What is the calculated spindle speed N before applying the 60% reduction in the given example?

• A. 460 rpm
• B. 760 rpm
• C. 600 rpm
• D. 1000 rpm

Answer: (B)

When a 60% reduction is applied, the reduced spindle speed is 0.6 times the original speed. To find the original speed, divide the reduced value by 0.6.

In the example, the speed after reduction is 460 rpm, so the original spindle speed is 460 ÷ 0.6 ≈ 766.7 rpm, which is typically rounded to 760 rpm in machining practice.

So the calculated spindle speed before applying the reduction is about 760 rpm. The other numbers don’t fit the given reduction relationship: 460 rpm is the reduced speed, not the original; the remaining options would require a different reduced value than the one shown in the example.