GB2070986A

GB2070986A - Apparatus for the shear-force processing of material placed in rotatable barrels

Info

Publication number: GB2070986A
Application number: GB8006465A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1980-02-26
Filing date: 1980-02-26
Publication date: 1981-09-16

Abstract

Bulk material or workpieces that require shear-force processing by being placed in a rotating barrel (26) are subjected to enhanced shear forces by rotating the barrel not only about its own axis (27) but also about a parallel axis (22) displaced from the barrel axis. <IMAGE>

Description

SPECIFICATION Apparatus for the shear-force processing of material placed in rotatable barrels This invention relates to apparatus that uses shear forces to process material placed in rotatable barrels. This processing may take the form of abrading workpieces placed in such barrels. In this instance the barrels may be provided with abrasive linings to their interiors such that material is abraded from the workpieces by a grinding action. Alternatively abrasive powder may be placed in the barrels together with the workpieces such that abrasion of the workpieces is promoted by a lapping action. Other forms of shear-force processing material placed in rotating barrels include the de-burring by a tumbling action of workpieces, the pulverising of material, and the mixing of material.Thus shear-force processing comprehends inter alia the method of processing commonly referred to as ball-milling.

In conventional shear-force processing of this sort the or each barrel is mounted for rotation about a horizontal axis that is fixed in space, and the shear forces developed depend upon the action of gravity. This limits the achievable cutting rate, pulverising, or mixing rate, as can readily be understood by considering for instance how a workpiece is abraded in a rotating barrel. When such a barrel rotates, any workpiece inside it is carried along with the moving surface until the limiting friction force existing between the workpiece and the barrel is reduced beneath the resolved component of the force of gravity acting on the workpiece acting in the tangential direction.If the rotation rate of the barrel is slowly increased from zero, the workpiece gets carried further up the wall of the barrel because the increased rotational speed produces an increased acceleration that presses the workpiece more firmly into contact with the barrel, and hence increases the maximum tangential frictional force. This gives an increased cutting rate. However a further increase of rotation rate will eventually lead to a reduction rate which diminishes to zero when the rotation is so fast that the workpiece is carried round with the barrel, being held so firmly against it that there is no relative movement between them.

According to the present invention there is provided apparatus for the shear-force processing of material which apparatus is provided with one or more barrels for receiving said material, the or each of which barrels is mounted for rotation about its own-axis on a frame mounted for rotation about an axis parallel with, but laterally displaced from, the axis of rotation of the or each barrel, and wherein the apparatus is provided with means for rotating the or each barrel about its own axis while the frame is being rotated about its axis.

The effect of the rotation of the frame is to provide an artificial enhanced gravity effect which enables greater shear forces to be experienced by material placed within the barrels.

The invention also provides a method of processing material by subjecting it to shear forces produced by placing said material in a barrel which is rotated about its axis while at the same time it is being rotated about an axis parallel with, but laterally displaced from, the barrel axis.

There follows a description of apparatus embodying the invention in a preferred form.

The description refers to the accompanying drawings in which: Figure 1 is a sectioned perspective view of A quartz resonator disc.

Figure 2 is a perspective view of the apparatus, and Figure 3 is a diagram showing the forces acting on a workpiece in a rotating barrel.

The apparatus now to be described with reference to Fig. 2 was designed for the particular purpose of shaping small discs of quartz for use as quartz crystal resonator elements. Initially such a disc has a cylinder form, being about 100 mm in diameter and 1 mm thick. Later the disc is attached at points in its periphery to a mount, and this inevitably produces a certain amount of unwanted mechanical damping. It is found that the amount of damping is reduced if the disc is thinned around its periphery before it is mounted. One method previously used for thinning the periphery of such discs has involved placing a number of them with a charge of abrasive in a barrel which is then rotated about its axis for a period of time. The rotation rate is adjusted so that there is only a limited amount of tumbling action.A certain amount of tumbling is required in order for both faces of the disc to be equally abraded, but too much tumbling is to be deprecated because it increases the risk of fracture damage and also reduces the cutting rate. The relative shapes of disc before and after a thinning operation are illustrated in Fig. 1 which shows the selectioned perspective view of a disc 1. Before the thinning operation, the disc has a uniform thickness D, as represented by the broken lines. The thinning operation produces spherical surfaces 2 and 3 having a radius of curvature approximately equal to that of the interior of the barrel in which they are abraded. As the thinning proceeds, the area of each spherical surface increases the thereby progressively reduces both the thickness d of the disc at its periphery, and diameter h of the residual plane surfaces 4 and 5.Generally the thinning operation is terminated before either dor h are reduced to zero. Nevertheless with conventional construction abrading barrel apparatus in which the axis of the rotating barrel is held fixed the time required to produce adequate thinning of the discs is typically measured in days rather than hours.

