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When we use an automatic screwdriver ie: screwdrilling machine, we set the controls of the machine in such a way so as to rotate the drill bit at a certain fixed speed.

But the point around which I can't wrap my head around is that if the screwdrilling machine is providing torque for the drill bit to rotate there has got to be an angular acceleration of the drill bit because of the equation $ T = I \alpha $ .

If there is angular acceleration then it ought to speed up it's rotation ( angular velocity) over time. Then why does the drillbit seem to be moving at a constant angular velocity.

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  • $\begingroup$ Don't forget about friction $\endgroup$
    – Triatticus
    Commented Feb 11, 2020 at 5:33
  • $\begingroup$ Closely related recent question: physics.stackexchange.com/questions/530198/… $\endgroup$
    – G. Smith
    Commented Feb 11, 2020 at 5:58
  • $\begingroup$ Consider the situation as if the screwdrilling machine with the drillbit is spun in mid air. Because then also we observe that the drill bit spins at a constant angular velocity $\endgroup$
    – TheChemist
    Commented Feb 11, 2020 at 6:07
  • $\begingroup$ @G.Smith I read those answers but I think that the question that even though an $\alpha$ exists why is there a constant angular velocity , still remains unanswered $\endgroup$
    – TheChemist
    Commented Feb 11, 2020 at 6:09
  • $\begingroup$ Yes but there is internal friction between the gears and other internals $\endgroup$
    – Triatticus
    Commented Feb 11, 2020 at 6:10

1 Answer 1

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The screw drilling machine has an automatic speed controller built in to it. When you set the speed you want, the control system maintains that speed for you by varying the voltage to the drive motor, and monitoring how fast it is turning at all times. In this regard, it's like the cruise control in your car.

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