1 Introduction

Stress at the work-piece/die boundary, in both bulk forming and sheet forming, is arguably the single most important physical parameter influencing the processing of metals, yet it remains the least understood parameter; hence the need for basic research into metal-die interface mechanisms. To gain a good understanding of the mechanisms at the interface and to be able to verify the friction and tribology models that exist, friction sensors are needed.

2 Friction sensors in general

Designing sensors to measure friction in metal working has been pursued by many researchers; see Schey [SCH 83]]. Two types of sensor which are used in laboratories are: (1) the pin-type sensor originally devised by van Rooyen and Backofen [ROO 57] and improved by Al-Salehi [ALS 73] for use in metal rolling and (2) the cantilever friction sensor [BRI 86], see Figure 1. The pin-type sensor {1} measures two forces at the interface, one orthogonal to the rolling direction and the other at an angle to rolling and is specially designed for measuring friction in rolling.; by converting the angular measurement, and comparing it to the normal force, the friction is eventually obtained. The cantilever sensor can be used in other situations, including rolling; it will be discussed in detail later in this chapter.

Figure 1: The pin sensor and cantilever sensor

Other devices which are not reviewed by Schey [SCH 83] but have been tried, are those by Jeswiet...