Workholding, Clamping and Fixtures
Clamp force and friction, vises, soft jaws, locating with pins, jigs and stops, and how to hold thin or awkward parts.
Why hold the work
A part must be held still (so the cut is accurate), safely (so your hands stay clear) and without damage (so the surface is not marked or the part not bent). Workholding fails when the cutting force exceeds what the clamps can resist, and it is the cause of many injuries: a drill that grabs a hand-held part spins it like a saw blade.
- Never hold small work by hand near a cutter, drill or grinder.
- Think about where the force goes before you clamp: the part should be pressed against something solid, not just pinched.
- Support the work close to the cut so it cannot flex or vibrate.
Holding by friction
A vise or clamp holds a part by squeezing it: the clamping force N on each side produces a friction force of up to μ N on each face, so the force that the grip can resist is F = 2 μ N.
Worked example: how much can a vise hold?
A vise clamping with 5 000 N (about 500 kg) holds a part against these sideways forces:
| Jaw surface | Friction coefficient μ | Holding force 2 μ N |
|---|---|---|
| smooth steel jaws (oily) | 0.1 | 1 000 N |
| smooth steel jaws (dry) | 0.2 | 2 000 N |
| serrated jaws | 0.4 | 4 000 N |
| soft aluminum jaws | 0.3 | 3 000 N |
- To resist a cutting force of 600 N with a friction coefficient of 0.3 the clamping force must be at least 600 ÷ (2 × 0.3) = 1 000 N; with a safety factor of 2, 2 000 N.
- Oil and cutting fluid on the jaws cuts the grip to a third. Clean and dry the faces for heavy cuts, or use serrated jaws.
- Friction cannot resist a force that pushes the part out of the jaws in the direction of the grip: add a stop.
Clamps
| Clamp | Good for | Notes |
|---|---|---|
| F-clamp / bar clamp | Gluing, assembly, holding to a bench | Parallel jaws; use pads to protect the work |
| G-clamp (C-clamp) | Metal, drill press, welding | Strong but slow; deep throat for reach |
| Quick-release / trigger clamp | Temporary holding | Light force; fine for sawing and drilling thin material |
| Toggle clamp | Jigs and repeated work | A lever that locks; fast and consistent |
| Spring clamp | Light holding, thin parts | Low force, quick |
| Pipe clamp | Long glue-ups | Any length of pipe |
| Hold-down and strap clamp | Machine table, bench dogs | Used with T-slots; see CNC Machining |
Worked example: clamp forces
- G-clamp: a 4 N·m tightening torque on a screw of lead 3 mm and efficiency 0.25 gives F = 2π × 4 × 0.25 ÷ 0.003 = 2 094 N (about 213 kg).
- Toggle clamp: a lever of ratio 120 ÷ 30 = 4 turns an 80 N hand force into 320 N at the work; the over-center action keeps the force when you let go.
- A clamp can bend the work. A 20 mm wide × 3 mm thick aluminum strip held 60 mm out from a clamp and pushed by 200 N deflects 4.57 mm (δ = F L³ ÷ 3 E I with I = 45.0 mm⁴): support thin parts or the cut will be out of line.
Vises and soft jaws
- Bench vise: a heavy cast vise for filing, sawing and bending; the jaw width sets the size (100 to 150 mm).
- Machine vise: a precision vise bolted to the drill press or mill table, with jaws ground square.
- Soft jaws of aluminum, copper, wood or leather cover the steel jaws and protect the part.
- Parallels: flat bars that lift the part above the jaws so its bottom is square and supported.
- Tighten once, firmly, then tap the part down onto the parallels with a soft mallet before the final tightening.
- Do not hammer on a vise except on the anvil face, and do not extend the handle with a pipe.
Worked example: jaw pressure on soft metal
A vise clamping with 5 000 N on jaws that touch 40 × 15 mm = 600 mm² of a part produces a contact pressure of 8.3 MPa. That is only 9 % of the 95 MPa yield strength of annealed 6061 aluminum, so smooth jaws do not mark it. Serrated jaws touch only on their tips, perhaps a tenth of the area, which multiplies the local pressure by ten and does mark it. Use soft or smooth jaws and less force for finished surfaces.
