US 3596704 A
Description (OCR text may contain errors)
United States Patent Forrest D. Roe Garden, Calif. 874,660
Nov. 6, 1969 Aug. 3, 1971 A 81 A Die Casting Co.
Inventor Appl. No. Filed Patented Assignee TRANSFER MECHANISM WITH FLOATING CAST- ARTICLE SUPPORT MANDRELS IN A DIECASTING AND TRIMMING MACHINE 20 Claims, 9 Drawing Figs.
1m. 01. 1122a 17/00, 822d 29/00 Field of Search  References Cited UNITED STATES PATENTS 2,427,712 9/1947 Casler et a1. 198/209 X 2,821,157 H1958 Boyd 198/209X 3,328,853 7/1967 Pekrol 164/262 Primary Examiner-Robert D. Baldwin Attorney-Seymour A. Scholnick ABSTRACT: An improved transfer mechanism in a diecasting and trimming machine comprising an indexable turntable with floating mandrels, which extend from the table's periphery. The function of the mandrels is to support runners which in turn support the cast articles. Each station of the machine includes means which accurately position each mandrel in the station irrespective of inaccuracies in the turntable indexed position.
Patented Aug. 3, 1971 3,596,704
3 Sheets-Sheet l W WJM Patented Aug. 3, 1971 3 Sheets-Sheet 2 TRANSFER MECHANISM WITI-I FLOATING CAST- ARTICLE SUPPORT MANDRELS IN A DIECASTING AND TRIMMING MACHINE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a machine for diecasting and trimming articles formed therein and, more particularly, to improvements in a multistation diecasting and trimming machine, to control the accurate positioning of cast articles in the various stations.
2. Description of the Prior Art The state of the art is best described in U.S. Pat. Nos. 2,848,770 and 3,328,853. In U.S. Pat. No. 2,848,770, issued to .l. R. Schuchardt on Aug. 26, 1958, a two-station diecasting and trimming machine, hereafter simply referred to as the machine, is disclosed. The two stations include a casting station and a trimming station. A transfer mechanism, which comprises a rotatable shaft with two support arms at opposite sides of its free end, is incorporated. During the casting cycle of the machines operation, one of the arms is exposed in the die cavity to support the end of a runner which extends from the article which isbeing cast. Then, during the transfer cycle of the machines operation, the arm is rotated to the trimming station carrying therewith the cast article by means of the runner which extends therefrom. At the same time, the other arm is transferred to the casting station. The precise positioning of the arms, which can only be achieved by accurately rotating or indexing the shaft, is an essential feature of the machine, disclosed therein. This requirement is clearly spelled out on column 5, lines 16 etc. of said patent.
The machine described in U.S. Pat. No. 3,328,853, issued to G. G. Pekrol on July 4, 1967 is likewise dependent, for its satisfactory operation, on the accurate positioning or indexing of its transfer mechanism. In the latter patent the transfer mechanism consists of a rotatable turntable with several, more than two, support areas. Each area consists of a notch in the tables periphery which is sequentially placed to be in communication with the casting station. During casting, a runner is formed, which extends from the notch in the table to the cast article. Then, during the transfer cycle, the table is rotatable by one increment, positioning a next notch in the casting station, while the previously formed runner transfers the formed article to a station following the casting station. These stations include cooling stations and trimming stations.
Although theoretically the Pekrol machine should operate fairly satisfactorily, experience with it has led to the conclusion that its notched turntable does not provide a satisfactory transfer mechanism, particularly for high-rate operation and/or accurate article-trimming requirements. This is primarily due to the fact that the positioning of the cast article in one or more trimming stations depends on a runner which fixedly extends from a fixed notch in the turntable. Thus, unless the incremental rotational movements of the table are very precise, i.e., table indexing is extremely accurate, an article, carried by a runner from the casting station, may not be accurately positioned in a subsequent trimming station for precise and close tolerance trimming.
Also, the formation ofa runner which is only connected to the turntable by means of a notch in the latter has been found to be unsatisfactory. The runners do not seem to insure the proper radial orientations of the cast articles with respect to the center of rotation of the table, since the notches in the table seem to be insufficient to control the radial alignments of the cast runners which carry the articles. For Pekrols machine to operate with any degree of satisfaction, it is necessary to reduce the tables rate of rotation and extend the cooling period in the casting station, thereby greatly reducing the casting production rate, which is the primary goal of such a machine. Thus, a need exists for improvements in a die casting and trimming machine to overcome the disadvantageous features of the prior art machines.
