Question

A drilling machine has to be designed with $8$ spindle speeds ranging approximately between $120$ to $100rpm$. The $5th$ spindle speed is

• A. $445$
• B. $480$
• C. $620$
• D. $865$

Answer 1

The correct option is A $445$

Given data:
$N_{min}=120,N_{max}=1200$
$N=8$ speeds

$r=\sqrt[n-1]{n-1\frac{N_{max}}{N_{min}}}=\sqrt[7]{7\frac{1200}{120}}=\sqrt[7]{710}=1.389$

$N_5=N_1{r}^4=N_{min}{r}^4=447rpm$