A METHOD AND APPARATUS FOR CORRECTING DEVIATIONS IN THE SHAPE OF HARDENED METAL OBJECTS

Abstract: A method for correction shape deviations in hard, hardened objects in which the objects are heated to a temperature which lies beneath the temperature at which the structure imparted to the objects during a preceding heating process (e.g. a temperature of ca 200 DEG C) is markedly affected, and then shape corrected. The invention also relates to an arrangement of apparatus, including an object conveyor and an object shape rectifying means, for carrying out the method.

Description: A method and apparatus for correcting deviations in the shape of hardened metal objects Technical field The present invention relates to a method for correcting deviations or errors in the shape of a hard, hardened metal object, particularly, but not exclusively, a ring- shaped object such as a ball race of a ball bearing assembly, by heating the object and subjecting said object to a shape correction in elevated temperature. The invention also relates to apparatus for correcting deviations or errors in shape of such objects. Background Prior Art U.S. Patent No. 4 531 391 (Enge en) describes an adaptive method and apparatus for correcting deviations in the desired shape of an object. The method comprises the steps of sensing the true or real shape of the object, determining the deviation from the desired shape and defining the dimensional values of said deviation, or correction length corresponding to the deviation, and subjecting the object in a straightening means to a shape correction corresponding to the correction length, or dimensional values. The apparatus by means of which the method is put into effect includes at least one measurement transmitter for sensing the true shape of an object and delivering a corresponding output signal, which is received by a data processing unit for comparison with stored data relating to the desired shape of the subject, therewith to determine the deviation in shape. The data processing unit delivers an output signal which defines a correc¬ tion length corresponding to the deviation and which steers a straightening device, which carries out cor- rective forming of the object determined by the cor¬ rection length. The method and the apparatus are adap¬ tive insomuch that the shape of the object is sensed after the shape correction has been completed and the relationship between correction length and shape devi- ation is modified in dependence on any remaining shape deviation. When shape deviations are corrected by means of methods and apparatus of this kind for instance, there is a serious risk that cracks will form in the object whose shape is being corrected, the shape or configuration of which object may well vary widely. The objects with which the present invention is concerned may be, for instance, ring shaped or rod shaped and symmetrical or asymmetrical. An example of asymmetric objects is found in the gear shifting fork of a vehicle transmission system or gear box. FR-A1-2 288 786 (Centre Technique des Industries Mecani- ques) describes a method of successively straightening an elongated object, such as a rod or shaft, in a straightening machine, in which the actual part of the rod, etc., to be straightened is heated inductively in conjunction with the straightening process. The object in this instance has not been hardened. ϋS-A-4 177 661 (Schwarzbach et al) and US-A-4 254 649 (Cervenka et al) disclose examples of earlier known methods in which a metal object is first heated and then worked. These methods, however, are concerned with the bending of an object and not with straightening of an object whose shape deviates from its intended shape. None of these known publications proposes a solution to the problem of avoiding the formation of cracks in a hard, hardened metal object when correcting deviations from the desired shape thereof. The present invention is based on the realization that the risk of crack formation can be eliminated, by intro¬ ducing an additional, simple procedure in the shape correcting method. Object of the invention Accordingly, one object of the invention is to provide an improved method of the aforesaid kind which will eliminate essentially the risk of crack formations in an object which is subjected to shape correction. Another object is to provide reliable apparatus for correcting deviations in shape of an object such as to avoid the risk of crack formations therein and such as to ensure that the shape corrected objects will be of uniform and high quality and that said shape corrected objects will have only small deviations in quality, or no deviations at all, even when the objects concerned form part of a large series of objects. Summary of the invention These and other objects are achieved by means of the inventive method and apparatus. The inventive method is mainly characterised by heating the whole of the object over a short period of time, to a temperature which lies beneath the temperature at which the structure and hardness imparted to the object in the preceding heat hardening treatment process would be affected, and by shape correcting the object in immediate association with said additional heating process, i.e. before said object has cooled to any appreciable extent. It has been found in practice, quite surprisingly, that when applying the inventive method the ability to cor¬ rect a deviation in shape of an object without the occurrence of cracks is increased tenfold to twentyfold. The reason for this is not fully understood, but is thought to be because the acceptance to shape changes of the material from which the object is made increases markedly, without the creation of appreciable stress concentration, when the material