OBJECTS AND SUMMARY OF THE INVENTION It is a primary object of the present invention to provide an improved diecasting and trimming machine in which the accurate indexing of the transfer mechanism is not an essential feature of the machine.
It is another object of the present invention to provide improvements in the transfer mechanism of a die casting and trimming machine to insure proper cast-article positioning in any of the machine's stations, irrespective of minor variations in the indexing of the transfer mechanism.
A further object of the invention is to provide an improved die casting and trimming machine with a plurality of trimming stations in which maximum utilization is made of the accurate positioning of the cast articles in each of the stations.
Still a further object of the invention is to provide a new improved transfer mechanism for accurately positioning a cast article in a succession of cast trimming stations in which coarse to fine trimming takes place.
These and other objects of the invention are achieved by providing, in a diecasting and trimming machine a novel transfer mechanism, comprising a turntable with novel floating cast-article-carrying elements. The floating elements are accurately positionable in each of the stations of the machine so that the position in each station of the cast article is independent of minor turntable indexing inaccuracies. In accordance with this invention an otherwise conventional turntable is provided with a plurality of slots. Each slot is used to accommodate one end of a cast-article-carrying element, hereafter referred to as a mandrel. The opposite end of each mandrel extends outwardly from the turntable periphery.
The dimensions of the slots and the mandrels are chosen so that the mandrels float within the slots. Springs are used to spring bias each mandrel within its slot, so that the mandrel's longitudinal axis coincides with a radial axis across the slot. In addition, each station of the machine is provided with a mandrel-positioning mechanism to control the mandrels position in the station. The turntable carries each mandrel from station to station. However, the position of each floating mandrel in each station is controlled by the mandrel-positioning mechanism and is not dependent on the turntable indexed position.
The free end of each mandrel extends into the die cavity when the mandrel is in the casting station. During casting, a runner is formed which extends from the cast article to the mandrels free end, so that the mandrel serves to carry the article from the casting station to each of a plurality of succeeding stations. In each of them the position of the cast article is controlled by the positioning of the floating mandrel, by means of the mandrel-positioning mechanism rather than as a function of table indexed position. Thus, minor inaccuracies in turntable indexing are tolerable without adverse effect.
The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention will best be understood from the follow ing description when read in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a top view of a novel transfer mechanism in accordance with the present invention;
FIGS. 2 and 3 are perspective views ofa novel slot and mandrel arrangement in accordance with the present invention;
FIG. 4 is a simple diagram useful in summarizing the advantages of the invention;
FIGS. 5a, 5b and 5c are cross-sectional side views of a casting station in a machine incorporating the present invention;
FIG. 6 is a top view of the novel transfer mechanism and a plurality of stations of a diecasting and trimming machine which incorporates the present invention; and
FIG. 7 is a simple isometric view of various elements in u runner-stripping station.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, in accordance with the teachings of the present invention, a turntable is provided, for use in a diecasting and trimming machine. The turntable 10 has a plurality of T-shaped slots 12, equidistantly positioned about the turntable's periphery 14. The leg of each slot 12 extends to the turntable periphery. The center line or longitudinal axis of each slot is along a radial line with the cross portion of the slot, designated by numeral 16, being in a direction perpendicular to the radial line.
Associated with each slot 12 is a T-shaped mandrel 20. The cross portion 21 and top portion 22 of the mandrels leg 23 are accommodated in the slots cross portion 16 and leg 15, respectively. The bottom portion 24 of the mandrels leg 23 extends outwardly through the slots leg 15 from the turntable periphery 14. It is the leg bottom portion 24 and, particularly, its tip 25 that is used to support the cast article and transfer it from station to station. The tip 25 may hereafter be referred to as the mandrels support tip 25. A portion of the table 10, a slot 12 and one mandrel 20, outside the shot, are shown in isometric view in FIG. 2, while FIG. 3 is an isometric view of a portion of the table with a mandrel in its corresponding slot. Preferably, portion 24 of each mandrel 10 which extends outwardly from the table's periphery is cylindrically shaped as shown in the various figures.
