Conventional Machining Techniques

Conventional machining techniques: The purpose of any machine tool is to remove metal. Each machine tool removes metal in a different way. For example, in one type (the lathe) metal is removed by a single point tool as the work is rotated, whereas in another type (the milling machine) a cutter is rotated and metal is removed as the work is progressed beneath it. Which machine tool is to be used for a particular job depends to a large extent upon the type of machining required. There is, however, a certain amount of overlapping and some machine tools can be utilized for several different operations.  In the illustrations which follow, typical machining operations are illustrated but it must not be assumed that the particular machine tool is restricted to the operation shown. 

The machine tools which will be found in the modem tool room are as follows: 

1. Lathes for turning, boring and screw cutting etc. 

2. Cylindrical grinding machines for the production of precision cylindrical surfaces. 

3. Shaping and planning machines for the reduction of steel blocks and plates to the required thickness and for ‘squaring up’ these plates.

4. Surface grinding machining for the production of precision flat surfaces.

5. Milling machine for the rapid removal of metal, for machining slots, recesses, boring holes, machining splines, etc. 

Tracer controlled milling machines for the accurate reproduction of complex cavity and core forms. 

Lathe: The primary purpose of the lathe is to machine cylindrical forms. The contour is generated by rotating the work with respect to a single point cutting tool. For machining the outside surface, the cutter is moved parallel to the axis of rotation.  This operation is called turning. Alternatively, metal may be removed from the inside of the work in which case the operation is called boring.  When the tool is moved across the face of the work it is called facing. The lathe is extremely versatile and is used for making a large variety of mould parts.

For example, guide pillars, guide bushes, circular support blocks, ejector rods, ejector rod bushes, push back pins, etc. are all manufactured on the lathe. In addition to this bolster work the cavity and core are also produced on a lathe if the moulding form is cylindrical. Turning is a relatively fast machining operation and for this reason moulds for circular components are cheaper to produce than corresponding moulds for components of any other form. Internal and external thread form are also easily generated, when required, as for example on the end of an ejector rod . A slight complication arises if the thread is required on the core or in the cavity to produce a complementary moulded thread as shown. In these cases it is necessary to make some allowance for the plastics material shrinkage on the mould thread pitch (i.e. the mould thread pitch must be machined slightly larger than required to allow for the material shrinkage on cooling). 

Cylindrical grinding machine: This machine tool is used for precision grinding cylindrical mould parts.  Metal is removed by the action of rotating abrasive grinding wheel which is brought into contact with a contra-rotating workpiece. The axes of both the grinding wheel and the workpiece are parallel for normal operation. An important feature of the grinding machine is that it can cut hardened metal. This characteristic, together with the close tolerances and the high surface finish obtainable, makes this machine tool an essential piece of tool room equipment. 

Shaping and planning machines: A mould normally includes a number of steel plates suitably secured together. Each of these plates must have parallel faces and ideally, the four sides should be square. Now, as the primary purpose of a shaping machine is to produce flat surfaces, this machine tool is used in the initial preparation of mould blocks. 

Surface grinding machine: We have previously discussed the cylindrical grinding machine for the grinding of cylindrical surfaces. Now the surface grinding machine performs a similar function for flat surfaces, and grinding normally follows the shaping or planning operation. An excellent surface finish combined with accuracy can be achieved on hard or soft steel with this machine tool. 

Milling machine: Milling is an operation in which metal is removed from a workpiece by a rotating milling cutter. The workpiece can be moved in three directions at right angles of each other, with respect to the cutter. The three directions are longitudinal transverse and vertical, respectively. There are two basic types of milling machine. In one the axis of the cutter is perpendicular to the surface of the workpiece and this is called a vertical milling machine. 

In the other, the axis of the milling cutter is parallel to the surface of the workpiece and this is called a horizontal milling machine. The operation of both types of milling machine is similar. The movement of the work in all three directions can be manually or automatically operated.  It is normal practice to operate the machine in only one direction at a time.  For example, consider the machining of a large slot in a bolster. While, as we have stated, these machines are used extensively in the manufacture of various parts of a mould, they cannot easily be used for the manufacture of three dimensional forms, which is often required for the cavity and core.  For these complex shapes some form of copy milling machine is needed.