It may not be advisable to clamp the workpiece in a jig by placing it directly on the jig base plate or resting it against one of the walls of the jig because the workpiece may not be machined at those places where it is most convenient to effect clamping. Presence of chip between the job and the jig wall or bottom may cause misalignment. Further, when locating faces of the jig are not at right angle to each other, the misalignment error will be more than the thickness of the chip adhering between the inclined surfaces. To avoid these problems, jack pins or other raised locators are used to support the workpiece. 

More than three locators when arranged in one plane do not improve the location in any way, rather may result in instability of the workpiece if its surface is not flat. However, for locating flat surfaces, four point location promotes stability due to extra support. Another advantage of using four locating points is that any stray chip on any locating button will be immediately noticed as it will cause the workpiece to rock on the four button support, whereas no warning of its presence will be given in the case of three button support. 

But a disadvantage of four locating buttons in one plane is that if any one button is worn out, then the workpiece will not lie in the plane determined by the other three locating buttons and the workpiece may warp if clamped too firmly and may shift under cutting forces if not properly secured. Locating devices are available in the market in large variety. A general description of the more common types is given in the following:

Rest buttons, supporting pads and jack pins are used for supporting the workpiece and are usually loaded in compression. The stop pins are used for locating flat surfaces where these are under shear. All the supporting pins or rest buttons may be rounded head type or flat headed type. While the rounded head buttons are employed for locating jobs with rough. surfaces, the flat rest buttons or pads find use in locating flat machined surfaces only.

Rest buttons (or supporting pins) are either press fit or screwed in the jig base and accordingly these are fixed type and adjustable type. Screwed type locators are used where more wear of the locator is involved as these are easily replaceable. Locators are raised a little

Fixed and adjustable type locator

Fixed and adjustable type locator. The fixed type locator as at (a) is used to locate or support against a machined surface and is easier to use. The adjustable type locator is. used when locating surface is rough and uneven such as that of a casting.

from the jig bottom face to allow some clearance below the workpiece to avoid hindrance due to the accumulation of chips

Supporting pads (or locators) are used for locating flat machined surfaces of the workpiece. These may be in the form of flat surface or flat headed type button locators.

Supporting pads

Flat locators for locating flat machined surfaces of the job: (a) Location by jig body, Location by button placed at right angle to job and (c) Location by button placed at one face of the job.

Jacks or pin locators are adjustable type locators. These are used for supporting uneven or rough workpieces such as castings. Bolt head type and knurled knob type jack pin locators. Spring loaded jack pin locators are used for workpieces having features subject to variation as in unmachined surfaces.

Wedge type jack pin locators are used for supporting and locating purposes.

Rest button and its typical use are shown in Fig.

Cylindrical (or pin) locators are used for workpieces having drilled holes which are used for accommodating the pin locator and thus quick location is possible.

Conical locators are the most commonly used type of locators and are similar to cylindrical locators in use except that they have a comical top with a large shoulder. They have the capacity to accommodate a slight variation in the hole diameter of the workpiece without affecting the accuracy of location.

Jack pin Locators

Jack pin locators: (a) Bolt-head type and (b) Knurled knob type. These are used for supporting rough workpieces from their bottom side as shown at (c).


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