US2726433A - Method for shaping ceramic articles - Google Patents

Description

Dec. 13, 1955 M. SKUNDA 2,725,433

METHOD FOR SHAPING CERAMIC ARTICLES Filed Sept. 18, 1951 2 Sheets-Sheet l Jnventor (Ittornegs Dec. 13, 1955 M. SKUNDA METHOD FOR SHAPING CERAMIC ARTICLES 2 Sheets-Sheet 2 Filed Sept. 18 1951 aw M 9 (Ittornegs United States Patent METHOD FOR SHAPING CERAMIC ARTICLES Michael Skunda, Davison, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application September 18, 1951, Serial No. 247,111

6 Claims. (Cl. -156) The present invention relates to ceramic articles and particularly to a method for shaping such ceramic articles as spark plug insulators by altering the contour of aceramic blank having a composition including some softenable material such as a thermoplastic resin.

To provide for better insulating properties, it has been found desirable to provide annular rings and grooves on the surfaces of spark plug insulators, thus allowing more surface distance between the spark plug lead connection and the grounded spark plug shell. To accomplish an insulator with this increased complexity of contour, various methods and apparatus for manufacture have been suggested. One of these methods is to mold the insulator from a composition of ceramic material and binder as disclosed in U. S. Patent 2,446,872. Another is to mold the insulator to finished contour at only some portions and then to provide additional material to the points where annular surface rings are desired, and remold to a finished product. Both of these methods, however, are disadvantageous in that in either case the molding process results in flash at the mold parting line. Such a flash line is particularly undesirable because in some cases it prevents the spark plug gaskets from sealing properly. These molding processes are also quite expensive, requiring precision tooling and yielding low product output. v

Another method for manufacturing spark plug insulators of complex contour is to mold a blank of a composition containing a ceramic material and resin binder, mount the blank for rotation along its longitudinal axis, heat the material to a plastic condition and then rotatably engage the surface of the blank with a series of rolls to cause annular rings and grooves at the desired places. This method is disadvantageous in that it can be used only to produce less complicated shapes and it is not readily adaptable to producing rings of different cross-sectional areas on the same insulator.

The insulator could also be made from a blank of suitable material by a grinding operation.

complicated shapes. There is also considerable waste of insulator material, thus making the process costly.

Another possible method is to mount an insulator blank for rotation on its longitudinal axis and engage the surface of the rotated blank with a surface cutting tool to trim away circumferential portions until an insulator of the desired contour is obtained. Due, however, to the inherent abrasiveness of the material, tool life is extremely short, thus making the process costly. This cold turning method is also disadvantageous in that frictional heat builds up in the insulator material and causes deformation.

It is therefore one of the objects of this invention to eliminate the difliculties of the above mentioned methods by providing an efiicient and low cost method for producing spark plug insulators. I

Another object of this invention is to provide an eflicient method for producing ceramic articles having complex contours. I

However, this; method, too, is adaptable only to the manufacture of'less Another object of this invention is to provide an eiiicient method for altering the contour of previously formed ceramic articles.

Another object of this invention is to provide an efiicient process for removing surface portions of ceramic articles.

Other objects and advantages will be seen more clearly from the following description of my invention and more particularly from the appended claims.

In accordance with my invention, a blank made of a ceramic material and containing a thermoplastic binder is rotated on its axis while its surface is engaged by any suitable cutting tool which is heated to above the softening temperature of the binder.

In the drawings:

Fig. 1 shows a side view of a ceramic material blank mounted for axial rotation.

Fig. 2 shows a top view of a surface skiving tool provided with a heating element and suitably mounted in cutting engagement with the rotated blank. v

Figs. 3 and 4 show top views of progressive steps in the contouring of the article as successive portions of the skiving tool engage successive portions along the length of the blank.

Fig. 5 shows a side view of a rotatably mounted ceramic spark plug insulator after the skiving operation has been completed.

Fig. 6 is a view of the apparatus along the lines 66 of Fig. 5 and shows a cross-sectional view of the mounted insulator, the skiving tool and its heating and mounting means.

Fig. 7 shows a modification of the skiving tool and skiving tool heating means.

In Fig. 1, a blank 1, made of a suitable thermoplastic bound ceramic material is shown mounted for rotation. The particular blank shown has a perfectly cylindrical shape; however, any suitable shaped blank will suffice, it being necessary only that suflicient material be provided to allow for the cross-sectional area requiredof thefinished article. The blank can be formed in any suitable manner known in the art, i. e. molding, the exact process or apparatus used in this blank forming step being no. The composition used to.

part of the present invention. make the blank can be any suitable ceramic material together with any of a variety of thermoplastic binders well known in the art. Compositions particularly adapted to the manufacture of spark plug insulators by my process and apparatus are described in U. S. Patent 2,446,872. The blank 1 is mounted for rotation on rotatable shafts 2 which are in frictional engagement at 3 with the blank 1 through grommets 4, these grommets being made of any suitable frictional material such as rubber. In the particular modification shown, the blank is provided With a longitudinal bore 5 into which the ends of the shafts'2 can be appropriately designed to fit; however, such a structure is not, of course, essential. The shafts 2 are in driving engagement with some driving means located in the casing 6.

and not shown, it being understood that either one or both of the shafts 2 can be driven by suitable driving The entire assembly is supported on base memmeans.

ber 7'.