object is heated in immediate association with the step of correcting the shape of the object concerned. When practicing the inventive method, it is important that the object is heated under controlled conditions and that the temperature to which the object is heated is not so high as to have a marked influence on the structure or degree of hardness imparted to the object in a preceding heat treatment process. Thus, the inventive heating process must not result in tempering of the material or in a change in the crystal¬ line structure or hardness thereof. In other words, it is important that the object is heated to a predetermined temperature which is not excessively high, said temperature varying with the different material from which the objects are made. The inventive method enhances the acceptance of the material to shape changes, such as to enable the material to be plastically reshaped in the absence of crack formation. The method also results in relaxation of the material during the heating process, which causes a decrease rather than an increase in stress concentration, which is of great benefit. This unexpected effect is contributory to some of the advantages afforded by the invention. In practical applications of the inventive method, the object is shape changed in immediate connection with the heating process, i.e. before the object as a whole is able to cool to any appreciable extent. Practical tests have shovn that when treating hardened steel objects, the objects should be heated to a temper¬ ature between 150' and 270 β C, suitably between 180° and 240 β C, and preferably to a temperature of ca 200 °C. It is difficult at present to recommend an exact tem¬ perature. The temperature will thus vary in dependence on the properties of the material concerned, the extent to which it has been tempered, etc., and some of the advantages afforded by the invention can be obtained with com ensurably lower and higher heating temper¬ atures. The shape and dimensions of the object are also liable to influence the final result, and the actual length of time over which the object is heated may also be sig¬ nificant. Another important factor in the present context is that the temperatures should be held as constant as possible. This is particularly applicable when shape correcting large or smaller series of objects and when said object^. are to exhibit mutually similar properties, to the best extent possible. It has been found in practice that when the temperature varies too widely, the objects will deform plastically to mutually different extents. When excessively high temperatures are applied, the structure of the material will change, with a subsequent risk of tempering and of a change in hardness. The object(s) can be heated in various ways, e.g. induc¬ tively or by convection. Practical tests carried out with inductive heating have been encumbered with not-readily solve problems, inter alia due to the fact that it is necessary to adapt the induction coil to changes in size of the objects whose shape is to be corrected. Furthermore, it is necessary to adapt the rate at which the object is heated to the general configuration of the objects concerned. Experiments hitherto performed have shown that convec¬ tion heating, e.g. in an oven or furnace, is to be preferred. The invention also relates to apparatus for correcting shape deviations in hard, hardened metal objects, com¬ prising an object conveyor for moving objects along a path, and a shape correcting device. The apparatus is mainly characterised by a heating device which is placed in connection with or adjacent to the shape correcting device and which is operative in heating the objects to a predetermined temperature. The invention is illustrated in the following examples and also in the accompanying drawings. Example 1 Figure 1 illustrates an investigation carried out on thin, hardened ball races having a hardness of 63 HRc and shape change at elevated temperature, in accordance with the present invention, to avoid the formation of cracks. The curves indicate a dramatic increase in ductility, when the races were tested at high tempera¬ tures. Example 2 The curves in Figure 2 show the importance of heating the objects in a correct manner, if the surface hardness of the objects is to be maintained at a high level. A short heating period, e.g. from 1 to 2 minutes, at a temperature of 200°C is important if the desired end result is to be achieved. Stabilized races, or other ring-shaped objects, (i.e. tempered at 220-240°C) can be heated and shape corrected at temperatures of up to 250°C without decreasing the hardness of said objects. Example 3 Hot tensile testing of hardened samples made of ball bearing steel shows a very large increase in ductility for test temperatures above 180°C (Figure 3). Example 4 The yield strength or the proof stress (0.05% elonga¬ tion) decreases at high test temperatures. The distance between proof stress and ultimate strength (safety agasinst cracking) increases at higher test temperatures (Figure 4) . Calculations made from the hot tensile test curves show that Youngs modulus decreases with increased test tem¬ peratures over 150 β C. However, this will be taken care of in the computer program. Example 5 Tests have shown that induction heating affords the advantage of short heating times, in practice times of only a few seconds. Furthermore, induction heating simplifies handling of the objects, e.g. ball races whose shapes are to be corrected. The drawbacks experienced with inductive heating are primarily that different heating coils must be used for races or rings of different kinds and of different dimensions. Furthermore, it is necessary to rotate the rings, races, during the short heating period, in order