Locating Principle Used for Jig and Fixture :

Principle Of Location in Jig And Fixture :

The principle of location is being discussed here with the help of a most popular example which is available in any of the book covering jigs and fixtures. It is important that one should understand the problem first.
Any rectangular body many have three axis along x-axis, y-axis and z-axis. It can more along any of these axes or any of its movement can be released to these three axes. At the same time the body can also rotate about these axes too. So total degree of freedom of the body along which it can move is six. For processing the body it is required to restrain all the degree of freedom (DOF) by arranging suitable locating points and then clamping it in a fixed and required position. The basic principle used to locate the points is desirable below.

See also: Introduction and Difference Between Jig And Fixture

Locating – principles and methods

3-2-1 Principle of Location used in Jig & Fixtures:

[1] It is also known as a six-pin or six-point location principle. In this, the three adjacent locating surfaces of the blank (workpiece) are resting against 3, 2, and 1 pin respectively, which prevent 9 degrees of freedom.

[2] The rest three degrees of freedom are arrested by three external forces usually provided directly by clamping.

3-2-1 Principle Statement : 

The 3-2-1 principle states that the six locators are sufficient to restrict the required degree of freedom of any workpiece. In this, motion is restricted using clamps and locators. A three-pin base can restrict five motions and six pins restrict nine motions.

Methodology of 3-2-1 Principle:

For this, refer the below figure;

[1] The workpiece is resting on three pins A, B, and C which are inserted in the base of the fixed body.
[2] The workpiece cannot rotate about the axes XX and YY and also cannot move downward.
[3] In this way, the five degrees of freedom 1,2,3,4 and 5 have been arrested.
[4] Two pins D and E are inserted in the fixed body, in a plane perpendicular to the plane containing pins A, B & C.
[5] Now the workpiece cannot rotate about the Z-axis and also it cannot move towards the left.
[6] Hence the addition of pins D and E restrict three more degrees of freedom, namely 6, 7, and 8.
[7] Another pin F in the second vertical face of the fixed body, arrests degree of freedom 9.

The above method of locating a workpiece in a fixture is called the 3-2-1 Principle.

Summary of 3-2-1 Principle:

A workpiece will be completely confined when blanked against:

• 3 points in one plane: The “3” in 3-2-1 refers to 3 locators (passive fixture elements) on the primary locating/datum surface.
• 2 points in another plane: The “2” in 3-2-1 refers to 2 locators on the secondary locating/datum surface.
• 1 point in a third plane: The “1” in 3-2-1 refers to 1 locator on the tertiary locating/datum surface

Some basic principles or rules need to be followed while planning for locating blanks in fixtures, such as;

• One or more surfaces (preferably machined) and/or drilled/bored hole(s) are to be taken for reference
• The reference surfaces should be a significant and important feature(s) based on which most of the dimensions are laid down
• Locating should be easy, quick and accurate In case of locating by pin, the pins and their mounting and contact points should be strong, rigid and hard
•  A minimum of three-point must be used to locate a horizontal flat surface
•  The locating pins should be as far apart as feasible
• Vee block and cones should be used for self-locating solid and hollow cylindrical jobs.

Types Of Locators used In Jig And Fixture For supporting :

The location arrangement depends on the type of operation, degrees of required accuracy.

• Flat locators: These are used for the location of the flat machined surfaces of a component.
• Cylindrical locators: These are used for the location of components with drilled holes.
• Conical locators: These locators are used to support the workpieces with cylindrical holes.
• Jack pin locators: These locators support rough workpieces.
• Drill bush locators: These locators hold and locate the cylindrical workpieces.
• Vee locators: These locators hold circular and semicircular workpieces.

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