Referring now to Fig. 2, apparatus providing an enhanced cutting rate consists essentially of a rigid stand 20 supporting a substantially rectangular frame 21 by means of a shaft 22 mounted for rotation in plummer blocks 23. The stand and frame are both constructed of square section tubular steel.

Bearings 24 are secured to the ends of the frame to support a pair of steel barrels 26 mounted on shafts 27. The barrels, which are typically about 50 cm long, are made of steel, and each is substantially cylindrical being provided with flanged ends to which are secured steel end plates 28. A seal between each end plate and its barrel is provided by a synthetic rubber gasket (not shown).

The frame 21 is rotated by means of a drive applied to a pulley 30 secured to the frame drive shaft 22. The two barrels are rotated by means of drive belts 31 acting on pulleys 32 secured to the barrel shafts 27. The drive belts 31 receive their drive from a ganged pulley 33 mounted on bearings (not shown) on the frame drive shaft 22. The ganged pulley 33 has a flange 34 by which it is secured with a bolt 35 to a plate 36 attached to the stand 20. In this way the ganged pulley is held fixed so that rotation of the frame in either sense causes the two barrels to rotate in the opposite sense at a rate proportional to the frame rotational rate.

This particular apparatus was designed to operate at a frame speed of 300 revolutions per minute and to provide an acceration for the discs in the barrels corresponding to fifty times gravity. (The choice of a factor of 50 is an arbitrary choice made having regard to the desire to produce a significant improvement in cutting rate while subjecting neither the discs not the apparatus to excessive stresses.) With a rotational rate of Q radians per second the acceleration of a point at radius R from the axis of rotation is 2R. Therefore the radial length of the arms of the frame 21 needs to be somewhat less than 49.7 cm to take account of the fact that the discs will reside at a distance approximately equal to the sum of the frame radius R and the barrel radius r.

The calculation of an appropriate rotational rate c for the barrels can be made by reference to Fig. 3. knowing an appropriate rate of rotation in the case of an equivalent barrel rotating about a fixed axis. Fig. 3 shows the forces acting on a disc of mass m inside a barrel of radius r rotating at G, radians per second about a fixed axis when the coefficient of friction between the disc and the barrel is . The disc is carried round with the barrel through an angle 9 from the vertical where the resolved component of gravity in the tangential direction just balances the limiting friction force. In actual practice since the static coefficient of friction is somewhat greater than the kinetic coefficient of friction the discs tend to execute an oscillatory motion about a mean angle.At this point the radial force acting on the disc is m g cos 8+ mw2r (where g is the acceleration due to gravity).

The resolved component of gravity in the tangential direction is m g sin 8. while the limiting friction force is p (m g cos 8+ m 2r).

Therefore g sin B = (g cos 8 + 2r).+ O+r).

Now it is found that with a barrel of internal radius r of 5.2 cm rotating at 100 revolutions per minute the mean value of 8 about 45 and therefore it can be calculated that the mean value for the coefficient of friction ,u is about 0.55. Turning attention now to the case in which the barrel is mounted in a frame that provides an acceleration of 509, it can be seen that, reflecting the effects of gravity which are small compared with the effects produced by the rotation rate 52 of the frame, the new equation relating the resolved component of the acceleration force on the disc in the tangential direction and the binding friction force is given by 50g sin 8 = y (50g cos 8 + 2r).

Therefore, assuming the same value for the coefficient of friction it is seen that a rotational rate of about 700 revolutions per minute should provide an angle 8 in the region of 45'.

On the basis of the above calculation, the ration of the diameter of the ganged pulley 33 to that of each of the two barrel pulleys 32 was chosen to be 7 = 3. It is not suggested that this is an optimum ratio, but merely that this is the ratio chosen on the basis that it was likely to work reasonably well, and found in practice to do so.

Using this apparatus to abrade quartz crystal discs it has been found that with a frame rotation rate of 300 revolutions per minute a batch of between 300-3000 discs is abraded using a charge of about 10-20 grams per barrel of 280 grade silicon carbide abrasive grit in a matter of four hours to the extent that would have required a period of about 100 hours if the abrading were carried out with the barrel rotating at 100 revolutions per minute about an axis that is fixed in space.

The ganged pulley 33 has been made large enough to accommodate a third belt (not shown) so that, if desired, the pulley can be uncoupled from the stand 20 and driven from a separate motor (not shown). This allows the relative speeds of the frame and the barrels to be adjusted independently of each other.

