EP2942164B1 - Knife with automatic blade return - Google Patents

Description

The invention relates to a knife with automatic blade return with a housing, a movable actuator, a rotatable blade unit, which is engageable with the actuating device and which is movable by means of the actuating device from a retracted position in the housing in an extended position of the housing, wherein the blade unit is rotatable by a cutting force in the extended position from an engagement position in which the blade unit is engaged with the actuator to a release position in which the blade unit is not engaged with the actuator, and a restoring element which is always one Restoring force on the blade unit in the direction of the retracted position applies.

Such automatic blade return knives permit automatic movement of the blade from the extended position, in which the blade at least partially protrudes from the housing of the knife, to the retracted position, in which the blade is usually located entirely within the housing of the knife, independently of the operating state of the actuator.

The movement of the blade is thus decoupled from the movement of the actuator, by means of which usually with the aid of the thumb or the index finger, the blade is pushed out of the housing. The decoupling of the movements blade and actuator is achieved by the action of a cutting force on the blade. The cutting force leads to a decoupling movement of the blade, which may be formed, for example, as a rotary movement. During decoupling, the actuator releases the blade so that the blade and actuator can move independently of each other.

After the cutting force is released on the blade after completion of the cutting operation, the blade is moved from the extended position to the retracted position immediately and independently of the state of the actuator by means of the return member.

Knife with automatic blade return are especially designed for situations where unintentional movements occur during the cutting process. Frequently, users of such knives misjudge the cutting ability of the blade or the resistance of the material to be cut so that the user of the blade loses control of the movement due to over-use of force as soon as the blade leaves the material to be cut. Because of these uncontrolled blade movements, a knife with the blade extended would pose a significant risk of injury to the user of the knife itself, as well as to nearby persons. It can also lead to accidental damage to surrounding objects.

The automatic blade reset after the removal of cutting force on the blade such injuries to persons and property damage are avoided by unintentional and uncontrolled knife guide. Due to the increased security during the use of such knives, these are also referred to as safety knives.

The safety knives known in the prior art regularly have a complex mechanism, which are exposed to excessive risk of high risk of malfunction. In particular, the blade guide and the decoupling mechanism known safety knife regularly suffer functional failures when the blade housing is damaged or deformed due to overuse.

For example, a safety knife with automatic blade reset is off EP 1 864 766 B1 known. The proposed safety knife has a blade unit which is rotated under cutting force in the housing and thus causes a decoupling of the manually operable actuator. The longitudinal guide of the blade unit is realized due to the limited space within the blade housing by a laterally arranged on the blade unit pin which is movable in a slot of the blade housing.

In longitudinal guides of the type described above, it may happen that if the blade is overstressed, e.g. with excessive lateral force acting on the blade housing, due to housing compliances, the guards will come out of engagement. This would significantly affect the functionality of the knife.

Furthermore, it is off DE 10 2011 007 234 B3 a safety knife is known, which relies on a separate coupling element to realize the coupling or decoupling of the manually operable actuator and the cutting blade. In the proposed safety knife, the mechanism is extended by an additional sensitive coupling mechanism.

WO 2014/125004 A1 discloses a safety knife with an automatic retraction mechanism. The knife has a housing, a blade carriage and a blade holder. The carriage is connected to an actuating button which is accessible outside the housing. The blade holder is arranged on the carriage, that it is mounted relative to this displaceable by a pin is guided through a corresponding slot on the carriage.

The object of the invention is therefore to be seen in providing an improved knife with automatic blade return. In particular, it is an object of the present invention to provide a knife, which has a reduced risk of malfunction and / or increased robustness against excessive stress compared to the known knives.

The invention solves the underlying object with a knife with the features of claim 1, namely in particular a movable carriage on which the blade unit is rotatably mounted.

The invention makes use of the knowledge that an additional movable carriage, the need to guide the blade unit using the housing is resolved. If there is a damage or a yielding of the housing, there is a significantly reduced risk of malfunction in the knife according to the invention over the known solutions. Furthermore, the knife according to the invention dispenses with a separate coupling element, as a result of which the knife mechanism is considerably simplified.

