Apparatus for transporting workpiece holders in circulation

The apparatus for transporting workpiece holders in circulation between a charging station (16), a work station (22) and a discharge station (26) has a carriage (20) with two sockets (36, 38) for workpiece holders (12) to be placed on it from above. The carriage can be moved between a front position and a rear position. In the first of these, one work piece holder (12) each with a workpiece can be set on the first and the second sockets. In the other position, the two workpiece holders (12) can be lifted from the carriage (20), and the workpiece holder (12) located in the work station (22) is clamped against the underside of a frame (24). From the discharge station (26), a conveyor system (14) transports the workpiece holders (12) back to the charging station (16).

The invention relates to an apparatus for transporting workpiece holders in 
circulation between a charging station, a work station, and a discharge 
station, disposed in a straight horizontal row behind these two stations. 
For many machining, processing and measurement operations, workpiece 
holders carried along through multiple stations are needed to assure 
precise fastening of workpieces, such as boards for electronic circuits. 
Since the workpiece holders repeatedly have to receive new workpieces as 
the production process continues, they have to be transported in 
circulation. This presents the problem of finding a transport apparatus 
that allows a short cycle, takes up little space, and can be manufactured 
economically. 
Revolving conveyors cannot adequately meet these demands. Turntables take 
up a relatively large space. Moreover, like conveyor belts on which all 
the workpiece holders carried in circulation are secured, turntables have 
the disadvantage that every time they are indexed forward, a comparatively 
large mass has to be accelerated and decelerated. This disadvantage of the 
known revolving conveyor systems becomes troublesome above all when the 
measurement or machining time, for instance for an exposure operation or 
for using lasers, is very brief. 
To overcome these disadvantages, according to the invention an apparatus is 
proposed which is characterized in that a carriage with a first and second 
socket, the latter offset in the direction of motion from the first, for 
workpiece holders to be set down on it vertically from above is movable 
back and forth between a front and a rear position, that in the front 
position of the carriage at the charging station a workpiece holder is 
placeable onto the first socket by a loading device and at the work 
station a workpiece holder is placeable on the second socket by a lifting 
device, whereas in the rear position of the carriage a workpiece holder is 
liftable from the first socket by the lifting device and at the discharge 
station a workpiece holder is liftable from the second socket by an 
unloading device, and that a conveyor system is present, by which the 
workpice holders can be transported back from the discharge station to the 
charging station. 
By means of the novel apparatus with the special feature of a reciprocating 
double transport element as an essential part of a circulatory system, it 
is achieved that the cycle time outside the machining time depends only on 
the times needed for the motion of a small, lightweight carriage with 
sockets for only two workpiece holders. Every time one workpiece is being 
processed, the carriage makes a fast reverse trip in the unloaded state, 
and is then again ready to carry a processed workpiece from the work 
station and at the same time bring an unfinished workpiece to the work 
station again. Even if only three workpiece holders are used, about twice 
the machining time is available for returning them from the discharge 
station to the charging station. 
Disposing the work stations in a row one after the other is favorable from 
a standpoint of production technology, and the space required is little 
since the motions take place in three planes one above the other. In the 
work station, the workpiece holders are held clamped in an uppermost 
plane, so that the carriage can move back and forth below them. Returning 
the workpiece holders is done by the conveyor system in a plane below the 
carriage. Thus the motions of the conveyor system and the carriage are 
also independent from one another, except for the transfer of the 
workpiece holders, which is preferably done by loading devices and 
unloading devices. 
In a preferred practical embodiment of the invention, the workpiece holders 
are platelike parts with pins on their top sides, of which at least one is 
mounted adjustably in position on the workpiece holder so as to receive 
printed circuit boards of different sizes. 
In a preferred embodiment of the invention, the carriage in plan view has 
the shape substantially of an H and is movable in the longitudinal 
direction of its legs. This shape offers the dual advantage that the 
carriage needs to have only a relatively small mass, and that in its front 
position the loading device and in its rear position the unloading device 
can enter into the space between the legs and can set down the workpiece 
holders on the carriage or lift them up from it. 
So that the workpiece holders will each be held by one socket in a 
predetermined position on the carriage and also secured in this position 
during the acceleration phases, the sockets are each formed by at least 
two pins protruding upward from the top side of carriage, and the pins 
cooperate with correspondingly disposed bores in the workpiece holders. 
Since the terminal positions of a reciprocatable carriage are also easily 
determined by end stops, the workpiece holders and thus the workpieces 
fixed on them arrive in the clamping device at the work station in a very 
precisely defined position. 
In a simple version, the carriage drive can comprise a pneumatic cylinder. 
