Jigs & Fixtures: Types, Materials, and Tolerances

A jig guides a tool. A fixture holds and locates a workpiece. Both are usually made as one-off or small-batch parts, built to fit one product, and both are bought like any custom part: with a model, a drawing, and a clear statement of what the tooling must hold. This page covers the common types, typical materials and tolerances, what drives the cost of custom tooling, and how to prepare the information a shop needs to build it well.

What a jig is, and what a fixture is

A jig holds the workpiece and guides the cutting tool. The classic example is a drill jig: a plate or box that locates the part and carries hardened bushings, so hole position comes from the jig rather than from layout lines. A fixture holds and locates the part but does not guide the tool. A milling fixture clamps the part against fixed locators while the machine program controls the cutter. The fixture’s whole job is to make sure every part sits in that program the same way.

Where the terms overlap

In real shops the words blur. A welding jig often guides nothing; it only holds parts in position, a fixture by the strict definition. The label matters less than the function: what locates the part, what clamps it, and what, if anything, guides the tool.

When dedicated tooling pays

Dedicated tooling earns its cost in three situations. First, repeatability: the fixture loads the part against the same locators every cycle, so the operator stops indicating and shimming each piece. Second, locational tolerance. When a hole pattern must hold a tight relationship to a datum, the fixture carries that relationship so the machine does not re-find it every time. Third, skill compression: a fixture that accepts the part only in the correct orientation lets a newer operator produce work that used to depend on an experienced setup hand.

Alternatives that come first

Dedicated tooling sits at the top of a ladder, and the lower rungs cost less. Soft jaws, aluminum cut in place in the vise to the part’s contour, are the cheapest step up from stock hard jaws. Modular fixturing, reusable grid plates and clamp towers assembled per job, suits low-volume, high-mix work because the components amortize across many parts. A common rule of thumb: high volume with a stable design favors dedicated fixtures; low volume with many part numbers favors modular or printed solutions.

The common types of jigs and fixtures

CNC workholding

CNC workholding spans soft jaws, dedicated vise fixtures, tombstones, and pallets. A dedicated vise fixture is usually an aluminum body machined to the part’s contour, bolted into a vise or to the table. Tombstones, the multi-sided columns on horizontal machining centers, put four faces of parts into one cycle. Pallet and zero-point systems let the next batch be set up offline and swapped on in seconds; makers commonly quote repeatability in the low micron range, around 0.002 to 0.005 mm.

Drill jigs and bushings

Drill jigs carry replaceable hardened bushings. Headless press-fit bushings, pressed flush into the jig plate, are the low-cost choice for a single drilling operation. Headed press-fit bushings stay put where heavy chip load could push a headless one through the plate. Renewable bushings fit into permanently pressed liners and lock with a screw, which suits longer runs and multi-operation holes. Common practice sizes the bushing outside diameter at roughly twice the drill diameter. The bushing also stands 1 to 1.5 drill diameters above the workpiece, so chips can escape.

Welding fixtures

Welding fixtures locate parts relative to each other and hold them through the weld heat. Heat is the design problem: the weld shrinks as it cools and pulls the assembly out of shape, and the fixture restrains that movement. Restraint has a price, because clamping hard against shrinkage locks in residual stress. Typical practice therefore combines restraint with a symmetric layout, a balanced weld sequence, and intermittent welds where a continuous bead is not needed. Copper chill details pull heat out of stainless welds. Mild steel bodies with replaceable stainless or copper-alloy locators keep spatter from sticking, and an anti-spatter coating is common.

Inspection and checking fixtures

Checking fixtures hold a finished part on its datums and carry pins, gauges, or dial indicators, so an operator can accept or reject a part in seconds. The fixture must mirror the drawing’s datum scheme, because a fixture that locates on features other than the drawing’s datums checks a different part. Fixed go and no-go gauges follow the metrology rule of ten, consuming about a tenth of the tolerance they check. CMM hold fixtures are a gentler cousin: light clamping, support on the datums, and open probe access, since clamping force that deflects the part corrupts the measurement.

Assembly fixtures

Assembly fixtures locate two or more parts relative to each other for bonding, fastening, or soldering. Clamping forces are low and geometry is everything. Poka-yoke features, such as an asymmetric pin that accepts the part only one way, prevent the mirrored load that ruins an assembly. Printed plastics are common, and electronics work often calls for ESD-safe filament, with dissipative grades commonly quoted at one million to one billion ohms of surface resistance.

What drives fixture design

Datum strategy mirrors the part drawing

The fixture should locate the part the way the drawing measures it. GD&T, standardized in ASME Y14.5, defines datums and the degrees of freedom they constrain. The 3-2-1 rule places three locators on the primary datum face, two on the secondary, and one on the tertiary, fixing all six degrees of freedom. When the fixture and the inspection share a datum scheme, a part that passes inspection also fits its assembly. When they differ, the two checks disagree.

Clamping that does not deflect the part

Clamps should push the part into its locators, toward support, not into thin walls or unsupported spans. A clamp that bows the part holds the bow through the cut, and the part springs back out of shape after release, wrong everywhere even though every programmed dimension was cut exactly. Sequence matters too: tightening the wrong clamp first can shift the part off its locators.

Chip and swarf escape

Fixtures that trap chips mis-load parts. Open corners, clearance slots around locators, and relieved clamp faces give chips somewhere to go. A bushing standing off the workpiece lets chips clear the drill instead of packing under the jig. A fixture that must be blown out between loads adds labor to every cycle.

Load and unload speed

The fixture is handled once per part, so ergonomics compound. One-motion clamping, quarter-turn screws, and room for the hands are the difference between a three-second load and a ten-second one, real money across ten thousand parts. If the part can load wrong, it eventually will.

