GB2124942A - Workpiece-exchanging apparatus for machine tools - Google Patents

Abstract

In work changing apparatus wherein workpieces (W) are positioned and clamped on each work pallet (C) or removed therefrom at a workpiece- exchanging station (Y) there is provided a transfer device to move the work pallet to a pallet exchanging station, where the pallet is shifted onto a work table 47 of the machine tool and positioned and clamped thereon ready for the workpieces to be machined. Each work pallet includes a base plate 5 in which hydraulic cylinders (H) are located, each including a vertically movable piston adapted to carry a clamping member for holding workpieces (W) to the plate 5. The base plate additionally including an oil path for feeding hydraulic pressure to each of the cylinders. A valve is provided for holding the oil pressure in the hydraulic cylinders to enable shutting off of the oil path so that it is also possible to transfer the work-clamp pallet with the oil hose disconnected and proceed with machining without replenishing hydraulic oil. <IMAGE>

Description

SPECIFICATION Workpiece-exchanging apparatus for machine tools The present invention relates generally to workpiece-exchanging apparatus for machine tools. More particularly, but not exclusively, it relates to a workpiece-exchanging apparatus devised to shorten the time required for changing workpieces while the machine tool is stopped for improving the working efficiency of the machine tool.

The most modern NC (numerical controlled) machine tools such as machining centres are of automatic operating type with their high speed, high precision cutting features and their productivity is quite high.

For automatic operation of such a machine tool, however, it is essential to have workpieces positioned with high precision and clamped in place.

As a means of clamping the workpieces on work table, higherto known is a system in which nuts are set in T-section grooves in the work table on mating bolts planted therein with clamp arms thereon for holding down the workpieces on the table as the nuts are fastened.

In this conventional method, however, it takes time to fasten the nuts on the bolts and the required time of inactivity of the machine tool for exchaning the workpieces is prolonged, resulting in lowering of the working efficiency of the machine tool and lowering of its productivity as well.

The time required for clamping the works on the table increases with increasing number of workpieces, resulting in a drop ofthe machine tool's working efficiency in proportion thereto.

A method for overcoming this drawback is disclosed in U.S. Patent No. 4,174,828.

This method consists in embedding a multiplicity of hydraulic cylinders in the machine tool's table spaced longitudinally as well as laterally with a clamp shaft detachably connected to the piston of each cylinder, and the clamp shaft is fitted with a clamp arm at the upper end thereof.

According to the method, workpieces are first placed on the table at the desired positions, the hydraulic cylinder nearest to each workpiece has the clamp shaft connected to its piston and then each hydraulic cylinder is driven toward the clamping side for the clamp arm to hold down each workpiece on the table to be thus clamped in place.

In this method, however, the position for each workpiece to be clamped on the table is not predetermined, hence it takes time to position the workpiece on the table with high precision, and still no sufficient improvement of the machine tool's working efficiency is attainable.

The time required for positioning and clamping the workpieces on the table increases with increasing number of workpieces and the corresponding increase of the idle time of the machine tool means no substantial improvement of its working efficiency by this method, either.

Moreover, the table for this method is of a special shape and construction with a multiple of hydraulic cylinders built therein and, when it is used for an existing machine tool, it means replacement of the standard table with T-sectioned grooves, which results in additional expense.

Further, the new table is required to be custommade for each model of machine tool as regards its outside dimensions as well as the shape, construction and dimensions of the fixture required for its attachment to the machine tool, and its manufacturing cost is by no means insignificant.

