US2487091A - Cutting corundum rod - Google Patents

Description

Nov. 8, 1949 M. H. BARNES 2,487,091

CUTTING CORUNDUM ROD Filed July 15, 1944 I U) K i so .1 so :1 m 40 a, I 3 2O 0 IO 4o 6O a0 OPTIC ORIENTATION-DEGREES INVENTOR MALCOLM H. BARNES BYM ATTORNEY Patented Nov. 8, 1 949 UNITED STATES PATENT. OFFlE zaa'zmr oo'rrmc OOBUNDUM ROD Malcolm 1!. Bones, Kenmore, N. Y., assignor to The Linde Air Products Company. a corporation of Ohio Application my it, messes: No. 545,011

,15 cam. (01. 125-12) bycuttingandgrindingsoastoimprovethe' yield of usable products obtained. Primarily, my method is concerned with increasing the yield of usable products obtained when cutting thin slices from such corundum rods. In addition, my invention pertains to the marking of unicrystalline corundum rods in a novel manner so that a rod can be positioned properly for cutting.

It has been customary to cut jewel bearing blanks from unicrystalline synthetic corundum boules of large diameter, such as about threefourths inch, which may be white sapphire, blue sapphire, or ruby, depending on the presence or absence of coloring matter with the alumina of the boule. Such a boule is first split longitudinally along a central plane either by nicking the crown or by pinching the loot at the bottom of the boule, and the half-boules are then sliced transversely by a saw to form relatively large slices which are cut again and ground to produce bearing blanks of the desired size. In another procedure, thin rods are first cut fromthe halfboules and then sliced transversely. Both of these procedures are expensive, laborious, and.

wasteful of the large quantity of raw material which is removed by the saw in making the many necessary cuts.

Recently, there has been developed a method for growing unicrystalline synthetic corundum bodies in the form of elongated cylindrical rods of small diameter which can be controlled to approximate the desired diameters of difierent types of jewel bearing blanks. The rods as grown are clear, homogeneous, somewhat translucent in appearance due to the existence of minute crystal projections on their surfaces, and are quite stable when their diameters do not exceed three-sixteenths inch. Rods of greater diameter are usually unstable and tend to crack and split. In conventional boules the splitting plane contains the axis of growth so that rods machined from a halfboule obviously do not have the growth axis within their boundaries. In rods as grown by the recently developed method the growth axis is arranged substantially centrally within the rod. When stablesynthetic corundum rods were first made, their suitability for jewel bearings was immediately recognized, but upon at- 3 lowerthan had been expected. From some rods,

.which were selected at random from a group of apparently identical rods, excellent yields were obtained, whereas from other rods, very poor tempting to fabricate them into jewel bearing 'blanksby slicing them transversely of their longitudinal axes with a saw, or upon'attempting to grind them in a centerless grinder, it was found that the average yield of usable product was much yields were obtained.

After extensive research. into the subject of cutting and grinding synthetic unicrystalline corundum rods I found, surprisingly, that the yield varied greatly with variations in the optic orientation, and that the reason behind the unpredictable results obtained from apparently identical rods was the existence of. different optic orientations in the crystals. 'Good to excellent yields were obtained upon cutting or grinding rods optically oriented within ,a certain range, whereas very poor' results were obtained upon rods oriented outside of this range, as discussed in detail hereinafter. This feature of my invention originally was disclosed and claimed in my application Serial No. 503,240, filed September 21, 1943, of which the present application is a. continuation-in-part.

Further research has shown that good cutting results depend not only upon the optic orientation of the rod being cut, but also upon the position of the optic ax s with respect both to the saw and to the direction of saw motion.

Among the objects of my invention, therefore, are to provide a novel method for improving the yield of usable products obtained by mechanically working unicrystalline corundum rods, as by cutting and grinding, which involves selecting rods having preferred optic orientations; which involves so arranging such a rod with respect to a cutting or grinding tool that the optic axis of the rod assumes-a predetermined preferred position relative to the tool: and which involves so arranging such a rod with respect to a cutting or grinding tool, that the fo ces applied to such rod by the tool are such as in ive a good yield of usable product.

