5. Locating Principle And Locators zw11

LOCATING PRINCIPLES AND LOCATORS

LOCATORS 

To perform properly, workholders must accurately and consistently position the workpiece relative to the cutting tool, part after part. To accomplish this, the locators must ensure that the workpiece is properly referenced and the process is repeatable.

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The devices that restrict a workpiece's movement are the locators. The locators, therefore, must be strong enough to maintain the position of the workpiece and to resist the cutting forces.

BASIC PRINCIPLES Positioning the locator  Accuracy & tolerances  Fool proofing  Duplicate location  Motion economy 

POSITIONING THE LOCATOR 

Locators should the work on a solid and stable point: 

This permits accurate placement of the part in the tool & ensures the repeatability of the jig and fixture

Position the locators such that they the workpiece on a machined surface. The machined surface usually offers a more-stable form of location.  The best machined surfaces to use for location, when available, are machined holes.  Proper spacing of locator is necessary 

LOCATING GUIDELINES

LOCATING GUIDELINES

ACCURACY & TOLERANCES The workpiece itself determines the overall size of a locating element.  Locators must be made to suit the MMC (Maximum-Material Condition) of the area to be located. (The MMC of a feature is the size of the feature where is has the maximum amount of material).  With external features, like shafts, the MMC is the largest size within the limits.  With internal features, like holes, it is the smallest size within the limits. 

MAXIMUM-MATERIAL CONDITION

FOOL PROOFING Ensures that the part fits into the tool in its correct position only.  The simplest and most cost effective method is positioning a fool proof pin. 

DUPLICATE LOCATION 

Redundant, or duplicate, locators should be avoided.

MOTION ECONOMY 

It involves use of easy, quick and economic loading of work pieces.

THE MECHANICS OF LOCATING 

A workpiece free in space can move in an infinite number of directions. For analysis, this motion can be broken down into twelve directional movements, or "degrees of freedom."

3-2-1 LOCATING PRINCIPLE

3-2-1 principle may be stated as: “To locate an object fully, place and hold it against three points in a base plane, two points in a vertical plane, and one point in a plane square with the first two.”

3-2-1 LOCATING PRINCIPLE

3-2-1 LOCATING PRINCIPLE

3-2-1 LOCATING PRINCIPLE Motion is restricted using clamps and locators.  A three pin base can restrict five motions. Rotation about X,Y axes.(4 motions) Translation along -ve z-axis (1 motion)  Directions nine, ten and eleven are restricted by a clamping device. (3 motions)  To restrict the movement of the part around the ZZ axis and in direction eight, two more pin type locators are positioned in a vertical plane.(3 motions)  A single pin locator in vertical plane restricts motion along direction 7.( 1 motion) 

3-2-1 LOCATING PRINCIPLE Motion in all 12 directions are thus restricted.  This is the most common locating method employed for square or rectangular parts.  Flat bases may also be used, but these should be installed rather than machining into the base.  But pin/button type locators offers more accuracy as the area of is less. Moreover, they raise the work above the base so that chips wont interrupt the operation. 

3-2-1 LOCATING PRINCIPLE 

A workholding rule defining the minimum number of 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.

WORKPIECES WITH HOLES: Holes provide an excellent method for locating.  A round pin inside the hole(primary) and a diamond pin(secondary) can restrict 11 motions! 

LOCATING METHODS: 

Locating from a Flat Surface: There are three primary methods of locating work from a flat surface: solid s  adjustable s,  equalizing s 

SOLID S: Easiest and cheapest.  Less accurate.  Used where machined surface acts as a locating point. 

ADJUSTABLE S: Used where surfaces are uneven (casting, forging..)  Threaded style is the easiest and more economical.  Adjustable locators are normally used with one or more solid locators to allow any adjustment needed to level the work. 

EQUALIZING S: They provide equal through two connected points.  As one point is depressed, the other raises and maintains with the part.  This feature is especially necessary on uneven cast surfaces. 

LOCATING FROM AN INTERNAL DIAMETER Locating a part from a hole or pattern is the most effective way to accurately position work.  Nine of the twelve directions of movement are restricted by using a single pin, and eleven directions of movement are restricted with two pins.  When possible, it is logical to use holes as primary part locators. 

LOCATING CYLINDRICAL SURFACES (EXTERNALLY) “V”-LOCATORS Vee locators are used mainly for round work.  They can locate flat work with rounded or angular ends and flat discs.  Two types: Fixed and Adjustable 

LOCATING FROM AN EXTERNAL PROFILE Nesting locators position a part by enclosing it in a depression, or recess, of the same shape as the part.  Nesting is the most accurate locating device for profile location.  Obviously, the height of the nest should be lesser than the height of the workpiece.  In case of sheet-metals or thin workpieces, finger slots or ejector pins should be provided. 

TYPES OF LOCATORS Pin and button locator  Rest pad and plates  Diamond & round pin locator  Nesting locator or cavity locator etc. 

PIN AND BUTTON LOCATOR Locator used to or hold the workpiece in position.  Pins locators are longer and for horizontal locations. Button locators shorter, vertical locations.  Locating buttons-press fit and screwed(wear and tear more – replaceable) 

PIN AND BUTTON LOCATOR

REST PADS AND PLATES Used with heavier and larger workpiece.  To and locate the work vertically.  Hold jig or fixture base plate by socket-head cap screws. 

DIAMOND PIN LOCATOR Work piece with the drilled holes use two round pins  If dimension variation between center to center distance of the holes, one round and diamond pin locators are used  Prevents movement around the pin and are relieved on two sides to allow variation 

DIAMOND PIN LOCATOR

DIAMOND PIN LOCATOR

DIAMOND PIN LOCATOR 

There are other ways of utilizing diamond locating pins. This is one example, where you need to perfectly align the center of the part, but the angular orientation is not critical.

NESTING LOCATOR OR CAVITY LOCATOR Used to position the work piece  Accurate method for profile location  No need of supplementary locating devices  But it is difficult to lift out of cavity  Common types:

Ring nest  Full nest 