Rake angle is the measure of inclination of rake surface of the cutter from reference plane. Basically reference plane (πR) is an imaginary fixed plane assumed at the direction perpendicular to cutting velocity vector (Vc). Different cutting tools may have varying three-dimensional orientations of rake surface. Moreover, based on the direction it is measured, rake angle can also be different for same cutter. Thus rake angle (γ) can be defined as the angle of orientation of cutting tool’s rake surface from reference plane and measured on some other plane. Based on the plane (it indicates direction) on which it is measured, rake angle may have different name like side rake (γX), back rake (γY), orthogonal rake (γO), normal rake (γN), maximum rake(γmax), etc.
Master line: Master line (ML) is the line of intersection between reference plane and any one tool point surface. There exist three tool point surfaces for a single point turning tool, namely rake surface, principal flank surface and auxiliary flank surface. So line of intersection between reference plane and rake surface is termed as master line for rake. Similarly master line for principal flank surface is obtained by its intersection with reference plane. Master line for auxiliary flank is also obtained by its intersection with reference plane. ML is very useful for tool angle conversion from one designation system to another by graphical method.
Maximum rake angle: It is the rake angle measured on a plane located perpendicular to ML for rake surface. By definition maximum rake angle the angle of orientation of rake surface from reference plane and measured on a plane perpendicular to master line for rake. One important feature of master line for rake is—along the master line, rake angle will be zero (because it merges with reference plane) and perpendicular to master line, rake angle will be maximum. Since maximum rake is measured in a direction perpendicular to master line, so it is the measure of maximum value of rake angle of a particular cutting tool. This angle is also displayed on MRS (Maximum Rake System) of tool designation.
Can maximum rake be zero?
It is worth mentioning that, unlike clearance angle that should have a positive finite value, rake angle of a cutting tool may be positive, negative or zero. This actually indicates orientation of rake surface from standard reference plane—in one direction (towards flank surface) it is assumed as positive while in other direction it is assumed as negative. Usually rake angle varies in between +15° to –15°. So apparently one may think that maximum rake can be zero but there is a cache. It is possible only in a special rare case as discussed in following sections.
What will happen if it becomes zero? Imagine clearly. Rake angle becomes zero only when rake surface merges with reference plane. This is practically feasible. However, we consider master line as the line of intersection between reference plane and rake surface. So in this case maximum rake is directed along the master line because the concerned angle is zero. But geometrically one should get maximum rake in a direction perpendicular to master line (along the master line rake angle is always zero). Here this maximum rake angle is also zero. Therefore, the situation directly indicates: rake angle is zero along the ML and also rake angle is zero perpendicular to ML.
When maximum rake can be zero? Note that rake surface of a single point turning tool (SPTT) is a flat surface (not a curved one) but not necessarily horizontal. So the above situation can happen only if rake surface becomes parallel to reference plane. In such a case, in any direction, rake angle will be zero. Accordingly master line can be considered in any direction because entire rake face merges with reference plane. Typically certain thread cutting tools may have flat rake and for them maximum rake can be zero.
Therefore, it can concluded that for a turning tool maximum rake angle can be zero only if entire rake surface merges with reference plane. In any other case it is not possible.