The blade unit according to the invention comprises a blade and a blade carrier. The blade is connected to the blade carrier in such a way that a rotational movement between blade and blade carrier is substantially prevented. In particular, the blade has one, two or more recesses, in which engage one, two or more pins of the blade carrier. The connection between the blade and the blade carrier can also be designed as an adhesive connection, stamped connection, screw connection or clamping connection. By such a two- or multi-part blade unit, it comes to a function decoupling. This is to be understood that the blade is designed to take over the cutting function and the blade carrier is designed to realize the mechanical coupling to other blade elements, such as the restoring element. By such a function decoupling functional materials can be used, so that the blade unit is formed, for example, stainless steel knife to improve the cutting properties of the knife, and the blade carrier is formed of a plastic material, which by means of injection molding advantageous for integration into the knife mechanism and possibly . complex geometry is gebbaren.

In a first preferred embodiment of the knife according to the invention, the carriage is displaceable together with the blade unit between the retracted position and the extended position along a longitudinal axis. Preferably, both the carriage and the blade unit in the retracted position are arranged completely in the housing of the blade. By also moving the blade unit from the retracted position to the extended position when the carriage is moved between the retracted position and the extended position, contact between the blade unit and the housing can be completely avoided. It comes only to a contact of the carriage with the housing of the knife. Due to the avoidance of contact between the blade unit and the housing, the risk of a functional failure in case of a housing damage, such as a housing break or a housing deformation, further reduced.

In a second preferred embodiment of the knife according to the invention, the carriage is guided in such a way that a rotational movement of the carriage is substantially prevented. It is thus avoided tilting of the carriage or on the carriage rotatably mounted blade unit.

In a further preferred embodiment of the knife according to the invention, the carriage has a stop which cooperates in the extended position of the blade unit with the housing or the actuating device in such a form-fitting manner that further extension of the blade unit is prevented. Since the limitation of the movement of the blade unit is not made by the blade unit itself, but by the carriage on which the blade unit is rotatably mounted, the safety-related function of limiting the movement of the blade unit is performed by a member which is not rotatable within the housing. Due to the non-rotation of the carriage can be done by means of the stop a secure limitation of the movement of the blade unit.

In a further preferred embodiment of the knife according to the invention, the blade unit has a driver, which in the engaged position of the blade unit can be brought into positive engagement with the actuating device. In particular, the driver is designed such that it is engaged behind in the engaged position of the blade unit by a part of the actuator. Thus, the blade unit can be moved by the actuator from the retracted position to the extended position, but not from the extended position to the retracted position. The driver is preferably designed as a projection, which in the engagement position of the blade unit has a substantially orthogonal standing on the longitudinal axis engagement surface. The driver is preferably formed on the blade carrier of the blade unit.

In a further preferred embodiment of the knife according to the invention, the return element brings a restoring force in the direction of the engagement position on the blade unit. The return element is thus adapted to move the blade unit in the release position, in which the blade unit is not engaged with the actuating device, from the extended position to the retracted position and to move the blade unit in the retracted position from the release position to the engaged position , In particular, the restoring force on the blade unit in the direction of the retracted position is the longitudinal force component and the restoring force on the blade unit in the direction of the engagement position, a transverse force component of the restoring force applied by the restoring element on the blade unit.

In a particularly preferred embodiment of the knife according to the invention, the housing or the actuating device with the blade unit form a holding mechanism, by means of which a rotational movement of the blade unit in
Direction of the engagement position after taking the release position upon movement of the blade unit is prevented from the extended position in the direction of the retracted position. There is thus a retention of the set rotation angle of the blade unit during which the blade unit is moved by the return element from the extended position in the retracted position.

The knife according to the invention is advantageously further developed in that the holding mechanism releases the rotational movement of the blade unit in the direction of the engagement position only when reaching the retracted position of the blade unit again. Thus, the rotational angle of the blade unit is maintained throughout the retraction process from the extended position to the retracted position. Furthermore, in the context of the invention, it should be understood that the holding mechanism can release the rotational movement of the blade unit in the direction of the engaged position also a few millimeters before reaching the retracted position, to ensure a safe functioning even with fluctuating manufacturing tolerances.

