METHOD FOR MANUFACTURING A DIRECTIONALLY DEPENDENT REFLECTIVE SURFACE TECHNICAL FIELD The invention relates to a method for manufacturing a directionally dependent reflective surface on a substantially cylindrical body, and in particular to a method for manufacturing a directionally dependent reflective surface on a substantially cylindrical grating element or code disk having regions of high and low reflectivity. Such code disks are found in torque sensors and angle sensors for use in electric power steering systems in vehicle applications.

BACKGROUND Torque and angle sensors of the type suitable for use in vehicle electric power steering systems are described in International Patent Publication Nos.

W099/09385 and WO00/06973, both in the name of Bishop Innovation Pty Limited. These type of torque and angle sensors utilise"grating elements"or "code disks"that have regions of high and low reflectivity on their outside surface that, in use, are illuminated by a source of electro-magnetic radiation (EMR). One method of manufacturing such regions of high and low reflectivity on the outside surface of grating elements or code disks of a torque sensor by means of a laser patterning process is disclosed in International Patent Publication No. W099/20427, also in the name of Bishop Innovation Pty Limited.

The present invention provides a method that facilitates the manufacture of a directionally dependent reflective surface on the outside surface of a substantially cylindrical body. In particular this method is suitable for manufacture of a directionally reflective surface on the outside surface of a grating element or code disk, prior to laser patterning regions of high and low

reflectivity thereupon, such as that described in International Patent Publication No. W099/20427.

SUMMARY OF INVENTION In a first aspect, the present invention consists in a method for manufacturing a directionally dependent reflective surface on a substantially cylindrical body, said method comprising the engagement of a tool with the outside surface of said body, thereby forming at least one groove therein with a cross section, when measured parallel to the longitudinal axis of said body, that is asymmetric and wherein said groove is formed without removal of material from said body.

In one embodiment the method preferably comprises said tool relatively moving along said outside surface in a direction parallel to the longitudinal axis of said cylindrical body whilst said cylindrical body is rotated relative to said tool, thereby imparting a helical path to said tool relative to said body and hence forming said at least one groove as a helical groove.

Preferably said outside surface of said body is stainless steel or nickel silver.

Preferably the tool engages with said outside surface of said body to a depth in the range of 20-200 um.

Preferably said body is rotated at an angular velocity in the range of 20-200 rev/min relative to said tool.

Preferably said tool is relatively moved along said outside surface at a feed rate in the range of 20-200 um/rev.

Preferably said body has a diameter in the range of 20-80 mm.

In another embodiment the method preferably comprises rotating said body whilst said tool is held axially stationary relative to said body.

BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a schematic end view of a tool engaging with a cylindrical body in accordance with a first embodiment of the present invention.

Fig. 2 is a schematic elevation view of the tool engaging with the cylindrical body of Fig. 1.

Figs. 3a and 3b are enlarged sectional views of the outside surface of the cylindrical body within circle A of Fig 2, when viewed in a cross sectional plane parallel to and passing through the longitudinal axis of the body.

MODE OF CARRYING OUT THE INVENTION Figs. 1 and 2 schematically depict one embodiment of the present invention.

A substantially cylindrical body 1, preferably of grade 316 or 316L stainless steel, is rotated (in a direction indicated by arrow 3) about its longitudinal axis 2 at a speed in the range of about 20-200 revs/min. Rotation of body 1, may be carried out in a machine having a rotating spindle such as a lathe.

Tool 4, preferably made of tungsten carbide coated with either titanium nitride or titanium carbon nitride, is tangentially introduced into the cylindrical outer surface 8 of body 1 to a depth typically in the range of 20-200 um, and moved axially relative to body 1 in a direction 5, parallel to longitudinal axis 2, at a fixed feed rate in the range 20-200 um/rev.

When tool 4 is introduced into the outside surface 8 of body 1, a corner of tool 4 engages with body 1, such that tool face 22 is disposed at angle 6 relative to the outside surface 8 of body 1, of about 24 deg.

Axial movement of tool 4 relative to rotating body 1, results in a helically disposed groove 23 of asymmetrical form (when measured in cross section parallel to longitudinal axis 2), having a major face 9 and a minor face 10, formed in the outer surface 8. The major face 9 is also at angle 6 to longitudinal axis 2, which in this embodiment is about 24 deg.

Helically disposed groove 23 has a pitch 21 that is equal to the axial feed rate of tool 4 relative to body 1. It has been found advantageous that this axial feed rate lies in the range of 20-200 um/rev, and hence the axial pitch 21 of helically disposed groove 23 also lies in this range of 20-200 um, in order to generate an optimum directionally dependent reflective characteristic on the outside surface 8 of body 1.

When viewed in cross section as shown in Figs. 3a and 3b, helically disposed groove 23 appears to a have an asymmetrical saw tooth form.

