NRS System of Tool Designation – Normal Rake System

Cutting tool geometry basically refers to some specific angles pertinent to the cutting edges. Certain features of the cutting tool, written in a specific and standardized manner, are used to designate that tool. There are various standard systems for designation of a single point turning tool; each has specific advantages and disadvantages. The commonly used systems for turning tool designation include ASA system, ORS system, NRS system, etc. as enlisted below.

Planes used as reference in NRS system of tool designation

Normal Rake System (NRS) utilizes three planes as reference for measuring various tool angles. These three planes are enlisted below.

  • Reference Plane (πR)—It is a plane perpendicular to the cutting velocity vector (Vc).
  • Cutting Plane (πC)—It is a plane perpendicular to reference plane (πR) and contains the principal cutting edge of the tool.
  • Normal Plane (πN)—It is a plane perpendicular to the principal cutting edge of the tool. Normal plane may not be perpendicular to the reference plane (πR) and Cutting Plane (πC). However, normal plane is always perpendicular to the principal cutting edge.

Unlike ASA and ORS systems of tool designation where all three planes of reference are mutually perpendicular, in NRS system three planes of reference may not be mutually perpendicular. They will become mutually perpendicular only when the cutting tool has zero inclination angle (λ). In such case, NRS system and ORS system will become same. So difference between ORS and NRS systems can be realized if the cutting tool has a non-zero finite (positive or negative) value of inclination angle. In fact the angle between the Orthogonal Plane (used in ORS system) and the Normal Plane (used in NRS system) is equal to the inclination angle (λ) of the tool.

Representation of tool angles in NRS system of tool designation
Representation of tool angles in NRS system of tool designation.

Various features displayed in NRS system of tool designation

Similar to ORS system, NRS system of tool designation also specifies two different rake angles, two different clearance angles, two different cutting edge angles, and the nose radius value in mm. Various features of the single point turning tool (SPTT) that NRS system specify are provided below.

  • Normal Rake Angle (γN)—It is the angle of orientation of tool’s rake surface from the reference plane (πR) and measured on normal plane (πN).
  • Inclination Angle (λ)—It is the angle of inclination of tool’s principal cutting edge from the reference plane (πR) and measured on cutting plane (πC).
  • Normal Clearance Angle (αN)—It is the angle of orientation of tool’s principal flank surface from the cutting plane (πC) and measured on normal plane (πN).
  • Auxiliary Normal Clearance Angle (αN’)—It is the angle of orientation of tool’s auxiliary flank surface from the auxiliary cutting plane (πC’) and measured on auxiliary normal plane (πN’).
  • Principal Cutting Edge Angle (Φ)—It is the angle between cutting plane (πC) (which contains principal cutting edge) and the longitudinal feed direction, measured on reference plane (πR).
  • Auxiliary Cutting Edge Angle (Φ1)—It is the angle between auxiliary cutting plane (πC’) (which contains auxiliary cutting edge) and the longitudinal feed line, measured on reference plane (πR).
  • Nose Radius (r)—This is nothing but the curvature at the tool tip. Similar to ORS system, in NRS system, nose radius value is expressed in mm.

Tool nomenclature in NRS system

The above mentioned seven features of the cutting tool are specified in sequence as shown above. The sequence of designation should be followed strictly. However, different people may use different notations for various angles maintaining the original sequence unchanged.

Tool Signature in NRS

Example for NRS system of tool designation

Few points should be considered for giving examples of tool nomenclature. First and foremost one is the value of clearance angles. Clearance angles are always positive—it cannot be zero or negative. Usually it ranges from 3º – 15º. Also auxiliary cutting edge angle is usually lower than principal cutting edge angle; however, for thread cutting tool, they can be same. A zero inclination angle indicates ORS and NRS system are same. One example of NRS system of tool designation and interpretation of tool angles from such nomenclature is illustrated in the following figure.

NRS system of turning tool designation

Let us consider another example. Say, a typical turning tool can be specified in NRS system as:

–10º, –6º, 10º, 5º, 15º, 60º, 0.8 (mm)

Therefore, upon interpretation, we may write:

  • Inclination Angle (λ) = –10º
  • Normal Rake Angle (γN) = –6º
  • Normal Clearance Angle (αN) = 10º
  • Auxiliary Normal Clearance Angle (αN’) = 5º
  • Auxiliary Cutting Edge Angle (Φ1) = 15º
  • Principal Cutting Edge Angle (Φ) = 60º
  • Nose Radius (r) = 0.8 mm.

References

  • Book: Principles of Mechanical Engineering by S. Singh (S. Shand).
  • Book: Machining and Machine Tools by A. B. Chattopadhyay (Wiley).
  • Book: Metal Cutting: Theory And Practice by A. Bhattacharya (New Central Book Agency).
  • Book: Manufacturing Processes by J. P. Kaushish (PHI).
  • Book: Manufacturing Science by M. I. Haque and S. Khan (PHI).