Hub · Interactive
How Jigs and Fixtures Hold a Part
A jig or fixture has one job: put the part in the same place every time and keep it there while work is done on it. This page covers how that is done, with two small simulations you can click through.
Start here
1Jig or fixture?
Both hold a part so the work comes out the same on every piece. The textbook difference is whether the tool is guided too.
Fixture
Locates and holds the part. The machine finds its own way to the work. A bracket clamped on a CNC table is in a fixture: the program knows where the part is because the fixture always puts it in the same spot.
Jig
Locates and holds the part, and also guides the tool. The classic case is a drill jig. A hardened bushing sits over each hole position and the drill goes through it, so the hole lands in the right place even with a hand drill.
A checking jig (or gauge) is a third kind. It holds a finished part so an operator can see quickly whether it is in or out of tolerance, often with go/no-go pins.
Core idea
2Six ways a part can move
A loose part on a table can move in six independent ways, called degrees of freedom. Three are slides and three are turns. Engineers shorten it to DOF.
Slides (translation)
Along X: left and right.
Along Y: toward you and away.
Along Z: up and down.
Turns (rotation)
About X: tipping forward or back.
About Y: tipping side to side.
About Z: spinning flat on the table.
Each one has two directions, and that matters. A locator only stops the part moving toward it. A stop on the left end keeps the part from sliding left and does nothing if it slides right. Holding it against the stop is the clamp’s job.
Locating · Interactive
3The 3-2-1 rule
The standard way to locate a block-shaped part uses six fixed points: three under it, two on one side and one on the end. Switch the locators on below and see which motions are left.
Primary: 3 base pads
Secondary: 2 side pins
Tertiary: 1 end stop
Clamps
Try this
B4 can be switched on once B1, B2 and B3 are in.
Free Stopped by a locator Held Turn stopped one way only
- Slide along X
- Slide along Y
- Slide along Z
- Turn about X
- Turn about Y
- Turn about Z
Three points always sit on a surface without rocking, for the same reason a three-legged stool never wobbles. Spread them as far apart as the part allows, so a small error at one pad tips the part less.
The three faces have names. The face on the pads is the primary datum, the face against the two pins is the secondary, and the end face is the tertiary. Make the primary the largest stable face, or the face the drawing dimensions from. That is the face the part will sit on most accurately.
Round parts and parts with holes use the same idea with different hardware. A V-block takes the place of the side pins. A round pin in a hole stops sliding in X and Y, and a second, diamond-shaped pin in another hole stops the turn.
Clamping
4Locate first, then clamp
Locators decide where the part is. Clamps only keep it there. If closing a clamp moves the part, the jig needs fixing.
Push toward the locators
Every clamp should press the part onto a locator: down onto the pads, sideways into the pins, endways into the stop. A clamp pushing away from a locator drags the part off it.
Clamp over support
Put the clamp directly above a pad or a solid section. Clamp between two pads and a thin part bends while clamped, gets machined flat, then springs back crooked when released.
Cutting force into the locators
Arrange the jig so the cutter or press pushes the part into a fixed locator, not against a clamp. Clamps can slip under load. Fixed stops don’t move.
Clamp in order
Seat the part on the pads, push it against the side pins and end stop, then apply the full down clamp. Clamp down hard first and friction keeps the part from sliding over to the pins.
Common mistake
5Too many locators
Over-constraining means using more locating points than the six the part needs. It looks safer. In practice the part can only touch all of them if the part and the jig are both perfect, and neither one is.
| Case | What happens | Usual fix |
|---|---|---|
| Four pads under a cast or sawn face | The part sits on three and rocks onto the fourth. Parts come out different depending on which three they landed on. | Three pads. Or four pads ground flat together, under a machined face only. |
| Two tight round pins in two holes | Hole spacing varies a little from part to part. Some parts jam, some won’t go on. | One round pin and one diamond (relieved) pin. The diamond pin only locates in the direction that stops the turn. |
| Locating on both sides of a slot | Fits only when the slot is exactly the nominal width. | Locate on one side and clamp toward it. |
| A clamp that also locates | The part’s position changes with clamp force and wear. | Separate the jobs: a fixed locator and a moving clamp. |
To see the first case, switch on B1, B2 and B3 in the simulator, then the 4th pad. The DOF list doesn’t change, but the part starts to rock.