In order to ensure that material it is not removed from body 1 whilst tool 4 is engaged therewith, suitable lubrication may be provided in the form of a soluble oil or an extreme pressure (EP) type synthetic or mineral oil. Ideally a certain level of hydrodynamic lubrication is maintained between tool face 22 and outside surface 8 of body 1 during the forming of groove 23, and hence it is possible to minimize wear on tool face 22 of tool 4. The use of lubrication ensures the forming of major face 9 of groove 23 in a smooth and consistent manner with minimum wear on tool face 22.

As groove 23 is asymmetric when viewed in cross section parallel to longitudinal axis 2 of body 1, it ensures that EMR reflected thereupon from EMR source 12 and 17 is directionally dependent. With reference to Fig 3a, EMR emanating from source 12, as represented by arrow 13, strikes major face 9 at angle 16, relative to line 30, and is reflected back towards EMR detector 15 shown by arrow 14. However, with reference to Fig. 3b, EMR

emanating from source 17 as represented by arrow 18, strikes major face 9 at the same angle 16 relative to line 30 normal to longitudinal axis 2 and is reflected away from line 30 (ie. the normal to longitudinal axis 2) represented by arrow 19, in the opposite direction, thereby not being detected by EMR detector 15.

In both instances, whilst EMR detector 15 is arranged to detect EMR propagating normally to longitudinal axis 2 of body from the same point of reflection 20, the ability to detect EMR reflected from major face 9, is dependent on the location of the EMR source (ie. either source 12 or source 17) and the angle of major face 9.

Groove 23 thereby provides body 1 with a directionally dependent reflective characteristic. Such a body 1, preferably having a diameter in the range of 20-80 mm, is suitable as a grating element or code disk to be laser patterned in accordance with International Patent Publication No.

W099/20427, for use in a torque sensor or angle sensor.

It should be understood that an important aspect of the present invention is that groove 23 is formed without removal of material, and the forming action of tool 4 upon body 1 is akin to a burnishing, planishing or rubbing action. In Figs. 3a and 3b outside surface 8, prior to the forming of groove 23, is shown as chain dotted line 11 which will lie at approximately the mean radial depth between tips 24 and roots 25 of groove 23.

Whilst in the abovementioned embodiment angle 6 between tool 4 and body 1 is held at substantially 24 deg. , it should be understood that in other not shown embodiments angle 6 may be some other angle, depending on the directionally dependent reflective characteristic that is desired for the particular application.

In another not shown embodiment angle 6 may be varied during the process as tool 4 is moved axially relative to body 1 in direction as shown. This generates a directionally dependent reflective characteristic on outside surface 8 that varies along the length of body 1. In one case this embodies groups of predetermined different angles as a function of axial position along outside surface 8 of body 1, or alternatively, angle 6 of groove 23 may vary continuously as a function of axial position along outside surface 8 of body 1.

Whilst the above mentioned embodiment is directed to the forming of helical groove 23, it should be understood that, in another not shown embodiment, body 1 may have one or more circumferential grooves, of asymmetric form when viewed in cross section parallel to longitudinal axis 2, imparted thereon and spaced axially apart at relatively small intervals in the range of 20-200 um. To achieve this, tool 4 is brought into engagement with body 1 as body 1 is rotated relative to tool 4, thereby forming a continuous (or closed) circumferential groove. Tool 4 is then radially withdrawn from body 1, moved axially relative to body 1 by a small distance, and the process repeated. As with the earlier embodiment, an important feature is to ensure that material it is not removed from body 1 whilst tool 4 is engaged therewith and forming the asymmetric groove. This non-removal of material has been found to maximize the smoothness (and hence reflectivity) of major face of groove 23, and hence further benefit the optical properties of outside surface 8 of body 1.

In the abovementioned embodiment the outside surface 8 of cylindrical body 1 is preferably manufactured from stainless steel eg. grade 316 or 316L. Grade 316L has a lower carbon content to that of the standard grade 316, and therefore has greater malleability. It is therefore easier to form groove 23 therein without removal of material. However, in other embodiments still other grades of stainless steel, or indeed other malleable materials such as nickel silver, may be used for the outside surface 8 of body 1.

Whilst in the above mentioned embodiment the tool is preferably made of tungsten carbide coated with either titanium nitride or titanium carbon nitride, it may in other embodiments be made of high speed steel or any other suitable hard wearing forming/cutting tool grade material.

Whilst in the abovementioned embodiment the body 1 is rotated about longitudinal axis 2 in a lathe and tool 4 is moved therealong, it shoud be understood that the operation may also occur in a milling machine, where similar relative movements between the tool 4 and body 1 may be carried out.

It should be understood that the symbol"um"as used herein represents micrometers.