What is the definition of "primary datum"?
The first plane of the 3-2-1 method determined by three points. The primary datum usually coincides with the largest surface of the workpiece.

Learn more about primary datum in the class Supporting and Locating Principles 106 below.

Workholding Training

Class Information
Tooling U-SME classes are offered at the beginner, intermediate, and advanced levels. The typical class consists of 12 to 25 lessons and will take approximately one hour to complete.
Class Name:Supporting and Locating Principles 106
Description:This class describes the fundamental theory to properly supporting, locating, and clamping a workpiece. Includes an Interactive Lab.
Prerequisites: none
Number of Lessons:15
Language:English, Spanish, Chinese
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Class Outline
  • Objectives
  • Workholding in Manufacturing
  • Cartesian Coordinates
  • Supporting Workpieces
  • Locating Workpieces
  • Clamping Workpieces
  • The 12 Degrees of Freedom
  • 3-2-1 Location
  • The Datum Planes
  • Redundant Location
  • Using Holes for Locating
  • Machined and Unmachined Surfaces
  • Preferred Locating Surfaces
  • Locating Alternatives
  • Summary
Class Objectives
  • Describe workholding.
  • Identify the axes of the Cartesian coordinate system.
  • Describe workpiece support.
  • Describe workpiece location.
  • Describe workpiece clamping.
  • Describe the 12 degrees of freedom.
  • Describe the 3-2-1 method of location.
  • Describe datum planes.
  • Describe redundant location.
  • Identify a concentric locator.
  • Distinguish between locating with machined and unmachined surfaces.
  • Identify preferred locating surfaces.
  • Identify locating alternatives.

Class Vocabulary

Vocabulary TermDefinition
12 degrees of freedom The collective range of possible directions that a workpiece could move. An unrestricted workpiece has twelve degrees of freedom.
3-2-1 method A workholding rule defining the minimum number of contact points necessary to properly locate a rectangular part in three planes. The primary datum requires three points, the secondary datum two points, and the tertiary datum one point.
adjustable locator A locator that can be adjusted to accommodate variations in workpiece dimensions. Adjustable locators are less accurate than fixed locators.
auxiliary locating feature A part of a workpiece included for the purposes of workholding that is not related to the use or function of the workpiece.
axes Imaginary lines that pass through the center of an object. Axes are used to define the location of objects in the Cartesian coordinate system.
axial degrees of freedom The potential direction of linear workpiece movement along its axes.
Cartesian coordinate system The system that describes the location of an object by numerically expressing its distance from a fixed position along three imaginary linear axes.
clamp A workholding device that maintains the position of a workpiece by holding it in place against locators.
concentric locator A locator whose center aligns with the center of a circle in a workpiece.
conical locator A locator used to position a cylindrical workpiece by self-centering the workpiece on a cone.
cutting force A force generated by the motion of the cutting tool.
datum A surface that is assumed to be geometrically perfect or that acts as a reasonable surface for the purpose of workpiece location.
fixture A customized workholding device used on machine tools to position and hold a part during various machining operations. A fixture is built to hold a specific part design.
holding Maintaining the positioning of a workpiece. Workholding devices maintain, or hold, the workpiece in its desired location usually with the aid of clamping devices.
locating The process of positioning the workpiece in a designated location. Locating is also used to describe the precise positioning of the workpiece in the horizontal plane.
machine tool A power-driven machine that uses a cutting tool to create chips and remove metal from a workpiece.
machined hole A hole that has been shaped by a cutting tool. Machined holes are more accurate than cast holes.
machined surface A surface created by the removal of metal during a machining process. Machined surfaces are generally accurate and are preferable for locating a workpiece.
metal cutting A machining process that uses a tool to create chips and remove metal from a workpiece.
milling machine A machine that uses a multi-toothed milling cutter to remove metal from the workpiece surface to create flat and angular surfaces and grooves.
perpendicular An angle formed by two lines at a right angle. The corner of a piece of paper is formed by perpendicular lines.
positive stop A locator into which the part is directed by primary cutting forces generated by the cutting tool.
primary datum The first plane of the 3-2-1 method determined by three points. The primary datum usually coincides with the largest surface of the workpiece.
radial degrees of freedom The potential rotational direction of workpiece movement around its axes.
redundant location A situation in which a workpiece has more locating points than it actually needs. Redundant locators increase the chances of errors in manufacturing processes.
secondary datum The second plane of the 3-2-1 method determined by two points. This plane must be perpendicular to the primary plane and is usually the second largest surface of the workpiece.
sight location A rough locating method used as a first step. Machined details are usually created in this step for further use and better precision.
support The process of locating from underneath the workpiece. Supports generally restrict motion down along the Z-axis.
tertiary datum The third plane of the 3-2-1 method determined by a single point. This plane must be perpendicular to both the primary and secondary planes and is usually the smallest surface of the workpiece.
unmachined surface A surface that is less accurate than a machined surface. Cast parts have rough, unmachined surfaces.
vise A workholding device with one fixed jaw and one moveable jaw. Vises are often used to hold simple rectangular or cubic workpieces on a mill or machining center.
workpiece A part that is being worked on. It may be subject to cutting, welding, forming or other operations.
X-axis The linear axis representing motions and positions along a line parallel to the longest edge of the worktable.
Y-axis The linear axis representing motions and positions along a line parallel to the shortest edge of the worktable.
Z-axis The linear axis representing motions and positions along a line perpendicular to the worktable.