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#094 Work Holding Technology - 2: Locating Datum Surface Basics

Category : Locating Technology
November11, 2011

There are various shapes and dimensions devised for locating datum features in order to correspond to many types of work-piece shapes. The datum basics are explained here.

Locating datum basics

(1) Integrated locating datum method ([Fig.1])
  • Locating datum surfaces machined directly on the work holding fixtures.
  • Disadvantages in requiring labor to machine the datum surfaces, and in wear resistance.
  • Requires extra material in order to achieve an integrated configuration.

[Fig.1] Integrated locating datum method

(2) Assembled datum surface method
  • This method also requires the machining labor for the locating datum tools.
  • The datum surface parts can be replaced.
  • Since the datum parts are separate, materials of high wear resistance can be used.

[Fig.2] Assembled datum surface method

(3) Locating datum pin method
  • The simplest and most widely applicable locating method.
  • Many standardized locating pins are commercially available in various configurations and are easy to obtain.
  • In this method, locating pin layout rule "3-2-1 rule * ([Fig.4])" is applicable.
  • For applications where the work-pieces can move due to external forces, the pin shape [fig.5] must be considered and number of pins used will need to be increased to prevent the movements. [Fig.6] although it is only possible to extract the work-piece upwards, secure holding is possible with this method.

[Fig.3] Locating datum pin method

"3-2-1 Rule *"

Three pins are used to form the datum on the bottom (Z axis direction). Two pins form one direction (X axis direction), and one pin forms the remaining other direction (Y axis direction).

[Fig.4]

[Fig.5] Example of Clearance-free locating pin layout, [Fig.6] Various configurations of locating pins

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