The present invention provides workpieceexchanging apparatus for a machine tool, the apparatus comprising: work-clamp palletforthe clamping of workpieces thereon, transfer bed means for transferring work-clamp pallets thereon between a workpiece-exchanging station and a work-clamp pallet exchanging station on the side of the machine tool, the work-clamp pallet being transferable between a work table of said machine tool and said transfer bed means at said pallet exchanging station, clamping means for releasably clamping said workclamp pallet on said work table of said machine tool, a plurality of hydraulic cylinders spaced longitudinally and laterally within a base plate of said workclamp pallet and so arranged to enable exchange of multiplicity of said workpieces on said work-clamp pallel at said work-exchanging station, each of said hydraulic cylinders including a piston vertically movable beneath the top surface of said base plate, said piston having means for enabling a clamping member to be secured thereto so as to effect a unitary movement of said piston and said clamping member; and said base plate including an oil path for supplying each of said hydraulic cylinders with hydraulic pressure.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view showing a machine tool incorporating a workpiece-exchanging apparatus according to the present invention; Figure 2 is a side view of the machine tool in Figure 1; Figure 3 is a plan view of a work clamp pallet of the apparatus; Figure 4 is a vertical sectional side view of the base plate showing the conveyor system; Figures 5 and 6 are vertical sectional front views of one castor and one spring therefor respectively; Figure 7 is a vertical sectional front view taken through the main section of the auxiliary table; Figure 8 is a vertical sectional front view showing the guide means; Figure 9 is a vertical sectional front view of the positioning device; Figure 10 is a vertical sectional front view of the clamping device;; Figure 11 is a vertical cross-section on an enlarged scale through the main section of the work-clamp pallet; Figure 12 is a bottom view of the portion indicated by the arrow XII in Figure 11; Figure 13 is a front view of the work clamp pallet on which workpieces of the same size and shape are loaded; Figure 14 is a partial plan view of the work-clamp pallet of a first alternative embodiment; Figure 15 is a front view of a partially modified version of the embodiment in Figure 14; Figure 16 is a front view of a further modified version of the work-clamp pallet of a second alternative embodiment; Figure 17 is a plan view of the embodiment in Figure 16; Figure 18 is a partial front view of a still further modified version of the work-clamp pallet of a third alternative embodiment; Figure 19 is a plan view of the embodiment in Figure 18;; Figure 20 is a plan view of another modified version of the work-clamp pallet of a fourth alternative embodiment; Figure 2 1 is a vertical cross-section through a further modified version of the hydraulic cylinder of a fifth alternative embodiment; Figure 22 is a partial vertical sectional front view of the work-clamp pallet of a sixth alternative embodiment; Figure 23 is a plan view of the embodiment in Figure 22; Figure 24 is a partial vertical sectional front view of the work-clamp pallet of a seventh alternative embodiment.

Referring to Figures 1 to 13, described below is a preferred embodiment of the present invention.

In front of a table 1 of an NC (numerically controlled) vertical machine tool (M) is provided a rotary transfer bed 47, and on the table 1 is placed an auxiliary table 1A fixedly bolted thereto. On the transfer bed 47 there is mounted a turn table 47A which can rotate horizontally about the centre shaft by half a turn in either direction, and the turn table 47A can carry on it two work-clamp pallets (C).

Of the spaces on the turn table 47A for two work-clamp pallets, the one immediately in front of the table 1 is a work-clamp pallet-exchanging station (X) and the other on the opposite side is the workpiece-exchanging station (Y).

After a multiplicity of workpieces (W) have been positioned and clamped on the top surface 2 of the base plate 5 of the work-clamp pallet (C) placed on the work-exchanging station (Y), the turn table 47A is turned by half a turn to bring said work-clamp pallet (C) to the work-clamp pallet-exchanging station (X), and then the work-clamp pallet (C) is caused to roll on rails 48 onto the auxiliary table lAto be positioned and clamped thereon.

Machining of the workpieces (W) clamped on the work-clamp pallet (C) is now proceeded with, while at the workpiece-exchanging station (Y) the finished workpieces (W) are removed and workpieces to be machine (W) are set (positioned and clamped).

When one cycle of machining is over on the machine tool (M), the work-clamp pallet (C) on the auxiliary table 1 A is withdrawn to the work-clamp pallet-exchanging station (X), the turn table 47A is turned by half a turn, and then the abovementioned procedure is repeated again.

As seen from Figures 4 to 6, castor means 110 are built in two, right and left, longitudinal rectangularsectioned grooves 111 in the under-surface of the base plate 5 of the work-clamp pallet (C). A main frame 113 carrying each castor 112 is urged downward by a spring 1 14 so that the tread of the castor 112 is slightly projected beyond the under-surface of the base plate 5.

As seen from Figure 1 and Figures 7 to 10, the auxiliary table 1A is provided with a pair of left and right positioning devices 49 as well as pairs of left, right and central clamping devices 50.

The reference numeral 115 designates a guide means for guiding the work-clamp pallet (C) and the reference numeral 116 a supporting base thereof.

For bringing the work-clamp pallet (C) from the work-clamp pallet-exchanging station (X) onto the auxiliary table 1A and for positioning and clamping it thereon, the first guide means 115 is set in the guide groove 117 in the base plate 5 to slide therein and, with this stopped at the predetermined position, a stop plug 49a of a hydraulic positioning device 49 is inserted into a positioning hole 49b in the undersurface of the base plate 5 for the base plate 5 to be accurately positioned thereby.

