end user, and we manufacture the locating and clamping elements to that design.