GB2210817A - A centrifugal casting device - Google Patents

Abstract

The device comprises a rotary arm (2) mounted on a shaft (1). One end of the arm has a balance weight (3) that is adjustable in the axial direction and which balances a casting crucible (41) and mould (51). A casting material melting means (6) comprises two units (60) that separate along a split. In operation, material (M) to be melted, especially for forming a dental casting, is placed in the crucible (41) which is closely surrounded by the two units (60) of the induction melting means (6). When the material is molten, the units (60) are drawn away laterally so as to lie outside the rotation path of the arm (2). The latter is rotated to cast the melt in the mould (51). <IMAGE>

Description

CENTRIFUGAL CASTING DEVICE This invention relates generally to a centrifugal casting device for molten glass and similar materials, and more particularly to a device suitable for centrifugally casting an artificial dental crown and other dental members.

It is well known to use a centrifugal casting device to cast members from glass or metal material for use in dental prosthesis and repair. In this known type of centrifugal casting device a molten casting material is poured into a mould connected to a casting crucible to form the cast. The crucible is held at one end of a rotary arm and the casting material is placed in the crucible and melted by means such as a torch or high frequency or arc melting , and the arm is rotated so that under centrifugal force of the rotating arm the molten casting material is poured into the mould.

Alternatively a melting furnace is installed around the crucible and adapted to be rotated along with the rotary arm during moulding.

The rotary arm has means adapted to cause the arm to hold a casting crucible with a mould in contact with the crucible at one end of the arm and to instantaneously rotate the arm at high speed by the stored resilience of a coil spring.

In these casting devices the former device has the disadvantage that it is difficult to regulate temperature, especially by torch heating, and in practice the proper melting temperature of a casting material is determined by the naked eye. Also, in the case of high frequency heating or arc melting, the device is enlarged in scale, and temperature detection must also be dependent upon an optical means with the result that a great error in temperature measurement tends to be produced.The latter device with the furnace around the crucible enables a precise measuring means for temperature detection and makes it possible to provide the device in compact form, but because the melting furnace has to rotate along with the rotary arm, not withstanding the fact that the furnace is not directly related to the casting stage, a drive source (for example, an electric source) for rotating the furnace and the arm together requires a considerable output, and because the arm is also subjected to a powerful moment, the arm must be thick and strong. It is desirable for the rotary arm to attain the required speed of rotation instantaneously after the arm starts rotation, but when the arm is equipped at one end with a heavy melting furnace, it is very difficult to obtain ideal initial velocity.

Also, in the driving means for the rotary-arm, charging of the coil spring is effected by manually rotating the arm and the charged state of the spring is maintained by an engaging pin that is manually drawn in and out.

Accordingly, the charging of the spring takes time for preparatory work before casting and is accompanied with danger. The arm is also able to rotate freely even after the end of casting, and therefore time is taken before demounting and mounting of the next mould or before the step of filling a casting material into a crucible. Additionally because the rotary arm stops at any given position, the operator is not free from the troublesome task of restoring the arm to its normal position before starting the next step of operation.A further problem is that because the coil spring is subject to fatigue due to the effect of time, charging the spring manually in the manner described above renders it very difficult to make the charge of the spring invariably constant by adjusting the zero level (horizontal position of the rotary arm) and the extent of the charge of the spring has a great effect on the speed of rotation of the rotary arm. Difficulties of this kind proved a serious hindrance to the automatization of centrifugal casting devices.

The present invention is directed towards providing a casting apparatus that accurately controls the melting temperature but requires minimum out put drive force to achieve ideal initial velocity. The invention is also directed towards bringing the rotary arm to rest as soon as possible and returning the arm to a normal position as soon as possible ready for the next casting procedure so as to enable a series of automatic operations.

Accordingly the invention provides a centrifugal casting apparatus for dental articles, the apparatus comprising a rotary arm mounted for rotation on a shaft, one end of the arm having a balance weight which is adjustable in the axial direction of the arm and the other end of the arm having means for holding a casting crucible that is adapted to receive casting material and means for holding a mould in communication with the centrifugal side of the casting crucible, the apparatus also comprising a casting material melting means adapted to enclose the casting crucible to melt the casting material and which separates along a split for removal from the casting crucible out of the path of the rotary arm.