A T-shaped clamper 50a of the clamping device 50 is engaged in a mating T-sectioned groove 50b in the under-surface of the base plate 5 for relative sliding therein and, after positioning of the base plate 5 by said positioning device 49, the clamping device 50 is actuated and the base plate 5 is pulled down thereby to be clamped on the supporting bed 116.

As best shown in Figure 3, the work-clamp pallet (C) includes a base plate 5 as a matrix, made of a thick steel in which 15 hydraulic cylinders (H) are totally embedded at equal spaces so as to form 3 lines and 5 rows.

As shown in Figure 11, the hydraulic cylinders (H) are of the single-acting type, each consisting of a stationary section 6 and a movable section 7. The stationary section 6 is the cylinder itself, and the movable section 7 is a piston. The cylinder 6 is fitted in a hole 5a formed in the under-surface of the base plate 5. The piston 7 is inserted in the cylinder 6 and vertically moves within the limit that its rod 7a never rises up the top surface 2 of the base plate 5.

When oil is introduced into an oil chamber 8, the piston 7 is caused to descend against a spring 9, and when the oil is withdrawn, the piston 7 restores under the action of the spring 9. The cylinder 6 has a shoulder 6b, and the piston 7 has a shoulder 7b.

When the piston 7 ascends, these shoulders come into abutment, and the piston 7 is prevented from further upward movement. The hydraulic cylinders (H) in the same line are communicted by an oil path 10 passing through the base plate 5 such that the oil chambers 8 in each cylinder are communicated one after another. The oil path 10 is supplied with oil at a port 11 at which the pipe 4 is connected to the base plate 5.

The piston rod 7a includes a hole 12 so threaded as to receive a threaded portion of a clamping bolt 3, thereby effecting their associated movement. The clamping bolt 3 includes a head (P), which is intended to hold a workpiece (W) against the surface 2 of the base plate 5. In this case an auxiliary clamping plate 3a strengthens the clamping force.

The workpiece (W) is held by clamping plate 3a at its edge portion. Each hydraulic cylinder (H) is protected against a possible instrusion of machining chips by means of a ring-shaped rubber packing 3b.

In the hydraulic cylinder (H) where a clamping bolt is lacking, a plug 13 is screwed in the threaded hole 12 of the piston rod 7a through an opening 14 in the top surface 2, the plug 13 including a shoulder 13b adapted to engage the cylinder 6. In this way the piston 7 is prevented from descending when oil is introduced into the oil chamber 8. The plug 13 has a flat top surface flush with or alternatively slightly beneath, the top surface 2 of the base plate 5. The plug 13 is also useful in protecting the hydraulic cylinder (H) against a possible instrusion of machining chips. By making the anused hydraulic cylinder inactive, the hydraulic action in the used hydraulic cylinders (H) advantageously becomes more active, thereby securing a quicker action of the pistons 7.In addition, the life of the hydraulic cylinders (H) can be prolonged.

When the clamping bolt 3 is screwed in the piston rod 7a, the piston tends to rotate. To prevent it, a stopper 16 is embedded in a recess 15 produced in the bottom of the base plate 5, the stopper 16 being fastened by means of bolts 17. The stopper 16 is also engaged in recesses 6c and 7c produced in the cylinder 6 and the piston 7, respectively, as shown in Figure 12. The stopper 16 also serves to fasten the cylinder to the base plate 5.

As shown in Figure 3, the top surface 2 of the base plate 5 is provided with T-sectioned grooves 18 arranged in the pattern of a fretwork. The grooves 18 are helpful for holding the workpiece (W) on base plate 5. The work-clamp pallet (C) is fastened to the table 1 by means of bolts through holes 19.

Figure 13 shows, as an example, that a plurality of workpieces (W) of the same size are held.

Hereinafter, descriptions will relate to alternative embodiments.

I. The first alternative embodiment illustrated in Figures 14 and 15 provides rectangular grooves 20 instead of the T-sectioned ones. In addition holes 21 for receving clamping bolts are arranged with equal spaces along horizontal and vertical lines as best shown in Figure 14. In this embodiment a jig 22 as a side support can be used to support the workpiece (W) at its side when it is clamped by means of the clamping bolts 3.

II. Figures 16 and 17 show the second modification in which a fork-shaped or U-shaped jig 22 is used to oppress adjacent workpieces (W) against the top surface 2 of the base plate 5. This is particularly effective when the workpieces (W) have steps as shown in Figure 16.