In a further preferred embodiment of the knife according to the invention, the holding mechanism is designed as a latching mechanism, wherein the housing or the actuating unit and the blade unit have mutually cooperating latching elements. By means of a latching mechanism, the rotation angle of the blade unit can be maintained in a particularly simple manner. The necessary components of the locking mechanism can be produced for example by injection molding. Locking mechanisms do not require expensive materials or component geometries, so that such mechanisms are also advantageous from an economic point of view.

In a further preferred embodiment of the knife according to the invention, the latching element of the housing or of the actuating device has a hook section and the latching element of the blade unit has a latching edge. The hook portion and the locking edge are formed and arranged such that during a rotational movement of the blade unit in the extended position of the engagement position in the release position of the hook portion engages over the locking edge such that a rotational movement of the blade unit is prevented by the release position back into the engaged position.

In a further preferred embodiment of the knife according to the invention, the latching element of the housing or the actuating device during the rotational movement the blade unit in the extended position of the engagement position in the release position elastically deflected. Such elastic locking mechanisms are also known as snap connections. Snap connections have a low risk of malfunction and are quick and inexpensive to produce.

The knife according to the invention is further developed in that the latching element of the housing or the actuating device has a web which is in the engaged position and / or the release position of the blade unit in contact with the blade unit.

In a further preferred embodiment of the knife according to the invention, the actuating device between a passive position and a cutting position is movable and a second return element brings a restoring force on the actuating device in the direction of the passive position. The restoring force of the second return member causes the actuator to be automatically returned from the cutting position to the passive position as soon as the application of force to the actuating device, which is usually achieved by means of a human thumb or another finger, is interrupted.

In a further preferred embodiment of the knife according to the invention, the actuating device has an actuating housing and / or a manual actuating slide. Preferably, the actuating housing or the actuating slide has a plurality of grooves or grooves arranged substantially transversely to the direction of movement, which support the transmission of force between the actuating device and a human finger, for example the thumb.

Fig. 1:

eine perspektivische Darstellung eines erfindungsgemäßen Messers, wobei sich die drehbare Klingeneinheit in einer eingefahrenen Position befindet;

Fig. 2:

eine perspektivische Darstellung eines erfindungsgemäßen Messers, wobei sich die Klingeneinheit in einer ausgefahrenen Position befindet;

Fig. 3:

eine Seitenansicht von Teilen eines erfindungsgemäßen Messers, wobei sich die Klingeneinheit in einer Eingriffsstellung befindet;

Fig. 4:

eine Seitenansicht von Teilen eines erfindungsgemäßen Messers, wobei sich die Klingeneinheit in einer Freigabestellung befindet;

Fig. 5:

eine Explosionsdarstellung von Teilen eines erfindungsgemäßen Messers;

Fig. 6:

eine Seitenansicht von Teilen eines erfindungsgemäßen Messers;

Fig. 7:

eine perspektivische Darstellung von Teilen eines erfindungsgemäßen Messers;

Fig. 8:

eine Seitenansicht der Teile des erfindungsgemäßen Messers aus Fig. 7;

Fig. 9:

eine perspektivische Darstellung von Teilen eines erfindungsgemäßen Messers;

Fig. 10:

eine Seitenansicht der Teile des erfindungsgemäßen Messers aus Fig. 9;

Fig. 11:

eine Seitenansicht von Teilen eines erfindungsgemäßen Messers;

Fig. 12:

eine perspektivische Darstellung der Teile des erfindungsgemäßen Messers aus Fig. 11; und

Fig. 13:

eine perspektivische Darstellung von Teilen eines erfindungsgemäßen Messers.

Further features and advantages of the invention will become apparent from the appended claims and the following description of the figures, are explained in the embodiments of the knife according to the invention with reference to drawings in detail. Showing:

Fig. 1:

a perspective view of a knife according to the invention, wherein the rotatable blade unit is in a retracted position;

Fig. 2:

a perspective view of a knife according to the invention, wherein the blade unit is in an extended position;

3:

a side view of parts of a knife according to the invention, wherein the blade unit is in an engaged position;

4:

a side view of parts of a knife according to the invention, wherein the blade unit is in a release position;

Fig. 5:

an exploded view of parts of a knife according to the invention;

Fig. 6:

a side view of parts of a knife according to the invention;

Fig. 7:

a perspective view of parts of a knife according to the invention;

Fig. 8:

a side view of the parts of the knife according to the invention Fig. 7 ;

Fig. 9:

a perspective view of parts of a knife according to the invention;

Fig. 10:

a side view of the parts of the knife according to the invention Fig. 9 ;

Fig. 11:

a side view of parts of a knife according to the invention;

Fig. 12:

a perspective view of the parts of the knife according to the invention Fig. 11 ; and

Fig. 13:

a perspective view of parts of a knife according to the invention.