A detailed description will now be given of embodiments of the invention with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a centrifugal casting device; Figure 2 is a longitudinal sectional view taken along the line II-II of Figure 1; Figure 3 is a longitudinal sectional view taken along the line III-III Figure 1; Figures 4a, b, c and d diagrammatically show in progressive steps, respectively, how melting and casting are effected by the use of the device of the invention; Figure 5 is an exploded perspective view of an embodiment of the invention; Figure 6 is a longitudinal sectional view taken along the line VI-VI of Figure 5; Figure 7 is a plan view, partly broken, showing a rotating mechanism of an arm in Figure 1 with a sensor plate 16 being intentionally inclined with respect to the axis of a rotary driving shaft 1 for easier understanding.

Figure 8 is a perspective view of the line VIII of Figure 7; Figure 9 is an enlarged sectional view taken along the line IX-IX of Figure 8; and Figures 10a and lOb are an explanatory view showing the state of operation of a clutch pin used in the invention.

As shown in Figures 1 to 6, a centrifugal casting device comprises a rotary driving shaft 1, a rotary arm 2 mounted at the end of the shaft 1 and rotated around the axis of the shaft 1 by the driving force of the shaft 1, a balance weight 3 adjustably mounted at one end of the arm 2, a crucible holding means 4 disposed at the other end of the arm 2 for holding a casting crucible 41 adapted to receive a melting material for casting therein, a mould holding means 5 connected to the centrifugal side of the crucible holding means 4, and a casting material melting means 6 mounted so as to lie outside the path of rotation of the arm 2 at least when the rotary arm 2 is rotated.

In Figure 1, the character A designates a casing for a driving means for rotating the rotary arm 2 and for apparatus related with a control system. B represents a control panel, and C designates a safety cover hinged to the side of the casing A and adapted to cover rotating parts when the device is operated. The rotary arm 2 is attached in a T-shape to the end of the driving shaft 1. The shaft is rotated around the axis of the shaft 1 by the driving force of an electric motor (not shown) mounted in the casing A or by the resilient force of a spring 12 (see Figure 3) fitted around the shaft 1.

Figures 1 to 4 show the melting furnace with a casting material melting means 6 positioned close to the crucible holder 4, which is at rest. The melting material M is melted in the melting furnace 6 and is transferred into a casting crucible 41 by tilting the melting furnace, and it is then poured into a mould 51 by rotation of the rotary arm 2 and becomes cast therein. The casting crucible 41 which receives the molten casting material from the furnace 6 is shaped like a wooden shoe and has a small passageway 411 communicating with a cavity 511 of the mould 51 on the centrifuging side of the furnace.

The melting furnace 6 comprises a heat-resisting core tube 61 and a heating element 62 wound around the tube 61, and is disposed in the casing A so as to be tiltable towards the side of the crucible holder 4. A fingerstall-shaped melting crucible 63 is inserted into the core tube 61 and a melting material (an ingot) N is melted in the crucible 63. The numberal 64 designates a cover over the upper opening of the crucible 63, and the cover is designed to be opened in interlocking relation with the tilted furnance 6. The numeral 631 designates a crucible locator designed to engage with the opening of the crucible 63 and the plate 631 functions to prevent the crucible 63 from being drawn out of the core tube 61 even when the furnace 6 is tilted by the resilience of a leaf spring 632.The numeral 65 designates a thermocouple for detecting the temperature in the furnace 6 and connected to a temperature control apparatus in a control panel B The driving shaft 1 has a coil spring means 12 fitted thereabout. The spring is charged by an electric motor (not shown) or the like, and the rotary arm 2 is turned in the direction of arrow A by the recoil strength of the spring means 12.

A series of operations for the members above are programmed in the control panel B and are automatically conducted by manipulation of buttons on the surface of the panel B . Procedures for this operation are briefly described with reference to Figure 4a to 4d.

In Figure 4a, a casting crucible 41 is set on a crucible holding means 4 and a mould 51 is set on a mould holding means 5 and a start button on the control panel B is pressed. Thereupon, the melting furnace 6 is brought into a preheated state.

