The invention relates to a power drill which comprises a housing, a drill spindle supported in the housing, a drive unit arranged to rotate and move the drill spindle

longitudinally, and a control system including cam

activated switching devices for controlling the

longitudinal movements of the drill spindle between forward and rear end positions.

In power drills of the above described type there is a problem to increase the productivity by bringing down cycle time for each drilling operation. One way of doing this is to increase the advance speed of the drill spindle to obtain a quicker approach of the drill bit to the object to be drilled. However, this also means a higher feed speed during the drilling operation. An obvious drawback when using an increased advance speed is that when hitting the surface of the object the drill bit is exposed to a severe impact force very often resulting in damaging the drill bit. Instead of improving the productivity of the drilling operations this previously used method results in undesired interruptions of the drilling operations caused by damaged drill bits and following replacement of the latters.

Another drawback of this known method is that the feed rate during drilling will be too high for most materials to be drilled, which results in an undesirably high wear of the drill bit and hence, frequent stops in production for replacing drill bit also for this reason.

A power drill of the above mentioned type is previously described in for instance US Patent No. 4,591,299. In advanced drilling machines having numerically controlled operations this problem is solved by providing a high advance speed up to a predetermined drill spindle position determined by certain coordinates, whereafter a correct and favourable feed rate is employed to get a proper drilling operation. This means that a quick advance phase of the drill bit is provided up to a position very close to the object to be drilled without influencing on the proper low feed rate to be applied during the following drilling operation. No severe impact forces on the drill bit will arise, and there is a no risk for drill bit damages.

However, in less advanced drill units where the drilling operations are not controlled via numerically based control systems there is still a problem to increase efficiency by reducing cycle times without getting problems with drill bit being damaged and/or excessively worn.

It is an object of the invention to solve the above problem by providing a power drill with a control system including switching devices to be activated by cam elements on the drill spindle to initiate desirable changes in the advance speed of the drill spindle. Further objects and advantages of the invention will appear from the following specification and claims.

A preferred embodiment of the invention is described below with reference to the accompanying drawings.

In the drawings Fig. 1 shows a side view, partly in section, of a shortened power drill according to the invention and illustrates the drill spindle its rear end position.

Fig. 2 shows the same view as in Fig. 1 but illustrates the drill spindle in its advance speed shifting position.

Fig. 3 shows the same view as in Figs. 1 and 2 but

illustrates the drill spindle in its forward end position.

The power drill illustrated in the drawings is in fact a portable power drill intended to be firmly attached to an object during drilling. For that purpose, this type of drills are always provided with anchoring means for

attachment to an object, but in order not to complicate the illustrations these anchoring means are left out from the drawings .

The power drill illustrated in the drawings comprises a housing 10, a pneumatically powered drive unit 11 located in the housing 10 and including a pneumatic rotation motor, and a control system for controlling the operation of the drive unit 11 and, thereby, the movements of a drill spindle 12. The latter is rotatable as well as

longitudinally movable relative to the housing 10 and carries a drill bit 13 at its forward end. The drill spindle 12 is provided with an external thread 15 for co ^¬ operation with a non-illustrated threaded nut element in the drive unit 11 for accomplishing longitudinal advance and feed movements of the drill spindle 12. The latter is also provided with a couple of axially extending grooves 16 for transfer of rotational movement from the drive unit 11 to the drill spindle 12. This is a well known technique for driving a drill spindle and is not described in further detail. The housing 10 comprises a cover tube 14 which encloses and protects the rear end of the drill spindle 12.

In power drills of this type the housing 10 there is also provided a non-illustrated socket portion for mounting of an anchoring means. Such anchoring means is intended for attachment of the power drill to an object to be drilled for obtaining an accurate guidance of the drill during drilling operations. Since these features do not form parts of the invention but belong to a known technique

previously, described in for instance US Patent No.