As shown in Fig. 1 the knife 1 a housing 2, in which an actuator 4 is arranged. The actuating device 4 comprises a movable actuating housing 6, which is guided manually displaceable within the housing 2 of the knife 1. The actuating device 4 is in the illustrated state of the knife 1 in the passive position. In the passive position of the actuating device 4, a gap between the movable actuating housing 6 and an inner edge of the housing 2 is formed.

A blade 8, which is arranged completely inside the housing 2, extends over this free space, so that the blade 8 is visible from the outside. The blade 8 has a cutting edge 10. By means of the actuator 4, the blade 8 can be moved from the illustrated retracted position to an extended position.

In order for the blade 8 to be able to leave the housing 2 of the knife 1 during the movement from the retracted position into the extended position, the housing 2 has an opening slot 12 at one end face. The width of the opening slot 12 is greater than the thickness of the blade 8. The opening slot 12 includes a first portion 14a and a second portion 14b. The second portion 14b is tapered outwardly in the direction of the housing center, so that when the blade 8 is moved by the actuator 4 in the extended position, a larger portion of the blade 10 of the blade 8 protrudes from the housing 2 of the blade 1 as would be the case if the second section 14b were not bevelled.

The housing 2 further has a recess 16 on the side opposite the opening slot 12 side. The recess 16 is formed continuously and has a substantially rectangular cross-section with rounded corners. The recess 16 can be used, for example, to receive a loop, so that the knife 1 can be fastened to the clothes of the knife operator, for example, for transport purposes. Furthermore, the recess 16 can be used to hang the knife 1 on a receiving device. Thus, the knife 1 can be stored, for example, hanging on a tool wall, so that it is readily accessible if necessary.

The housing 2 also has two opposite rounded portions 18a, 18b. The rounded portions 18a, 18b of the housing 2 create a positive subjective perception of the user of the knife 1, since the palm does not come into contact with distinct edges. The rounded portions 18a, 18b of the housing 2 further allow the user of the knife to apply high compressive forces to the housing 2 without a subjective sensation of pain. The curves of the rounded portions 18a, 18b each extend to an angular range of about 180 degrees.

Between the rounded portions 18a, 18b, a substantially planar surface 20 is arranged on both side surfaces of the blade 1. The plane surface For example, 20 may be used to display information such as a knife name or a manufacturer's name.

Fig. 2 shows an inventive knife 1, in which the blade 8 is in an extended position and the actuator 4 in a cutting cutting position. In the extended position of the blade 8, this passes through the first and second sections 14a, 14b of the opening slot 12th

Fig. 3 shows parts of a knife 1 according to the invention in side view. The movable actuator housing 6 of the actuator 4 has a plurality of grooves 22. The grooves 22 support the transmission of force between the movable actuator housing 6 of the actuator 4 and a human finger, such as the thumb. By the grooves 22, which are substantially slightly bent and arranged at right angles to a longitudinal axis of the knife 1, the frictional and positive connection between the finger or thumb surface and an outer side of the movable actuating housing 6 is improved.

On the movable actuating housing 6 a formed in the form of a hook fastening device 24 is arranged. On the fastening device 24, a return element 26 is attached. The restoring element 26 is designed as an elastically deformable helical spring. Furthermore, the return element 26 is connected via a fastening device 28 with the housing 2. In the illustrated cutting position of the actuator 4, the return element 26 brings a restoring force on the actuator 4 in the direction of the passive position.

