AU632688B2

AU632688B2 - Internal threads in ceramic articles

Info

Publication number: AU632688B2
Application number: AU76170/91A
Authority: AU
Inventors: Matthew Baird
Original assignee: Gainsborough Hardware Industries Ltd
Current assignee: Gainsborough Hardware Industries Ltd
Priority date: 1991-04-29
Filing date: 1991-04-29
Publication date: 1993-01-07
Anticipated expiration: 2011-04-29
Also published as: AU7617091A

Description

I

632688 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION (Original) FOR OFFICE USE Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published.

Priority: Related Art: Name of Applicant: Address of Applicant: Actual Inventor(s): Address for Service: Gainsborough Hardware Industries Limited 275 Canterbury Road CANTERBURY VIC 3126 Matthew BAIRD Nigel Paul BERRY DAVIES COLLISON, Patent Attorneys, 1 Little Collins Street, Melbourne, 3000.

Complete Specification for the invention entitled: "Internal Threads in Ceramic Articles" The following statenment is a full description of this invention, including the best method of performing it known to us: -1- 910429,GJNDAT.040,GainsborPro,1

I

-2- "INTERNAL THREADS IN CERAMIC ARTICLES" This invention relates to the formation of ceramic articles, or mouldings therefor, with internal threads, and has particular, though by no means exclusive, application to the fixing of porcelain door furniture. For convenience, the 10 invention is described herein primarily in connection with door knobs but has 'it r, general broa 5 application to ceramic articles, especially porcelain products such as insulators, spark plugs and fuse plugs.

Porcelain door knobs cannot readily be drilled to provide a facing thread o O 0 and are therefore usually fitted with an internally thr:. oded coaxial metal insert.

o 0 S 15 This insert is pressed with adhesive into a matching aperture in the knob after moulding and firing, and typically has a shape to also provide a physical keying against rotation in the knob. It has not to date been considered practical to O* o include a threaded former in the mould since the feed clay is a soft material, and since the initial moulding must be extracted and handled for firing.

It is an object of the invention to provide a process or apparatus for furming a. ceramic article, or a moulding therefor, with an internal thread.

To this end, the invention provides a process for forming a moulding for a ceramic article with an internal thread comprising:- S: moulding ceramic feed material under pressure about an externally a i t threaded former to produce a moulding with an internal thread moulded by and complementary to said former; and rotating said former and thereby withdrawing the former from the moulded fyed material by threadably disengaging the former from said complementary internal thread without stripping said internal thread.

The process preferably further includes treating the moulding to produce a ceramic article with an internal thread.

910429,G3NSPE.004,CERAMIC.SPE,2 -3- Preferably, the feed material is selected to have a shrinkage within a predictable range during said treatment, and the threaded former is selected in relation to its diameter and pitch whereby, after shrinkage of both diameter and pitch in the moulding, the internal thread in the ceramic article is a standard size.

The invention further affords apparatus for forming a moulding for a ceramic article with an internal thread, comprising:a mould for moulding ceramic feed material under pressure; an externally threaded former moveable by rotation between a withdrawn position and an operative position in which the former projects into the mould so that the latter may mould ceramic feed material about the former to "t produce a moulding with an internal thread moulded by and complementary to said former; and #*too means for rotating said former and thereby withdrawing the former from the moulded feed material by threadably disengaging the former from said 15 complementary internal thread without stripping said infernial thread.

The feed material may be a clay material, and the ceramic article produced may be a porcelain article. To achieve the preferred predictable shrinkage, the .clay feed material is preferably a blend of several different clays, for example 4 c. lays from different sources.

The mould may include a base plate with an aperturx through which said t former projects in its operative position. The former preferably co-axially projects from a body which defines a shoulder which is flush with the inside face of the base plate when the former is in its operative position. In this position in the o aforesaid aperture, the body and the aperture preferably co-operate to prevent or S' 25 minimise clay leakage through the aperture during said moulding under pressure.

The invention will be further de.ccribed by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a schematic axial cross-section through apparatus for forming a porcelain doorknob in accordance with an embodiment of the invention, shown during operation, after moulding but before withdrawal of the former; and Figure 2 is a cross-section of the line 2-2 in Figure 1.

910429,GJNSPE0o04,CERAMC.SPE,3 L t_-l II LI -4- The illustrated apparatus 10 includes a mould 12 having a base plate 14, respective die blocks 16, 17 moveable into and out of mutual face-to-face contact on plate 14, and a generally cylindrical punch 18. The apparatus further has an externally threaded former 20, and a drive 22 for translating the former The article to be formed is an integral porcelain doorknob of the overall axially symmetric shape of moulding 7, including a convex outer face 7a on a head 7b, a base 7c and an intervening neck 7d. This shape is determined at the inner end by base plate 14, about the side margins by die recesses 16a, 17a of blocks 16, 17 and at the outer end of head 7b by the concave tip of punch 18. Punch 18 is slidable in a firm fit within a matching cylindrical bore 13 of the same diameter as the head of moulding 7.

Threaded former 20 projects co-axially and integrally from a larger, 0 1 cylindrical body 32, also carrying an external thread 33 of pitch equal to the pitch of the thread 21 of the former. This former assembly is rotatable and thereby 15 axially translatable through an aperture 36 in base plate 14 by drive 22, which includes appropriate gearing an a member in threaded engagement with thread 33. The drive may include a suitable electric or air motor. In an alternative arrangement, the drive may rotate former 20 via suitable gearing and thread 33 may simply engage a fixed threaded guide. Aperture 36 is a close sliding fit with body 32.

