Patent Application
20110172783
This application claims the priority of Swiss patent application no. 141/08, of which the entire contents of disclosure are included herewith by reference.
The present invention relates to the manufacture of precision instruments such as tweezers.
Such instruments have two legs, which are connected to one another at the one end, by spot welding for example. The other ends of the two legs are both free and are located with a small distance between them, and may resiliently be pressed together. In the case of a tweezers, for example, the free ends of the legs may be shaped into fine tips, which must fit together perfectly in order for the tweezers to perform its function properly. Since the two legs are joined together at the other end thereof only after their free tips have been produced, the tweezers must usually be aftertreated by hand to ensure the fit of the tips.
On the other hand, as is known, continuous metal parts such as rails, profiles or wires are produced using two-cylinder rolling mills. These exert a symmetrical squeezing force on the metal. The crystal lattice axes of the newly layered structure are oriented in a star formation extending from the middle towards both sides in the direction of rolling; a “roll ridge” is formed in front of the rolls. As a result, the rolled material is spread both widthwise and lengthwise. It is not possible to prevent the rolling material from spreading widthwise by providing lateral delimiters or openings in the rolling cylinders, because the laterally oriented forces in the spreading material would create an explosive effect with burr formation. This would lead to a breaking of the rolling cylinders or would cause the entire machine to break or seize up. In order to prevent the rolling material from spreading laterally in two-cylinder rolling mills, a traction device exerting a pull large enough to ensure that the rolling material can only undergo insignificant lateral spread must be provided behind the rolls.
In paragraph 5 of EP 1 275 472 A it is mentioned that complex contours can be created by rolling with single-roll mechanical tools, wherein the rolling process takes place at an effective point between the roll and the tool surface.
WO 01/13756 A discloses a tweezers made of light metal which consists of a single part and has no welded spot. It is produced without rolling by separating an extruded light metal profile into a plurality of such tweezers.
The first objective of the present invention is to provide an intermediate product in the form of a metal strip having new material properties, which is suitable for manufacturing instruments of the kind described in the introduction. A further objective is to provide a process for producing such a metal strip and the associated device, wherein this process in particular produces in the metal strip the resilient parts of the legs of instruments such as those described in the introduction. A further objective of the invention is to provide a process for producing other metal objects that contain a resilient region or have complex profile shapes.
The first objective is solved according to the invention by a single-piece metal strip having no weld seams and made of a polycrystalline metal, comprising at least one region in which the crystallites have a comparatively more pronounced anisotropic orientation, and at least one region in which the crystallites have a comparatively less pronounced anisotropic orientation; and wherein Θ-2Θ X-ray diffractograms measured at two arbitrary points of the strip using CuKα radiation do not show statistically significant differences with respect to the position and shape of the respectively corresponding pikes. The comparatively more pronounced anisotropic orientation of the crystallites in the one region is more pronounced than the comparatively less pronounced anisotropic orientation of the crystallites in the other region.
This region of comparatively more pronounced anisotropic crystallite orientation which is resilient is obtainable by a rolling process for deforming an initial shaped body of metal, wherein the rolling process is performed between a roll having an axis of rotation and a rolling surface on the one hand, and a support having a support surface on the other hand; characterized in that the angular velocity ω of the roll is controlled in such manner that
applies for at least one point of the roll surface that contacts the initial shaped body in rolling manner, and in which formula ν is the rolling velocity and R is the distance between the axis of rotation and the described point on the roll surface measured perpendicularly to the axis of rotation of the roll.
Preferred embodiments of the metal strip according to the invention, of the production process and of the associated device are described in the dependent claims. The rolling device for performing the process according to the invention as well as tweezers obtainable using the process according to the invention, are likewise objects of the invention.
“Rolling velocity ν” in the above formula (1) is the speed at which an imaginary point on the axis of rotation of the roll, lying equidistantly between the points where the axis of rotation intersects the two frontal faces of the roll, moves relative to the initial shaped body before it enters the rolling zone between the roll and the support.
The process according to the invention is carried out in such manner that at least one point on the roll surface contacting the initial shaped body in rolling manner the angular velocity ω of the roll is less than ν/R, wherein ν and R have the meanings defined previously. This rolling point may be the point or points whose distance R from the axis of rotation is or are minimal compared to the distances R from the other rolling points on the rolling surface. It is preferred that for each rolling point of the rolling surface ω is smaller than ν/R, wherein ν and R have the meanings defined previously. These features differ from a conventional rolling process with two counter-rotating rolls, wherein for each rolling point of the roll surfaces of both rolls the angular velocity ω of the roll in question is greater than the quotient ν/R, wherein ν and R have the meanings defined previously.