Ill. Figures 18 and 19 show the third alternative embodiment in which L-shaped jigs 22 are used to hold the workpieces. This is also effective when the workpieces have steps at their bottom portions as shown in Figure 18. In this case, the terminating end of each L-shaped jig 22 is engaged in the groove 20, thereby avoiding slipping on the top surface 2 of the base plate 5.

IV. Figure 20 shows the fourth alternative embodiment characterized by an arrangement of oil paths 10 formed along the horizontal and the vertical lines communicatingly crossing at right angle. Owing to this fretwork pattern arrangement, a single pipe 4 can be enough. In Figure 20 the oil paths along the horizontal line are designed by the reference numeral 10X, and the ones along the vertical lines by the reference numeral 10Y. The reference numeral 23 designates a plug for closing the oil path. The reference numerals 24X and 24Y designate keys inserted in the rectangular grooves 20, which keys are designed to locate the workpiece exactly at the clamping position. The sides of the keys, designated by the reference numerals 25X and 25Y, constitute defining walls against the workpiece (W).

V. Referring to Figure 21, the fifth alternative embodiment of the hydraulic cylinder (H) will be explained.

In the illustrated embodiment the stationary section 6 has no cylinder but is constituted by the hole 5a in which a ring 6A is fitted in its upper section. The piston 7 is provided with a tail 26 at its bottom, which is secured in a groove 28 produced in a bottom plate 27. Thus the tail 26 can prevent the piston 7 from rotating in the hole 5a during its up and down movement. The piston 7 is provided with a centering cap 13A at its top, wherein the centering cap is provided with a cylindrical centering projection 29 adapted to locate the centers by fitting in the holes 30 of the workpieces (W). When the hole 30 is too large for the projection 29, an inserter 29a is additionally fitted around the projection 29.

The arrangement of the hydraulic cylinders (H) is not limited to the illustrated embodiment, but it can be formed in triangle or any other desired patterns.

The shape and dimension of the base plate 5 and the distances between the hydraulic cylinders (H) and the capacity thereof are determined, as desired, in accordance with the size and type of the machine tools.

VI. Referring now to Figures 22 and 23, described below is the sixth embodiment.

In each hydraulic cylinder (H) is set the piston 7 slidable up and down and above the piston 7 there is formed the oil chamber 8 and loaded under it is the spring 9.

Each oil chamber 8 communicates with a hydraulic oil source (not shown) through the oil path 10 in the base plate 5, and it is so arranged that the hydraulic oil is supplied from the hydraulic oil source to every oil chamber 8 so that the pistons 7 are caused to descend all at once.

Of course, it is not necessary that every hydraulic cylinder (H) is then operated, and the hydraulic cylinders (H) under the workpieces or remotest therefrom are kept out of operation with their pistons 7 arrested by the respective plugs 13.

Disposed above each hydraulic cylinder (H) is a clamping member 32 which is interlockingly connected over a connecting rod 3 with the piston 7 of the hydraulic cylinder (H) thereunder.

On the top surface 2 of the base plate 5 are arranged the work carrying means 31, each thereof made up of two stationary beds 31a and one movable bed 31 b.

The stationary beds 31a are disposed under each clamping member 32 for the hydraulic cylinders (H) in the lefthand row and the movable beds 31 b under each clamping member 32 for the same in the righthand row respectively so that each pair of the stationary bed 31a and the movable bed 31b cooperate to carry one work piece thereon. The stationary bed 31 a is positioned with its base fitted in the opening 14 in the top surface 2 of the base plate 5.

On the base plate 5 are also provided 3-point type positioners 33, each set thereof serving to position each workpiece straddling on the work carrying means 31 longitudinally as well as laterally. That is, each set of the positioner 33 is made up of a positioning element 33a which is secured to the top surface of the base plate 5 for determining the longitudinal position of the work piece and another positioning element 33b which is formed in the stationary bed 31a for the hydraulic cylinder H in the lefthand row for determining the lateral positioning of the workpiece.By the way, since the movable beds 31 b are arranged in a beam-like formation with a streaky projectiom provided longitudinally along one edge of the work carrying surface 34 thereof, hence by moving the movable beds 31 b toward the stationary beds 31 a with the workpiece stradding the stationary beds and the movable beds, it is possible to have either the left or right end faces of the workpieces in each row checked by the positioning elements 33b simultaneously.