In Fig. 2 the ceramic blank 1 is shownsupported by the shafts 2 and frictionally engaged for rotation by grommets 4. Skiving tool 8 provided with a cylindrical shank portion 9 is held adjustably positioned in clamp 10 by the screw 11, the screw 11 allowing for angular. adjustment of the tool 8. A heating element 12 sur-Q rounds the shank portion 9 of the tool 8, the heat from this heating element being conducted through the shank 9 and the tool 8 to the tool edge 13. The-heating element 12 is adapted to deliver tothe tooledge 13 sufiicient heat to maintain it above the softening-"tern perature of the particular binder used in blank 1. The edge of the tool 13 is contoured so that it will give the desired shape to the ceramic article. In the preferred embodiment of the process and apparatus, as shown by the accompanying drawings, the tool 8 is set at an angle to the axis of rotation of the blank so that only one portion of the tool edge 13 engages the surface of the blank 1 at any one time. Tool edge 13 can be of any suitable material, tungsten carbide for example, proving very satisfactory.

The edge 13 of the tool 8 engages the surface of the rotating blank 1 and shaves or chips away portions of the surface of the blank according to the contour with which the tool edge is provided. In the preferred embodiment shown on Figs. 2, 3 and 4, as the heated skiving tool 8 is moved across the blank 1, continuously successive portions of the tool edge 13 move into cutting engagement with continuously successive circumferential portions along the length of the blank 1.

Fig. 2 shows the process at a stage where portion 14 of the tool edge 13 is in cutting engagement with the blank 1. Fig. 3 shows a later stage in the process, portion 15 of the tool edge 13 at this time being in cutting engagement with the surface of the blank 1 as the tool is caused to move across the blank. Fig. 4 shows a still later stage in the process, at this time portion 16 of the tool edge 13 being in cutting engagement with the blank 1. After portion 16 of the tool edge 13 has completed its skiving of the corresponding portion 17 of the ceramic article, the contouring operation is comp ete.

In accordance with my invention, therefore, a ceramic article having a complex contour is simply and economically produced. An example of an article such as is produced by my invention is best shown in Fig. 5 wherein a spark plug insulator 18 provided by the contouring operation with circumferential rings and grooves 19, and tapered portions 20 and 21, is shown still mounted in the apparatus.

While in the preferred embodiment of my invention, as shown in the accompanying drawings and as described above, the contouring operation is performed with a skiving tool, it is to be understood that other types of forming tools may of course be used. The tool can be positioned to engage the blank either radially or tangentially and it may be set so that the cutting edge is parallel with the work centerline rather than at an angle. When, for example, a very small circumferential area of the blank is to be contoured, it is sometimes desirable to use this latter modification wherein the tool edge is positioned parallel to the work centerline.

After the contouring operation is complete the article can be fired to remove the binder and sinter the ceramic material into a coherent mass.

In Figs. 2 to 6 the tool heating means shown consists of an electrical heating element 12 surrounding shank 9 and in heat conducting relationship with the tool edge through shank 9 and the body of the tool 8. This heating apparatus is best indicated in Fig. 6 which shows a cross-sectional view of the apparatus. Fig. 7 shows a modified tool and tool heating system wherein the heating element 22, having electric leads 23, is located within a recess 24 provided on the tool 8' adjacent the cutting edge 13. Fig. 7 also shows a thermocouple 25 located near the tool edge for close temperature control. Such a thermocouple can, of course, also be used with the apparatus shown in Figs. 2 to 6 for closer temperature control. Controlled heating of the tool edge 13 can be accomplished by either heating apparatus. However, I have found that for closer control of the tool temperature, the apparatus as shown in Fig. 7, wherein the heating element is closer to the cutting edge, is desirable.

As has been previously stated, the temperature of the tool edge 13 must be maintained above the softening temperature of the binder used in making the blank.

