Work-holding on a small mill is two jobs, and a lot of setups only solve the first one. Job one is holding the part still while the cutter tries to move it. Job two is putting the part back in the same place next time, without touching off X, Y and Z again by hand. A vise, step clamps and direct T-slot clamping mainly do job one. A fixture plate with locating pins does job two. A vacuum table does job one for thin sheet the others cannot grip. Pick by which job is costing you time.
If your setup only solves the first job, every repeat run starts with touch off, touch off, touch off.
The five options, and the problem each one solves
| Method | Solves | Good at | Weak at |
|---|---|---|---|
| Machinist's vise | Holding still | Fast setup, rectangular stock, rigid grip | Returning to the same zero after a teardown; blocks two faces |
| Toe clamps and step clamps | Holding still | Odd shapes, thin stock, full top-face access | Clamp placement has to be planned around the toolpath for each part |
| Direct T-slot clamping | Holding still | Large or awkward stock, no extra hardware | Slowest setup of the group; nothing defines position but your indicator |
| Fixture plate with locating pins | Putting it back | Rebuilding a setup at the same zero, several small parts at once | Upfront cost; a badly designed fixture wastes the same time a bad setup does |
| Vacuum table | Holding still (thin sheet) | Flat sheet, no clamps in the cutter's way | Tall or narrow parts, soft or slippery stock, anything that will not seal |
Holding it still without bending it
Carr Lane's rule is one line: clamping force has to be high enough to counter the cutting forces and not so high that it deforms the workpiece. Where the force goes matters as much as how much. Carr Lane says to position clamps so the force goes into the supports or locators, because force against an unsupported area bends the part. A thin plate pulled flat by a clamp springs back once the clamp comes off, and the face you just cut flat is now a curve. That taper gets blamed on the spindle.
Strap-clamp force is published, so you do not have to guess. Carr Lane's table gives roughly 2,300 lb for a standard strap clamp on a clean, dry 1/2-13 stud at 38 ft-lb of torque. That assumes a 2:1 lever ratio, with the stud midway between the workpiece and the heel block; a different stud position is a different ratio, and the same page has a calculator for it.
OSHA's milling-machine trainer script adds the direction rule for a vise or jig: place the locking arrangement so the force is exerted away from the cutter. The same script sits the work on parallels about 1/8 inch above the jaws, with both parallels snug once the vise is tight.
Putting it back in the same place
Carr Lane's definition of a locator is the whole second job in one sentence: the primary function of any locator is to reference the workpiece and to ensure repeatability. A clamp holds. A locator positions. A plain vise gives you one locator, the fixed jaw, plus whatever stop you add against it. A fixture plate with dowel pins on a known grid gives you as many as the job needs, and the grid, not your memory of where the vise sat, defines the coordinate system. Tormach says the same from the shop floor: the benefit of a fixture is reusing it without re-referencing everything every time, and its tip for the vise is to leave it attached or mark its holes, so it goes back where it was.
When the job is thin sheet
A 3 mm sheet gives a vise nothing to grip, so vacuum holds it with the atmosphere instead. Pierson Workholding's rule for its vacuum chuck: multiply the part's surface area over the vacuum, in square inches, by 14 to get the holding force in pounds. A 6 by 6 inch part is held with about 504 lb, and that arithmetic is why vacuum is a sheet tool. Pierson says a part 1 inch square and 4 inches tall will not stand up to cutting forces, and very thin or very soft material can flex at the edges and peel off. Any through hole needs a gasket, or the vacuum is lost when the cutter breaks through. Low-friction plastics such as nylon, PTFE and acetal need pins, side rails or a shallow pocket to sit in, or they walk sideways.
Before you press cycle start
- Every clamp pushes into a support or locator, never into an unsupported span (Carr Lane).
- The vise or jig is locked so the force is exerted away from the cutter (OSHA).
- The work sits about 1/8 inch above the jaws and both parallels are snug (OSHA).
- Chuck keys and wrenches are out of the spindle (OSHA).
- On vacuum: through holes gasketed, slippery plastics held sideways by pins, rails or a pocket (Pierson).
Common questions
Can a vise handle a part that needs both faces machined?
Yes, if the flip is a second setup with its own locators. Carr Lane's 3-2-1 method uses three locators on the primary face, two on a side and one on an end. In vise terms: the parallels are the three, the fixed jaw the two, and a hard stop against the fixed jaw the one. Without that stop, the flipped part lands wherever you slid it.
How tight is tight enough on a vise?
The manual for your vise model states the handle torque it was designed for; the handle it shipped with is the lever that delivers it. More handle is not more accuracy: Carr Lane's warning is that too much force bends or marks the workpiece.
Takeaway
Buy for the job you run, not the job you might run someday. A vise and step clamps cover one-off rectangular work, vacuum covers thin sheet, and a plate with pins covers the job that comes back. That last one is the difference between scrambling on deadline and just running the job again. For a boat fitting, the CNC-or-print guide covers whether it should be machined at all; the tolerance guide covers which features the fixture has to hold true.
Checked against Carr Lane, Pierson Workholding, Tormach and OSHA help pages on 2026-09-21: Locating and clamping principles, Clamping force calculations, Approximate clamping force of standard clamp straps, Fixture design principles, SmartVac 3 vacuum chuck, Proper workholding adds efficiency to your shop, A few need-to-knows for simplifying your CNC workholding, OSHA milling machine train-the-trainer script (PDF)
