Design principles of Jigs and Fixtures

The art of metalworking has a primary concern, which is locating the part to be machined relative to the platform. A CNC machine starts machining at a specific point corresponding to the fixture and proceeds from there. Therefore, the preciseness with which a job is machined is very dependent on the accuracy with which it is held in the fixture.Accurate locating of every part loaded into the fixture is essential. Any deviation in part location adds to the dimensional tolerance that must be assigned to the finished pieces. Furthermore, improper supporting and securing of the part in the fixture affects surface finishes as well, by temporarily or permanently deforming it. Hence, techniques for supporting, clamping, and locating must be considered together to assure repeatability from part-to-part.

The 3-2-1 principle

Locating a part to be machined involves mainly three steps: Supporting, Positioning, and Clamping.

Two main intentions of when placing a job on a jig/fixture are:

  • Precisely positioning the part at the desired coordinates.
  • Curbing all six degrees of movement so that the part cannot budge.

An extensively used method for obtaining these objectives is known as the 3-2-1 principle or six degrees of freedom for part location.

The 3-2-1 method is a work holding principle where three pins are located on the 1st principle plane, i.e. either XY, YZ, ZX. And two pins are located on the 2nd plane which is perpendicular to the 1st plane, and at last one pin on the plane which is mutually perpendicular to the 1st and 2nd planes. The ultimate goal is to constrain the movement of the workpiece along all the three axes.



Design objectives of Jigs and Fixtures

Before sitting down to design jigs/fixtures, the designer must consider the following points:

  • The tool must be fool proof to prevent any mishandling or accidental usage by the operator
  • Easy to operate for increasing efficiency
  • Easy to manufacture using the lowest costs
  • Can weather the tool life instead of appropriate materials
  • Must be consistent at producing high-quality parts
  • Must be safe and secure to use

The designer must know the basic of the process and tools associated in it for which the jig/fixture is to be designed. Overall objectives to look out for a while designing such tools are:

  • Cycle time
  • Type of Jig/Fixture 
  • Part Assembly sequence or Machining locations
  • Number of parts to be  
  • Type of joining or machining process
  • Clamping method and clamping sequence 
  • Required output accuracy
  • Type of equipment to be used with the jig
  • Method of ejecting finished output and transferring it to the next platform, whether the manual or automatic mode
  • The type of material, recommended weight, number of spots involving welding