In operation of the apparatus 10, the former is disposed so as to project above base plate 14, the shoulder 31 at the end of body 32 being flush with the inner face 14a of plate 14. The die blocks 16, 17 are brought together on plate 14 in a firm contact at parting faces 38, and open-topped cavity 40 defined by recesses 16a, 17a, filled from above by clay feed material. Punch 18 is then driven under pressure into the cavity 40 along bore 13 so as to mould the clay feed material under pressure in the cavity about former 20, thereby producing a moulding 7 with an internal thread 8 moulded by and complementary to the former. The fit of former assembly body 32 in aperture 36 and of punch 18 in cavity 40 is such as to prevent unacceptable leakage of clay material at these interfaces during the moulding step.

910429,GJNSPE.004,CERAMIC.SPE.4 1 9 1 The punch is then withdrawn, or at least application of its drive pressure stopped, and former 20 is rotated by drive 22 and thereby withdrawn from the moulding by being threadably disengaged from internal thread 8. The withdrawal is controlled so as not to strip internal thread 8: this requires an initial torque sufficient to overcome the surface adhesion between the clay and former threads, and a subsequent speed of rotation high enough to achieve withdrawal within an acceptable time, but not so high as to strip or otherwise significantly damage the internal thread.

The thread on former 20 may have been polished to facilitate movement of the former in the moulding. During withdrawal of former 20, die blocks 16, 17 are retained in closed engagement with the moulding to prevent its rotation.

It may be desirable to hold the punch in contact with the moulding for optimal ,q a 4 support.

a ag Once the former is withdrawn, die blocks 16, 17 are retracted away and a Oa 15 suitable device, eg a vacuum pick-up head, brought in to extract the moulding for subsequent treatment, including firing, to produce the porcelain doorknob with internal thread 7 intact. This treatment incurs shrinkage of the moulding. It is desirable that the final internal thread be a standard thread size. If the original :feed material is selected to give a predictable shrinkage, the former can be sized It I t in relation to diameter and pitch so that the internal thread 7, after the expected shrinkage, will be a specific standard thread size. The selection may be done by using a specific blend of different clays, eg of between 5 and 10 clays from different sources, so that variation in any one clay will not substantially effect the properties of the clay blend. This is a known practice in the industrial manufacture of porcelain articles. An exemplary such blend familiar to the applicant has a predictable shrinkage through the process of 14%.

910429,GJNSPEG4,CERAMICSPE,5

Claims

1. A process for forming a moulding for i ceramic article with an internal thread, comprising:- moulding ceramic feed material under pressure about an externally threaded former to produce a moulding with an internal thread moulded by and complementary to said former; rotating said former and thereby withdrawing the former from the moulded feed material by threadably disengaging the former from said complementary internal thread without stripping said internal thread.

2. A process according to claim 1 further including treating the moulding to produce a ceramic article with an internal thread. 1' 15

3. A process according to claim 2 wherein the feed material is selected to have a shrinkage within a predictable range during said treatment, and the threaded former is selected in relation to its diameter and pitch whereby, after shrinkage of both diameter and pitch in the moulding, the internal thread in the t ceramic article is a standard size. ti%

4. A process according to claim 2 or 3 wherein said ceramic feed material is a clay material, and said treatment includes firing said moulding. Se

5. A process according to claim 4 wherein said clay material and said 6J treatment are such that the ceramic article produced is a porcelain article.

6. A process according to claims 3, 4 or 5 wherein said clay feed material is a blend of several different clays, for example clays from different sources.

7. A process according to any preceding clain 'herein said moulding is effected in a mould which includes a base plate with an aperture through which 910429,OJNSPE.004,CERA.MIC.SPE,6 -7- B said former projects.

8. Apparatus for forming a moulding for a ceramic article with an internal thread comprising:- a mould for moulding ceramic feed material under pressure; an externally threaded former moveable by rotation between a withdrawn position and an operative position in which the former projects into the mould so that the latter may mould ceramic feed material about the former to produce a moulding with an internal thread moulded by and complementary to said former; and means for rotating said former and thereby withdrawing the former from 9 '094the moulded feed material by threadably disengaging the former from said complementary internal thread without stripping said internal thread. 6 a

9. Apparatus according to claim 8 wherein said mould includes a base plate with an aperture through which said former projects in its operative position.

44410. Apparatus according to claim 8 or 9 wherein said former co-axially projects from a body which defines a shoulder which is flush with the inside face of the base plate when the former is in its operative position, in which position in the 4 4 aaforesaid aperture, the body and the aperture preferably co-operate to prevent or minimise clay leakage through the aperture during said moulding under pressure. 11. Apparatus according to any one of claims 8 to 10 wherein said mould comprises a plurality of die blocks moveable apart to release said moulding, 12. A process or apparatus for forming a moulding for a ceramic article with an internal thread, substantially as hereinbefore described with reference to the accompanying drawings. 91O429,GJN.' PE.004,cERAMIC.SPE,7 13. The steigs or featur1es dise'zhri o n obnaintee I Dated this 29th day of April 1991 GAINSBOROUGHIHARDWARE INDUSTRI] S LIMTED By its Patent Attorneys

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