Calculate the Actual Machining Time
Estimate the actual machining time required for the component of diameter 42 mm and length 120 mm. The available speeds are 70, 110, 176, 280, 440, 700, 1100, 1760 and 2800. Use a roughing speed of 30 m/min and finish speed of 60 m/min. the feed roughing is 0.24 mm/rev. while that for finishing is 0.10 mm/rev. The maximum depth of cut for roughing is 2 mm. Finish allowance might be taken as 0.75 mm. Blank to be utilized for machining is 50 mm in diameter.
Solution
Stock to be removed = (50 - 42)/4 = 4 mm
Finish allowance = 0.75 mm
Roughing
Stock available = 4 - 0.75 = 3.25 mm
Since maximum depth of cut to be taken is 2 mm, there are two roughing passes.
Given cutting speed V = 30 m/min
Average depth of cut = (50 + 42)/2 = 46 mm
Spindle speed N = (1000 × 30) / (π× 46) = 207.59 rpm
The nearest rpm available from the list is 176 rpm as 280 is very high compared to 207 as calculated.
Machining time for one pass =(120 + 2) / (0.24 × 176) = 2.898 minutes
Finishing
Given cutting speed V = 60 m/min
Spindle speed N = (1000 × 30)/ (π× 42) = 439.05 rpm
The nearest rpm available from the list is 440 rpm
Machining time for one pass = (120 × 2 )/ (0.10 × 440)
= 2.77 minutes
Total machining time = 2 × 2.888 + 2.77 = 8.546 minutes