Locating a part: 3-2-1
A free part has six degrees of freedom (three translations and three rotations). To repeat a position, remove them all with the minimum number of contacts: the 3-2-1 rule (the same idea as a datum reference frame in GD&T).
| Contacts | Points | Removes |
|---|---|---|
| Primary plane | 3 points | Removes 3 degrees of freedom (1 translation, 2 rotations) |
| Secondary line | 2 points | Removes 2 (1 translation, 1 rotation) |
| Tertiary stop | 1 point | Removes the last degree of freedom (one translation): the part is fully located |
Worked example: pins and clearance
- A 5.98 mm pin in a 6.00 mm hole has a clearance of 0.02 mm. Two such pins 80 mm apart allow the part to rotate by about 2 × 0.02 ÷ 80 = 0.0005 rad = 0.03°.
- Use one round pin and one diamond (relieved) pin: the round pin sets the position, the diamond stops the rotation without over-constraining.
- A stop block that is repeatable to ±0.1 mm gives parts within about that range; if the error of 0.1 mm is random, 20 parts stacked end to end vary by 0.45 mm (the root of the sum of squares).
- Locate from the finished datum edge, not the rough one, and clamp so that the force pushes the part against the locators.
Jigs, stops and fixtures
- A stop block clamped to a fence makes repeated cuts the same length, and it saves hours of marking.
- A drilling jig (a block with hardened bushings) puts holes in the right place every time.
- A bench hook holds a board against the edge of the bench for sawing.
- A fixture for a machine holds the part in a known place relative to the tool: a flat base, locating pins and clamps.
- Build it from scrap and keep it: a good jig is a drawn setup you can reuse.
- Make the jig stiffer than the part; it should be the most accurate thing in the setup.
Thin, soft and awkward parts
| Part | Problem | Solution |
|---|---|---|
| Thin sheet | Bends and vibrates | Sandwich between two sacrificial boards; clamp close to the cut; double-sided tape |
| Round bar or tube | Rolls and crushes | V-block, split jaws, or a fixture that matches the diameter; don’t over-tighten a thin tube |
| Soft metal and plastic | Marked or deformed by jaws | Soft jaws, light force, wrap in paper or leather |
| Large and heavy | Hard to move and tips | Support on rollers or a second stand; use two clamps |
| Irregular or curved | No flat face | Wooden cradle, hot-melt glue, or a custom fixture |
| Small parts | Cannot be held by hand safely | Hold in a vise or a pin vise; use a clamp, never your fingers |
Holding for drilling and cutting
Worked example: clamping a plate for a 12 mm hole
- The drill (see Power Tools and Shop Machines) puts 8.1 N·m of torque on the plate. A clamp 25 mm from the hole resists with a force of 8.1 ÷ 0.025 = 324 N tangentially.
- With a safety factor of 3 the clamp must resist 972 N: at a friction coefficient of 0.3 that is a clamping force of 972 ÷ 0.3 = 3 240 N on one face, or about 1 620 N if you clamp both faces.
- In practice, clamp the plate to the table and put a stop against the opposite edge, so the drill pushes the plate against the stop and the clamp only has to hold it down.
- Back the work with scrap so the drill does not break out and catch.
Troubleshooting
| Problem | Likely cause | Try |
|---|---|---|
| Part slips in the vise | Oil on the jaws, force too low, no stop | Clean the jaws; serrated or soft-grip jaws; add a stop |
| Part marked by the jaws | Hardened jaws on soft metal | Soft jaws; less force; paper shim |
| Part is not square after clamping | Clamp forces bend it, burr under the part | Deburr; use parallels; tap down; unclamp and recheck |
| Cut chatters or vibrates | Poor support, overhang | Clamp closer; support the free end |
| Drilled holes in different places each time | Part not located | Use pins or a stop block; clamp against a datum |
| Clamp leaves dents in wood | No pads | Use scrap pads or clamp pads |
Related
- Shop Tools: the overview of this guide.
- Workbench and Shop Setup: the bench and vise.
- Power Tools and Shop Machines: drill press, bandsaw and the forces they put on a part.
- CNC Machining (workholding for mills and lathes) and GD&T (datums).
FAQ
How tight should a vise be?
Tight enough that the friction force 2 μ N exceeds the cutting force with a margin of at least two; no more, to avoid marking or bending the part.
Why do I use soft jaws?
They spread the load and protect finished surfaces; they can also be machined to the shape of the part.
What is the 3-2-1 rule?
Three points on a plane, two on a line and one at a stop remove all six degrees of freedom of a part with the fewest contacts.
Can I hold a small part by hand while I drill?
No: use a vise, clamp or pin vise. A drill that grabs the part will spin it faster than you can let go.
References
- Oberg et al., Machinery’s Handbook (workholding, jigs and fixtures).
- Hoffman, E. G., Jig and Fixture Design, Cengage.
- ASME Y14.5-2018 (datum reference frames).
Last reviewed: October 2026. Forces and friction coefficients are typical: test your own setup.