Question

A through hole of 10 mm diameter is to be drilled in a mild steel plate of 30 mm thickness. The selected spindle speed and feed for drilling hole are 600 revolutions per minute (RPM) and 0.3 mm/rev, respectively. Take initial approach and breakthrough distances as 3 mm each. The total time (in minute) for drilling one hole is ______. (Rounded off to two decimal places) 
 

Hint:

In drilling time calculations, never forget to include the approach and breakthrough distances to the plate thickness to find the total length the drill must travel. The time is simply total distance divided by speed (feed rate).

Answer 1

Step 1: Analyzing the Total Drill Path:
In a drilling operation, the tool must travel a distance greater than the actual plate thickness to ensure a clean cut. The total travel length ($L$) consists of the work-piece thickness ($t$), the initial approach distance ($A$), and the breakthrough distance ($B$) required for the drill point to exit the other side completely. \[ L = t + A + B = 30 + 3 + 3 = 36 \text{ mm} \]
Step 2: Calculating the Material Removal Feed Rate:
The table feed or axial velocity ($f_m$) of the drill is determined by how much the drill advances per revolution multiplied by the number of revolutions it makes in one minute. \[ f_m = \text{Feed per revolution} (f) \times \text{Spindle Speed} (N) \] \[ f_m = 0.3 \text{ mm/rev} \times 600 \text{ rev/min} = 180 \text{ mm/min} \]
Step 3: Calculating Total Machining Time:
The time required to complete the hole is the total distance the drill travels divided by the speed at which it moves axially. \[ T = \frac{L}{f_m} = \frac{36 \text{ mm}}{180 \text{ mm/min}} = 0.2 \text{ minutes} \]