Designing good fixtures

1. Fixture Design

The gauge maker must create a complete 3D CAD design of the proposed fixture, in an agreed file format.

The materials selected and the structure design must be fit for purpose, and for the environment in which the gauge will be used and stored.

2. Location and Measurement

The component should be held in car-line, particularly if it is a flexible piece of trim, to avoid any distortion due to gravity/its own weight.

The component should be held using the same locator strategy as will be the case for its fitment to the vehicle.

The cut planes for the component, as specified by the OEM/supplier, should be used to determine the measurement points and corresponding CMM measurement programme.

Measurement points and CMM measurement requirements must be determined prior to fixture design and clamp positioning to avoid clashes.

3. Datums

The fixture must have a minimum of 2 master datums on the base plate, which must, therefore, be a machined surface.

The use of 3 master datums eliminates the need for the base to have an accurately machined surface.

Bushes are recommended for the datums, with spheres and D-pads also being suitable.

Using the edges of the base plate is not recommended as they can easily be damaged when handling/transporting the fixture.

All alignments should be taken off the master datums on the fixture base and using "best fit" alignments should be avoided wherever possible as they do not give errors from a definitive point and exact replication cannot be achieved.

4. Form Bars and Simulators

Form bars and simulators must be NC cut from the vehicle manufacturer's CAD data.

Gapping between form bar and component to be clearly specified - typically 3mm, occasionally 5mm.

If the fixture is to be used in a production environment, recommended materials are Obomodulan 1000g/m3 or Ciba 5166.

5. Tolerances

The following tolerances should be specified (where feasible)*, relative to the master datums:

Base flat/parallel (if used as datum surface) ?0.05mm
Locator net pads ?0.10mm
Locator pin positions ?0.05mm
SPC data capture bush ?0.10mm
Form bars (gap and flush measurement) ?0.15mm (0.15/400 waviness)

* On larger fixtures (over large distances), whilst the above tolerances may be possible, accuracy may need to be sacrificed to a certain extent in order for the fixture to remain economically viable. Individual cases should be discussed with the vendor.

6. Clamping

Clamping sequence must be determined by the gauge maker and clearly identified on the fixture.

Clamps should be sourced from a recognised and established manufacturer.

7. Inspection

Inspections and R&R studies performed at the gauge maker's site should be carried out on a CNC CMM to guarantee repeatability.

Inspection reports should include a graphical illustration of the fixture, based on the 3D CAD design, with annotated measurement points.

All reports must be accompanied by a copy of the measurement machine calibration certificate.

8. Identification

Identification plate(s) must be attached to the fixture base plate or structure and must clearly display the following information:

  • Part number
  • Part level
  • Tool/asset number
  • Weight and lifting method
  • Datum co-ordinates
  • Gap and flush measurements (if applicable)
  • Owner
  • Vendor contact details

9. Handling and Health & Safety

Lifting and handling aids appropriate to the fixture weight and customer facilities must be provided.

Relevant safety information must be displayed clearly on the fixture base. Suitable storage racks/tables etc. should be quoted for and provide by the gauge maker.

Click the small boxes at the top left for more information on each element.

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