The workpieces in each row are set to straddle the comprising beds (31 a, 31 b) of the work carrying means 31, and their longitudinal as well as lateral positions determined by means of the 3-point type positioners 33, the pistons 7 of the hydraulic cylinders are caused to descend all at once so that the workpieces are clamped on the base plate 5 by the clamping means 32 acting over each set of work carrying means 31.

VII. Referring to Figure 24, now described is the seventh embodiment.

In this embodiment the workpiece (W) is clamped on the base plate 5 by stationary positioning means 36 locked by the hydraulic cylinders (H) in the work-clamp pallet (C) and side wedge type clamping means 37 arranged to be driven thereby.

Said positioning means 36 has its shape not particularly defined but generally comprises a work carrying portion 36a for carrying the workpiece in a lifted state and a pair of positioning faces 36b. Each positioning face 36b is saw-toothed downwardly to ensure against lifting of the workpiece (W).

The positioning means 36 has set therethrough along its centre axis a clamp bolt 3, which is screwed into a tapped hole 12 made in a piston rod 7a thereunder, and is locked on the base plate 5 as the piston rod 7a ascends to the upper dead point.

Across the space for accommodating the workpiece (W) from the positioning means 36, the opening 14 has inserted therein a downwardly projecting portion 39 of a stationary frame 38 of said slide wedge type clamping means 37 for positioning of said frame 38.

This stationary frame 38 has, on the one hand, a function to support a work pressing means 41 over a spring 40, and has, on the other hand, set through it the clamp bolt 3 with its head (P) engaged from above in the recess of the work pressing means 41 which extends downwardly through the clamp bolt hole 42 running through 41 and is screwed into a tapped hole 12 made in a piston rod 7a thereunder so that the work pressing means 41 can be driven downwardly by a hydraulic cylinder (H) thereunder.

The clamp bolt hole 42 of the work pressing means 41 therethrough is formed oblong-sectioned to allow displacement thereof toward or away from the work positioning means 36.

The work pressing means 41 has its outer surface on the side away from the work positioning means 36 forming a downwardly inclined cam face 43. The stationary frame 38 has its inner surface away from the work positioning means 36 forming also a cam face 44a inclined downwardly in the opposite sense to be in close contact with the cam face 43 of the work pressing means 41. This face-to-face contact between these two parts is advantageous for the reduced planar contact pressure.

The vertical surface of the work pressing means 41 on the side toward the work positioning means 36 has, as its face of contact with the workpiece, a downwardly saw-toothed face 45.

The half of the stationary frame 38 on the side toward the work positioning means 36 constitutes a work carrying bed 46 for carrying the workpiece at a predetermined height above the top surface of the base plate 5.

When, with the workpiece set to straddle the work carrying bed 36a of the work positioning means 36 and the work carrying bed 46 to the clamping means 37, the hydraulic oil is supplied to the oil chamber 8 from the hydraulic oil source, the pistons 7 of the hydraulic cylinders (H) descend and, at the work pressing means 41 comes down, the mating inclined cam faces 43/44a cooperate to displace it toward the work positioning means 36, and the workpiece, positioned precisely by the work positioning means 36 automatically, is clamped between it and the work pressing means 41.

An advantage of the above embodiments is that they provide a method for positioning workpieces on a work table of a machine tool with the additional possibility of exchanging them quickly so as to improve sufficiently the working efficiency of the machine tool for sizable enhancement of its productivity; it is possible to exchange the workpieces on the work-clamp pallet taken out to the workpiece exchanging station while machining is underway on the workpieces clamped on the work-clamp pallet releasably clamped on the machine tool's table.

When machining is over, the work-clamp pallet with the finished workpieces is pulled out from the table in exchange for another work-clamp pallet with the workpieces to be machined clamped thereon which is then shifted onto the table to be releasably clamped thereon.

The time required for exchanging the works on the machine tool's table can thus be shortened to the time required for exchanging the work-clamp pallets.

Moreover, the work-clamp pallet need only be placed on the machine tool table at the predetermined position, hence its positioning is feasible with a high precision readily with ease.

Another advantage is that exchange of an increased number of workpieces on the table at a time is enabled with no increase of the time required therefor for further improvement of the working efficiency of the machine tool.

Still another advantage is that the described methods are applicable to existing machine tools readily and economically, by virtue of the multiplicity of the hydraulic cylinders being not in the machine tool's table but in the work-clamp pallet and by the releasable clamping of the latter on the table.

