Machining Time of Facing Operation in a CNC Turning Centre

Estimation of Machining Time in Facing in a CNC Turning Centre using constant spindle speed and constant cutting velocity

Estimation of Machining Time in Facing in a CNC Turning Centre using constant spindle speed and constant cutting velocity

Facing is the machining process of removing metal from the end of a workpiece to produce a flat surface. It is done to create a smooth, flat face perpendicular to the axis of a cylindrical part.

Machining time is the time needed to process or machine a material under some operation(s). The machining time needs to be determined or assessed from time to time for several purposes, such as, assessment of productivity, evaluation of machining cost per piece, determination of man-machine hour cost per piece, assessment of relative performance or capability of any machining methods, machine tool, cutting tool, or any specific techniques in terms of saving in machining time.

The machining time required for a particular operation can be determined either by approximately calculation (estimation) or by actual measurement. Measurement generally provides accurate and precise information but it is tedious and expensive. On the other hand, though estimation by simple calculation may not be accurate, is much more simple, quick and inexpensive. Therefore, machining time is regularly evaluated for different purposes by simple calculation.

Facing of a disc in a CNC turning centre can be done in two cases. One with constant spindle speed and another with constant cutting velocity.

Consider a cylindrical shell with outer diameter d₂ and inner diameter d₁ and other parameters as shown in figure.

(a) Facing with constant Spindle Speed (n):

If the spindle speed (n) is constant, then the machining time (t_m) can be estimated neglecting the approach and over-travel as:

 Where d₂ (mm) is outer diameter of the cylinder workpiece, d₁ (mm) is the inner diameter of the workpiece, s is feed in (mm/rev) and n is spindle speed in (rpm).

(b) Facing with constant Cutting Velocity:

If the spindle speed is automatically compensated by the CNC system and the cutting velocity is kept constant, then the machining time (t’_m) can be estimated as:

Also, we can estimate this machining time using another method as shown below:

Let for a infinitesimally small change of time, dt, the change in length be –dR =  –dD/2

Here, t_m is machining time in minutes, d₂ (mm) is outer diameter of the cylinder workpiece, d₁ (mm) is the inner diameter of the workpiece, s is feed in (mm/rev) and v_c is cutting velocity in mm/min.