A rotatable blade unit 30, which also includes the blade 8, also has a fastening device 32 designed as a hook. On the fastening device 32, a return element 34 is arranged, which is also designed as an elastically deformable helical spring. The restoring element 34 is connected to the housing 2 at a further fastening device 36. The rotatable blade unit 30 is also rotatably supported on a movable carriage 38. However, the storage point is hidden by the movable actuator housing 6 of the actuator 4.

The rotatable blade unit 30 is in the illustrated state of an engagement position in which the blade unit 30 with the actuator 4 in Engagement is rotatable in a release position in which the blade unit 30 is not engaged with the actuator 4.

In Fig. 4 is the movable blade unit 30 in the release position, so that the blade unit 30 is not engaged with the actuator 4. The rotational movement of the blade unit 30 from the engaged position (FIG. Fig. 3 ) in the release position is caused by the action of a cutting force Fs. The cutting force Fs is generated essentially by the pressing of the blade 8 or the cutting edge 10 of the blade 8 onto the material to be cut. The magnitude of the cutting force acting on the blade 8, which causes the rotational movement of the blade unit 30 from the engagement position to the release position, is dependent on the force applied by the user of the blade to the housing. The rotatable mounting of the blade unit 30 on the movable carriage 38 is designed such that even cutting forces which are sufficient for cutting thin paper webs cause a rotational movement of the blade unit from the engagement position to the release position.

Fig. 5 shows an exploded view of parts of a knife according to the invention. The rotatable blade unit 30 comprises the blade 8 and the blade carrier 40. The blade 8 is connected by means of the blade attachment 42 with the blade carrier 40 such that a rotational movement between the blade and blade carrier is substantially prevented. The blade carrier 42 is formed from plastic and has a driver 44 designed as a projection. The driver 44 is in the engaged position of the movable blade unit 30 in engagement with the actuator 4. If there is a rotary movement of the movable blade unit 30 from the engaged position to the release position, the engagement of the driver is dissolved.

The blade carrier 40 also has a latching edge 46, which forms a lateral projection on the blade carrier. The projection adjoins a recess 48 of the blade carrier 40. The movable carriage 38, on which the rotatable blade unit 30 is mounted, has two opposing and substantially parallel surfaces 50a, 50b. The surfaces 50 a, 50 b interact with inner surfaces of the actuating housing 6 of the actuating device 4 in such a way that a longitudinal guide of the movable carriage 38 is made possible within the actuating housing 6. The movable carriage 38 also has a stop 52, which limits the longitudinal movement of the carriage 38 relative to the actuating device 4 in one direction. The limitation of the longitudinal movement of the carriage 38 via a positive cooperation of the stop 52 of the carriage 38 with an outer edge or outer surface of the actuating housing. 6

The actuating device 4 has a hook portion 54, which forms with the latching edge 44 of the movable blade unit 30 designed as a latching mechanism holding mechanism. The holding mechanism is configured to prevent rotational movement of the rotatable blade unit 30 in the direction of the engagement position after taking the release position upon movement of the blade unit 30 from the extended position toward the retracted position. The holding mechanism designed as a latching mechanism is set up to release the rotary movement of the blade unit 30 in the direction of the engagement position only when the blade unit 30 has reached the retracted position. At the hook portion 54, a web 56 connects, which is connected via a material bridge 58 with the remaining parts of the actuator 4. If there is a rotational movement of the blade unit 30 in the extended position from the engagement position to the release position, the web 56 and the hook portion 54 is elastically deflected. The elastic deflectability is achieved in particular by the torsional ability of the material bridge 58.

Fig. 6 shows parts of a knife according to the invention in side view. The rotatable blade unit 30, which comprises the blade carrier 40 and the blade 8, is rotatably connected to the movable carriage 38 via the linkage 60. The driver 44 is in the illustrated engagement position with the movable actuator housing 6 of the actuator 4 in engagement. The engagement is achieved by a positive cooperation of the abutment surface 62 of the actuating housing 6 and the driver 44 of the rotary blade unit 30.

