L 4.1 Machining Time

n this section we study how to calculate Machining Time in different machining operation like Turning, Drilling, Shaping and milling operation. 

Let us start with Turning operation on lathe machine.  

Turning, on a lathe, is the removal of excess material from the workpiece by means of a pointed tool, to produce a cylindrical or cone-shaped surface. From cutting speed, r.p.m. of the job is calculated by using the formula.  

 

where  

N = r.p.m. of job  

V = cutting speed in meters/minute  

D = Diameter of the stock to be turned (in mm)  

if f = Feed per revolution (in mm)  

L = Length of stock to be turned (in mm)  

T = Time required for turning (in minutes) 

 

Length of cut: It is the distance travelled by the tool to machine the workpiece and is calculated as follows: 

Length of cut (L) = Approach length + Length of the workpiece to be machined +Over travel 

The approach is the distance a tool travels, from the time it touches the workpiece until it is cutting to full depth. Overtravel is the distance the tool is fed while it is not cutting. It is the distance over which the tool idles before it enters and after it leaves the cut. These terms are explained in Fig. for a cutting operation on the lathe. 

 

Q. 1.  A mild steel bar 100 mm long and 38 mm in diameter is turned to 35 mm dia. And was again turned to a diameter of 32 mm over a length of 40 mm as shown in Fig. The bar was machined at both the ends to give a chamfer of 45° × 5 mm after facing. Calculate the machining time. Assume cutting speed of 60 m/min and feed 0.4 mm/rev. The depth of cut is not to exceed 3 mm in any operation.