The angular velocity ω is the angle (measured in radians) that the roll rotates per unit of time. The unit of the angular velocity ω is thus s−1.
The angular velocity ω preferably has a value of 30 to 95%, more preferably of 50 to 80% of the quotient ν/R.
The reduced angular velocity ω in the rolling process according to the invention may be achieved as follows: The roll is pressed against the initial shaped body to be rolled and lying on the support with a normal force F sufficient to deform the initial shaped body. The roll is then pushed or pulled in this pressed state over the initial shaped body at a desired rolling speed ν. To ensure that the angular velocity ω remains less than ν/R as provided for in the invention, the rotation of the roll is simultaneously hindered or braked. As a result of the braked rotation of the roll in the process according to the invention the initial shaped body is not simply rolled flat, but a compressed bulge of material is formed in front of the roll, which is pushed ahead of the roll like a pressure wave.
This hindering or braking of the roll's rotation may be effected via a suitable braking device, which acts on the roll during the rolling process. On the other hand said material bulge also has a braking effect on the angular velocity ω of the roll. If the parameters of the rolling process, such as contact pressure, rolling speed and so on are selected appropriately, it may be possible in favourable situations to dispense with the explicit braking of the roll as soon as this material bulge is formed. If a brake is used, then it may be any known type of brake, for example a friction brake, such as a drum brake, a disc or wedge brake, an eddy current brake, or a brake that is operated or actuated hydraulically (viscosity brake). The angular velocity of the roll may also be controlled by providing an electric or hydraulic motor which by itself may drive the roll but whose speed is selected such as to enable the said control of the angular velocity of the roll. The braking force may be set or controlled with a suitable braking force adjuster. The minimum required braking force is defined such that the roll's rotation is braked until ω<ν/R is achieved at least one rolling point of the roll surface. On the other hand, however, the braking force is also large enough to stop the rotation of the roll partially or even completely (i.e. ω≧0). The braking force may be varied between these two threshold values, thus giving an angular velocity ω of the roll that is less than ν/R and equal to or greater than zero.
The magnitude of the pull (push) depends on the contact pressure of the roll against the initial shaped body, on the pressure wave created thereby in front of the roll, and on the rolling speed ν. The contact pressure must be consistent with the desired degree of forming of the rolling procedure, but it must be less than the pressure that would result in approaching or exceeding the yield strength of the material of the initial shaped body. The size of the braking material bulge that is pushed ahead of the roll depends directly on the magnitude of the rolling pressure. Its braking effect on the angular velocity ω and on the rolling speed of the roll, and thus also on the pull (push) may be increased by designing the roll as a profiled roll, which results in a greater degree of cold forming and thus requires from the roll a greater working effort. The rolling speed ν affects the height of the material bulge in conjunction with the other factors.
In the process according to the invention the roll may be guided over the initial shaped body to be rolled in a predefined linear or curved path according to the desired profile. In this context, the roll is preferably pushed according to the invention.
The process according to the invention is not a continuously operating process, since the support only has finite dimensions, and a rolling cycle ends at the latest after the roll has rolled over the entire support. On the other hand, this also offers the possibility of rolling an initial shaped body in several consecutive cycles, whereby these cycles may be performed one directly after the other, e.g. using the same roll every time, or with replacement of the roll between two consecutive cycles.
According to the invention, the roll does not have to be a cylindrical roll; it may also be a roll with a non-cylindrical shape. The axis of rotation of the roll, from which the distances R to the rolling points of the roll surface are measured, does not have to be located inside the roll; however, it is preferably inside the roll. These distances R are greater than zero, and in extreme cases may go towards infinity; the latter case corresponds to a roll with a flat roll surface. For the purposes of the invention, however, the roll is preferably a cylindrical roll, or a rotationally symmetrical profile roll, or a sector roll having a rolling surface that is cylindrical or has a rotationally symmetrical profile. In the case of a cylindrical roll, a rotationally symmetrical profile roll, or a sector roll having a rolling surface that is cylindrical or has a rotationally symmetrical profile, the roll's axis of symmetry is preferably coincident with the central axis of the roll. In the cases of a cylindrical roll, a sector roll, a rotationally symmetrical profile roll, or a sector roll having a surface with rotationally symmetrical rolling profile, the said distance between the axis of rotation and the rolling point of the roll surface is identical with the radius R of the roll at this point. The roll profile that may be optionally applied to the roll surface is preferably such that the baseline of its cross sectional profile corresponds to the strip width of the initial shaped body to be rolled (i.e. to the rolled region of the metal strip according to the invention). In order for a rolling process according to the invention to be carried out, i.e. with no more than negligible lateral spread, the cross-sectional area of the rolled metal strip preferably remains constant during the reshaping of its own profile. During the process according to the invention, the roll simultaneously performs two functions: 1) the pressing function to reduce the thickness of the initial shaped body, and 2) the pulling function to lengthen the rolled material, which corresponds to a reduction in the cross-section whereas the width thereof remains unchanged or increases only insignificantly.
