Workholding and Fixturing

What workholding must do

Workholding (also called fixturing) holds the part so that the tool can cut it. A good fixture does four things: it locates the part in a known position, it holds it against the cutting forces without letting it move or vibrate, it leaves access for the tool to reach every feature, and it does all this without distorting the part. Most scrapped parts and many broken tools trace back to the workholding, not to the program.

• Locate from a flat surface, a corner or a pair of pins, the same way each time. See locating below.
• Hold with enough force to resist the cutting force (and the heavy forces at the start of a cut) with a safety margin of 2–3.
• Access. The cutter and its holder must clear the clamps, the vise and the jaws. Check in simulation.
• Do not distort. Clamping a thin or uneven part too hard bends it; when the clamp is released, it springs back and the cut surface is no longer flat.
• Repeat. For more than one part, make the loading quick and the position repeatable: stops, pins, dedicated jaws.

Forces: what the cut asks of the clamp

The cutting force is the power at the cutter divided by the cutting speed. A part is held by friction at the jaws and clamps, so the holding force must be larger than the cutting force divided by the friction coefficient.

F_cut = P_c ÷ v_c    F_hold ≥ safety factor × F_cut ÷ µ

• Direction matters. Cutting forces push the part away from the tool: place stops and the fixed jaw on that side, so that the force presses the part against them rather than against the clamp.
• Climb milling pulls the part into the cut and can drag it if the hold is weak. A rigid vise handles it; vacuum and tape do not.
• Heavier cuts need more holding. A hobby machine’s light cuts allow simple tape or small clamps; a mill’s heavy roughing cuts need a vise or clamps.

Vises, parallels and soft jaws

• Milling vise. A precision vise with hardened, ground jaws is the main workholding on a mill. Clamp the part against the fixed jaw, with a thin rod or a strip between the movable jaw and the part, so that the part is pushed down onto the parallels.
• Parallels raise the part so that the cutter does not hit the vise. Use matched, ground parallels, tap the part down with a soft mallet while tightening so that it seats, and check that the parallels are not loose.
• Grip depth. Hold at least one-third of the part’s height in the jaws (more for tall, thin parts), and cut only the part that sticks out. If you must cut deeper, use a taller vise, a different set-up, or a fixture.
• Soft jaws (aluminum or steel) are machined to the part’s shape. They give even contact over a large area, which holds odd shapes and finished surfaces without marking, and can be machined in place for exact repeatability.
• Vise stops and pins locate the part from the fixed jaw, so that the next part is in the same place.
• Torque. Do not extend the handle with a pipe; over-tightening bends the vise’s jaws and distorts the part, and can damage the screw.
• Heat. A part heats as it is cut: 100 mm of aluminum warms by 20 °C and grows 47 µm, which can lock it in the vise. Allow for that on long parts.

Strap clamps, toe clamps and T-slots

Large and odd-shaped parts are held to the table with clamps. The standard clamp is a strap (step) clamp: a bar rests on the part at one end and on a block (the step block) of the same height at the other, with the clamping bolt between them. The bolt’s force is shared between the two ends by the lever principle.

F_clamp = F_bolt × b ÷ (a + b)

Here a is the distance from the part to the bolt and b the distance from the bolt to the step block. With a bolt force of 5 000 N, a = 20 mm and b = 60 mm, the clamp force is 3 750 N; with the bolt centered (a = b = 40 mm) it is 2 500 N.

• Bolt close to the part. The nearer the bolt, the more force reaches the part.
• Step block the same height as the part, so that the strap lies level. If the block is lower, the strap tilts and touches only the edge.
• Use the shortest stud and a nut with a washer, and make sure the stud is well engaged in the T-nut.
• Clamp over the supported area (above a parallel or a flat table), not over an unsupported overhang, or the part bends and lifts at the other side.
• Place clamps outside the toolpath, and mark them on the CAM model so that the simulation can check them.
• Toe clamps (hold-downs with a serrated tip) press the part down and sideways as the screw is tightened; they take little height and are good for rough stock in a vise or on a plate.
• T-slot table. Use T-nuts that fit the slot, and clean the slot of chips so the nuts seat.

Tape, vacuum, fixture plates and tabs

Method	Best for	Limits	Notes
Double-sided tape	Thin, flat sheet; light finishing cuts	Low holding force; heat softens it	Use cloth-backed tape; clean the surfaces; press the part down hard; remove by sliding a thin blade
Vacuum table	Flat sheet and panels on a router	Needs a large flat area, a seal and a good pump; small parts fall off	Holds against sideways force by friction; gasket and spoilboard needed
Spoilboard with screws	Wood and plastic sheet	Screws can be hit by the tool	A sacrificial MDF board under the work allows cutting through; skim it flat
Fixture (grid) plate	Repeat parts, many set-ups	Needs accurate holes and tools	A plate with a grid of threaded holes and dowel holes; clamps and stops go anywhere
Tabs / onion skin	Sheet parts cut free of the sheet	Tabs must be removed afterwards	Small bridges hold the cut part to the sheet; a thin final layer (onion skin) can hold parts too
Magnetic chucks	Flat steel parts for grinding and light milling	Only for ferrous metals; low side force	Fast loading; keep the tool away from the magnet
Hot glue, wax, CA glue	Tiny or very thin parts	Weak; residue	Common for small plastic and wood parts; heat releases them

Locating: repeatable positions

To put a part in the same place every time, locate it against fixed features. The classic rule is 3-2-1: three points on a base surface stop three motions (one translation and two rotations); two points on a second surface stop two more; one point on a third stops the last. This is the same logic as the datum reference frame in Datums and Datum Reference Frames.

