Vices & Soft Jaws
The machine vice is the workhorse of the machining-centre table, and the soft jaw is what turns it from a tool for rough stock into a tool for finished parts. A vice — the “vise” of American usage — is the general-purpose workholder of the mill and the machining centre: two jaws that close on a prismatic part and hold it while it is machined. This entry, part of the workholding group, treats the vice after the clamping entry treated clamps: its jaws, and the soft jaws machined to fit the part.
The vice and its jaws
A machining vice is a precision screw clamp, and its accuracy is what makes it a machine tool rather than a bench tool. The body of the vice carries a fixed jaw and a movable jaw; the movable jaw slides on the vice’s ways, driven by a screw and nut, usually with a quick-release lever that opens it wide in one pull. The jaws are hardened and ground, and the whole vice is machined square and true: the base sits flat on the machine table or on the face of a tombstone, the fixed jaw is square to the base and parallel to the ways, and the jaws close parallel to each other, so that a part clamped between them is held squarely and repeatably. The vice is held to the table by its own bolts or by the table’s T-slots and keys, and it is set true with an indicator when it is fitted, for everything machined in it inherits the vice’s truth.
Hard jaws and soft jaws
The vice is fitted with jaws of two kinds, for rough stock and for finished work. The hard jaws are the steel jaws that come with the vice — hardened, often serrated or with hardened inserts — and they grip raw stock firmly: the serrations bite the rough surface of bar or billet, and the hard jaws stand up to the dirt and the force of first operations. Their bite is their weakness, for they mark the work, and a hard jaw cannot grip a shape: it holds only flat, parallel, reasonably clean surfaces. The soft jaws are the opposite: jaws of soft material — usually aluminium, sometimes mild steel or plastic — made as blanks and machined by the user to fit the part. A set of soft jaws is bolted to the vice in place of the hard jaws, and the shop machines them, cutting into the jaw faces a pocket, a step or a full outline that matches the part, so the part sits in the jaws as in a form.
Machining the soft jaws
The shop machines soft jaws itself, in the very vice in which they will be used. Soft jaws arrive as a pair of blanks that bolt to the fixed and movable jaws, and the shop closes the vice, then mills across both jaw faces together, cutting a pocket into the pair: a round pocket for a cylindrical part, a square step for a block, a contour for an irregular part. Cutting the two jaws together is what makes them true: because the fixed and movable jaws are machined in one pass, the pocket they form is aligned, and a part clamped between them is seated evenly. When the jaws wear or the part changes, the shop machines them again, cutting the old pocket away and recutting a fresh one.
Soft jaws in use
The soft jaws are chosen for the work the hard jaws cannot do well, and the cases are the everyday ones of the finishing shop. A finished part that has already been machined on its other faces must not be marked or distorted, and a soft jaw grips it without a scar. A contoured part — round, angled or shaped — cannot be held between flat hard jaws, but a soft jaw machined to its outline holds it on the shape itself. A thin or delicate part is held by a soft jaw that spreads the grip over a wide area instead of concentrating it on a serrated line. And soft jaws hold several parts at once: the shop cuts a row of pockets across a long pair of jaws, loads a part into each pocket, and machines a whole batch of parts in one setup — the way production work fills a vice from end to end, cutting the top faces of six, eight or ten parts at a pass. The soft-jaw setup turns the one-part vice into a small fixture, which is why the soft jaw is the bridge between the general-purpose vice and the dedicated fixture.
Setting the vice for a job
The vice is only as good as its setting, and the setting follows the principles of the workholding group. The part sits on parallels — the precision blocks that raise it above the vice bed, so that the tool can pass beneath it — and the part is pressed down onto the parallels as the vice tightens, its underside seated on the parallels and its side against the fixed jaw; a part that lifts off the parallels, or rocks on a chip, is not being held where it seems. The jaws must be clean — a chip between the part and the jaw, or between the part and the parallels, throws the part out of square, the setup mistake that hides in almost every bad first part — and the clamping force must be enough to hold the part firm against the cutting forces but not so great that it bows a thin part or springs it off the parallels. The tightening should be consistent from part to part, so that every part is held the same. And when every part must sit in the same place, a vice stop — a bar or pin at the end of the jaws — is set, so each part loads to the same position without re-setting.
The vice in the shop
The vice earns its place by its flexibility, and a machining centre will carry several of them at once. The shop bolts two, three or more vices side by side on the table or on a tombstone, each set and indicated, and runs a batch of parts from one end of the row to the other; the vices hold the parts for the first operations, the soft jaws for the finishing, and the same machining centre machines the soft jaws that hold them. It is the least specialised of workholding — no fixture to design, no permanent tooling to store, a pair of aluminium blanks machined in minutes for a new part — and for that reason it is the default for prismatic work in any quantity short of a full production run: the vice with its hard jaws for the stock and its soft jaws for the finished shape is the workholding most shops reach for first, a precision screw clamp on the table made to hold one part and remade in minutes to hold the next.