In the particular embodiment, shown in these figures, the mandrel leg portion 22 defines a pair of slots 31, designed to accommodate ends of leaf springs 32 which engage the sidewalls of the leg 15 of slot 12. Thus, when the mandrel is in its slot, these springs tend to bias the mandrel, so that its central or longitudinal axis coincides with the slots longitudinal axis. A pair of coil springs 34 are accommodated in bores 35 in cross member 21 of the mandrel to spring bias the latter to be centrally positioned in the cross portion 16 of slot 12. After the mandrel is located in its slot, a cover plate 38 is placed thereover and is secured to the top of the turntable 10 by means of bolts 39. The plate 38 is shown only in FIG. 2.
In the absence of the application of forces to the mandrel 20, other than the forces which the biasing springs 32 and 34 apply, each mandrel is radially aligned in its respective slot 12 so that the longitudinal axes of both coincide. In this position, the mandrel is assumed to freely float in its slot. The dimensions of the slot 12 are selected to enable the mandrel to assume other than such a radially aligned free floating position, when it is subjected to external mandrel-positioning forces. Such forces are applied to each mandrel in each of several stations of the diecasting and trimming machine by a separate mandrel-positioning mechanism, associated with each station. Consequently, in each station the mandrel is positioned at a selected accurate position which is defined or controlled by the mandrel-positioning mechanism, rather than by the turntable indexed position. This permits the machine to operate satisfactorily irrespective of small variations or inaccuracies in table indexing.
As shown in FIG. 3, the mandrel-positioning mechanism provided at each station includes a positioning yoke 40 and a shot pin 41 with a tapered end 42. In operation, as the table is indexed and each mandrel is in a difierent station, the stations shot pin 41 is lowered, as indicated by arrow 43, causing the pin 41 to engage a bore 44 in the crossmember 21 of the mandrel 20. Similarly, the yoke 40 is lowered, as indicated by arrow 45, until its semicircular slot 46 engages and surrounds the cylindrically shaped mandrel part 24. Thus, the yoke 40 and the pin 41 control the mandrel's position in the station. The pin controls the mandrel's radial position and the yoke 40 controls the mandrels lateral position.
The advantages, realizable with the present invention, may best be summarized in connection with the simple diagram of FIG. 4. Let it be assumed that for proper operation, it is necessary for tip of a mandrel 20 to assume the position, shown by the solid-line mandrel 20, with respect to a machine station 50. The station may be the diecasting station or any one of a plurality of trimming stations, which characterize a diecasting and trimming machine, in which the present invention is assumed to be incorporated. Let it further be assumed that due to improper table indexing, the turntable and, more particularly the slot 12 assumes the position, as shown by the dashed lines. Clearly, if the mandrel were fixed to the table or if the slot 12 were to fit tightly around the mandrels top and the mandrel would be forced to assume the improper position as shown by dashed lines. However, by designing the slot 12 with sufficient clearance around the mandrel top so that the mandrel floats therein and by providing the mandrel-positioning mechanism, comprising yoke 40 and pin 41, even though the table is improperly indexed the mandrel is movable within slot 12 to assume its proper position with respect to station 50.
The manner in which each mandrel 20 and, more particularly, its tip 25 is used to carry or support a cast article may best be explained in connection with FIGS. 5a, 5b and 5c. Each of the latter-mentioned figures is a cross-sectional side view of a casting station, assumed to be part of the diecasting and trimming machine, in which the present invention is incorporated. The casting station 55 includes a stationary platen 56 and a movable platen 57. When the two are in a closed position as shown in FIG. 50, they define cavities which include a casting cavity 60, and a runner or holder cavity 62.
One section of the cavity 62 extends from cavity 60 to a sprue 65, through which molten metal is supplied and forced through cavity 62 into cavity 60. Another section of runner cavity 62 extends from sprue 65 to tip 25 of one of mandrels 20 which extends into runner cavity 62 when the mandrel is in the casting station. The movable platen 57 includes a coneshaped portion 67 with the tip of the cone disposed adjacent the sprue 65. Consequently, when molten metal enters runner cavity 62, it spreads in both directions toward the tip 25 of the mandrel 20 and the main casting cavity 60. The latter assumes the shapes of one or more articles to be cast. Tl-Ius, the rectangular shape of cavity 60 should be regarded as an example only.