Materials for jigs and fixtures

Aluminum: 6061-T651 and cast tooling plate

Aluminum is the default: it machines fast, stays put, and is light enough to handle. Rolled plate in the T651 temper is stress relieved by stretching after quenching. Heavy milling relieves residual stress and warps the plate on its own, and the stretched temper minimizes that movement. Cast tooling plate, sold under names such as MIC-6 and ATP-5, is cast near net thickness with very low internal stress and is commonly quoted within about 0.005 in of nominal; it machines flat and stays flat, the standard for jig plates and bodies with locational features.

Steel and stainless

Mild steel, 1018 for machined details and A36 for welded frames, is the material of weld fixtures and large tooling: it welds, it is stiff, and it is cheap. Wear surfaces get harder treatment: prehard 4140 or case-hardened inserts at locator contacts, and hardened bushing steel, commonly around 60 HRC, wherever a tool passes through. Stainless details appear where spatter sticks to steel or corrosion matters.

Printed plastics

Printing earns its place at the light end of the duty range. PLA suits one-off handling aids. Nylon, printed by FDM, SLS, or MJF, suits assembly fixtures that see real handling, and carbon-fiber nylon adds stiffness. The limits are real: printed parts creep under sustained clamp load, hold looser tolerance than machined metal, and should not carry heavy cutting forces. A printed fixture that survives a hundred assemblies may sag in a thousand.

Tolerances on fixtures

Locating features

The features that locate the part, and any bushings that guide a tool, carry the tight work. A common shop heuristic holds them several times tighter than the part tolerance they control. There is no single published number, because the value depends on the tolerance held and on the stack between fixture, machine, and part. It is worth settling with the shop before the fixture is cut.

Bushing bores and press fits

Catalog bushings arrive ground, so the fit work happens in the jig plate. Mounting holes are jig-bored or reamed. Press-fit practice commonly cited by bushing makers calls for about 0.0005 to 0.0008 in of diametral interference for headless bushings and 0.0003 to 0.0005 in for headed ones. More interference is not better: an oversize press distorts the plate and closes the bore, and the drill then guides crooked. Renewable bushings run a small clearance, commonly 0.0001 to 0.0006 in, inside their liners. Catalogs typically list bore-to-outside-diameter concentricity at 0.0003 to 0.0005 in total indicator reading.

Bodies and non-critical faces

Everything else can be loose. Clearance holes, body edges, lightening pockets, and non-locating faces do not need tight tolerance, and specifying it there only buys machining hours. Tolerance is a budget: spend it on locators, bushings, and clamp paths.

What drives the cost of custom tooling

Fixture cost breaks into a handful of drivers, and knowing them helps a buyer shape a quote. Material follows stock size: a body machined from plate close to the finished envelope costs less than one hogged from oversize bar, and cast plate saves roughing time. Machining hours follow the feature count, and especially the number and accuracy of holes, since each jig-bored hole is an operation. Purchased hardware, bushings, liners, clamps, locators, and pins, is often a meaningful share of the cost.

Programming and setup are fixed, and quantity is the lever: a set of identical fixtures spreads one program across many bodies, so the second and third cost far less than the first. Jigs generally cost more than plain fixtures for the same part, because the bushings, liners, and hole-position accuracy add operations.

How to specify a jig or fixture for quotation

Tooling requests need more context than part requests, because the shop is quoting a device built around your part. A complete package contains:

• The part model, and a drawing with the datum scheme and critical tolerances marked.
• The operation the tooling serves, and the machine or station it runs on.
• Which tolerances the fixture must hold, versus which the machine or operator will find.
• The expected cycle count or tooling life, per year and across the program.
• Where clamping is allowed, and which surfaces must stay clear for the cutter, torch, or probe.
• How the part loads and unloads, and any foolproofing requirement.
• The quantity of fixtures, since duplicates amortize the programming.

Agreeing on the datum scheme before cutting metal is the highest-value single step. Stating what the tooling must not do, such as marking a finished surface, shapes the clamp materials.

Wear, repair, and re-bushing

Renewable bushings and liners

Bushings are consumables. A long-run jig presses hardened liners into the plate permanently and runs renewable bushings inside them, so a worn bore means the bushing is released, slipped out, and replaced in minutes; the body is untouched. Press-fit bushings are pressed out and replaced, and the light interference fits above let the plate survive many replacements.

Wear parts and hardening

Locators, clamp faces, and wear pads are the other consumables. Replaceable pads protect the body, and hardened or plated inserts protect high-contact locators. A locating face that wears out of true is usually re-machined or given a new insert, not replaced. A fixture designed with its wear parts in mind lasts the life of the program.

When dedicated tooling is the wrong buy

Dedicated tooling is a commitment, and there are good reasons to skip it. If the run is a handful of parts, the fixture usually costs more than the labor it saves. If the part design is still moving, tooling locks geometry that is about to change, and every revision taxes the fixture. And if the machine can hold the tolerance without help, soft jaws, modular workholding, or printed parts often close the gap for a fraction of the cost. The common mistakes come from the same place:

• Ordering a dedicated fixture for a batch that runs once.
• Freezing tooling before the part design is stable, then paying for fixture revisions.
• Locating on features the drawing does not use as datums, so parts pass inspection and fail assembly.
• Clamping into unsupported walls, so parts spring back out of tolerance.
• Ignoring chip escape, then adding labor to every cycle.
• Skipping the wear plan, so the first worn bushing becomes a rebuild.
• Over-tolerancing the body, paying for precision on faces that never touch the part.