The support may be flat but it may also have a surface that is suitably curved or profiled in three spatial dimensions. In this case, the initial shaped body then not only undergoes rolling deformation but also form rolling. If the surface is to be profiled, this profile is preferably also selected such that by itself it results in reshaping wherein the cross-section remains constant.
In the process according to the invention, explicit heating is preferably not done, so that the only heat source acting on the initial shaped body is the heat that is generated within the shaped body itself during rolling. This means preferably that the temperature of the initial shaped body and the resulting rolled metal strip during the entire rolling process does not exceed 100° C. at any point.
The initial shaped body that can be rolled by the process according to the invention may consist of any sufficiently ductile metal. If a metal strip according to the invention is to be produced, the initial shaped body also consists of a polycrystalline metal. Examples of such metals would be steel alloys, bronze, aluminium, copper, titanium, or brass. It is preferably a metal that crystallises according to a body-centred cubic (bcc) crystal lattice, that is to say a ferritic, martensitic, or mixed ferritic/martensitic steel. Among the martensitic or mixed ferritic/martensitic steels, the α′ modification of the martensite is preferred. Strictly, speaking, this crystallises in a body-centred tetragonal lattice, but for practical purposes it may also be considered to undergo body-centred cubic crystallisation. According to the invention, a martensitic steel that is free from nickel and molybdenum is preferred (in this context, “free” means having a content of less than 0.01 percent by weight). More preferably, it has the composition Cr 12.50-14.50 percent by weight, C 0.42-0.50 percent by weight, Si max. 1.00 percent by weight, Mn max. 1.00 percent by weight, P max. 0.045 percent by weight, S max. 0.030 percent by weight, the rest essentially consisting of iron and unavoidable contaminants. In particular, it may be a steel corresponding to material number 1.4034. The initial shaped body is preferably already in the form of a metal strip or of sheet metal.
To enable the rolling deformation to take place more easily and with lower pressures, the metallic initial shaped body, if consisting of a steel alloy, may undergo a beforehand heat treatment, preferably with subsequent quenching in cold water. This treatment reduces the metal's strength isotropically. In the field of steel alloys, this thermal treatment is known as solution heat treatment, and typically a temperature range from 1050-1080° C. is selected, the heat treatment typically lasts from about 10 minutes to about 1 hour, preferably half an hour.
The method according to the invention is suitable for production all types of tweezers, needle holders, springs, dissectors, clamps, scissors (for example hairdressing scissors), knives (in which case rolling may be performed along the axis of the blade instead of transversally to the blade axis, as has been done so far), or special profiles of any kind (for example for constructing façades or pipes). It is particularly suitable for producing parts that have at least one resilient region, thus for instance for manufacturing tweezers, needle holders or springs, and especially also for tweezers with an additional forward- or backward-acting cutting function (tweezer scissors), or for surgical or orthopaedic implants.
The device according to the invention, with which the process according to the invention is performed, includes at least one roll, one support, and one brake as described for exemplary purposes in the preceding, which is capable of braking the angular velocity ω of the roll during rolling. The support itself may be movable or non-movable, it is preferably non-movable. The device according to the invention further includes the bearings and associated guides that are necessary to press the roll against the material, and to move it (push or pull). These roll guides preferably include hydraulic cylinders or mechanical systems which are capable of guiding the roll over the support accurately and with a constant or controlled distance curve (the thickness of the resulting rolled metal strip is then constant or variable, respectively). These hydraulic or mechanical roll guides are known in the field of rolling and do not need to be described further. By the appropriately controlled braking of the roll and the simultaneous forward pulling or pushing thereof an additional drawing device for the rolled material is unnecessary in the device according to the invention.
The result of the process according to the invention and other, optional prior shaping steps, is an at least partly rolled metal strip. This metal strip does not necessarily have a regular or flat shape. It consists of one single piece, i.e. it does not consist of two or more separate parts that have been screwed, riveted or stuck together. It also has no weld points.
