Method and apparatus for clamping a tool in a metal cutting machine

An ultrasonic machining device 10 is attached to a movable carriage 3, and has a borehole 11 to whose free end a differential screw 14 is threaded. To clamp a tool 20, first the device is returned to a neutral position whereat a wrench 25, which is mounted stationarily in a frame 2, engages the differential screw which rests on a shoulder of the wrench with low prestress. The wrench is then rotated until the screw projects a specific amount beyond the end of the device. The tool is then threaded onto the screw until it abuts the device 10, and finally the screw is tightened by a pneumatic cylinder 32.

BACKGROUND OF THE INVENTION 
An ultrasonic machining tool is known from Swiss patent No. 665,784, where 
the tool or the sonotrode is attached by means of a differential screw. 
The device has an axial borehole, to which a wrench is permanently 
installed in order to tighten and loosen the differential screw. The 
wrench can be moved axially relative to the device and is prestressed by 
means of a spring into its retracted position, where it is disengaged with 
the differential screw. To rotate the screw, the wrench is engaged by a 
lifting magnet and subsequently rotated. This kind of manual connection 
gives satisfactory results for ultrasonic machining, but does not lend 
itself to automated control. 
SUMMARY OF THE INVENTION 
The present invention is based on the problem of providing a process and a 
metal cutting machine, with which the clamping of a tool can be automated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings, a carriage 3 can be moved in a feed direction A 
on longitudinal guideways 1 of a machine frame 2 by a pneumatic cylinder 
4. With an arrangement of valves 5 the force can be adjusted separately 
both in the direction of feed and in the return direction. To this end, 
there exist two separate pressure reducing valves 6, 7 for the two 
cylinder connections. 
An ultrasonic machining device 10, whose axis extends parallel to the 
guideways 1, is mounted on the carriage 3. The device 10 has an axial 
borehole 11. On one end the borehole has an inner thread 12 (FIG. 2), into 
which a first thread 13 of a differential screw 14 is screwed. On its end 
the device 10 has a stop face 15 and a centering shoulder 16. 
The tool 20 or the sonotrode is screwed with another inner thread 21 on the 
second thread 22 of the differential screw 14, with its end face 23 
bearing on the stop face 15, and centered by the shoulder 16. The threads 
21, 22 have a greater pitch than the threads 12, 13. Thus, the connection 
can be tightened by rotating the differential screw 14 without relative 
rotation between device 10 and tool 20. The screw 14 has a hexagonal 
recess 24. 
A wrench 25, which is permanently installed in the borehole 11, serves to 
tighten and loosen the screw. The wrench has a hexagonal head 26 followed 
by a stop shoulder 27 for the screw 14 to abut. The wrench can be mounted 
freely rotatable but axially immovably in the frame 2. It can be rotated 
either manually by a rotary knob 30 or by a servomotor 31 to adjust the 
screw 14. To tighten the screw, there is another pneumatic cylinder 32, 
which rotates a gear wheel 34 coaxial to the wrench via a rack 33. The 
pressure of the cylinder 32, and attendantly the torque of the gear wheel 
34, is adjustable. The gear wheel can be coupled to the wrench 25 by a 
pneumatically or electrically operated claw coupling 35. 
Attached to the carriage is an arm 39, which actuates a sensing element 40 
in the retracted or neutral position of the carriage 3, shown in FIG. 1. 
The sensing element 40 measures the carriage stroke and is connected to a 
control unit 41. 
To clamp the tool 20 the carriage is first moved into its retracted neutral 
position, whereat the hexagonal head 26 engages the hexagonal recess 24 of 
the differential screw 14, and the end face of the screw rests on the 
shoulder 27. The carriage 3 is pulled against this stop with a low force, 
adjusted by means of the valve 6. At this stage the screw 14 is rotated by 
the handwheel 30, or automatically by the control unit 41 and the 
servomotor 31, until the sensing element 40 reports a position whereat the 
second thread 22 of the differential screw 14 projects beyond the stop 
face 15 by an exactly defined amount. At this stage the tool 20, which is 
controlled manually or also automatically by the control unit 41, is 
gripped and screwed onto the thread 22 by a tool changer 42, which is 
indicated schematically as chuck teeth in the drawing, until it 
friction-locks against the stop face 15. Finally, the coupling 35 is 
engaged and the cylinder 32 is loaded with an adjusted pressure, so that 
the screw 14 is tightened with a precisely defined torque. 
During the ultrasonic machining the carriage is advanced in the feed 
direction A. The wrench 25 remains in its fixed axial position, so that 
when the carriage 3 returns, the wrench again engages the differential 
screw 14. To release the connection, the cylinder 32 is loaded in the 
opposite direction until it strikes its stop. The tool 20 is then 
unscrewed from the thread 22 manually or by the changer 42. 
With the described connection, the differential screw is screwed both into 
the device 10 and into the tool 20 with an exactly defined number of 
threads. Thus, a highly reliable connection is achieved. The invention 
enables the tools to be changed automatically, a feature that was not 
possible to date for ultrasonic machining systems.