As is appreciated by those familiar with the art, after the hot molten metal, which is forced into cavities 60 and 62 during casting, cools sufficiently, a cast article is formed in cavity 60 and a metal support member, often referred to as a runner, is formed in the runnercavity 62. In the present invention, the runner extends from the tip 25 which is surrounded by the runner to the cavity 60. In FlG. 5b the cast article is designated by numeral 70 and the cast runner by numeral 72.
As is appreciated by those familiar with the art, after proper metal cooling; the casting operation or step is completed by moving the platen 57, together with the turntable and mandrel 20, away from the stationary platen 56. At some point, while the movable platen 57 continues to move away from platen 56, the table movement is stopped, while ejector pins (not shown) are used to eject the cast article 70 from cavity 60.
Ejector pins may also be used to eject runner 72 from the runner cavity 62. Consequently, the mandrel 20, the cast article 70 and the runner 72 are separated from the movable platen 57, as shown in FIG. 5c.
Thereafter, the movable platen 57 is stopped and the table is indexed, i.e., rotated a fixed angular increment. As the table rotates, the cast article is removed from the casting station 55 to another succeeding station and a new mandrel enters the casting station, so that when the movable platen again abuts against the stationary platen 56 for a subsequent casting step, the tip 25 of the new mandrel is in position to engage a new runner to be formed.
The cast article is removed by the mandrel to another station such as a trimming station, wherein trimming is performed while a new article is cast in the casting station. It should again be stressed that each station in which the accurate positioning of the mandrel and, more particularly, the cast article carried thereby, is required, a yoke 40 and shot pin 41 are included. Consequently, when the mandrel, carrying an article, reaches the station, it is accurately positioned therein by the yoke-pin combination.
FIG. 6, to which reference is now made, is a top view of the table with its mandrels shown at a plurality of different stations of a diecasting and trimming machine, constructed in accordance with the teachings of the present invention. The stations include the casting station 55 in which two articles in the form of toy cars 70a and 70b are assumed to be cast, simultaneously, and carried by a single multibranch runner 72. The machine is shown to include three trimmingstations 75, 76 and 77, a runner-stripping station 78 and a runner test or check station 79. The number and the types of stations shown in FIG. 6 are presented to explain the invention rather than to limit its scope or application.
In operation, assuming a clockwise rotation of the turntable 10, as indicated by arrow 81, after the articles 70a and 70b are cast, they are moved to the'first trimming station 75 through a quenching zone 82. A source 83 of cooling matter is assumed to be located at zone 82. Source 83 may produce a cold spray of water or the like to quench the cast articles as they pass through zone 82 in order to cool the metal of the articles, so that when they arrive at station 75 the articles are sufficiently solidified for trimming. Thus, is the table is indexed by one position the cast articles are sufficiently cooled to be trimmed. This eliminates the disadvantage of Pekrols machine in which the cast articles have to be indexed several positions before they are sufficiently cool for trimming.
Station 75 may be used to trim article 70b, station 76 may be used to trim both articles while station 77 may be used to trim article 700. Also, these stations may be used to perform coarse to fine trimming after station 77, all that is left on the mandrel is the runner 72. The runner is stripped off the mandrel by a stripping fork in station 78.
The stripping of the runner 72 from the mandrel which is assumed to occur in station 78 may be explained in connection with FIG. 7. Station 78 includes a stripping fork 90 which defines a cavity 92. The fork 90 which is pivotably supported about a pivot point 93 is coupled to an actuable cylinder 94 by means of a bracket 95. The cylinder 94 when actuated applies a force to the fork 90, causing it to pivot about point 93 in a selected direction. In the diagram it is assumed that an upward force is applied so that the fork 90 pivots in a clockwise direction.
In operation, each runner-carrying mandrel enters the stripping fork 90 through the open end of cavity 91. The width of the cavity 92 is chosen to fit snugly around the circular mandrel. The cylinder is then activated causing the fork to pivot about point 93. As the fork pivots in the clockwise direction, its upper surface first engages the runner 72 and thereafter strips it off the mandrel, so that the mandrel's tip is cleared of any metal previously cast thereto in the casting station. Thereafier, the fork 90 is pivoted in the opposite direction in order to be in a position to accommodate the next runner-carrying mandrel.