In general, a strip according to the invention consists of a polycrystalline metal that includes a rolled region such as is described in the preceding, in which region the anisotropic orientation of the crystallites is comparatively more pronounced. In the region or regions that is(are) not rolled, it retains the crystallite orientation of the initial shaped body, that is to say the orientation of the crystallites is comparatively less pronounced, or in some cases may be essentially isotropic. The term “not rolled” also refers to a pretreatment of the initial shaped body under rolls, provided that the last step of such pretreatment has been a heat treatment such as is described in the preceding, and which undoes any changes in the lattice that may have been caused by the rolling.
The orientation of the crystallites is defined for the purpose of the present invention in terms of the orientation density function, also known in the technical community by the abbreviation ODF. In English it is called the “orientational distribution function”, which is also abbreviated as ODF.
For the purposes of the present application, an orthogonal coordinate system is used for the rolled strip, the X-axis of which extends parallel to the rolling direction of the strip region rolled according to the invention; the Y-axis of which is perpendicular to the rolling direction, and extends to the left when the strip is viewed from above, and the Z-axis of which is the normal as the cross product of the X-axis and the Z-axis.
Said orientational distribution function ODF is determined within the scope of the present application from X-ray structural data obtained with CuKα radiation (λ=1.54 Angström). On the one hand, for example, disc-shaped samples having a flat surface to be irradiated may be cut out of the strip to be examined. In this case, the sample to be examined is cut out of the strip in such manner that the surface to be irradiated stands vertically on the Z-axis. If the strip already has a flat surface that stands vertically on the Z-axis, the strip might also be measured directly at this surface.
This ODF may be defined on the one hand as
In formula (2a),
This ODF may be calculated as f(φ1, Φ, φ2) from pole figure measurements. The pole figures and the ODF are approximated as series expansions of generalised spherical harmonic functions, these two approximations are, inserted in the fundamental equation of the texture analysis, and the coefficients of the series expansion are calculated from that. This method is described in sections 11.4.1 and 11.6.5 (“Harmonic method”) of the textbook “Moderne Röntgenbeugung” (Modern X-ray diffraction), L. Spiess, R. Schwarzer, H. Behnken, G. Teichert, October 2005, B. G. Teubner Verlag, Wiesbaden, Germany. This description is included in this document by reference.
Alternatively, the ODF may also be defined with formula (2b):
In formula (2b),
This orientational distribution function W(θ,φ) includes a polar angle θ measured from the z axes of the internal crystallite coordinate systems and an azimuthal angle φ measured from the x-axes as variables. This ODF is obtained by the following steps, a)-c) (see also J. Appli. Cryst. 1970, 3, p. 313ff.):
a) The sample is fixed on the sample holder of the diffractogram such that the surface normal of the surface to be irradiated is perpendicular to the axis of the diffractometer. The goniometer angle 2θ is selected such that a diffraction at a crystal lattice plane family with a given Miller index (hkl) is detected. The sample (or the strip itself) is then inclined such that the surface normal of the surface to be irradiated is rotated by an angle α from the normal of this plane family towards the diffractometer axis. With this inclination, and simultaneous rotation of the sample through 360° about the surface normal of the surface to be irradiated while retaining the goniometer angle 2θ selected in advance, the cumulative diffraction intensity Ihkl(α) is measured. This measurement is carried out for a total K of different angles α, but always with the same θ.
b) Each Ihkl(α) determined in a) is assumed to be representable by a series expansion of the form
In this equation, Kvw(θhkl,φhkl) is the value of the “symmetry-adapted spherical harmonic” (SASH) Kvw adapted to the crystal lattice symmetry of the metal in question at the angle pair (θ,φ) describing the direction of the normal vector of the crystal plane family having Miller Index (hkl) in the internal crystal coordinate systems. The index v only runs over the even numbers greater than zero up to the maximum considered number V. The larger the value of V, the higher the accuracy. The index w runs over all such linearly independent spherical harmonics of order v. The number K of Ihkl(α) measured in a) must be one greater than the total number of summands in the double sum of formula (3). Pv(cos α) is the value of the Legendre polynomial of order v at cos α. Formula (3) also includes:
wherein:
represents the atomic form factor of the j-th atom in the unit cell depending on sin(θ)/λ. λ is 1.54 Angström. These atomic form factors are known. The sum in which these atomic form factors occur runs over all N atoms in the unit cell;
From equations (3) the coefficients Cvw and also Q are determined in turn.
c) The orientational distribution function W(θ,φ) is obtained via the coefficient Cvw contained in b) by using formula
wherein Kvw(θ,φ) are again the symmetry-adapted spherical harmonics discussed above, the angle pair (θ,φ) within the internal crystallite coordinate system is as described above, and v, V, w and W have the meanings defined above.
