BROACHING MACHINE

Abstract: A broaching machine (l0) having an upper crosshead (ll), a worktable (l4) and a lower crosshead (l2) is provided with drive interruption means to reduce the risk of breaking a broach during broaching of an undersized or misplaced work piece. The upper crosshead (ll) descends on a pair of rams, one of which has its piston rod (20) non-positively coupled to the crosshead (ll). A bobbin (25) rigid with the rod (20) may undergo movement rela­ tive to the crosshead (ll) if the latter is obstructed and this movement is detected by a proximity sensor (27) providing a signal to interrupt the crosshead lowering drive. The arrangement can optionally be used for the setting of upper and lower limit switches (3l) and (34) for the machine which are positioned by electric cylin­ ders (32) having a lead screw and gear roller arrangement and incorporating pulse generating signal means. The upper limit switch is set for a particular broach by lowering the upper crosshead (ll) until the broach con­ tacts the pull chuck (9). The electric cylinder (32) seeks the position of the crosshead (ll) and generates a control signal to position the upper limit switch accor­ dingly. The same control signal can automatically be used to set the position of the lower limit switch (34) mounted on a second electric cylinder (35).

Description: This invention relates to broaching machines. In the following description by way of example, reference will be made primarily to an internal broaching machine but certain aspects of the invention may be more widely applicable, for example to external or surface broaching machines. In an internal broaching operation, a tool known as a broach, which is made of hardened high speed tool steel, is forced through a bore to cut formations such as splines in the walls of the bore. The broach is usually pulled through the bore by hydraulic rams. Clearly, this involves the generation of consider­ able stresses in the broach. If the bore is undersized or the work piece is misplaced so that it obstructs the broach, the stresses in the tool can become excessive and the broach may shatter. This can be dangerous to the operator of the machine in spite of the use of machine guards. It inevitably means the loss of the expensive broaching tool and a time delay while the broken tool is removed and a new broach is inserted to continue operation of the machine. For this reason, it has been considered necessary for every work piece being passed to a broaching machine to be gauged for the accuracy of the internal bore. In spite of this, breaking of the broach is a fairly frequent accurance. The need for l00% gauging also reduces the scope for automating the process, for example by robot loading of the broaching machine. It has been proposed to provide a safety device in which, as the broach is lowered into the bore, any build up of hydraulic pressure in the power rams is detected and serves to cut off the drive to the broach. However, this is a very insensitive method and does not satisfac­ torily prevent breaking of the broach in every instance. It is an object of a first aspect of the invention to provide a broaching machine in which this disadvantage is overcome or reduced. According to this aspect, the invention provides a broaching machine comprising a work piece holder; a crosshead adapted to carry a broaching tool and arranged for translational movement towards the work piece holder under the operation of a pair of fluid operated ram assemblies, one ram assembly being rigidly secured to the crosshead and the other ram assembly being non-positively coupled to the crosshead; proximity sensing means arran­ ged to detect relative movement between the crosshead and said other ram assembly; and drive interruption means operable in response to a signal of the proximity sensing means, whereby, when travel of the crosshead is resisted, any resulting relative movement detected between the crosshead and said other ram assembly serves to stop further travel of the crosshead. The drive interruption means may comprise drive reversal means adapted automatically to reverse the direction of drive to the broach. The invention has a further aspect, concerned with the setting of limit switches required for cyclic opera­ tion of the broaching machine. Since a broaching machine may be 4 or 5 metres tall, the manual setting of limit switches at the top and bottom of the power stroke of the broach may be a fairly lengthy process, involving repeated adjustments at the upper and lower parts of the broach travel. Because of the lengthy setting up time for each new broach, machine down time becomes a signifi­ cant factor except where long runs of identical broaching operations are to be carried out. In a broaching machine of the type which can be cycled automatically an upper crosshead or "retriever crosshead" holds the broach above the work piece holder or table while the work piece is inserted into position. The retriever crosshead is lowered until the lower end of the broach projects below the work piece, where it can be gripped by a chuck of the lower or power crosshead. The changeover point at which the power crosshead grips the broach must be indicated by a limit switch to control proper cycling of the machine. Clearly, the retriever crosshead has to be arranged to release the tool before the power crosshead is operated. When the entire operative length of the broach has been pulled by the power crosshead through the work piece, it is necessary for the power crosshead to be returned slightly and the retriever crosshead must then grasp the upper end of the broach to draw it back to the start position. In order to prevent the return stroke from being excessively long, since it represents idle time for the machine, a lower limit switch needs to be set at a posi­ tion such that the power stroke ends at a distance below the work piece holder related to the broach length. This will obviously also be related to the retriever crosshead stroke above the work piece holder. It is an object of a further aspect of the present invention to provide means for setting the position of upper and lower limit means for the broaching machine. According to this aspect of the invention there is provided a broaching machine comprising a work piece holder; a