Error-proofing · Interactive
6A jig that refuses the wrong part
Poka-yoke is the Japanese term for error-proofing. You design the jig so the mistake can’t be made, or so the machine won’t cycle if it is. Below is a pressing nest for a connector housing. Load a part, then press CYCLE START.
S1 part seated OFF
S2 color correct OFF
Ready to cycle
Fault
Good parts: 0 · Refused cycles: 0
Nest empty. No part, no cycle.
Two kinds of protection are at work here. The offset pin is physical: a reversed housing can’t sit down in the nest, whoever is loading it. The color sensor catches what the nest can’t, because the blue housing is the same molding. Where you can, make the wrong part physically not fit. Use a sensor where the parts look the same to the nest.
Other common ones: a pin pattern that only fits one way round, a pin that matches a hole only the right variant has, a part-present sensor on every cavity of a multi-part nest, and a step counter that stops the jig if a step is skipped.
Clamping hardware
7Hand clamps, air clamps and semi-automated jigs
The locating rules stay the same. What changes is where the clamping force comes from and whether a PLC can check it.
| Type | Good for | Watch out for |
|---|---|---|
| Toggle clamp (hand lever) | Quick loading, low to medium volume, no air needed. It locks over center, so holding force is set by the adjustment, not by how hard the operator pulls. | The operator can forget to close it. No signal to a PLC unless a switch is added. |
| Screw and swing clamps | Heavy parts and machining, where a lot of force is needed for little cost. | Slow to load. Force depends on the operator. |
| Pneumatic clamps | Production. Same force every cycle, and a reed switch on the cylinder tells the PLC the clamp has closed. | Needs clean, dry air, a valve and a regulator. Force is limited by bore size and pressure (see How Pneumatics Move a Machine). |
| Semi-automated jig | The operator loads and unloads. Air clamps, a press or a dispenser do the rest under PLC control. | More to build and maintain than a manual jig. Worth it when volume or consistency calls for it. |
Quick sizing: cylinder force is air pressure times piston area. A 32 mm bore at 0.5 MPa (5 bar) pushes about 400 N before friction. Designers usually count on only 50 to 70 percent of that, to cover friction and drops in line pressure.
Materials
8What jigs are made of
Most jigs mix materials. Hard steel goes where the part touches, lighter material everywhere else.
| Part of the jig | Common material | Why |
|---|---|---|
| Locating pins, pads, bushings | Hardened tool steel, for example SKD11 at about 58–62 HRC | Contact points wear first. Hardened steel holds its size for a long time and can be replaced when it does wear. |
| Base plates and bodies | Aluminum (6061 is common) or steel | Aluminum is light, machines fast and doesn’t rust. Steel for heavy press loads or where stiffness matters more than weight. |
| Guides, nests and covers | Acrylic or POM (acetal) | Acrylic is clear, so the operator can watch the part seat. POM is slippery and soft enough not to mark plated or painted parts. |
| Clamp tips | Urethane or POM pads | Grip without denting or scratching the part. |
| Checking jigs and trial nests | 3D-printed plastic | Fast and cheap for checking fit or low volume. It wears and creeps under clamp load, so it is not for long production runs. |
Applications
9Where this shows up in our builds
Large Profile Setting Jig
Positions and holds large components for assembly or inspection. Built with a clear acrylic guide column and a hand-lever clamp.
Semi-Automated Production Jigs
The operator loads the part and the jig does the pressing or the grease application. This is where sensor interlocks like the one in section 6 come in.
Fabricated Production Tools
Non-automated tools such as pressers and forming tools. The part still has to sit in the same place on every press, so the same locating rules apply.
Jigs, fixtures and harness tools
Precision-machined jigs and fixtures, plus wiring-harness tools such as blades and crimpers.
Photos and details are on our Projects page.
Check yourself
10Quick quiz
1. Strictly speaking, what does a jig do that a fixture doesn’t?
2. In 3-2-1 locating, the three pads under the part stop…
3. Where should a clamp push?
4. A part with two holes goes on two tight round pins and sometimes jams. The usual fix?
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