In Figure 4b, pressing of second operation button on the panel B (for example, named a crucible key) opens a cover 64 and tilts the furnace 6 at an angle of about forty-five degrees so that a melting crucible 63 having a casting material M therein is inserted into the core tube 61 of the furnace 6. Then, pressing the second operation button again restores the furnace 6 and the cover 64 to their original positions and sets the furnace to a melting temperature and charges a coil spring means 12.

In Figure 4c, pressing of a third operation button (for example, a casting key) on the panel B after having covered a front rotary portion with a cover C tilts the furnace 6 at an angle of about one hundred and thirty-five degrees, and pours the melting material M into the crucible 41 and then restores the furnace 6 to its original position.

In Figure 4d, immediately after the furnace 6 returns outside the path of rotation of the rotary arm 2, the arm 2 is rotated to effect moulding. After the moulding is over, the arm stops at the initial position, and returns to the state in Figure 4a (a warning of the return may conveniently be made by a buzzer).

Accordingly, another batch of moulding can be started by the above operation.

In this manner, rotation of the arm 2 around the axis of the driving shaft 1 pours the melting material M from the crucible 41 into the mould 51 by centrifugal force.

At this time, a balance weight 3 is suitably shifted axially on the arm 2 to adjust balance relative to the weight of the members on the casting side (crucible 41, mould 51, etc.). Since the load applied to the arm 2 is only that of the members on the casting side and of the balance weight 3, a small amount of power is sufficient for the drive source for driving the members and balance weight, and the arm can attain the required speed of rotation as soon as rotation starts so that casting is performed efficiently. Also, since melting of the casting material is carried out by the furnace 6, a suitable combination of a temperature detecting means and control means for the furnace 6 readily provides proper temperature setting in accordance with each casting material.

As described above, the series of operations are carried out by button manipulation and a small amount of power, so that the device can be provided in compact form, and hence installation of the device at a dental office is highly useful from the point of space. Furthermore, preprogramming a melting temperature, or staying time of a melt in the furnace, in accordance with the kind of casting material makes it easy for the operator to conduct a series of operations from melting to casting by simple button operation.

Figures 5 and 6 show a split-type furnace 6 consisting of two units 60 and 60. The rotary arm 2 is disposed vertically in a halted state, and a core tube 61 having a fingerstall-shaped melting crucible 41 inserted thereinto and a mould 51 are fixed along the arm 2 by a crucible holding means 4 and a mould holding means 5, the core tube 61 being encircled by the split type units 60 and 60 to melt the casting material H in the crucible 41. Once the casting material H has been melted, the parts 60 of the furnace 6 are separated and the furnace is outside the path of rotation of the arm 2. The arm 2 is then rotated in the direction of arrow B to pour the material M in the crucible 41 into the mould 51 via its opening 511 by centrifugal force, wherein the melting material M is cast.Other parts of the apparatus are as described above and their series of operations can be likewise automatically preprogrammed by button operation of control apparatus disposed inside the control panel.

Use of a platinum alloy heating element for the furnace 6 is most preferred.

Referring now to Figures 7 to 10a and 10b, another embodiment of the invention further comprises means for driving the rotary arm 2 which includes a coil spring means 12 fitted around the driving shaft 1 and secured at one end to the shaft 1 and fixed at the other end to a driving wheel 11 fitted around the shaft 1 and rotatable relative to the . shaft 1; a drive source 13 connected to the driving wheel 11 for urging the coil spring means 12 in the direction of its winding; a clutch pin 7 resiliently mounted in parallel to the driving shaft 1 by a compression spring 71 so as to be freely movable in and out of the rotary arm 2; a clutch member 8 fixed to the shaft 1 in a position nearest to the arm 2, and which in the direction of rotation of the shaft 1 is adapted to come into contact with the periphery of the clutch pin 7 and to bring the shaft and the arm 2 into co-operative rotation, but which, in the opposite direction of rotation, is adapted to retreat the clutch pin 7 to slide over the top of the pin; a stopper 9 for stopping the rotary arm 2 in the normal position; a first sensor 14 for detecting the position of rotation of the shaft 1 and setting the zero level of the arm 2; and a second sensor 15 for detecting the rotated position of the driving wheel 11.

