15 is kept open and no power is supplied to the motor the second safety unit 32 comprises a push button 321 a transmission system 322 and a pusher 323 the pusher 323 protrudes from the periphery of the inlet 25 of the processing part 2 close to the bottom side thereof when the inlet 25 is put in the insertion hole 243 on the casing 24 and turned into its final angular position the pusher 323 displaces the transmission system 322 indicating that the inlet 25 sits tightly on the casing 24 the transmission system 322 is mounted on the casing 24 it has a horizontal bar 324 and a vertical bar 325 the horizontal bar 324 is glidingly mounted below the top side of the casing 24 it extends to the periphery of the insertion hole on the casing 24 contacting the pusher 323 when the inlet 25 is inserted and brought into its final angular position the other end of the horizontal bar 324 contacts the upper end of the vertical bar 325 the vertical bar 325 is vertically glidable it extends down to the upper side of the protruding seat 14 of the main part 1 when the casing 24 is set.