How much of the part does the vice need to grip?

As a guide, at least the equivalent of one diameter or the width of the part, and never less than 8-10 mm of grip on small parts. With too little grip the part shifts or chatters, and what looks like a tooling problem is a workholding problem.

When is a fixture worth making instead of using a vice?

When the batch is big enough that the repeated set-up time outweighs the cost of making the fixture, when the part is irregular and does not seat well, or when the same reference has to be guaranteed across several operations. Past a few dozen identical parts it almost always pays.

Why does a properly clamped part still distort?

Usually from over-clamping on thin walls or tubes, or because the set-up concentrates the force at two points instead of spreading it. It is fixed with soft or form-machined jaws, intermediate supports, and a measured clamping force rather than the maximum.

Workholding: why the part moves and how to clamp it properly

📅 Published 28 September 2026 · A practical guide from mecanizador.com

There is a line you hear in every workshop: "the part moved". It is almost never quite true. What happened is that the cutting force exceeded what the set-up could take, and that is decided before cycle start, not during it.

Workholding does not resist weight, it resists cutting force

A twenty-kilo part held in two fingers' worth of vice jaw feels solid when you push it by hand. But when a 16 mm cutter goes in three millimetres deep, the force that appears has nothing to do with weight: it is sideways, pulsing, and it runs in the direction of feed. That is the one that moves parts.

The practical rule is to think about three things every time: which way the tool pushes, what stops that movement, and what happens if the tool grabs. If the answer to the second one is "friction in the vice", you are running close to the edge.

The vice: the basics that get done badly

The milling vice is the most used and the most abused workholding there is. Four things make the difference:

Grip depth. At a minimum, the jaws should take 5 to 8 mm of the part. Less than that and you are relying on friction.
Seating on the base. The part has to sit down on the parallels, not hang above them. A sharp tap with a nylon mallet before the final nip seats it; if the parallels still slide by hand, it is not seated.
Direction of cut. Orient the part so the main force pushes against the fixed jaw, not against the moving one.
Clamping force. More torque is not more safety: a thin part distorts as you clamp, gets machined distorted, and springs back out of size when released. That is clamping distortion, and it is behind a lot of rejected parts that "measured fine on the machine".

Clamps: when they are worth it

When the part is large, flat or awkwardly shaped, the vice stops making sense. With clamps, what matters is where you support and where you tighten:

The heel block should sit at the same height as the part or slightly higher. If it sits lower, the clamp works like a wedge and the part tends to lift.
The bolt goes as close to the part as possible. The further away, the less real clamping force and the more the clamp flexes.
Spread the load: three or four well-placed points hold better than two done up to death.

Thin-walled parts and sections

Here the problem is not that they move, it is that they distort. Shop tricks that work: pack the inside with a sacrificial material, use side stops that do not clamp but do stop the part shifting, spread the pressure with aluminium or copper shims, and reduce the depth of cut even though it costs time. On tube and section, jaws machined to the profile are worth what they cost.

The lathe: chuck jaws, tailstock and steady

On the lathe the picture changes. The part turns, and with it come centrifugal force and bending moment. Three rules:

Keep the overhang short. As a guide, if the part sticks out of the chuck more than three times its diameter, you need the tailstock.
Tailstock, but not too hard. Over-tightened it bends the part and heats it; under-tightened it chatters. It should turn freely but with no play.
A steady rest for long slender parts, above all for finishing.

And one safety point people forget: take the chuck key out before you start. It sounds obvious until somebody does not.

Workholding that gives no trouble and costs little

For a batch, a simple fixture pays for itself fast: a plate with stops and two clamps that locates the part the same way every time removes the set-up between parts and cuts repeatability error. For large flat parts, vacuum. For magnetic parts in grinding, the magnetic chuck. And for prototypes, high-tack double-sided tape holds more than you would think on light aluminium roughing.

The signs that the clamping is not holding

Before anything breaks, the set-up warns you:

A noise that changes pitch, that irregular chatter. Stop and check.
Vibration marks on the surface, regular waves that were not in the program.
Dimensions drifting in the same direction part after part.
Odd burrs, or an edge that breaks for no apparent reason.

If any of these turns up, the problem is rarely the program. Check the workholding before you touch the feeds.

The twenty-second check

Before cycle start, three questions: is the part really seated on the parallels? Does the cutting force push against the fixed support? Is there room for the tool to pass without touching clamps or vice anywhere along the path? That last one is answered by a dry run, which takes thirty seconds.

If you are starting out in the trade, this fits right after understanding what a CNC program is and before getting into the mistakes that break the part or the tool.

Need a part machined? The directory of Spanish machine shops lists more than 700 workshops across Spain with their processes, materials and certifications verified, filterable by province. And if you are learning, the CNC Path takes you from G-code to your first part.