The clutch member 8 is formed with pawls 81 and 81 extending centrifugally symmetrically relative to the centre of rotation of the member 8. One surface of each pawl 81 is a flat cam face 811 so as to make co-operative rotation with the arm 2 when it comes in contact with the clutch pin 7, while the other surface is an inclined cam face 812. When the cam face 812 comes in contact with the clutch pin 7, the face 812 retreats the pin 7 and slides over the top of the pin to make it possible for the arm 2 to rotate relative to the clutch member 8 without being regulated by the member 8.

The clutch pin 7 is made of a columnar member and is resiliently mounted by a compression spring 71, in the hole 21 formed in the base of the arm 2 so that the pin 7 is freely moveable in and out of the hole 21. The top of the pin 7 is made hemi-spherical so as to impart smooth slide to the face 812 when the inclined cam face 812 comes into contact with the spherical top of the pin 7. In Figure 10, the numeral 731 designates a buffer member made of felt or rubber for softening an impulsive sound produced between the arm 2 and an E-ring 73 when the clutch pin 7 is reciprocated within the hole 21.

A governor plate 72 is rotatably journalled on a shaft 721 on the base of the arm 2 near the clutch pin 7 and when the arm 2 is rotated for casting at high speed, the governor plate 72 is designed to swing centrifugally by centrifugal force with the shaft 721 as a fulcrum and along the base of the arm 2. When the arm 2 is brought into high speed rotation by being relieved of control of the stopper 9 and the clutch pin 7 is retreated into the hole 21 by the action of the inclined cam face 812, the governor plate 72, as described above, is shifted by centrifugal force, engaged with the top of the clutch pin 7 tending to project again, and functions to prevent the pin 7 from projecting.

A coil spring means 12 is fitted around the shaft 1 in such a manner that one end of the spring means 12 is caught by the slit 121 formed at the base end of the shaft 1 and the other end of the spring 12 is fixed to the driving wheel 11. The coil spring means 12 is engaged in the slit 121 in such a manner as to slide axially of the shaft 1 and not to rotate circumferentially of the shaft 1. A sensor plate 16 is fixed to the base end of the shaft 1, the plate 16 being circumferentially formed with a multiplicity of through-holes 161 so as to make it possible to detect the rotated position of the shaft 1 in combination with the first sensor 14.Furthermore, a sensor ring 17 having a multiplicity of through-holes 171 circumferentially formed therein is fixed coaxially with the driving wheel Il to the side of the wheel 11 so as to make it possible to detect the rotated position of the driving wheel 11, i.e. the degree of charge of the coil spring 12, in combination with a second sensor 15.

The driving wheel 11 is shown in the form of a gear wheel in the embodiment illustrated and is connected through a pinion gear 131 to a brake-fitted electric motor used as a drive source 13, and the rotary arm 2 is set in normal horizontal position or the coil spring 12 is charged in the direction of its winding by the driving force of the motor 13. The connected relation between the driving wheel 11 and the drive source 13 is not limited to the embodiment illustrated but gearing such as a timing belt or chain may also be used.

In the embodiment illustrated, a push-pull plunger disposed in parallel to the driving shaft 1 is used as a stopper 9. In the plunger 9, an electromagnetically actuated mechanism is operated by an external signal (on-off signal controlled by the control apparatus in the control panel B ) and a rod piston 91 is expanded and contracted in parallel to the shaft 1 in such a manner that when the piston 9 is expanded, it extends inside the path of rotation of the rotary arm 2 to prevent the rotation of the arm. A stop rod 22 is fixed to that position of the arm 2 corresponding to the piston 91 and this rod 22 bears against the expanded piston 91 to prevent rotation of the arm 2 and hold the arm in normal horizontal position for a casting preparatory step.

A description will be given of how the device of the invention constructed in the manner described is operated.