to ensure that the rings are heated uniformly. Small inclinations to the plane of the coil can result in large temperature differences in the ring being heated. Finally, heating must correspond to the shape correcting operation concerned, e.g. the nature of the misshape. The diagram in Figure 5 illustrates conventional heating processes by convection, with the aid of infrared lamps used to accelerate the heating effect. In this case, there is no need to arrange different types of rings in particular mutual settings. The temperature difference between different rings is small and this method of heating is preferred to induction heating in the ma¬ jority of cases. Example 6 Figure 6 illustrates the importance of heating the rings to a predetermined temperature, and the importance of ensuring that only a small temperature difference will prevail between mutually different rings and batches of rings, if a successful shape correction result is to be achieve . Figure 5 illustrates a crack forming limit at room temperature and that an elevated temperature during the shape correcting process will afford additional security against crack formation. Example 7 When a ring is placed under compression (cf. Figure 7), the internal stresses will increase and when reaching a certain level will result in initial plastic deformation of the material at its surface zones. The structure of a low tempered ring which has been hardened throughout its cross-section will include martensite, carbides and (10-15%) unstable residual austenite, which is soft and ductile and has a low yield point. Plastic deformation will commence in the aus- tenite. In the case of stabilized rings with low aus¬ tenite retention, plastic deformation will occur in the martensite. Conventionally hardened rings will have a given level of residual compressive stresses in their surface zones. Plastic deformation will occur in the ring core (and/or in the surface zones). It is difficult, if not im¬ possible, to predict the magnitude of the stresses which remain after correcting the shape of the rings. Rings which have been hardened throughout must be shape corrected at high temperatures (180-250°C). Grinding tests have indicated a low residual stress value. It is quite probable that relaxation occurs (cf. Figure 8). No differences were found in between the non-roundness of shape-corrected rings measured at high ring-tempera¬ tures and the same rings measured after cooling to room temperature. Example 8 The rings will have a non-round shape (oval) subsequent to being hardened. The extent to which the rings depart from a truly round shape will depend on many different factors, such as design, the type of oven or furnace used, the stresses in the material, etc. The curve reproduced in Figure 9 is representative of the dif- ferences in non-roundness (oval) of a batch of car- burized rings. When the rings are to be ground, the grinding tolerance will be set for the ring which has the most pronounced oval shape (no black spots can be accepted) . This grinding tolerance can be reduced by 50% or more, when the rings are first subjected to a shape correcting operation in accordance with the invention. Example 9 Carburized rings which have a diameter in excess of 70 mm, such as the ring illustrated in Figure 10, will in many cases have a pronounced oval shape after being hardened. When the ring is ground, an excessive part of the car¬ burized (hard) surface will be ground off. The Figure shows a carburized ring of oval shape. These examples illustrate two important advantages which are obtained when correcting deviations from the desired shape of primarily ring-shaped objects, such as ball- bearing races. Firstly, the hardened layer need not penetrate the material as deeply as is otherwise the case, but can be a relatively shallow surface zone. Secondly, when the object is subsequently ground, to bring it to its final shape, only a very small amount of material need be removed.

Claims: Claims 1. A method for correcting deviations in the shape of a hardened metal object, particularly, but not exclus- ively, a ring-shaped object, such as a ball race of a ball bearing assembly, including heating the object and subjecting the object to a shape change at elevated temperature in means herefor, characterised by heating the object for a short period of time to a temperature which lies beneath the temperature at which the struc¬ ture and hardness imparted to the object in the pre¬ ceding hardening heat-treatment process is markedly in¬ fluenced; and by subjecting the object to the shape cor¬ rection in immediate connection with the additional heating process, i.e. before the object has cooled to any appreciable extent. 2. A method according to Claim 1 in which the object is made of hardened steel, characterised by heating said object to a temperature between 150 * and 270°C, suitably between 180° and 240"C and preferably ca 200°C. 3. A method according to Claim 2, characterised by a heating period of 1 to 2 minutes. 4. Apparatus for correcting shape deviations in hard, hardened objects, comprising a) a conveyor path for advancing the objects to be shape corrected, and b) an object shape correcting means, characterised by c) a heating device which is placed in connection with or adjacent to the object shape correcting means and which is operative to heat the object over a short period of time to a temperature which lies beneath the temperature at which the structure and hardness imparted to the object during a preceding heat treatment process when hardening said object is markedly influenced.

Inventors: GUDMUNDSON ARNE, GUDMUNDSON, ARNE
Applicants: UVA AB, AB UVA
Application year: 1989
Main classification: B21D3
IPC codes: B21D3, C21D7, C21D8, C21D9, F16C33