The orientation of the crystallites in the region rolled according to the invention is more strongly anisotropic than in another region that has not been rolled or has been rolled conventionally. The anisotropy of the crystallite orientation in the region where the said anisotropy is more pronounced, is preferably such that the ODF described in the preceding formula (2b) contains at least one Cvw in its approximate series expansion that is of a magnitude of at least 0.050; this Cvw is particularly preferably of a magnitude of at least 0.100, and especially preferably of at least 0.200. On the other hand, the ODF described according to formula (2b) above in the region with comparatively less pronounced anisotropic crystallite orientation, is preferably such that in the said ODF series expansion is such that none of the Cvw is of a magnitude of greater than 0.050, i.e. it is essentially isotropic (for purely isotropic crystallite orientation, all Cvw, would be zero).
In the case of strips that include both a region that has been rolled according to the invention and a region that has not been rolled, and which are made from a ferritic, martensitic, or mixed ferritic/martensitic steel as described above, this anisotropy of orientation is expressed particularly as follows: If θ-2θ diffractograms are recorded of samples of the rolled and not rolled regions of such a strip such that the axis of the diffractometer is parallel to the surface of the sample to be irradiated, it is found that the crystallites in the region that has been rolled according to the invention are comparatively more frequently orientated so that their plane family having Miller Index (200) is parallel, to the irradiated surface. To a lesser extent, the crystallites are also orientated such that their plane family having Miller Index (211) is more frequently parallel to the irradiated surface. This special orientation of the crystallites becomes more pronounced as one progresses from the edge zones of the region that has been rolled according to the invention (from the outside) towards the middle (towards the inside). In contrast, on a strip made from the same metal, but which includes a region that has been rolled according to a conventional method (with two rolls), no such special orientation is noted.
Also in general terms, the following is observed in strips that are made of a polycrystalline metal, preferably of a metal that crystallises in a body-centred cubic (bcc) crystal lattice, particularly preferably of a ferritic, martensitic, or mixed ferritic/martensitic steel as described above (in particular corresponding to material no. 1.4034), and which include a region that has been rolled according to the invention and a region that has not been rolled: When they are examined at two arbitrary points in accordance with the diffraction method described in the previous paragraph, no statistically significant differences are found in the position or shape of their pikes if the corresponding pikes of the two diffractograms are compared with each other, i.e. the two pikes never differ from one another to a statistically significant degree in terms of the position and shape of their pikes. This again is different from a strip that is made of the same metal but includes a region that has been rolled according to a conventional method (with two rolls): in that case, the pike from a diffractogram of the rolled region may be shifted to a statistically significant degree and/or its shape may be have been distorted to a statistically significant degree with respect to the corresponding pike from a diffractogram of the region that has not been rolled.
For the purposes of the present application, the “shape” of a pike is understood to be the symmetry of the pike with respect to its maximum and the sharpness thereof (ratio of half-width to maximum intensity). The intensity of the pike is not understood to be the “shape” of the pike.
In order to determine whether two pikes differ from one another “to a statistically significant degree in terms of their position and shape”, the following steps 1)-4) are carried out for the purposes of the present application:
1) The two diffractograms for comparison are provided in a form in which the absolute intensity curves of the two pikes for comparison have been plotted as a function of 2θ in discrete counting intervals having width 0.05°. Each of these counting intervals will be identified in the following by an associated Index i.
2) In each of the two diffractograms the largest possible contiguous, matching 2θ region is searched which contains the maxima of the two pikes to be compared, and in which always either the absolute intensity of the i-th counting interval from the first diffractogram is at least twice as large as the associated baseline, or the absolute intensity of the corresponding i-th counting interval of the second diffractogram is at least twice as large as the associated baseline, or even both i-th intensities are each at least twice as large as the associated respective baseline. If no such region exists, the two pikes are considered to be “differ from one another to a statistically significant degree with regard to position”, and the remainder of the test is not carried out anymore.
3) However, if such a region having a total of k contiguous counting intervals does exist, the characteristic chi-square is calculated from all these counting intervals:
In this formula, 1bi and 2bi are the intensities of the first and second pikes, respectively, in the i-th counting interval, their base-line intensity having been removed and having been normalised to a pike maximum of 100 (one hundred) counts. This normalisation of the pikes to be compared to a maximum of 100 counts is done, firstly, because the number of irradiated crystallites is not constant from one sampling site to another (this results in differing intensities in the diffractograms taken at the two sampling sites), and, secondly, because differences may exist in the anisotropy of the crystallite orientation (this results in variations in the intensity ratios of the pikes within the same diffractogram; at least one region with relatively more pronounced anisotropic crystallite orientation and at least one region with relatively less pronounced anisotropic crystallite orientation is is in particular required in the case of a strip according to the invention). The statistical test described here however should only show, without consideration of and independently from the anisotropy of the crystallite's orientation, statistically significant differences in the properties of the crystallites themselves, e.g. a lower average size (visible as a widening of the pike), or tensions within the crystal lattice (visible as a shift of the pike maximum, pike broadening or asymmetry in the pike shape). The above formula (6) is derivable from the formula known in mathematics for the Chi-Square test variable for observed counting intensities 1bi or 2bi, if one takes the average from 1bi and 2bi, μi, as the associated i-th expectation value, and the square root of this average value as the standard deviation σi.