Alternatively, other drives, such as an electric motor with a threaded 
spindle, are also possible. 
The loading device and the unloading device preferable are lifting 
cylinders, disposed upright, with support plates on their upper end, the 
surfaces of the support plates being movable between a plane located above 
the carriage and a plate located below it and being provided with upward 
protruding pins, which cooperate with correspondingly disposed bores in 
the workpiece holders. Just as these simple lifting cylinders can be 
effective between the legs of the H-shaped carriage, they also are no 
hindrance to the function of the conveyor system, if the latter in a 
preferred version comprises parallel conveyor belts extending below the 
carriage but above the lower plane of the surface of the support plates, 
between which belts the loading device and the unloading device are 
disposed. 
If workpieces of different thickness and which possibly also tend to warp, 
such as printed circuit boards, are resting on a tabletop or workpiece 
holder plate in a work station and an optical measuring instrument or 
machining device is to be employed, care must constantly be taken to 
maintain an exact focus. According to the invention, changes and 
adjustments in focusing can be dispensed with if in the work station a 
frame is mounted above the transport plane of the printed circuit boards 
or other workpieces and if the workpiece resting on the workpiece holder 
can be pressed from below against the frame by a lifting device. In this 
way a platelike workpiece can be fastened between the frame and the 
workpiece holder, and the top side of the workpiece, regardless of its 
thickness and any warping that may have been present before it was 
fastened but is eliminated by its fastening, extends in the plane of the 
underside of the frame. The optical instruments and devices are focused on 
this plane, which remains constant, and need not be constantly readjusted. 
In the preferred embodiment of the invention, such a frame is mounted in 
the work station above the carriage. One workpiece holder at a time can be 
lifted from the carriage by the lifting device and pressed against the 
underside of the frame, and the printed circuit board carried by the 
workpiece holder is fastened between the workpiece holder and the frame. 
However, it will readily be understood that in a work station in which 
precise spacing between the surface of the workpieces and a measuring 
instrument or tool is critical, the basic principle described above, of 
bracing the workpieces against a bearing face not by their underside in 
the usual way but instead bracing them by their top side against the 
underside of a frame, can also be employed in workpiece transporting 
systems other than that described and claimed here. For instance, in a 
work station, above a turntable or delivery belts, a workpiece whose top 
side is to be machined and which is intrinsically dimensionally stable can 
be pressed only against the underside of a frame. If more unstable 
workpieces or such that have a greater tendency to warp are transported 
with conventional conveyor systems of the kind last mentioned above, and 
the clamping devices that pull them against the underside of the frame 
effect only an inadequate alignment, intrinsically rigid workpiece holders 
can be employed, which are transported with the turntable or other 
conventional conveyor system and are used in the work station for bracing 
the workpieces against the frame. 
In a preferred further feature of the invention, so as not to have to 
change the entire frame every time a different size of printed circuit 
boards to be machined on a mass production basis has to be handled, the 
frame comprises a base frame and an adapter frame that can be braced 
detachably against the underside of the base frame and that corresponds in 
size to the printed circuit boards to be processed. In that case, when 
production changes over to a different size of printed circuit board, only 
the adapter frame has to be changed. The transport apparatus for the 
workpiece holders is expediently used for this purpose. In the setting-up 
operation, the carriage instead of transporting a workpiece holder 
transports the adapter frame to the work station, and the lifting device 
then presses the adapter frame against the underside of the base frame, 
where it is held by clamping means that are present in addition to the 
clamping means that later keep the workpiece holders, with the printed 
circuit boards resting on them, braced tightly against the underside of 
the adapter frame during the machining operation. 
Because it would normally present difficulties to press the workpiece 
holders against the frame from below, tension claws mounted on opposite 
sides of the frame act as a lifting device in the work station; they pull 
the workpiece holders upward against the underside of the frame. 
The frame against which the workpiece holders with the workpiece are firmly 
braced in the work station, may be mounted in stationary fashion. In that 
case, the measuring instrument or machining tool disposed above the frame 
in order to be positioned relative to the workpiece, must be movably 
guided under control in two axes. The invention also allows an embodiment 
in which the measuring instrument or machining tool is held in stationary 
fashion, and the frame carrying the workpiece can be positioned under 
control in two axes. 
As already noted, the novel apparatus normally makes do with three 
workpiece holders transported in circulation, since for the return 
transport of the workpiece holders from the discharge station to the 
charging station, for instance by means of conveyor belts, about twice as 
much time as the machining time is available. However, if the machining 
time is very brief, then advantageously four workpiece holders in 
circulation can be employed, each of which is transported back to the 
loading device in two steps by the conveyor system. 