Our reason for wishing to have independent speed controls for the two rotational rates is so that before the frame starts to rotate the barrels can be accelerated to a rotational rate which holds the charges inside them so hard against the barrel walls that no slippage, and more importantly no tumbling occurs, while the frame is being accelerated up to its working rotational rate. Once the frame has been brought up to speed the rotational rate of the barrels is reduced so that abrasion under the enhanced gravity can commence. At the end of the processing the sequence of operations may be reversed in order to prevent tumbling while the frame is being brought to rest.

While the foregoing specific description has relaxed exclusively to the shaping of quartz crystal oscillator elements it should be apparent that the invention finds much broader application and the above described apparatus may be used with little alteration for, for instance, shaping bi-convex optical lens elements. The particular radius of barrel exemplified would produce a silica lens having a power of approximately 10 diotres. For the production of spherical surface lenses the abrasion would of course be continued until the diameter of the residual plane surfaces 4 and 5 (Fig. 1) is reduced to zero.

Claims

1. Apparatus for the shear-force processing of material which apparatus is provided with one or more barrels for receiving said material, the or each of which barrels is mounted for rotation about its own axis on a frame mounted for rotation about an axis parallel with but laterally displaced from the axis of rotation of the or each barrel, and wherein the apparatus is provided with means for rotating the or each barrel about its own axis while the frame is being rotated about is axis.

2. Apparatus as claimed in claim 1 having a pair of barrels mounted at opposite ends of the frame.

3. Apparatus as claimed in claim 1 or 2 wherein the or each barrel is belt driven from a stationary pulley.

4. Apparatus as claimed in claim 1 or 2 wherein the or each barrel is belt driven from a pulley provided with a separate drive pulley provided with a separate drive from that applied to rotate the frame.

5. Apparatus substantially as hereinbefore described with reference to Figs. 2 and 3 for the shear-force processing of material.

6. Material that has been subjected to shear-force processing using apparatus as claimed in any preceding claim.

7. A quartz resonator disc made by the method claimed in any preceding claim.

7. A method of processing material by subjecting it to shear forces produced by placing said material in a barrel which is rotated about its axis while at the same time it is being rotated about an axis parallel with but laterally displaced from the barrel axis.

8. A method as claimed in claim 7 wherein while the rotational rate of the barrel about said axis is displaced from the barrel axis is being increased from zero up to working rate the barrel is rotated about the barrel axis at a sufficient speed to ensure substantially no relative movement between the barrel wall and the material placed in the barrel.

9. A method as claimed in claim 7 or 8 wherein, after the barrel has been rotated at working rate for a predetermined time about said axis displaced from the barrel axis, said rotation about this axis is reduced to zero while the barrel is rotated about the barrel axis at a sufficient speed to ensure substantially no relative movement between the barrel wall and the material placed in the barrel.

10. A method as claimed in claim 7, 8, or 9 wherein the material to be processed takes the form of a batch of workpieces which workpieces are loaded into the barrel together with a charge of abrasive grit.

11. A method as claimed in claim 7, 8 or 9 wherein the material to be processed takes the form of a batch of workpieces and said workpieces are abraded by an abrasive layer lining the wall of the barrel.

12. A method as claimed in claim 7 wherein the material to be processed is placed in the barrel together with a charge of balls.

13. A method of subjecting material to shear-force processing substantially as hereinbefore described with reference to Figs. 2 and 3 of the accompanying drawings.

14. A workpiece that has been subjected to shear-force processing using the apparatus claimed in any claim of claims 1 to 6 or the method claimed in claim 7, 8, or 9.

15. A workpiece as claimed in claim 14 that is a quartz crystal resonator element.

16. A workpiece as claimed in claim 14 that is an optical lens.

CLAIMS (8Aug1981)

1. A method of making quartz resonator elements including the step of abrading a plurality of flat discs of quartz placed in a barrel which is rotated about its axis while at the same time it is being rotated about an axis parallel with but laterally displaced from the barrel axis, wherein the resulting abrasion is such as to thin the peripheries of the discs.

2. A method as claimed in claim 1, wherein, while the rotational rate of the barrel about said axis discplaced from the barrel axis is being increased from zero up to working rate, the barrel is rotated about the barrel axis at a sufficient speed to ensure substantially no relative movement between the barrel wall and the discs placed in the barrel.

3. A method as claimed in claim 1 or 2, wherein, after the barrel has been rotated at working rate for a predetermined time about said axis displaced from the barrel axis, said rotation about this axis is reduced to zero while the barrel is rotated about the barrel axis at a sufficient speed to ensure substan tially no relative movement between the barrel wall and the discs placed in the barrel.

4. A method as claimed in claim 1, 2, or 3, wherein the discs are abraded by a charge of abrasive placed in the barrel with the discs.

5. A method as claimed in claim 1, 2, or 3, wherein the discs are abraded by an abrasive layer lining the wall of the barrel.

6. A method of making quartz resonator elements as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.