4) The Chi-Square characteristic calculated in step 3) is compared with the value of the Chi-Square distribution for k degrees of freedom according to the following table 1 with a significance threshold of 0.001% (k is the number of counting intervals of the contiguous region identified in step 2):
If the Chi-Square characteristic calculated in step 3) is greater than the value listed in table 1 for the applicable number of degrees of freedom k, the two pikes are “different from each other to a statistically significant degree with regard to either position or shape”, otherwise they are “not different from each other to a statistically significant degree with regard to position and shape”.
This Chi-Square characteristic from two corresponding pikes from two diffractograms, from two arbitrary sites of strip that has been partly rolled according to the invention, but of which no part has been rolled conventionally, is preferably always so small that the two pikes may still be evaluated as “not different from each other to a statistically significant degree with regard to position and shape” even if the values used from the table are not those for the significance threshold 0.001%, but (in increasing levels of preference) for 0.01%, 0.1%, 1%, 5% or 10%.
In the case of a metal that crystallises according to a body-centred, particularly a body-centred cubic (bcc) crystal lattice (including the preferred ferritic, martensitic, or mixed ferritic/martensitic steels), or according to a surface centred lattice (for example the austenitic steels) statistically significant differences are most readily observed between the two diffractograms when the pikes with Miller. Index (200) are compared.
For example, diffractograms from two strips A and B from a steel with material number 1.4034, 70 mm long and 10 mm wide, and of which one end had not been rolled and had a constant thickness of about 1.5 mm, and of which the other end had been rolled either according to the invention (strip A) or conventionally (strip B) to a constant thickness of about 0.85 mm, were examined with a computer program according to the statistical test described above. The pike at 2θ approximately 64.8° (the (200) pike) was tested for statistically significant differences. The program required manual input of the positions of the pike maxima, and of baseline regions to the left and right of both pikes. As the baseline intensities the software calculated the averages of all intensities from both of these baseline regions. The program then identified the largest possible contiguous 2θ area in accordance with step 2) above, and the number of degrees of freedom k, and then calculated the Chi-Square value using formula (6) above. The following Chi-Square values and associated degrees of freedom k were obtained:
It can be seen that the conventionally rolled strip B shows pikes at 64.8° that “differ to a statistically significant degree in terms of either position or shape” when diffractograms of the not rolled and rolled areas are compared with each other. In contrast, no such statistically significant differences are observed for strip A, which was rolled according to the invention, regardless of the measurement sites of the two compared diffractograms. The same result is also obtained if any of the other significance thresholds from table 1 is chosen.
In the strip according to the invention, the region with relatively more pronounced anisotropic crystallite orientation preferably also has a less homogeneous microstructure than the region of relatively less pronounced anisotropic crystallite orientation, which is also caused by the rolling method according to the invention. These differences in homogeneity of the microstructures may be observed directly by comparing microphotographs of sections of the strip material from the regions in question.
At the same time, the rolling method according to the invention does not cause any internal stresses in the material, which may be recognized by the fact that the rolled region or regions do not show any tendency to warp during subsequent machining steps. Like any body, this metal strip has three principal axes of inertia. Since the metal strip according to the invention is rather elongated, the moment of inertia associated with one of the three principal axes of inertia is smaller than the two moments of inertia associated with the other two axes. This smallest principal moment of inertia is preferably at least 10 times smaller, particularly at least 50 times smaller, than the other two principal moments of inertia.