4,591,299, they are not described in detail in this

specification . The drive unit 11 comprises a pneumatic rotation motor 17 which is supplied with pressure air via a pneumatic conduit connection 18 at the rear end of the housing 10, and a throttle valve operable by a tangent 19. The drive unit 11 further comprises a power transmission including a speed controlling means by which the rectilinear movements of the drill spindle 12 are controlled. The speed controlling means comprises a gearing shiftable between a rapid advance speed condition and a slower drill feeding speed condition used during the actual drilling operation. However, since the power transmission and the speed controlling means are not parts of the invention they are not described in further detail.

The control system comprises valves and other pneumatic components for controlling the operation of the motor and the speed controlling means. For governing the longitudinal movements of the drill spindle there are provided switching devices arranged to be engaged and activated by cam elements carried on the drill spindle. Accordingly, on the rear portion of the drill spindle 12 there is adjustably mounted a first cam element 20 arranged to engage and activate a first switching device 21 mounted on the cover tube 14 for determining the drilling depth or the forward end position of the drill spindle 12. The first switching device 21 is locked to the cover tube 14 by a screw 24. See Fig. 3. The first cam element 20 is formed by a nut which is threaded onto the drill spindle 12 and lockable in a desired position by a lock screw 19 engaging one of the longitudinal grooves 16, thereby setting a desired forward end position of the drill spindle 12.

On a forward portion of the drill spindle 12 there is mounted a second cam element 22 arranged to engage and activate a second switching device 23 mounted on the housing 10. The forward end position is determined in view of the intended hole depth to be drilled on the actual object or the thickness of the object when drilling through the object. The second cam element 22 is rigidly secured to the drill spindle 12 and defines in co-operation with the second switching device 23 the fully retracted or rear end position of the drill spindle 12. See Fig. 1. On the cover tube 14 of the housing 10 there is adjustably attached a third switching device 25. This third switching device 25 is arranged to be activated by the first cam element 20 at a certain longitudinal position of the drill spindle 12 to thereby generate a signal to activate the speed controlling means for changing the advance speed of the drill spindle 12. The third switching device 25 is adjustable along the cover tube 14 and locked by a screw 26 in a position corresponding to a position of the drill spindle 12 in which the drill bit 13 is just about to engage the object to be drilled. By activating the third switching device 25 in that very position of the drill spindle 12 the advance speed of the latter may be changed from a rapid advance speed during the approach of the drill bit 13 to the object to be drilled to a slower proper drilling feed speed suitable for the actual drilling operation. See Fig. 2. A drilling cycle is started with the drill spindle 12 in the position illustrated in Fig. 3a, i.e. the fully

retracted position of the drill spindle 12. The drilling operation is started by the control system being activated by an operator whereby the drive unit is powered to start rotating and advancing the drill spindle 12. The control system is set to give the drill spindle 12 a rapid advance speed during the initial approach phase, i.e. when the drill bit 13 is travelling towards the object to be

drilled. This results in a desirable quick initial phase of the drilling operation. When however, the drill bit 13 gets close to the object the first cam element 20 gets into contact with and activates the third switching device 25, which means that a signal is generated by the latter to accomplish a speed change by the speed controlling means of the control system. This position is illustrated in Fig. 2. So, just before the drill bit 13 gets into contact with the surface of the object to be drilled the advance speed of the drill spindle 12 is reduced to the slower feed speed suitable for the actual drilling operation. Thereafter, the drilling operation is performed until the desired drilling depth is reached. This forward end position of the drill spindle 12 is determined by the setting of the first cam element 20 on the drill spindle 12, which means that when the cam element 20 gets into contact with the first switching device 21 a signal is generated to initiate stop of the longitudinal feed movement and commence a reverse retracting movement of the drill spindle 12. See Fig. 3.

It is to be noted that instead of a speed shifting gearing the speed controlling means may comprise a motor speed shifting means, particularly in electrically powered power drills. In such embodiment of the invention the switching devices comprise electric switches instead of pneumatic valves for co-operation with the cam elements on the drill spindle .