The above and other objects of the invention, and the best manner for attaining them, will become apparent from the following detailed description having reference to the annexed drawing. wherein:

Fig. l is an enlarged vertical midsectional view of a unicrystalline corundum rod illustrating the meaning of optic orientation;

Fig. 2 is a graph showing the relation between the optic orientation of corundum rods and the percentage yield of usable jewel bearing blanks cut from such rods;

Fig. 3 is an enlarged perspective view of a unicl'ystalline corundum rod marked in accordance 3 with my invention, and illustrating the meaning of various terms used in describing my method. Fig. 4 is an end elevational view showing a circular saw in position for slicing bearing blanks from a corundum rod by plunge cutting;

Fig. 5 is a diagrammatic view in perspective showing a circular saw in the process of plunge cutting a slice from a corundum rod, and indicating the character of the forces applied to the rod by the saw;

Fig. 6 is a diagram analyzing the forces exerted by the saw on the rod, and their relation to the optic axis during a cutting operation; and

Fig. 7 is an end view showing diagrammatically a circular saw in position for cutting a slice from a corundum rod in such a way that the relative translational movement is eifected along a nondiametrical chord of the saw.

Extensive research and experimentation on the mechanical working of synthetic unicrystalline corundumrods not exceeding a diameter of three-sixteenths inch, as by cutting or grinding them, have shown that there exists an important correlation between the crystallographic orientation of such rods and the yield of usable prodexerts the greatest influence on the results obtained in cutting or grinding, but the orientation of the a-axes also probably exerts an as yet undetermined effect on the results.

As shown in Fig. 1, the optic orientation of a synthetic unicrystalline corundum rod II is expressed in degrees as the angle 0 between the longitudinal or growth axis gg of the rod and the optic or c-axis 0-0 of the rod, the latter of which lies in the direction in which light can be passed through the crystal without being doubly refracted. In a single crystal of synthetic corundum, which crystallizes in the hexagonal system, there is only one such direction, and no light ray traveling through the crystal in a direction parallel to 0-0 will be doubly refracted.

When fabricating synthetic unicrystalline corundum rods, either clear or colored, by abrading material therefrom as in a sawing or grinding operation wherein pressure is applied transversely of the longitudinal axis of the rod, high yields of usable products such as bearing blanks are obtained from rods wherein the optic orientation is between and 80, and preferably between 40 and 80, whereas poor yields are obtained outside of these ranges because the rods often crack and split. When the optic orientation is less than 30, synthetic corundum rods even may split spontaneously before any attempt at fabrication has been made but, in any event, splitting or cracking almost invariably occurs when the rods are sawed or ground. Rods wherein the orientation is above 80 have many lateral notches, formed during rod growth, which frequently cause lateral cracking during grinding. Moreover notched rods require an excessive amount of grinding to eliminate the notches.

Since the optic orientation of a synthetic corundum rod changes gradually as the rod increases in length if the initial orientation is less than 90 when grown by the process described in my copending application Serial No. 503,240, it has thus far been impossible to obtain a rod which has absolutely uniform optic orientation throughout its length unless the initial orientation is 90. As a practical matter the optic orientation change in a rod of the short 3 feet commonly grown, and of the preferred optic orientation of'30" to 80, is practically negligible, so that if the orientation of the por- That is to say, within the broad optic orientation range of 30 to there may be a narrower preferred range within which the corundum rod of modified composition gives the best results.

The'critical nature of the effect of optic orientation on the fabrication of unicrystalline corundum rods is graphically illustrated in Fig. 2, in which the percentage yield of usable jewel blanks cut one at a time from synthetic white sapphire rods is plotted against the optic orientation in degrees. at some orientations were slightly above or below the curve of Fig. 2, which represents a fair average of the results obtained. As shown by the curve, a good yield of usable product was obtained with synthetic corundum rods optically oriented between 30 and 80, and a still better yield was obtained in the preferred range of 40 to 80.

The critical optic orientation range for cutting corundum rods holds true for all cutting procedures known to me, including those wherein the rod is rotated and wherein it is not rotated, as

well as for both single slicing technique and gang sawing technique wherein many slices are cut simultaneously. Furthermore, this range is critical for the cutting of circular slices and elliptical slices from rods.

To recapitulate, one phase of my novel method for increasing the yield of usable product obtained by mechanically working unicrystalline corundum rods comprises selecting for fabrication those rods wherein the angle between the c-axis 0-0 and the longitudinal axis g-g is between 30 and 80, and preferably between 40 and 80, and mechanically working the selected rods by applying thereto a tool such as a saw or grinder. Since the novel method of growing synthetic corundum rods of predetermined crystallographic orientation described in my above mentioned copending application makes it possible to.

confine within any desired range the optic orientation of all rods grown, it is apparent that by growing rods within the preferred ranges most of the raw material appears in the final product, and very little is wasted.