The movable carriage 38 is movable within the actuator 4 in the longitudinal directions of movement 64, 66. In the illustrated state, the blade unit and carriage are in the extended position so that they are prevented by the stop 52 from moving further in the direction of movement 66. The return element 34 (not shown in FIG Fig. 6 ) allows longitudinal movement of the carriage 38 together with the blade unit 30 along the longitudinal axis 70 in the direction of the retracted position 64 when the blade unit 30 has been rotated from the engaged position to the release position. The rotational movement of the blade unit 30 into the release position takes place in the direction of rotation 72. The direction of rotation 74 corresponds to the movement of the blade unit 30 from the release position into the direction of rotation Engaged position. The actuator 4 further comprises guide surfaces 68a, 68b for the carriage, which cooperate with the surfaces 50a, 50b of the carriage.

Fig. 7 shows the actuator housing 6 of the actuator 4, the rotatable blade unit 30 and the movable carriage 38. With respect to the actuator 4, the blade unit 30 is in an extended state. Further, the blade unit is in the engaged position, so that the blade unit 30 is engaged with the actuator 4. The hook portion 54 of the actuator 4 is disposed below the locking edge 46 within the recess 48. By the action of the cutting force Fs on the blade 10 of the blade 8, there is a rotational movement 72 of the blade unit 30 in the release position.

Fig. 8 shows that upon a rotational movement of the blade unit 30 in the direction of the release position (direction of rotation 72) of the housing (not shown) arranged in the rear part of the blade unit 30 is deflected in the direction of the housing bottom. During the rotational movement of the blade unit 30, the hook section 54 engages with respect to the latching edge 46. During the rotational movement of the blade unit 30 in the direction of the release position, the web 56 and the hook section 54 are deflected elastically. After reaching over the locking edge 46, the elastic deflection of the web 56 and the hook portion 54 is dissolved again. Due to the overlapping of the hook portion with respect to the latching edge, a rotational movement of the blade unit from the release position is prevented back into the engaged position.

Fig. 9 shows the rotatable blade unit 30 in the release position in which the blade unit 30 is not engaged with the actuator 4. If it comes to interrupting a cutting operation to cancel the cutting force on the blade 8, the blade unit 30 can be moved together with the movable carriage 38 in the retracted position (direction of movement 64). The movement of the blade unit 30 and the carriage 38 is caused due to a restoring force applied to the blade unit 30 by a return member.

Fig. 10 shows that the driver 44 is no longer engaged behind by a portion of the actuator 4, so that a movement of the blade unit 30 in the direction of the retracted position is no longer prevented. Due to the holding mechanism, which by an interaction of hook portion 54 and Locking edge 46 is implemented, the rotational movement of the blade unit 30 in the direction of engagement position is released only when reaching the retracted position of the blade unit 30 again.

11 and FIG. 12 show the state in which the blade unit 30 has reached the retracted position, and thus the rotational movement of the blade unit 30 is released from the release position to the engaged position. The rotational movement of the blade unit 30 is therefore no longer prevented by an interaction of hook portion 54 and locking edge 46.

As in Fig. 13 can now be brought by retraction of the actuator 4, for example by a further return element, the blade unit 30 again with the actuator 4 into engagement. This results in a relative movement between the blade unit 30 and the actuating device 4, so that the driver 44 of the blade unit 30 is engaged behind again by a portion of the actuating device 4. The relative movement between the blade unit 30 and the actuating device 4 is caused by the fact that retraction of the blade unit by the housing (not shown) of the blade is prevented during retraction of the actuator 4. Thus, the initial state of the knife is restored, so that by pressing the actuator 4, ie by moving the actuator housing 6, a new cutting process can be started and the automatic blade return is ready to operate.

LIST OF REFERENCE NUMBERS

1

knife

2

casing

4

actuator

6

movable actuator housing

8th

blade

10

cutting edge

12

opening slot

14a, 14b

first and second sections of the opening slot

16

Recess in the housing

18a, 18b

rounded sections of the housing

20

essentially plane surface

22

cried

24

fastening device

26

Return element

28

fastening device

30

rotatable blade unit

32

fastening device

34

Return element

36

fastener

38

movable slide

40

blade carrier

42

blade attachment

44

takeaway

46

catch edge

48

recess

50a. 50b

Exterior surfaces of the carriage

52

attack

54

hook section

56

web

58

material bridge

60

linkage

62

Anschlangsfläche

64

Movement direction in the retracted position

66

Movement direction in the extended position

68a, 68b

Guide surfaces for the carriage

70

longitudinal axis

72

Direction of rotation in the release position

74

Direction of rotation in the engaged position

fs

cutting force

Claims (14)

