GlossaryKind of term: Manufacturing termSoft jaws
What are soft jaws?
Soft jaws are jaws of unhardened steel, aluminium or plastic that you mill or bore to the shape or diameter of the workpiece.
They fit a chuck or vice and are brought to size in the shop itself. The soft material machines easily and is less likely to mark the workpiece. The part bears over a larger area than in hard jaws, the clamping force is spread better and the position repeats as long as the jaws are not moved. A set can be bored again for the next part, provided enough material is left.
How accurately do soft jaws centre?
How accurately soft jaws centre depends on how they were bored. Chuck jaws are bored in the chuck itself, at roughly the clamping force you will use, with a ring or plug between the jaws so they sit in the position in which they will hold the part. The bored diameter should match the workpiece diameter, so the jaw bears over its full width rather than on one edge. That takes out most of the play between jaw and chuck. If the jaws are removed or moved to another machine, that accuracy is not guaranteed. In a vice you mill the contour into the jaws on the machine where they will be used.
How do soft jaws work in practice?
Soft jaws are often used for a second setup on an already machined side, for free-form shapes, thin-walled parts and soft materials that hard jaws would damage. Making them costs material, machining time and sometimes programming time for the contour. If you keep them for a repeat order, you can save that time next time, provided the drawing has not changed. Whether you charge the cost to one order or spread it over repeat orders is an estimating choice.
What are soft jaws often confused with?
Soft jaws are not a dedicated fixture: a fixture is complete workholding for one part, soft jaws fit an existing chuck or vice. Nor are they jaw covers: covers protect the workpiece from marks but do not take on its shape.
Frequently asked questions
When are soft jaws worth making?
On a second setup against a machined side, on free-form shapes, thin-walled parts and batches or repeat orders. On a one-off, making the jaws can take more time than they save; jaw covers or other workholding are sometimes enough.
Why do you bore soft jaws under clamping force?
So they sit in the same position as when they hold the workpiece. That takes out most of the play between jaw and chuck, and they centre better than jaws bored loose or without clamping force.
Can you keep soft jaws for a repeat order?
Yes, if the drawing does not change. Record which part and revision they belong to. When you refit them, check runout or position again, because it can shift when the jaws are refitted.