Each stripped solid metal runner is returned through a chute to the melting pot of the diecasting machine, wherein they are melted for use in subsequent casting steps. After the stripping station the mandrel then passes through the runner check station 79 whereat the tip of the runner is checked to insure that all runner metal has been stripped therefrom. The check or test may be accomplished by placing a switch 85 in station 79. The switch is assumed to be actuated by any metal still remaining on the tip 25 of the mandrel which is being checked.
From the foregoing it should thus be appreciated that by providing the floating mandrels in the turntable l0 and by providing mandrel-positioning mechanisms at the various machine stations, the foregoing described objects are achieved. The accurate positioning of the cast articles in the various stations is made independent of minor inaccuracies of the turntable indexing. Clearly, the maximum inaccuracy in turntable indexing which can be tolerated depends on the relative tolerances between each mandrel 20 and its associated slot 12 which define the degree of freedom of position change of each mandrel in its associated slot.
Although particular embodiments of the invention have been described and-illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art and consequently it is intended that the claims be interpreted to cover such modifications and equivalents.
1. In a diecasting and trimming machine of the type including at least two stations which comprise one casting station in which an article and a runner are cast and one trimming station in which the cast article is trimmed, and indexable rotatable transfer mechanism for transferring each cast article to said trimming station, an improved transfer mechanism comprising:
an indexable rotatable turntable;
at least two mandrel means floatably coupled to said turntable, each mandrel means having an outer and extending from said turntable for relatively free limited radial and lateral movement with respect thereto; and
mandrel-positioning means in said casting and trimming stations for controlling the position of a mandrel means in said stations by applying radial, and lateral positioning forces thereto.
2. The apparatus as described in claim 1 wherein the outer end of mandrel means in said casting station is exposed in a runner-forming cavity of said casting station so that the runner, cast in said casting station, has one end thereof cast around the outer end of said mandrel means, with the other end of said runner being cast to said cast article.
3. The apparatus as described in claim 2 further including a stripping station comprising actuatable means for stripping the runner cast to a mandrel means in said casting station from the outer end of said mandrel means.
4. The apparatus as described in claim 3 wherein said stations include a plurality of trimming stations between said casting station and said stripping station and a cooling zone disposed between said casting station and a first one of said trimming stations, and means at said cooling zone for applying cooling matter to any cast article passing said zone, the number of said mandrel means equaling the number of stations, with one mandrel means in each station when said turntable is stationary.
5. The apparatus as described in claim 4 including a test station between said stripping station and said casting station, and test means at said test station for testing the absence of a runner from the outer end of a mandrel means in said test station.
6. The apparatus as described in claim 5 wherein the outer end of each mandrel means is cylindrically shaped and the mandrel-positioning means in each station includes a yokelike member for engaging the cylindrically shaped outer end of said mandrel means to control the lateral position thereof in the station in which said yokelike member is located.
7. The apparatus as described in claim 6 wherein each mandrel-positioning means include pin means for engaging said mandrel means at a point remote from said outer end to control the radial position of said mandrel means with respect to the center of rotation of said turntable.
8. In a diecasting and trimming machine of the type including a plurality of stations which include a casting station, in which at least one article is cast, and at least one trimming station in which a cast article is trimmed, and an indexable transfer mechanism for transferring each cast article from said casting station to said at least one trimming station, an arrangement comprising:
a plurality of article-supporting members floatably coupled to said indexable transfer mechanism for relatively free limited radial and lateral movement with respect thereto;
First member-positioning means associated with said casting station for controlling the radial and lateral position of one of said article-supporting members which is in said casting station during the casting of an article therein; and
second member-positioning means associated with said at least one trimming station for controlling the radial and lateral position of an article-supporting member, which supports an article in said at least one trimming station, so as to control the positioning of the article therein ir respective of the indexed position of said transfer mechanism.
9. A diecasting and trimming machine as described in claim 8 wherein said indexable transfer mechanism comprises a rotatable turntable defining a plurality of slots equidistantly disposed about the table's periphery, and each article-supporting member having a first end floatably disposed in a different one of said slots and a second end extending outwardly from the periphery of said turntable, said second end being controllably positionable in one of said stations.