• Base on the primary datum (the big flat face) and press the part against locating pins or the fixed jaw.
• Dowel pins in reamed holes give a position repeatable to a few micrometers; use two (one round, one diamond-shaped) so the part cannot be forced.
• Probe the part. A touch probe finds the part’s edges or a hole and sets the work offset automatically, accounting for any variation in how it was loaded.
• Second operations. To machine the second side, locate on features made in the first set-up (holes, a machined flat), not on rough stock. A flipped part needs its location reset to ±0.02 mm or better.
• Design the part for fixturing. A sacrificial tab or extra stock to grip, and datum features that are machined first, makes the part easy to hold; see Designing for CNC Milling.

Thin, soft and awkward parts

• Support thin walls while cutting. Leave them thick until the last pass, cut the finishing pass lightly, and keep a fillet or a rib that you remove last.
• Do not squeeze. Plastic and thin aluminum distort: use tape or vacuum, wider jaws, or soft jaws that spread the load.
• Residual stress. Plate and bar contain internal stress. When you machine away one side, the part bends. Rough both sides first, let it rest, then finish; or use stress-relieved plate (T651 aluminum, for example).
• Fixture the awkward shape with a custom plate: cut the plate to the part’s outline (or its inverse) and clamp the part in it.
• Very small parts. Use a sacrificial tab or machine from a larger blank, and cut them off last.
• Heavy parts need to be supported against sag and moved with lifting equipment, not by hand.

Chucks, collets and rotary work

• Lathe chucks and collets are described in Turning: Lathes and Operations. Soft jaws for the lathe are bored to the part’s diameter in the same chuck so that they run true.
• Mill collet blocks and square collet fixtures hold bar stock for milling flats and cross holes.
• 4th-axis (rotary table) work. Hold the part in a chuck or between centers on the rotary axis; mind the extra overhang and the weight, and support long parts with a tailstock.
• Balance. A part or fixture that is out of balance on a lathe or rotary axis vibrates at speed; add a counterweight.

Set-up checklist

Check	Why
Part is seated on the parallels or the plate, and clean	A chip under the part tilts it
Clamps tightened in the right order, evenly	Avoids bending the part
Clamps and the vise are clear of the toolpath and the holder	Prevents a crash
Enough of the part is gripped for the planned cuts	Prevents the part moving or lifting
Work zero is set on the held part, not on the stock before clamping	Clamping can move the part
Tool is short enough and the holder clears the work	Stiffness and access
Run an air cut at a safe height	Shows crashes before they cost a tool
Re-check after the first roughing pass	The part may have shifted or sprung

Related

• CNC Machining: the overview of this guide.
• Milling: Operations and Tooling: the cutting forces the clamp resists.
• Designing for CNC Milling: design the part so that it can be held.
• Datums and Datum Reference Frames: the same locating logic on drawings.

FAQ

How tight should I clamp a part?

Tight enough that the cut cannot move it, no tighter. Excess force bends thin parts and damages jaws. Do a trial cut and check for shift; leave a margin of about 2 to 3 times the expected cutting force divided by the friction.

Why did my part move during the cut?

The holding force was less than the cutting force (often a peak at the start of a cut), the surfaces were oily or polished, chips lay under the part, or the cut pulled the part out of the vise in climb milling. Check each.

Can I use double-sided tape on a CNC router?

For thin sheet and light finishing cuts, yes. For heavy roughing or climb cuts it will release. Combine tape with clamps or screws where you can, and cut with a stable, small step-down.

How do I hold a round part for milling?

Use a V-block in a vise, soft jaws bored to the diameter, or a collet block. Clamp on the round surface with a strap clamp pressing into the V.

What is a spoilboard?

A sacrificial sheet (often MDF) on the router table. It lets the cutter go through the part into the board without cutting the table, and can be re-surfaced flat with the machine.

References

• Oberg, E. et al., Machinery’s Handbook, 31st ed., Industrial Press: workholding, jigs and fixtures, friction coefficients.
• Sandvik Coromant, Workholding for Milling, and vise manufacturers’ data (Kurt, Schunk, Gerardi): clamping force and torque.
• ASME Y14.5-2018, Dimensioning and Tolerancing: datum reference frames.
• Hoffman, E., Jig and Fixture Design, Cengage.

Last reviewed: October 2026. Cutting data are starting points: machines, tools and materials differ, so test on scrap and follow the tool maker’s data.