It has also been found that the regions of a metal strip rolled using the process according to the invention have improved resilience by up to a factor of 6 compared with conventionally rolled regions. Strips that have been rolled to a given, constant thickness by the rolling process according to the invention, have a flatter spring characteristic in the rolled region than a strip of the same metal that has been rolled to the same thickness partly by a conventional rolling process with two rolls, i.e. less force is required to obtain a given bending in the strip rolled according to the invention than is the case for a strip having the same thickness but which has been rolled conventionally. Moreover, the spring characteristic in the strip that has been rolled to constant thickness with the process according to the invention is slightly degressive, i.e. with increasing bending less force is required to bend the strip even further. For example, if the strips A and B described previously were clamped at the ends thereof which are 1.5 mm thick, the following deflections were observed upon suspending various weights from the 0.85 mm thick, free ends thereof at a distance of 70 mm from the end (average values from 5 strips each):
The difference between the spring characteristics of strip A that was rolled according to the invention and strip B that was rolled conventionally becomes more pronounced as the strip is rolled more thinly, i.e. as the degree of quenching (the ratio between the height of the strip after rolling and the height of the strip before rolling) becomes smaller. It is possible that this change in spring characteristic is attributable to the earlier described increase in microstructure inhomogeneity in comparison to the not rolled region of the strip, when using the rolling process according to the invention. On the other hand, it is possible to create a leaf spring part with a progressive spring characteristic using the process according to the invention by rolling the required region to a variable thickness.
If strips A and B described in the preceding are cut through at their thicker ends with a rolled thickness of 1.5 mm and at their thinner ends with a rolled thickness of 0.85 mm transversely to the direction of rolling, and these two cut surfaces for each strip are tested for Vickers hardness according to ISO 4516 and ISO 6507/1 using a Leitz Miniload 2 microhardness tester, the following values for Vickers hardness (in MPa) are determined:
A relatively thick region that has been rolled according to the invention thus has a significantly higher Vickers hardness than a region that has been rolled to the same thickness by conventional means. The difference becomes smaller for thinner rolled strips.
If the metal strip is made from a steel alloy, as a rule a substantial fraction of deformation martensite is evident at the rolled sites, typically in the range from about 5 to about 50 percent by volume of the metal.
In a first preferred embodiment, the metal strip according to the invention is approximately straight and at least a part of its length is rolled in the method according to the invention. In this context, the term “length” is understood to mean the projection of the metal strip onto its said principal inertia axis with the smallest moment of inertia.
In another preferred embodiment, the at least partly rolled metal strip is bent in a U-shape such that it has two legs. Each of these legs has one or more (preferably exactly one) region that bois adjacent to the U-shaped bending point, which is obtainable by the rolling process according to the invention, and which has the properties indicated in the preceding. The length of this preferred, U-shaped metal strip projected onto the principal axis of inertia having the smallest moment of inertia as described previously is preferably about 90 to about 200 mm, particularly preferably about 100 to about 160 mm. The length of the regions of the two legs rolled according to the invention projected onto this principal axis of inertia is preferably about 30 to about 90 mm particularly preferably about 40 to 80 mm. The thickness of the two legs of the U-shaped metal strip for rolling is preferably in the range from about 1 to about 3 mm, preferably in the range from about 1.25 to about 2.75 mm. The thickness of the regions of the two legs rolled according to the invention is preferably in the range from about 0.5 to about 1 mm, particularly preferably in the range from about 0.7 to about 0.9 mm. The degree of deformation φ, calculated according to the formula
wherein l1 is the thickness of the rolled region of the leg and l0 is the thickness of the same region before rolling, is preferably in a range from about 50% to about 120%. The U-shaped metal strip may either be bent into the U-shape first, and then a region may be rolled on each leg at the same time in a rolling process according to the invention with two rolls and a support positioned between them. Alternatively, two regions on a starting shaped body that has not yet been bent into a U-shape may first be rolled individually with just one roll in a process according to the invention and a support, and the initial shaped body may then be bent into a U-shape between the two rolled regions. In this case, the support on which rolling is to be performed preferably has a surface contour that exactly matches the inner contour of the initial shaped body that has already been bent into a U-shape including the two regions of the two legs to be rolled. The initial shaped body for rolling may then be placed with total accuracy on the support, so that the two legs to be rolled hang down on the two surface sides of the support. If the two legs are then rolled at the same time, preferably with a rolling device according to the invention equipped with a pair of identical devices each having a roll; and preferably such that the roll device works from top to bottom, the initial shaped body is prevented from slipping during the rolling operation.
The metal strip according to the invention is suitable as an intermediate product in the manufacture of various objects such as were exemplified above. For this purpose, the metal strip may be processed further in subsequent processing steps, such as stamping, drilling, milling, bending, grinding, or even using the rolling process according to the invention to create a desired end product.
If the metal strip according to the invention is straight, it may be processed further to produce for example springs, particularly leaf springs, helical springs or watch springs, or knife blades. In the latter case, it is the knife blade that has been rolled using the process according to the invention.