10. A diecasting and trimming machine as described in claim 9 wherein each memberpositioning means includes a yokelike element for engaging said article-supporting member when said article-supporting member is in the station, with which said yokelike element is associated, to apply positioning forces thereto to control the position of the second end of said article-supporting member in the station.
11. A diecasting and trimming machine as described in claim 10 wherein each article-supporting member includes spring means for biasing the article-supporting member in said slot so that, in the absence of the application of positioning forces by said yokelike element to said article-supporting member, the member is centrally positioned in its associated slot.
12. A diecasting and trimming machine as described in claim 11 wherein each slot is T-shaped having a leg section in a radial direction with respect to a center of rotation of said turntable and a cross section in a direction perpendicular to said radial direction, said leg section extending from said cross section to the periphery of said turntable, and the first end of each article-supporting member is T-shaped, floatably located in its associated T-shaped slot, the second end of each articlesupporting member being cylindrically shaped and each yokelike element having an elongated semicircularly shaped yoke cavity for engaging the cylindrically shaped second end of said article-supporting member to control the position thereof in the station.
13. A diecasting and trimming machine as described in claim 12 wherein the T-shaped first end of each article-supporting member defines a central bore and the member-positioning means further includes shot pin means for controlling the radial position of said first end of said article-supporting member in a station when said shot pin means engage said article-supporting member at the central bore of its T-shaped first end.
14. In a diecasting and trimming machine of the type having a plurality of stations including a casting station which comprises first and second platens movable relative to one another and defining facing cavities, forming a runner support cavity and a casting cavity for an article to be diecast therein, rotatable means having a plurality of circumferentially arranged areas, each for supporting an article cast in said casting cavity by means of a runner, cast in said runner support cavity, and means rotating said rotating means unidirectionally in indexed increments of movement, each increment corresponding to the circumferential extent of each support area, the improvement comprising:
a plurality of mandrel means each associated with a different one of said support areas, each mandrel means having a first end floatably coupled to said rotatable means for relatively free limited radial and lateral movement with respect thereto and a second opposite end extending outwardly from said rotatable means, the second end of one of said mandrel means being disposed in said runner support cavity when said platens are in engagement to form said runner support cavity and said casting cavity; and
mandrel-positioning means in each of said stations for engaging said mandrel means at the first and second ends thereof to control its radial and lateral position in said station.
15. A diecasting and trimming machine as described in claim 14 wherein each support area defines a slot and each mandrel means has a first end floatably disposed in a different one of said slots and a second end extending outwardly from the periphery of said rotatable means, the position of said second end of each mandrel means being controlled in each station by mandrel-positioning means associated with said station.
16. A diecasting and trimming machine as described in claim 15 wherein each mandrel means includes spring means for biasing the mandrel means in said slot so that, in the absence of the application of positioning forces to said mandrel means, the mandrel means is centrally positioned in its associated slot.
17. A diecasting and trimming machine as described in claim 15 wherein each mandrel-positioning means includes a yokelike element for engaging said mandrel means when said mandrel means is in the station with which said yokelike element is associated, to apply positioning forces thereto to control the position of the second end of said mandrel means in the station.
18. A diecasting and trimming-machine as described in claim 17 wherein said rotatable means comprises a turntable and wherein each slot is T-shaped having a leg section ina radial direction with respect to a center of rotation of said turntable and a cross section in a direction perpendicular to said radial direction, said leg section extending from said cross section to the periphery of said turntable, and the first end of each mandrel means is T-shaped, floatably located in its associated T-shaped slot, the second end of each mandrel means being cylindrically shaped, and each yokelike element of each mandrel-positioning means having an elongated semicircularly shaped yoke cavity for engaging the cylindrically shaped second end of said mandrel means to control the position thereof in the station.
19. A diecasting and trimming machine as described in claim 18 wherein the T-shaped first end of each mandrel means defines a central bore and the mandrel-positioning means in each station further includes shot pin means for controlling the radial position of said first end of said mandrel means in the station when said shot pin means engage said mandrel means at a central bore of its T-shaped first end.
20. A die casting and trimming machine as described in claim 19 wherein each mandrel means includes spring means for biasing the mandrel means in its associated slot so that, in the absence of the application of positioning forces to said mandrel means, the mandrel means is centrally positioned in its associated slot.