The 3-2-1 principle – the basis of all fixture design
A free body has six degrees of freedom: three translations and three rotations. The 3-2-1 principle locks them with six locating points. Three points in the primary plane lock three degrees of freedom, two points in the secondary plane lock two more and a final point locks the sixth. More fixed locators than that make the fixture over-constrained, and the part's dimensional variation then sets its position instead of the fixture.
The locating points are placed on the faces that are datums on the part drawing, and as far apart as possible. The clamping force must push the part against the locators, never away from them, and act where the part is stiff enough not to distort. Two locating pins in holes are solved with one cylindrical and one relieved pin, so the part does not jam.
In practice the points are often replaced by small rest pads or hardened rest buttons, so the part is not pressed into the fixture. For cast or welded parts with uneven surfaces, adjustable supports are used in the primary plane. What matters is that the principle is followed: each degree of freedom is locked once, and only once.
Drill jigs and drill bushes
A drill jig guides the drill to the right position without the operator having to measure and centre-punch. We design the jig with standard drill bushes to DIN 179 (headless) or DIN 172 (headed), pressed into a hole with H7 tolerance. The bush bore has F7 tolerance and gives the drill a suitable clearance. As a guide value, the bush should be at least one to two times the drill diameter long to guide the drill reliably.
The gap between the underside of the bush and the part decides how the chips get out. The guide value for steel is roughly 0.5–1 times the drill diameter. A shorter gap gives better guidance but risks chip packing. For longer runs, renewable bushes are used, so a worn bush can be replaced without scrapping the whole jig.
The jig should also be impossible to use the wrong way. An asymmetric stop or an extra pin that only fits one way, known as foolproofing or poka-yoke, prevents the part being drilled mirror-inverted. It costs almost nothing to design in but saves a lot of scrapped parts.
Welding, gluing and assembly fixtures
A welding fixture must hold the parts in position despite heat and shrinkage. We place the locators so the weld joint is accessible and leave room for shrinkage, sometimes with a pre-bend that trial welds show is needed. Toggle clamps are placed so the operator can reach them without changing grip. Locators near the joint are made of copper or protected against spatter, so they do not burn in or wear out.
Gluing jigs and assembly fixtures rarely need the same stiffness, but they must position the parts exactly and be easy to clean. Printed fixtures work very well here. A typical solution is a base plate with recesses following the part's outline, with 0.2–0.3 mm clearance per side, and locating pins that fix the position. The assembly sequence can be engraved directly in the fixture.
Ergonomics affect both pace and quality. The fixture is placed at the right working height, heavy parts slide in rather than being lifted, and clamps are positioned so the operator does not have to stretch. A fixture that is comfortable to use also gets used correctly.
Inspection fixtures and go/no-go gauges
An inspection fixture compares the part with the drawing in seconds. The part is placed on the same datums as during machining, and gauging surfaces or pins show whether the critical dimensions are within tolerance. Go/no-go plug gauges for holes are designed to the tolerance limits in ISO 286: the go end at the smallest permitted size and the no-go end at the largest.
For an inspection fixture to be reliable, it must itself be considerably more accurate than the part. A rule of thumb is that the gauge's uncertainty should be no more than a tenth of the tolerance. That decides whether the fixture can be printed or has to be machined and calibrated. We explain how tolerances are specified on drawings in the guide tolerances and fits.
Inspection fixtures for plastic and sheet metal parts often work as a negative form of the part, where deviations show as gaps against the fixture edges. The gap is checked with feeler gauges, which makes the inspection quick and the same for every operator.
3D printed jigs and fixtures or machined?
Printed fixtures in PETG, PA or carbon-fibre-reinforced PA are produced in one or two days. They suit short runs, assembly and inspection with tolerances of a few tenths of a millimetre. Wear surfaces are reinforced with pressed-in bushes, threaded inserts and replaceable inserts. Read more about the possibilities under design for 3D printing.
Where the fixture takes cutting forces or heat, or needs accuracy to hundredths, we design it for milling in aluminium or steel with hardened pins and bushes. You then receive a toleranced drawing and a bill of materials with standard components, set out like our technical drawings. Each fixture is marked with part number and revision, so the right version is always used.
One advantage of printed fixtures is that they are cheap to replace. When the part changes, we print a new version the same week, and the old one can be kept for older variants. That also makes it possible to have several identical fixtures at different stations.
Guide values for jigs and fixtures
| Feature | Guide value or standard |
|---|---|
| Location | 3-2-1: three + two + one locating point lock six degrees of freedom |
| Locating pins in two holes | one cylindrical pin + one relieved (diamond) pin |
| Headless drill bush | DIN 179 / ISO 4247 – bore F7, outside n6 |
| Headed drill bush | DIN 172 – for higher axial forces |
| Hole for the bush in the fixture | H7 |
| Gap between bush and part | approx. 0.5–1 × drill diameter (steel) |
| Clearance in a printed outline | approx. 0.2–0.3 mm per side |
| Gauge uncertainty | max. approx. 1/10 of the tolerance |
| Screws in plastic that are undone often | brass threaded inserts |
Guide values. Final choices depend on the part material, batch size and accuracy requirements.
What we need from you
- The part's CAD model in STEP or a toleranced drawing
- Which dimensions and faces are critical and which datums the drawing uses
- The operation: drilling, welding, gluing, assembly or inspection
- Machine or workstation: fixture plate, vice, T-slots or robot
- Batch size and number of cycles per day
- How the part is loaded and unloaded – and by whom
- Standard components you already use, such as toggle clamps and pins
Typical projects
- Drill jig for a hole pattern in box section
- Welding fixture for a bracket or frame
- Gluing jig for positioning magnets
- Assembly fixture for circuit board and enclosure
- Inspection fixture with go/no-go pins
- Soft jaws for a vice
- Fixture for engraving and laser marking
- Holder for a robot gripper
- Bending template for tube and flat bar
- Template for a hole pattern in a wall or cabinet
What affects price and lead time
- Number of variants the fixture must handle
- Tolerance requirements and whether the fixture is milled or printed
- Number of clamping and locating points
- Need for replaceable wear parts and bushes
- Whether the part's CAD model exists or has to be created
- Documentation: drawings, bill of materials and instructions
What you get
- Fixture as an assembly with the part
- Collision check against tools, drill and welding torch
- Bill of materials with standard components and part numbers
- Drawings for milling or files for 3D printing
- Printed fixture ready to use (optional)
- Marking with part number and revision
Frequently asked questions
What is the difference between a fixture and a jig?
A fixture holds the part in place during machining or assembly. A jig also guides the tool, like a drill jig with bushes. In everyday speech the words are often used interchangeably.
Does a 3D-printed fixture last in production?
For assembly, gluing and inspection, often for thousands of cycles. Wear surfaces are reinforced with metal bushes and replaceable inserts. Fixtures that take cutting forces or heat we prefer to design for milling.
What is the 3-2-1 principle?
It is a way of locking a part's six degrees of freedom with exactly six locating points: three in one plane, two in a second and one in a third. It gives the same position every time without over-constraining the fixture.
Do you need the part's CAD file?
It is quickest with a STEP file. We can also work from a drawing, a sample or dimensions and model the part first.
Can one fixture handle several variants?
Yes. With replaceable inserts, movable stops or indexed supports, the same base can be used for several sizes. That reduces the number of fixtures on the shelf.
How accurate can a printed fixture be?
With calibrated printing and finishing of critical holes, accuracy is typically around ±0.1–0.2 mm. If more is needed, we press bushes into reamed holes or design the fixture for milling.