By this, the invented methods can be applied to any existing machine tool with no modification to its table of the standard T-sectioned groove design.

Moreover, the work-clamp pallet can then be standardized in size, shape, construction and dimensions easily regardless of the machine type or model, and hence can be manufactured economically.

Claims (17)

1. Workpiece-exchanging apparatus for a machine tool, the apparatus comprising: a workclamp pallet for the clamping of workpieces thereon, transfer bed means for transferring work-clamp pallets thereon between a workpiece-exchanging station and a work-clamp pallet exchaning station on the side of the machine tool, the work-clamp pallet being transferable between a work table of said machine tool and said transfer bed means at said pallet exchanging station, clamping means for releasably clamping said work-clamp pallet on said work table of said machine tool, a plurality of hydraulic cylinders spaced longitudinally and laterally within a base plate of said work-clamp pallet and so arranged to enable exchange of a multiplicity of said workpieces on said work-clamp pallet at said work-exchanging station, each of said hydraulic cylinders including a piston vertically movable beneath the top surface of said base plate, said piston having means for enabling a clamping memberto be secured thereto so as to effect a unitary movement of said piston and said clamping member; and said base plate including an oil path for supplying each of said hydraulic cylinders with hydraulic pressure.

2. Apparatus as defined in Claim 1, wherein said base plate is provided with holes in its bottom for accommodating said hydraulic cylinders therein.

3. Apparatus as defined in either Claim 1 or Claim 2, wherein said base plate of said work-clamp pellet is provided with T-sectioned grooves in its top surface on which the workpieces are clamped.

4. Apparatus as defined in either Claim 1 or Claim 2, wherein said base plate of said workclamping pallet is provided with rectangular grooves in its top surface on which the workpieces are clamped, and additionally with threaded holes along horizontal and vertical lines in said top surface.

5. Apparatus as defined in any preceding claim, wherein said oil path formed in said base plate is a single path leading from one cylinder to another so as to feed hydraulic pressure to each of said cylinders from a single port.

6. Apparatus as defined in any preceding claim, wherein said piston in said hydraulic cylinder embedded in said base plate can be optionally made inoperative under hydraulic pressure by replacing said clamping member with a plug secured to said means of said piston.

7. Apparatus as defined in any preceding claim, wherein said piston of said hydraulic cylinder comprises a body portion and a rod portion with a shoulder interposed, said shoulder being adapted to limit the upward movement of said piston.

8. Apparatus as defined in any preceding claim, wherein each hydraulic cylinder comprises a spring located under said piston so as to allow said piston to return when hydraulic pressure is withdrawn.

9. Apparatus as defined in any preceding claim, wherein said means in each piston is a threaded hole.

10. Apparatus as defined in any preceding claim, wherein said base plate includes means for preventing each of said pistons from rotating during its up and down movement.

11. Apparatus as defined in any preceding claim, wherein said base plate is constructed with a solid thick steel plate.

12. Apparatus as defined in any one of Claims 1 to 10, wherein said base plate is constructed with plural steel structural members built up with welding.

13. Apparatus a defined in any preceding claim, wherein the base of a stationary bed engages with a fitting hole formed in said base plate as the upper side of said hydraulic cylinders, thereby carrying and positioning said workpieces in a horizontal plane.

14. Apparatus as defined in any preceding claim, wherein there is disposed on said base plate above said hydraulic cylinder a slide wedge clamping device having a stationary frame the base portion of which frame is engaged with a mating hole formed in said base plate at the upper side of said hydraulic cylinder for positioning thereof, said stationary frame suppors a work pressing means over a spring, a clamping means is set through said stationary frame vertically displaceable with its head engaging said work pressing means from above and its stem extending downward through a through-hole therein to be screwed into a threaded hole in a piston rod at its lower end, and wherein a work pressing face is provided on one side of said work pressing means and its opposite side is formed as an inclined cam face in close contact with the mating face of said stationary frame.

15. Apparatus as defined in any preceding claim, wherein said base plate is provided with plural castor means in grooves in its under surface.

16. Apparatus as defined in any preceding claim, wherein said base plate is provided with a pair of positioning holes.

17. Workpiece-exchanging apparatussubstantially as hereinbefore described with reference to and as shown in Figures 1 to 13, or in Figures 1 to 13 as modified by Figures 14 and 15, 16 and 17, 18 and 19,20,21,22 and 23 or 24.