I have also found that the yield of usable prod-- not obtained by cutting slices from a synthetic corundum rod with any suitable type of cutting device, such as a rotating circular saw, can be greatly increased by mounting the rod in such a position with respect to the saw that, of the total force applied by the saw to the rod, the force component parallel to the c-axis of the rod is approximately the minimum possible at any position of the rod with respect to the cutting device (ideally zero). In general, it can be stated that the force component parallel to the c-axis should not exceed 40% of the total force applied by the saw summed vectorially. Relative translational movement between the rod and the cutting device toward one another is then eifected in a direction across the rod by moving the cutting delength of 2 or Of course, the actual results of cutting The rod I I and saw I9 of Fig. 4 are shown in perspective in Fig. 5 to illustrate the relative magnitude and direction of the forces applied by the saw to the rod in plunge cutting. There are three mutually perpendicular forces acting upon the rod as it is being cut, these being a downward force D due to the downward movement of the saw, a longitudinal force L due to the wobble of the saw, and a transverse force T due to the frictional drag of the saw against the rod. Although the saw I9 exerts longitudinal forces in two opposite directions on the two pieces of the rod I I on opposite sides of the cut, it is necessary only to consider the force toward the end from which the thin slice is being cut, because the force in the opposite direction is exerted on the well supported main body of the rod and is not effective to break or crack the thin slice being removed. The downward force D is roughly three times as great as the transverse force T although the mesh of diamond powder used may affect this ratio. The longitudinal force L does not bear a direct relation to D and T, but depends on the planeness of the saw, the tightness of the bearings, and similar factors affecting wobble.

In plunge cutting with gang saws the longitudinal force L is less significant because there are always two saw blades on opposite sides of each slice supporting the slice against breakage. This tends to meet the principal criterion of this invention bykeeping at a minimum the force parallel to the c-axls and directed toward the slice being removed.

Referring back to Figs. 3 and 4, the proper positioning of the rod II with respect to the saw I9 will be described. Actual experiments on plunge cutting have shown that an excellent yield of usable products in the form of thin jewel bearing blanks is obtained when the translational movement of the saw I9 relative to the rod II is in a direction between Oand 30 to the normal to the plane I3. It is apparent that this condition can also be expressed in these words: that the plane I3 makes an angle 17, measured in a specific direction as indicated in Fig. 3, of between 0 and 30 with the line EW when the latter is normal to the direction of relative movement.

The results of numerous plunge cutting tests on unicrystalline corundum rods having various orientations O, and positioned at different angles b, are tabulated below. Yields of circular usable jewel blanks are expressed as percentages of the total number of slices cut in each instance. Approximately the same number of test cuts were made at all the different combinations of optic orientation 0 and angle of inclination b.

Results of cutting tests Per Cent Per Cent

Per Cent in yields, it is apparent that both sets of test results support the 60 to range of optic orientation within which the best results are obtained.

v In off-center cutting wherein the relative movement between a rod II and a saw I9 is along a non-diametrical chord 3i of the saw, as shown in Fig. 7, the direction of progression 33 of the cut into the rod constantly changes as the depth of the cut increases and gradually approaches the normal 35 to the plane I3. However, the principles discussed previously herein apply, and experience has shown that the best cutting results can again be obtained by so positioning the rod Ii that the normal to the plane I3 makes an angle a with the constantly changing direction of progression 33 of the cut, which angle a remains within the 0 to 30 range as the cut progresses.

I have described the principles of my invention above as applied to specific cutting and grinding procedures by way of illustration only. It is apparent that changes in procedure can be made by those skilled in the art within the scope of the invention as defined in the appended claims.

I claim:

1. A method for increasing the yield of usable product obtained by mechanically working by abrasion unicrystalline synthetic corundum rods grown in rod form, which method comprises selecting for fabrication those rods wherein the angle between the c-axis and the longitudinal axis is between 30 and 80 degrees, and mechanically working the selected rods by abrading material therefrom with a tool such as a saw or grinder.

2. A method for increasing the yield of usable product obtained by mechanically working by abrasion unicrystalline synthetic corundum rods grown in rod form, which method comprises selecting for fabrication those rods wherein the angle between the c-axis and the longitudinal axis is between 40 and 80 degrees, and mechanically working the selected rods by 'abrading material therefrom with a tool such as a saw or grinder.