Since one of the great advantages to my process is that the chips from the blank can be used again to make other blanks and thus materially reduce costs of production, in order to use my process most advantageously it is desirable that the tool temperature not exceed the temperature at which the binder begins to char. When using a binder such as is described in U. S. Patent 2,446,872 and which consists generally of a mixture of ethyl cellulose, shellac and a suitable plasticizer such as N-butyl stearate, I have found that a tool temperature of between 350 F. and 650 F. is suitable for my process. When using such a binder and a tool temperature of 350 F., the cutting action is smooth and the chip is glossy, pliable and continuous. At 600 F. the chip is continuous and so plastic that it gives the appearance of a strip being extruded from a nozzle. Higher tool temperatures, i. e. 700 F. and above, can often be advantageously employed depending on the charring characteristics of the binder used. The cutting time required will of course depend on the speed with which the tool is moved into or across the work. The most advantageous tool speed which can be used depends on the type of binder and the tool temperature employed and the rate of work rotation.

It is thought that the hot tool plasticizes the binder and greatly reduces abrasiveness. The reduction of abrasiveness may be due to the binder acting as a lubricant for the cutting tool.

Whatever may be the cause, I have found that with my process frictional heat is minimized, thus dispensing with the problem of blank deformation which is due to a general softening of the binder throughout the blank. Also, a long tool life is obtained, thus resulting in low production costs. Ceramic articles with a complex contour, such as spark plug insulators with circumferential rings and grooves, can be efficiently produced with great savings in time and cost.

While my invention has been described in conjunction with certain specific examples, it is to be understood that the scope of my invention is not to be limited thereby except as defined in the appended claims.

I claim:

1. A process for manufacturing spark plug insulators comprising the steps of forming a mixture of ceramic material and heat-softening resin binder, forming an elongated blank from said mixture, rotating said blank on its longitudinal axis, heating the surface of the rotated blank to above the softening temperature of the resin binder, skiving portions of said heated surface to shape said blank into an insulator of desired contour, and firing said insulator to remove the binder and sinter said insulator into a coherent body.

2. In a process for manufacturing a spark plug insulator the steps of forming a mixture of ceramic ma terial and a thermoplastic binder, forming said mixture into an elongated blank, rotating said blank along its longitudinal axis, heating a cutting tool to above the softening temperature of said thermoplastic binder, and engaging the surface of the rotated blank with said cutting tool to remove portions of said surface.

3. In a process for manufacturing ceramic articles, the step of forming a mixture of ceramic and thermoplastic materials, shaping said mixture into a blank, heating an edged tool to above the softening temperature of the thermoplastic material and engaging the surface of said blank with said edged tool to remove portions of said surface.

4. A process for manufacturing ceramic articles including the steps of forming a mixture of ceramic and thermoplastic materials, shaping said mixture into a blank, heating the surface of said blank to above the softening temperature of the thermoplastic material and removing portions of said heated surface to shape said blank into an article of desired contour.

5. In a process for manufacturing ceramic articles the 5 6 steps of forming a mixture of ceramic and thermoplastic References Cited in the file of this patent materials, shapingsaid mixture into a blank, rotating said UNITED STATES PATENTS blank on its longitudinal axis, heatlng a cutting tool to above the softening temperature of the thermoplastic ma- 168'801 Stevens 1875 terial, and subjecting continuously successive portions of 716343 Locke 1902 the surface of said rotated blank to continuously suc- 5 914,816 Dqnton 1909 cessive portions of said cutting tool while maintaining said 9 9 9 Splller 1911 cutting tool heated to above softening temperature of 1240575 Jefiery Sept 1917 the thermoplastic materiaL Mccool p 51 6. In a process for manufacturing ceramic articles, the 1465157 Buckley 1923 step of forming a mixture of ceramic and thermoplastic 1O 1491079 chfa'mplon 1924 materials, shaping said mixture into a blank, heating an 1755308 Smith 1930 edged tool to above the softening temperature of the 2464'718 Potter et 1949 thermoplastic material, engaging the surface of said blank 2561050 Chamm et July 1951 with said edged tool to remove portions of said surface 2580092 Herbert et 1951 and thereby shape said blank into a body of desired 15 2,632,223 Jordan 1953 contour and thereafter firing said body to remove said FOREIGN PATENTS thermoplastic material and sinter said ceramic material. 278 181 Germany Sept. 22, 1914

Claims (1)

1. A PROCESS FOR MANUFACTURING SPARK PLUG INSULATORS COMPRISING THE STEPS OF FORMING A MIXTURE OF CERAMIC MATERIAL AND HEAT-SOFTENING RESIN BINDER, FORMING AN ELONGATED BLANK FROM SAID MIXTURE, ROTATING SAID BLANK ON ITS LONGITUDINAL AXIS, HEATING THE SURFACE OF THE ROTATED BLANK TO ABOVE THE SOFTENING TEMPERATURE OF THE RESIN BINDER, SKIVING PORTIONS OF SAID HEATED SURFACE TO SHAPE SAID BLANK INTO AN INSULATOR OF DESIRED CONTOUR, AND FIRING SAID INSULATOR TO REMOVE THE BINDER AND SINTER SAID INSULATOR INTO A COHERENT BODY.

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