If the workpieces are to be delivered automatically in a continuous 
processing line which includes additional work steps, then it is 
recommended that a charging device for placing printed circuit boards on 
the workpiece holders is disposed next to the charging station, and a 
discharge device for lifting printed circuit boards from the workpiece 
holders is disposed next to the discharge station. With the novel 
apparatus, however, it is also possible to work in such a way that both a 
charging device for placing printed circuit boards on the workpiece 
holders, and a discharge device for lifting printed circuit boards from 
the workpiece holders transported back by the conveyor system are disposed 
next to the charging station.

The exemplary embodiment, shown in simplified form in the drawing, of an 
apparatus according to the invention comprises a table-like stand 10, 
which can be set up on the floor and along which platelike workpiece 
holders 12 are transported from right to left, in terms of FIG. 1, at a 
level N1 and then back again from left to right at a lower level N2 that 
is defined by the upper run of conveyor belts 14. 
The workpiece holders 12 conveyed in circulation are placed onto the right 
half of a carriage 20 on the right hand end of the stand 10 in a charging 
station 16 by means of a loading device 18. In each case, they have 
previously been equipped with a workpiece, such as a board for an 
electronic circuit. By moving the carriage 20 toward the left in terms of 
FIG. 1, the workpiece holders 12 along with the printed circuit boards to 
be processed pass into a work station 22 disposed in the middle of the 
stand 10. There, the workpiece holders and workpieces are lifted from the 
carriage 20 and pressed against the underside of a frame 24. The carriage 
20 can then move back again to the right into the outset position and 
pickup a further workpiece holder, with a workpiece to be processed, in 
the charging station 16. In the meantime, the workpiece that has just 
before this been raised and pressed against the frame 24 has been 
processed in the work station 22. Before the carriage is put into motion 
again and moves toward the left, the workpiece holder that carries the 
processed workpiece is detached from the frame 24 and placed on the left 
half of the carriage 20. In the ensuing operation of transporting the 
carriage 20, thus on the one hand a workpiece holder 12 is again taken 
from the charging station 16 to the work station 22 and on the other at 
the same time the work-piece holder 12 previously braced in the latter is 
transported, with the finished, processed workpiece, from the work station 
22 to a discharge station 26 located on the left-hand end of the frame 10. 
After the carriage 20 has been stopped in its left-hand or rear terminal 
position, both of the transported workpiece holders, with all the 
workpieces seated on them, are lifted substantially simultaneously from 
the carriage 20, and directly after that the empty carriage 20 can move 
back into its right-hand or front terminal position again, in which the 
above-described loading operation, on the one hand at the loading station 
and on the other at the work station 22, is repeated. Meanwhile, in the 
discharge station 26, an unloading device 28 places the workpiece holder 
12, lifted there from the carriage onto the upper run of the conveyor 
belts 14. These belts transport the workpiece holder from left to right 
back to the charging station 16, where it is lifted from the conveyor 
belts 14 by the loading device 18 and placed on the carriage 20 for the 
next trip in circulation. 
The loading device 18 and the unloading device 28 are embodied identically 
in this exemplary embodiment. They each comprise vertically disposed 
lifting cylinders 30, which are provided on their upper end with a 
horizontally oriented support plate 32. Alternatively, instead of the 
lifting cylinders 30, electric motor drives with movable spindles may be 
used. In the upper terminal position of the support plate 32, marked 32a 
in FIG. 1, they are located at a level above the carriage 20, so that by 
raising the lifting cylinder 30 of the unloading device 28 by means of the 
support plate 32, workpiece holders can be lifted from the carriage 20 
and, when the support plate 32 of the loading device 18 is lowered, the 
workpiece holders can be set down onto the carriage 20. In the lower 
terminal position of the lifting cylinder 30, the support plate 32 is 
located somewhat below the level N2, so that when the lifting cylinder 30 
of the unloading device 28 is lowered, a workpiece holder 12 seated on the 
support plate 32 is set down onto the conveyor belts 14. 
From the above description of the exemplary embodiment, it can be seen that 
the workpiece holders 12 are placed on three different levels during one 
revolution. At the topmost level N3, suggested by dashed lines at 12a, at 
which level they are temporarily located in the charging station 16 and in 
the discharge station 26, and which can also coincide with their level 
during processing in the work station 22, they are raised far enough away 
from the carriage 20 that the carriage can move unhindered below them. 
The drive of the carriage 20 is merely suggested in the drawing in FIG. 2. 
It may be a pneumatic of hydraulic cylin-der, or an electric motor drive 
with a drive spindle. 
