Jigs (Model answer for a jig question)


Jigs are used to produce many copies of one item. Once the jig has been produced, production of the copies is quick because there is no need to measure and mark out the raw material or workpiece. Jigs can be used for cutting, drilling and gluing


There are two essential features of a jig. The workpiece must be located and it must be held. It must be located against only three surfaces, one in each of the X, Y and Z planes. Do not locate it against two surfaces in the same plane, as this requires all workpieces to be exactly the same size which they won’t be. Workpieces which are too large will not fit between the two locating surfaces and workpieces which are too small will not be able to locate against both surfaces at once


Holding the workpiece can be achieved by using commercially produced clamps which can be screwed to the base of the jig, or, the jig can be designed with a system of wedges which are tapped against fixed parts of the jig to produce holding forces. Secure holding is essential when cutting or drilling because a loose workpiece is a dangerous workpiece. Not only could you hurt yourself but damage to the workpiece or cutting machine could also result if the workpiece goes flying off the machine


If your teacher is using the bandsaw to cut equal lengths of wood from a strip or dowel, and the size of the material is not too heavy, the teacher may choose to use his hand to hold the material. The jig itself could be clamped if necessary. The jig would only be used to locate the material so that the lengths are equal


In the case of gluing jigs, gravity can sometimes be used as a holding force in at least one plane. Workpieces to be glued can sometimes be self-jigging. If the product is assembled in the right order, pieces already glued can be used to locate the next part to be added


Examples of Jigs questions are 2001 Paper 2 question 3c(i) and 2002 Paper 4 questions 4(b) and 4(c)