Machine tool

1. A machine tool for impacting a work piece comprises:

a pneumatic percussion mechanism having a cup-shaped exciter cylinder with an inner chamber open in a direction of impact, the inner chamber being defined, at least partially, by a shell-type wall, the shell type-wall having at least one ventilation opening and a recess, the at least one ventilation opening spaced at a distance from the recess in the direction of impact; and

a free piston movable in the inner chamber, the free piston including a sealing element located in the inner chamber and contacting an inner surface of the shell-type wall, the sealing element sealing off the at least one ventilation opening when the sealing element is oppositely situated from the at least one ventilation opening, the sealing element, when in the region of the recess, being separated at a distance from the shell-type wall when the sealing element is oppositely situated from the recess.

2. The machine tool as recited in claim 1 wherein a width of the recess is greater than a width of the ventilation openings.

3. The machine tool as recited in claim 1 wherein the recess has an annular shape.

4. The machine tool as recited in claim 1 wherein at least three of the ventilation openings are situated in a plane perpendicular to the direction of impact.

5. The machine tool as recited in claim 1 wherein the ventilation openings are provided in a uniform angular distribution around an axis of the percussion mechanism.

6. The machine tool as recited in claim 5 wherein the ventilation openings are situated in a spiral around the axis.

7. The machine tool as recited in claim 1 wherein further comprising a drive for periodically moving the exciter cylinder by one stroke height along the direction of impact, and a distance of the recess from the at least one ventilation opening is less than one-half the stroke height.

8. The machine tool as recited in claim 7 wherein the distance of the recess from the at least one ventilation opening is less than thirty percent of the stroke height.

9. The machine tool as recited in claim 1 wherein the movable free piston has an air channel for connecting oppositely situated sides of the free piston in the direction of impact, the air channel being interrupted by the sealing element.

10. The machine tool as recited in claim 1 further comprising a guide tube within which the exciter cylinder is movably situated, the guide tube having an outlet opening with a front edge in the direction of impact, and during a blank blow the sealing element being situated behind the front edge in the direction of impact, and in the impact position being situated in front of the front edge.

11. The machine tool as recited in claim 10 wherein the exciter cylinder has at at least one location an outer cross section forming an airtight seal with an inner cross section of the guide tube.

12. The machine tool as recited in claim 1 further comprising a drive mechanically connected to the exciter cylinder.

13. The machine tool as recited in claim 12 wherein the exciter cylinder has an articulated joint connected to the drive.

14. The machine tool as recited in claim 12 wherein the drive drives the exciter cylinder in a periodic linear motion.

15. The machine tool as recited in claim 1 wherein the piston has at at least one location an outer cross section forming an airtight seal with an inner cross section of the shell-type wall.

16. A machine tool for impacting a work piece comprising:

a drive;

a pneumatic percussion mechanism having a cup-shaped exciter cylinder driven by the drive and having a shell-type wall defining at least partially an inner chamber open in a direction of impact, the shell type-wall having at least one ventilation opening and a recess, the at least one ventilation opening spaced at a distance from the recess in the direction of impact; and

a free piston movable in the inner chamber, the free piston including a sealing element sealing off the at least one ventilation opening when the sealing element is oppositely situated from the at least one ventilation opening, the sealing element, when in the region of the recess, being separated at a distance from the shell-type wall when the sealing element is oppositely situated from the recess.

17. A machine tool for impacting a work piece comprises:

a pneumatic percussion mechanism having a cup-shaped exciter cylinder with an inner chamber open in a direction of impact, the inner chamber being defined, at least partially, by a shell-type wall, the shell type-wall having at least one ventilation opening and a recess, the at least one ventilation opening spaced at a distance from the recess in the direction of impact; and

a free piston movable in the inner chamber, the free piston including a sealing element sealing off the at least one ventilation opening when the sealing element is oppositely situated from the at least one ventilation opening, the sealing element, when in the region of the recess, being separated at a distance from the shell-type wall when the sealing element is oppositely situated from the recess;

wherein the movable free piston has an air channel for connecting oppositely situated sides of the free piston in the direction of impact, the air channel being interrupted by the sealing element.