Drilling machine

Claim 1

A portable drilling machine detachably and magnetically fixed to a huge mass of workpiece comprising in combination: a frame; an electric drill mounted on said frame in a linearly movable manner and having an electric driving motor; an electromagnetic base integrally attached to a lower portion of said frame for fixing the same to said huge mass of workpiece; a starting switch which combines a control switch of said electromagnetic base with a control switch for said electric drill; a manual feed handle for moving said electric drill toward and away from said huge mass of workpiece; a control circuit for said electric drill; sensor means operatively connected in said control circuit for detecting a load applied on said electric drill, and including a resistor coupled in series to said electric motor and a transformer, said resistor being operatively connected in parallel to a voltage rectifier; alarm means being operatively connected in said control circuit for sending out a signal when a load signal exceeding a first reference level is detected; and switch means being operatively connected in said control circuit for deenergizing said control circuit for said electric drill when a load signal exceeding a second reference level is detected; wherein an increase in load on said electric driving motor increases the current through said electric motor which current increases a potential difference across said resistor, an increase in said potential difference being boosted by said transformer and converted into a d.c. voltage by said rectifier, said voltage being operatively connected to a second resistor to produce a signal voltage proportionally distributed by a variable resistor being operatively connected to a first capacitor, said signal voltage above said first reference level flows through a constant-voltage diode to activate said alarm means.