For accurate positioning of the workpiece holders 12 on the carriage 20 and 
on the support plates 32, pins are used in each case which engage suitably 
disposed, fitting holes in the workpiece holders 12. In this example, two 
pins 36 on the right-hand half of the carriage 20 form a first socket, and 
two pins 38 on the left-hand half of the carriage 20 form a second socket 
for the workpiece holders to be placed on the carriage. The workpiece 
holders are also provided with pins on their top side, which engage 
correspondingly disposed, fitting holes in the printed circuit boards to 
be received. If batches of different-sized printed circuit boards are 
being processed, then preferably at least one pin is adjustable in its 
position and can be adjusted to the applicable size of printed circuit 
board. 
FIG. 3 shows a plan view in which one workpiece holder 12 each is located 
in the charging station 16 and in the discharge station 26. The support 
plates 32 located below the workpiece holders 12 are shown in dot-dash 
lines. 
The shape of the carriage 20 can be seen from FIG. 1 and FIG. 4. It can be 
seen that in plan view it is substantially in the form of an H, with a 
center portion 40 forming the crossbar and carrying the pins 36, 38, and 
with legs 42 extending in the direction of motion, on each of which legs a 
bearing block 44 is secured. Each of the two workpiece hol-ders 12 to be 
received by the carriage 20 rests on two bearing blocks 44 and on the 
middle part 40, with the position securing pins 36 and 38, respectively. 
The particular shape of the carriage 20 makes it possible for the support 
plates 32, in their vertical motion, to move freely between the legs 42, 
since the width of the support plates 32 is less than the crosswise 
spacing between the legs 42. This is correspondingly true for the conveyor 
belts 14. They are not shown in FIG. 2, but like the legs 42 of the 
carriage 20, they extend in the region of the longitudinal edges of the 
stand 10, so that the support plates 32 can also move unhindered up and 
down between the two parallel conveyor belts 14, which are driven in 
synchronized fashion. 
In the work station 22, the frame 24 is mounted in a stationary though if 
desired a position device 60 can be provided to position the frame under 
control in two horizontal axes fashion. If only printed circuit boards of 
a certain constant size are being machined, for instance being drilled 
with a laser, then a suitably dimensioned frame 24 suffices, against whose 
underside the workpiece holders 12 are pressed, with the printed circuit 
boards resting on them, in such a way that the boards are fastened all the 
way around and thereby straightened as well. If different sizes of printed 
circuit boards are to be processed, then a relatively large base frame 24 
to suit the largest printed circuit boards to be processed is used, and a 
smaller adapter frame is braced beneath it, the inside width of which 
adapter frame corresponds to the smaller printed circuit boards to be 
processed. To brace an adapter frame 46, for instance as shown in FIG. 5, 
against the underside of the base frame 24, tension claws 48 shown in FIG. 
3 are used, and for bracing the workpiece holders 12 with printed circuit 
boards resting on them against the underside of the adapter frame 46, 
further tension claws 50 are used. The tension claws 48, 50 are mounted on 
the base frame 24, optionally adjustably to accommodate different sizes of 
printed circuit board, and are for instance actuated by pneumatic or 
hydraulic cylinders. Since they engage the adapter frame 46 and then the 
workpiece holders 12, lift them from the carriage 20, and brace them to 
the base frame 24, all from above, they are as little hindrance to the 
motion of the carriage as the lifting cylinders 30 and support plates 32 
that operate between the legs 42 of the carriage 20. 
In the exemplary embodiment, the printed circuit boards are placed on the 
workpiece holders 12 in the charging station 16 by hand, or by a robot arm 
or other suitable charging device 62, while the loading device 18 keeps 
the workpiece holder in the uppermost position 12a shown in dashed lines. 
In the discharge station 26, the printed circuit boards are removed with 
the unloading device 28 raised upward, the removal again being done by 
hand, for instance, or by a robot arm or other suitable discharging device 
64. The workpiece holders 12 accordingly trans-port the printed circuit 
boards only from the charging station 16 to the discharge station 26, via 
the work station 22. After that, the empty workpiece holders are 
transported back to the charging station 16 by the conveyor belts 14. As 
an alternative it would also be possible to remove the printed circuit 
boards from the workpiece holders 12 at the charging station 16 either 
still at the level N2 or after they have been raised to the level N3 by 
discharging device 64, immediately before placing a further printed 
circuit board to be processed on them. 
If as described above the frame 24 is fixed in the work station 22, then 
the tool that acts on the workpieces, such as the laser, must execute the 
required motions for machining in the longitudinal and transverse 
directions to the stand 10 and the frame 24. Focusing needs to be done 
only once, because once the printed circuit boards have been fastened to 
the frame 24, the surfaces of these boards--regardless of their 
fluctuating thickness and any warping--coincide with the plane of the 
underside of the base frame 24 or adapter frame 46.