first crosshead adapted to carry a broaching tool and arranged for translational movement towards the work piece holder under the operation of a pair of fluid operated ram assemblies, one ram assembly being rigidly secured to the crosshead and the other ram assembly being non-positively coupled to the crosshead, proximity sen­ sing means arranged to detect relative movement between the crosshead and said other ram assembly; control means responsive to a signal of said proximity sensing means to actuate a first limit stop positioning means; said limit stop positioning means including measuring means adapted to measure the distance of the first crosshead from a datum point and to generate a signal related to said distance; and a first limit switch adapted to be moved to a first limit position determined by the distance signal such that, thereafter, the first limit switch can operate to prevent travel of the first crosshead beyond said first limit position towards the work piece holder. The machine may also comprise a second limit stop positioning means responsive to said distance signal to move a second limit switch positioned at the opposite side of the work piece holder from the first limit switch to a second limit position such that, thereafter, the second limit switch can operate to prevent travel of a second, power crosshead beyond said second limit position away from the work piece holder. A broaching machine embodying the invention will now be described by way of example only with reference to the accompanying drawings in which; FIGURE l is a plan view of a upper retriever cross­ head of a automatic cycle internal broaching machine. FIGURE 2 is a detail sectional view on the line 2:2 of Figure l. FIGURE 3 is a diagrammatic front elevational view of the broaching machine with parts of the machine broken away. FIGURE 4 is a partial side elevational view on the arrow 4 of Figure 3. Referring first to Figure 3 of the drawings, an internal broaching machine is generally indicated at l0 and comprises an upper, retriever crosshead ll and a lower, power crosshead l2 each of which is mounted for vertical reciprocating movement on respective columns such as l3, relative to a work piece holder or table l4. Each crosshead ll, l2 is equipped with one or more tool holding chucks l5 which can be operated to grip or release a broaching tool (not shown) in a tool holder such as l6. Considering the upper, retriever crosshead ll, this can be seen in more detail in Figures l and 2 of the drawings. The crosshead is raised and lowered by the action of a pair of hydraulic rams l7, l8. The ram l7 is rigidly secured to the crosshead ll by a nut l9 but the second ram l8 is merely arranged so that its piston rod 20 passes through a bush 2l in a cylindrical bore 22 of the crosshead ll. An enlarged collar 23 on the piston rod 20 carries the weight of the crosshead ll but permits relative sliding movement of the piston rod 20 and the crosshead ll should any obstruction be met by the cross­ head ll as it descends. An extension 24 of the piston rod 20 projects above the crosshead and is fitted with a flanged bobbin 25, spring loaded at 26 away from the upper surface of the crosshead ll. A proximity sensor 27 of generally known type is fixed at 28 to the upper surface of the crosshead ll and is arranged to detect any relative movement between the bobbin 25 and the sensor 27 (and hence the crosshead). The arrangement is such that, when the rams l7 and l8 are contracted to lower the crosshead ll, any obstruc­ tion to the downward movement of the crosshead will in effect tend to lift it off the supporting collar 23 and hence cause a relative movement between the bobbin 25 and the crosshead. This will be detected immediately by the proximity sensor 27 and a signal can then be sent to the control means for the apparatus to stop further lowering of the crosshead. The broach in the tool holder l6 may for example be obstructed by a misplaced or undersized work piece at the work table l4. This will serve to trigger the sequence of events just described and further lowering of the broach will be stopped immediately. The signal from the proximity sensor 27 can be arranged either simply to stop lowering of the tool, for example in a manual mode of the machine, or to reverse the lowering of the tool so that the crosshead ll is lifted again, enabling the undersized work piece to be removed and replaced. This prevents or reduces the danger of the broach being shattered by continuing the broaching cycle where the work piece is undersized or in some way displaced so as to obstruct the proper passage of the broach through the bore. In a broaching machine having this feature, it is possible automatically to set the upper and lower limit switches required for controlling the broaching cycle. Turning to Figures 3 and 4 of the drawings, the retriever crosshead ll has a normal top position defined by a fixed limit switch 29 and a normal fully lowered position defined by a lower fixed limit switch 30. How­ ever, a variable position limit switch 3l is also pro­ vided, the position of this being determined by the particular size of broach to be used. At the commencement of a broaching operation, the broach is held in the tool holder l6 of the retriever crosshead ll and is lowered until its lower end passes through the worktable l4 and work piece and can be gripped by the lower, power crosshead l2. It is necess­ ary to define the changeover point so that the upper retriever crosshead does not approach the worktable too closely at this stage. Clearly, the position of the upper movable limit switch 3l needs to be calculated in accordance with the position of the crosshead when the broaching tool engages in the lower tool holder or pull chuck 9. In order to set the upper movable limit switch 3l automatically, the broach is inserted into the tool hol­ der l6 and held in place by the retriever chuck l5. The upper, retriever crosshead ll is then lowered towards the worktable l4 and, as soon as the limit position is rea­ ched, and the broach contacts the bottom of the pull chuck 9, the proximity sensor 27 detects relative move­ ment between the crosshead ll and the bobbin 25. This simultaneously stops the lowering of the crosshead and initiates operation of the limit switch positioning