Manufacturing Processes - MFRG 315 - 2 Machining

2.1. Introduction
Whereas metal forming processes often result in little scrap but wide tolerances, machining can give good tolerances - and surface finish - by removing excess material in the form of chips. The scrap swarf may however be difficult to recycle efficiently.

Machining is a generic term covering all metal removal processes, but metal cutting is a process where metal is removed by a tool which is harder than the workpiece through a process of plastic deformation or controlled fracture.

In this module some theories of metal cutting are examined as well as practicalities such as tool materials and wear and cutting forces and power required.

For much of the work on metal cutting it is assumed for simplicity that the process being examined is:

i) 'orthogonal cutting' - where the edge of the tool is straight and perpendicular to the direction of motion.

ii) all deformation occurs within a shear plane of negligible thickness, length Ls

iii) the tool (or workpiece) moves at a velocity V m/minute

iv) the un-deformed chip thickness is h and the chip formed has a thickness hc

- see diagrams on the next page.

It is not possible to measure directly the forces acting on the shear plane or tool face, but by using a cutting force dynamometer and using the above assumptions, together with knowledge of the tool geometry, it is possible to resolve the forces measured and estimate the key parameters. This is described in later sections. Prediction of cutting forces is important as it enables cutting power to be determined and the the other critical factor in cutting economics, the tool life, depends substantially upon the cutting forces arising.

2.2 Cutting geometry

Reference:
'Metal Cutting Principles', by M C Shaw, OUP, 2nd Ed. 2005, ISBN: 0-19-514206-3.

Return to module introduction

David J Grieve, 10th March 2009, original, 17th December 2005.