3. A method for increasing the yield of usable Iv product obtained by mechanically working by abrasion unicrystalline synthetic corundum rods grown in rod form and having a diameter not exceeding three-sixteenths inch, said rods having their growth axes substantially centrally therein,

said method comprising selecting for fabrication those rods wherein the angle between the c-axis and the longitudinal axis is between 30 and 80 degrees, and mechanically working the selected rods by abrading material therefrom with a toolv and effecting relative movement between said rod and said saw in a direction across said rod.

5. A method for cutting a slice from a unicrystalline corundum rod comprising mounting said rod in such a position with respect to a saw that of the total force applied by the saw, the force component parallel to the c-axis of the rod and directed toward the slice being removed does not exceed 40%; and efiecting relative movement between said rod and said saw in a direction across said rod.

6. A method for cutting slices from unicrystalline corundum rods comprising selecting such a rod having an angle between its c-axis and its longitudinal axis of between 30 and 80; mounting said rod in such a position with respect to a saw that of the total force applied by the saw, the force component parallel to such c-axis and directed toward the slice being removed is approximately the minimum possible at any position of said rod with respect to said saw; and effecting relative movement between said rod and said saw in a direction across said rod.

7. A method for increasing the yield of usable product obtained by cutting a unicrystalline corundum rod with a saw, which method comprises effecting relative translational movement between said saw and said rod while holding said rod in such a position that the normal to the plane defined by the c-axis and the longitudinal axis of said rod makes an angle between and 30 with the direction of progression of the cut through said rod.

8. A method for increasing the yield of usable product obtained by cutting with a saw a unicrystalline thin synthetic corundum rod grown as a rod having a diameter not exceeding three-sixteenths inch and having its growth axis substantially centrally therein, which method comprises eflecting relative translational movement between said saw and said rod while holding said rod in such a position that the normal to the plane defined by the optic axis and the longitudinal axis or said rod makes an angle between. 0 and 30 with the direction of progression of the cut through said rod.

9. A method for increasing the yield of usable product obtained by cutting a unicrystalline corundum rod with a saw, which method comprises selecting such a rod having an angle between its tion substantially normal to the plane defined by the c-axis and the longitudinal axis of said rod.

12. A method for increasing the yield of usable product obtained by plunge cutting a unicrystalline corundum rod with a saw, which method comprises selecting such a rod having an angle between its c-axis and its longitudinal axis or between 30 and 80, and efiecting relative translational movement between said saw and said rod in a directon such that the normal to the plane defined by the c-axis and the longitudinal axis of said rod makes an angle between 0 and 30 3 with such direction of movement.

c-axis and its longitudinal axis of between 30 and and eflecting relative translational movement between said saw and said rod while holdin said rod in such a position that the normal to the plane defined by the c-axis and the longitudinal axis oi said rod makes an angle between 0 and 30 with the direction or progression 01' the'cut through said rod.

10. A method for increasing the yield of usable product obtained by plunge cutting a unicrystalline corundum rod with a saw, which method comprises eflecting relative translational movement between said saw and said rod in a direction between 0 and 30 to the normal to the plane defined by the c-axis and'thc longitudinal axis oi said rod.

' 11. A method for increasing the yield 01' usable product obtained by plunge cutting a unicrystalline corundum rod with a saw, which method comprises efi'ecting relative translational movement between said saw and said rod in a direc- 13. A method for increasing the yield of usable product obtained by plunge cutting a unicrystalline corundum rod with a saw, which method comprises selecting such a rod having an angle between its c-axis and its longitudinal axis of between 30 and 80, and effecting relative translational movement between said saw and said rod in a direction substantially normal to the plane defined by the c-axis and the longitudinal axis oi said rod. Y

14. A method for increasing the yield of usable product obtained by cutting with a saw a unicrystalline corundum rod, which method comprises eflecting relative movement between said rod and said saw in a direction along anon-diametrical chord of said saw while holding said rod in such a position that the normal to the plane defined by the optic axis and the longitudinal axis of said rod makes at all times an angle be-. tween 0 and 30 with the constantly changing tive translational movement between said saw and said rod while holding said rod in such a position, as indicated by the position or said mark, that the normal to the plane defined by said mark and the longitudinal axis of said rod makes an angle between 0 and 30 with the direction oi progression of the cut through said rod.

MALCOLM H. BARNES.

I REFERENCES CITED The following references are of file of this patent:

v UNITED STATES PATENTS Number .Name Date 1,766,037 Dawson Tilliev 24, 1930- 1326519 Bailey All. v10,1943 2,355,877

record in the I Le Van Aug. 15, 1044

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