1) Preparatory step before casting: When the rotary arm 2 is not in normal horizontal position (the position in which a molten casting material is poured into the casting crucible 41 by tilting the melting furnace 6) the push-pull plunger 9 is expanded to operate the electric motor 13 and to rotate the wheel gear 11 in the direction of arrow X By this time, the power of the gear 11 is transmitted to the driving shaft 1 by the coil spring-means 12 to rotate the shaft 1 in the same direction, whereupon the clutch member 8 is also rotated to bring the flat cam face 811 of the pawl 81 into contact with the clutch pin 7 to rotate the rotary arm 2. The rotary arm 2 turns round the axis of the shaft 1 to bring the stop rod 22 into abutment with the piston 91 of the push-pull plunger 9 to stop the rotation of the arm 2.At this time, a first sensor 14 detects the stop position and recognises zero level. In this state, rotation of the arm 2 in the above described direction of X is completely prevented but the arm is unstable in a direction opposite thereto and accordingly, the unstable arm 2 inconveniences the subsequent setting of the casting mould 51. Accordingly, when the wheel gear 11 is rotated in the direction of arrow X by further rotating the motor 13, the coil spring means 12 is slightly charged in the direction of its winding because the shaft 1 is made unable to rotate by the action of the clutch member 8 and clutch pin 7. The arn; 2 is, therefore, stabilized in both positive and negative directions by stored resilience of the spring means 12.

The position of the arm 2 which brings about this stabilized state of the arm 2 is detected and stored by a second sensor 15 and the wheel gear 11 is rotated by driving the motor 13 to charge the coil spring 12.

Difference between the detected position by the sensor 15 in this state and the zero level detected by the sensor 14 provides that amount of charge of coil spring means which corresponds to the force of rotation casting and which is properly determined depending on the properties or amount of the material to be cast. -Because the coil spring means 12 is subjected to fatigue over a period of time, mere mechanical charging of the spring 12 changes the amount of charge in the spring means, resulting in a different force of rotation of the rotary arm 2.In the invention, since zero level is detected by the first sensor 14 and the amount of charge of the spring means 12 is detected by the second sensor 15, programming of predetermined amount of spring charge makes it possible for the motor 13 to operate in accordance with the predetermined amount and always provides a constant amount of charge, namely, a predetermined force of rotation of the rotary arm 2.

2) Pouring of casting material and setting of a mould: The casting crucible 41 and the mould 51 are set in specified positions of the rotary arm 2, and the casting material premelted in the melting furnace 6 is poured into the crucible 41 by tilting the furnace 6. Since preparations for casting is completed by the above operation, the following steps.are immediately taken.

3) Casting: During the melting and transfer of material to the crucible 41, the stored resilience of the charged coil spring means 12 is prevented from rotating the arm 2 by the push-pull plunger 9. Contraction of piston 91 of the plunger 9 lifts this control and the arm 2 starts rotation rapidly in the direction of arrow X to reach a speed of about 1000 rpm so that the melting material in the crucible 41 is poured by centrifugal force into the mould 51 and cast.At this time the clutch member 8 is at rest and the clutch pin 7 comes in contact with the inclined cam face 812 opposite to the contacting face (i.e. flat cam face 811) of the clutch member 8 due to the direction of rotation of the arm 2 (see Figure 10a), but because the cam face 812 is inclined, the clutch pin 7 is retreated against the action of the compression spring 71 into the hole 21 by component of force at the time of its contact with the cam face 812, thereby enabling the arm 2 to continue rotating. When the governor plate 72 disposed near the clutch pin 7 swings centrifugally and the clutch pin 7 is retreated as described above, the governor plate 72 engages with the top of the clutch pin 7 and prevents projection of the pin (see Figure 10b). Thereafter rotation proceeds without the clutch member striking against the clutch pin 7.In this manner, casting is completed in 10 to 20 minutes, and at the same time, the speed of rotation of the arm 2 is reduced under the resistance of other structural members, whereupon centrifugal force applied to the governor plate 72 is also reduced and at a certain point of time the plate 72 is returned to its original position. Then, engagement of the governor plate 72 with the clutch pin 7 is released, with the result that the pin 7 again projects by the resilience of compression spring 71, recommences striking against the clutch member 8 intermittently.The governor plate 72 is preferably employed, because it functions to reduce resistance to the rotary arm 2 during casting as much as possible to thereby maintain speed of rotation, it prevents disagreeable sound of the clutch member 8 striking against the clutch pin 7 and also functions to reduce the speed of rotation of the arm 2 as soon as possible by contact resistance between the clutch pin 7 and the clutch member 8 at the end of casting. However it is to be understood that the structure described is not limited to the embodiment illustrated and various other modifications and amendments may be possible.