means to be described. An electric cylinder 32 is positioned at one side of the machine and carries a movable carriage on which is mounted the limit switch 3l. The electric cylinder is of a generally known type used for accurate positioning of components and incorporates a lead screw and gear roll­ ers. Rotation of the lead screw, driven by a d.c. motor, moves a roller nut linearly along the cylinder. The cylinder also includes an electric pulse transmitter which can send a pulse signal to the machine control means. The electric cylinder is used as a measuring system in that the limit switch 3l is moved to a position in which it detects the presence of the crosshead ll. It does this by contacting the switch cam 33 on the cross­ head. The electric cylinder 32 transmits a pulse signal to the control means indicating the distance between the cam 33 on the crosshead ll and a predetermined datum point. This signal is used to position the limit switch 3l so that, thereafter, it will stop the downward travel of the crosshead at the changeover position. A small amount of working clearance, typically a few millimetres, is added to the actual position of the crosshead where the broach contacts the pull chuck 9 since otherwise, on each cycle of operation, the broach would make contact with the bottom of the chuck. The signal to the control means is then used to position a lower movable limit switch 34 provided on a second electric cylinder 35 which is positioned below the worktable l4. This lower limit switch 34 is positioned a distance equivalent to that of the upper movable limit switch 3l above the table l4. The lower power crosshead has its uppermost limit of travel defined by the fixed limit switch 36 but the lower limit switch 34 is required to end the downward power stroke of the power crosshead l2, corresponding to the full passage of the broach through the work piece. It is therefore no longer necessary for the upper and lower movable limit switches 3l and 34 to be set manually by trial and error methods in setting up the broaching machine for the new broach. The broaches may be fitted to the machine and the limit switch setting cycle is initiated by lowering the retriever crosshead ll. The whole cycle can, in general, be completed and both limit switches set in less than one minute, compared with at least several minutes for manual setting of the switches. Furthermore, because of the necessary lubri­ cant reservoirs below the table level l4, it has hitherto been an unpleasant job to set the lower limit switch 34 and this is avoided by automatic setting. Since the positioning of the limit switches is achieved by means of a pulse generating electric cylin­ der, suitable pulse counting means in the control circui­ tary of the machine can be used to check the identity of the broach being used from the limit switch settings. This may be useful for monitoring production. A preferred form of electric cylinder for use in the apparatus is a type OC cylinder manufactured by Eline Engineering Limited.

Claims: 1. A broaching machine comprising a work piece holder; a crosshead adapted to carry a broaching tool and arranged for translational movement towards the work piece holder under the operation of a pair of fluid operated ram assemblies; and drive interruption means to stop further travel of the crosshead; characterised in that one ram assembly (l7) is rig­ idly secured to the crosshead (ll) and the other ram assembly (l8) is non-positively coupled to the crosshead (ll); in that proximity sensing means (27) are arranged to detect relative movement between the crosshead (ll) and said other ram assembly (l8); and in that the drive interruption means are oper­ able in response to a signal of the proximity sensing means (27) whereby, when travel of the crosshead (ll) is resisted, any resulting relative movement detected bet­ ween the crosshead (ll) and said other ram assembly (l8) serves to stop further travel of the crosshead. 2. A broaching machine according to Claim l further characterised in that the drive interruption means com­ prise drive reversal means adapted automatically to rev­ erse the direction of drive to the broach. 3. A broaching machine comprising a work piece holder; a first crosshead (ll) adapted to carry a broaching tool and arranged for translational movement towards the work piece holder under the operation of a pair of fluid operated ram assemblies characterised in that one ram assembly (l7) is rigidly secured to the first crosshead (ll) and the other ram assembly (l8) is non-positively coupled to the first crosshead (ll); in that proximity sensing means (27) are arranged to detect relative movement between the first crosshead (ll) and said other ram assembly (l8); in that control means are provided responsive to a signal of said proximity sensing means (27) to actuate a first limit stop (3l) positioning means; in that said limit stop positioning means include measuring means (32) adapted to measure the distance of the first crosshead (ll) from a datum point and to gene­ rate a signal related to said distance; and in that a first limit switch (3l) is provided and is adapted to be moved to a first limit position determined by the distance signal such that, thereafter, the first limit switch can operate to prevent travel of the first crosshead (ll) beyond said first limit position (3l) towards the work piece holder (l4). 4. A broaching machine according to Claim 3 and further characterised in that it comprises a second limit stop (34) positioning means responsive to said distance signal to move a second limit switch (34) positioned at the opposite side of the work piece holder (l4) from the first limit switch (3l) to a second limit position such that, thereafter, the second limit switch (34) can ope­ rate to prevent travel of a second, power crosshead (l2) beyond said second limit position away from the work piece holder (l4). 5. A broaching machine according to Claim 3 or Claim 4 further characterised in that said measuring means com­ prise an electric cylinder (32) generating a signal rel­ ated to the distance from a datum point to a cam (33) of the first crosshead (ll).

Inventors: BARKER, ANTHONY LESLIE
Applicants: ASSOCIATED MACHINES & TOOLS LIMITED
Application year: 1987
Main classification: B23D41
IPC codes: B23D41