1. A knife (1) with automatic blade return, having
   a housing (2),
   a movable operating device (4),
   a rotatable blade unit (30) which comprises a blade (8) and a blade carrier (40), wherein the blade (8) is connected to the blade carrier (40) in such a manner that a rotary motion between blade (8) and blade carrier (40) is prevented in that the blade (8) has two or more openings into which there engage two or more pins (42) of the blade carrier (40) or the blade (8) is connected to the blade carrier (40) by means of an adhesive connection, stamped connection, screw connection or clamped connection,
   wherein the blade carrier (40) can be brought into engagement with the operating device (4) and wherein the blade unit (30) is movable by means of the operating device (4) from a position in which it is retracted in the housing (2) into a position in which it is extended from the housing (2),
   wherein the blade unit (30) is rotatable, through cutting force action in the extended position, from an engagement position in which the blade carrier (40) engages the operating device (4) into a release position in which the blade carrier (40) does not engage the operating device (4),
   a return element (34) which always exerts a return force on the blade unit (30) in the direction of the retracted position, and
   a movable slide (38) on which the blade unit (30) is rotatably mounted.
2. A knife (1) according to claim 1,
   characterised in that the slide (38) together with the blade unit (30) is displaceable along a longitudinal axis (70) between the retracted position and the extended position.
3. A knife (1) according to claim 1 or 2,
   characterised in that the slide (38) is guided in such a manner that a rotary motion of the slide (38) is substantially prevented.
4. A knife (1) according to any one of the preceding claims,
   characterised in that the slide (38) has a stop (52) which in the extended position of the blade unit (30) cooperates with the housing (2) or the operating device (4) in a form-locked manner such that further extension of the blade unit (30) is prevented.
5. A knife (1) according to any one of the preceding claims,
   characterised in that the blade carrier (40) has an entrainer (44) which in the engagement position of the blade unit (30) can be brought into form-locked engagement with the operating device (4).
6. A knife (1) according to any one of the preceding claims,
   characterised in that the return element (34) exerts a restoring force on the blade unit (30) in the direction of the engagement position.
7. A knife (1) according to any one of the preceding claims,
   characterised in that the housing (2) or the operating device (4) forms a holding mechanism together with the blade unit (30), by means of which holding mechanism there is prevented a rotary motion of the blade unit (30) in the direction of the engagement position after assumption of the release position upon movement of the blade unit (30) from the extended position in the direction of the retracted position.
8. A knife (1) according to claim 7,
   characterised in that the holding mechanism releases again the rotary motion of the blade unit (30) in the direction of the engagement position when the retracted position of the blade unit (30) is reached.
9. A knife (1) according to claim 8,
   characterised in that the holding mechanism is in the form of a locking mechanism, wherein the housing (2) or the operating device (4) and the blade unit (30) have locking elements which cooperate with one another.
10. A knife (1) according to claim 9,
    characterised in that the locking element of the housing (2) or the operating unit (4) has a hook portion (54) and the locking element of the blade unit (30) has a locking edge (46), wherein upon rotary motion of the blade unit (30) in the extended position from an engagement position into the release position, the hook portion (54) engages over the locking edge (46) in such a manner that a rotary motion of the blade unit (30) from the release position back into the engagement position is prevented.
11. A knife (1) according to claim 10,
    characterised in that the locking element of the housing (2) or the operating device (4) is upon rotary motion of the blade unit (30) in the extended position elastically deflected from the engagement position into the release position.
12. A knife (1) according to claim 10 or 11,
    characterised in that the locking element of the housing (2) or the operating device (4) has a an elongated member (56) which in the engagement position and/or the release position of the blade unit (30) is in contact with the blade unit (30).
13. A knife (1) according to any one of the preceding claims,
    characterised in that the operating device (4) is movable between a passive position and a cutting position and a second return element (26) exerts a return force on the operating device (4) in the direction of the passive position.
14. A knife (1) according to claim 13,
    characterised in that the operating device (4) has an operating housing (6) and/or a manual operating slide.

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