If the metal strip is bent in a U-shape, as is preferred according to the invention, it may be processed further to produce single part instruments having a gripping function, such as tweezers, tweezer scissors, tongs (for example sugar or ice cube tongs). Tweezer scissors are tweezers on which the two free ends of the legs are shaped into scissor blades, and which slide past each other in a scissor action when the legs are squeezed together. This scissor effect may be directed forwards or backwards. Tweezer scissors with backwardly directed scissor action may be created by fashioning scissor blades on the ends of the legs and then bending the tips of the legs inwards and backwards.
For a tweezers with forward scissor action, the term “forward cutting tweezer scissors” may also be used for the purposes of this invention. For a tweezers with backward scissor action, the term “backward cutting tweezer scissors” may also be used for the purposes of this invention.
The U-shaped bending point makes the tweezers according to the invention, with or without scissor action, easier to sterilise and clean because there is no joining point at the rear end thereof, at which the two legs would meet at an acute angle. This acute, hard-to-reach joint on previously known instruments is where dirt and bacteria can collect. The tweezers with or without scissor action according to the present invention have longer and more resilient legs than the previously known instruments, and thus enable better adjustment of squeezing pressure when closing or releasing the two legs. The weld spot at the rear end of previously known instruments represents a site that is prone to corrosion, and is avoided in the tweezers according to the invention due the single step rolling and bending into a U-shape, with no welding, according to the invention (if a process variant with two paired devices, each furnished with one roll is selected). The ends of the two legs fit together more precisely, so that manual reworking of the legs, as is often necessary with the previously known manufacturing process including welding of the legs, is avoided.
The metal strip according to the invention may also be used to manufacture supporting implants and joint prostheses that are designed to support an impaired joint function. They promote the distraction of the cooperating parts of the joint (support implants, particularly for supporting a hip, knee, or other joint, for example) or replace a lost joint function (joint prostheses).
A common feature of all such support implants or prostheses is that leaf spring components which have been rolled using the process according to the invention ensure mobility in the primary load direction of the joint concerned. Depending on the movement type of the joint, i.e. flexion/extrusion, abduction/adduction, lateral flexion or interior and exterior rotation, one or more such leaf springs are present and are preferably loaded by traction, but may also sustain compression and torsion depending on their design.
Joint prostheses according to the invention may be used to replace a joint entirely, and in general may be adapted to any joint, for example the hip, spine, wrists and ankles, or the mandibular joint. The latter is a preferred example of a joint. The prosthesis may be attached either to the two remaining bone ends of the joint on the flexor side, to the two remaining bone ends on the extensor side, or crossed to one bone end on the flexor side and to the other bone end on the extensor side. In the case of the spine, the element may be attached on both sides in the area of the spinous processes/rib attachment sites.
Supporting implants according to the invention also include, besides the leaf spring components described previously, U-shape bending points that have not been rolled, and in which the anisotropic orientation of the crystallites is consequently again less pronounced. For the purposes of the invention, a “U-shaped” bending point does not necessarily mean that the bending point causes a change of direction through 180°; “U-shaped” typically means a deflection by 90° to 220°, preferably by 160° to 210°, particularly preferably by 170° to 200°, most preferably by 175° to 186°, or by exactly 180°. The leaf spring parts themselves may be flat or may present a certain curvature of constant or variable radius, or a bulging. The leaf spring parts may also be designed as a progressively or partially effective squib to stabilise its extensor effect. In the supporting implants according to the invention, leaf spring parts and bends are preferably arranged in an alternating sequence. Preferred examples of joints that may be supported by the supporting implants according to the invention may be ellipsoid joints (Articulatio ellipsoidea); hinge joints (Ginglymus, for example the finger joints), pivot joints (Articulatio trochoidea, for example the joint between the ulna and the radius); or bicondylar joints (Articulatio bicondylaris, such as the knee joint). Knee joints are a particularly preferred example.
The invention will now be explained in greater detail with reference to the figures. In the figures:
In a first preferred embodiment (
In a second preferred embodiment of the process according to the invention (
By virtue of its distraction effect, the supporting implant of
In this context, upper jaw part 191 is attached to the remaining part of the upper jaw via screws 199 and lower jaw part 196 is attached to the remaining part of the lower jaw via screws 198. The supporting part 194 fits against the upper jaw part 191 from below 191; supporting part 194 and the underside of upper jaw part 191 together ensure mobility approximating the natural movement of the jaw (rotation and displacement of the lower and upper jaws relative to one another). The leaf spring part 192, which is rolled according to the invention, provides the resilient flexibility that the prosthesis requires for this.
| Number | Date | Country | Kind |
|---|---|---|---|
| 141/08 | Jan 2008 | CH | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CH2009/000033 | 1/28/2009 | WO | 00 | 2/18/2011 |