Since in this manner a series of casting operations are completed and the arm 2 stops at any given position, starting of operation from the step in Item 1 above repeats the same casting operation efficiently when the succeeding casting is continued.

The above series of operations can be programmed as one cycle of operation by control apparatus incorporated into the control panel B and can be automatically performed by manipulation of buttons. Needless to say, it is possible to set the rotary arm 2 in normal horizontal position and to program the position as a starting position.

It should be understood that the invention is not limited to the embodiments illustrated and that for example, the first and second sensors 14 and 15 or the stopper 9 and also the drive source 13 may be provided in other modifications.

As described above, firstly, in the centrifugal casting device of the invention, not only is the setting of a proper melting temperature greatly facilitated by the casting material melting means but also a series of operations from melting to casting can automatically be performed by programming the operations beforehand. In casting, the melting means is positioned outside the path of rotation of the rotary arm with the result that load applied to the rotary arm and to the drive source for the arm is small. Accordingly, the whole of the device can be made very compact in construction. In addition, the fact that the load applied to the rotary arm is small due to nonrotation of the melting means provides the advantage that the rotary arm quickly attains the required speed of rotation desirable for casting and renders it possible to make rapid and efficient centrifugal casting.

In addition to the above advantage the invention makes it possible to set the rotary arm automatically and rapidly in a normal position for making preparation for casting and to reduce the speed of rotation of the arm rapidly by the action of the clutch pin and clutch member after casting is over, thus reducing intervals between the succeeding operations to provide highly efficient casting operations even when it is desired to perform a multiplicity of casting operations. Since the invention includes sensors for detecting the respective positions of rotation of the driving shaft and driving wheel, the invention is able to invariably properly set the zero level of the rotary arm and obtain the predetermined amount of charge of the coil spring means with good reproducibility by a combination of signals from both sensors in spite of fatigue produced by the effect of time in the coil spring means.

In addition thereto, the invention not only makes it possible to automatically control a series of operations but also automatizing by manipulation of buttons all the operations except for setting of a mould.

Claims (5)

1. A centrifugal casting apparatus for dental articles, the apparatus comprising a rotary arm mounted for rotation on a shaft, one end of the arm having a balance weight which is adjustable in the axial direction of the arm and the other end of the arm having means for holding a casting crucible that is adapted to receive casting material and means for holding a mould in communication with the centrifugal side of the casting crucible, the apparatus also comprising a casting material melting means adapted to enclose the casting crucible to melt the casting material and which separates along a split for removal from the casting crucible out of the path of the rotary arm.

2. A centrifugal casting apparatus according to any preceding claim further comprising coil spring means located on the shaft, the coil spring means being rotationally fixed to the shaft at one end and at the other end attached to a driving wheel rotatably mounted on the shaft, the driving wheel being connected to a driving source which rotates the wheel to torsion the spring.

3. A centrifugal casting apparatus according to Claim 2 further comprising a clutch pin resiliently mounted on the rotary arm and capable of reciprocation relative to the arm and parallel with the shaft and a clutch member fixed to the shaft which in one direction of rotation of the shaft with respect to the arm the clutch member is adapted to engage the clutch pin to co-operatively rotate the shaft and the arm and in the opposite direction of rotation tending to release torsion from the spring the clutch member 2 is adapted to retract the clutch pin towards the rotary arm and enable the rotary arm to rotate freely about the shaft.

4. A centrifugal casting apparatus according to Claim 2 or Claim 6 or Claim 7 further comprising a governor plate pivotally mounted on the rotary arm and adapted to swing centrifugally to hold the clutch pin in a retracted position.

5. A centrifugal casting apparatus for dental articles substantially as hereinbefore described with reference to and as illustrated in Figures 5 to 10 of the accompanying drawings.