Drilling Time:
A drill needs an approach distance (= a) before it might actually start eliminating material from the workpiece. This approach distance is required for positioning the drill above the hole and this is usually approximate 2 mm or so. As might be seen in Figure 8, the conical part of the drill must completely come out of the hole before the drilling is prevented. This additional distance A to be traversed is called as breakthrough distance and might be expressed as follows with the help of geometry of Figure 8 :
A = (D/2) tan a
Hence, total length of the drill travel (L) is specified by the following equation :
L= l +a + A
If the value of a for the drill is not specified then its normal value of 59o might be used.
Imagine the drill is being fed at 'fr' (mm/rev) and it rotates at N RPM, after that the time (td) for drilling a given hole is defined as
td = L / fr * N
and the volumetric material removal rate might be evaluated as following :
MRRv = Cross sectional area of the hole * Depth of the hole drilled in unit time
MRRv = (P/4) D2 (fr N)