Abstract
A surface finish stylus is described that includes an elongate stylus shaft and a contact element protruding from the elongate shaft for contacting a surface to be measured. The contact element is deformable and the stylus shaft includes a clamp for retaining the contact element, the contact element being deformed by the clamp. The contact element may comprise a metal, such as chromium steel or nitinol. The contact element includes one or more regions of weakness to cause a required deformation when retained by the clamp. The surface finish stylus may be used with a surface finish measurement probe or the like.
Claims
1. A surface finish stylus, comprising; an elongate stylus shaft, and a contact element protruding from the elongate shaft for contacting a surface to be measured, wherein the contact element is deformable and the stylus shaft comprises a clamp for retaining the contact element, the contact element being deformed by the clamp.
2. A surface finish stylus according to claim 1, wherein the contact element has an effective radius of less than 100 m.
3. A surface finish stylus according to claim 1, wherein the contact element comprises a sheet of material having a thickness less than 1 mm.
4. A surface finish stylus according to claim 1, wherein the contact element is formed from a material having a Youngs' Modulus less than 150 GPa.
5. A surface finish stylus according to claim 1, wherein the contact element comprises metal.
6. A surface finish stylus according to claim 1, wherein the contact element comprises at least one of chromium steel and nitinol.
7. A surface finish stylus according to claim 1, wherein the contact element comprises one or more regions of weakness to cause a required deformation when retained by the clamp.
8. A surface finish stylus according to claim 1, wherein the clamp deforms a portion of the contact element to immovably secure the contact element to the clamp.
9. A surface finish stylus according to claim 1, wherein the contact element comprises a disk.
10. A surface finish stylus according to claim 9, wherein the clamp deforms the disk to form a hollow cone having a peripheral edge for contacting a surface.
11. A surface finish stylus according to claim 1, wherein the clamp includes a pair of opposed clamping faces shaped to deform the contact element into the desired shape.
12. A surface finish stylus according to claim 1, wherein the clamp allows contact element to be releasably attached to shaft.
13. A surface finish stylus according to claim 1, comprising a plurality of contact elements and plurality of clamps for retaining each of the plurality of contact elements.
14. A machine tool scanning probe comprising; a probe body, a stylus holder moveably attached to the probe body, and a deflection sensor for measuring deflection of the stylus holder relative to the probe body, wherein a surface finish stylus according to any preceding claim is attached to the stylus holder.
15. A method for forming a surface finish stylus that comprises an elongate stylus shaft and a contact element protruding from the elongate shaft for contacting a surface to be measured, the method being characterised by a step of clamping the contact element to the elongate shaft, the clamping step deforming the contact element.
Description
[0038] The invention will now be described, by way of example only, with reference to the accompanying drawings in which;
[0039] FIG. 1 illustrates a prior art profilometer apparatus,
[0040] FIG. 2 illustrates a prior art surface roughness machine tool scanning probe,
[0041] FIGS. 3a and 3b illustrates a surface finish stylus of the present invention,
[0042] FIGS. 4a and 4b show in more detail the conical insert disk of the stylus described with reference to FIG. 3,
[0043] FIG. 5a illustrates a further surface finish stylus of the present invention and FIG. 5b is a photograph of such a surface finish stylus,
[0044] FIGS. 6a and 6b illustrate the disk of the surface finish stylus of FIGS. 5a and 5b,
[0045] FIG. 7 shows an alternative disk,
[0046] FIG. 8 illustrates the effect of effective stylus radius on surface finish measurements,
[0047] FIG. 9 shows the error in surface roughness measurement of various samples measured using the stylus of FIG. 5b,
[0048] FIG. 10 shows surface roughness measured using a machine tool scanning probe comprising the stylus of FIG. 5b compared with a reference measurement, and
[0049] FIG. 11 shows the contact pressure as a function of applied force for a variety of stylus materials and geometries.
[0050] Referring to FIG. 1, a prior art profilometer is schematically illustrated. The profilometer comprises a housing 2 from which extends an elongate shaft 4 having a longitudinal axis L. A contact element 6 extends perpendicularly from the shaft 4 along the direction P. The housing 2 includes a unidirectional transducer (not shown) that measures any deflection of the shaft 4 caused by motion of the contact element 6 back and forth along the direction P.
[0051] In use, the contact element 6 is brought into contact with the surface of an object 10 and lightly biased towards the surface. The object 10 is then moved in a direction M that is parallel to the longitudinal axis L. This relative motion between the object 10 and profilometer may be imparted by moving the object 10, the profilometer or both the object and the profilometer. The result of the relative motion is to cause the contact element 6 to move (i.e. to be pushed or pulled) along a path 12 on the surface of the object 10. The contact element 6 also moves up and down to follow the surface; this can be seen in the inset to FIG. 1 that provides an expanded view of the tip of the contact element 6 and the object's surface. The unidirectional transducer within the housing 2 outputs a deflection signal that is related to the vertical deflection of the contact element 6 (i.e. deflection along the direction P). Analysis of the variations in the deflection signal as the path 12 is traversed provides a measure of surface finish of that region of the object. For example, an average surface roughness or R.sub.a value may be calculated.
[0052] Referring to FIG. 2, the prior art surface roughness probe 20 of US2016/0231108 will be described in more detail. As mentioned above, the scanning probe of US2016/0231108 is a multi-directional scanning probe that can be mounted to the spindle of a machine tool to allow on-machine measurement of workpieces.
[0053] The scanning probe 20 comprises a probe body 22 and a stylus holder 21 for retaining a stylus. The stylus holder 21 is attached to the probe body 22 by a deflection mechanism (not shown) and a transducer 23 is also provided within the probe body 22 for measuring the magnitude of deflection of the stylus holder 21 (i.e. caused by deflection of an attached stylus) relative to the probe body 22. Deflection measurements taken by the transducer 23 are passed by a transmitter unit 25 to a probe interface 27. The probe body 22 is also attachable to the spindle of a machine tool via a tool shank (not shown). The probe 20 can thus be moved around the working volume of the machine tool and in particular the probe stylus can be brought into contact with the surface of an object to be measured.
[0054] A scanning probe 20 of this type is traditionally used with a stylus that enables the form of an object to be measured; e.g. such a stylus may comprise a ruby sphere of several millimetres diameter that is attached to the distal end of an elongate shaft. The amount of force required to ensure stylus engagement with the surface is relatively high. In the various examples described in US2016/0231108, a surface roughness stylus 24 is instead attached to the probe 20. The surface roughness stylus 24 comprises an elongate shaft 30 and a contact element 28 that extends perpendicularly P from the longitudinal axis L of the shaft 30. The contact element 28 has the form of a double-truncated cone with a rounded peripheral edge 26 (shown in the inset to FIG. 2) around its circumference.
[0055] In use, the perpendicular direction P along which the contact element 28 protrudes from the shaft 30 is aligned to be parallel with the surface normal N of a surface to be measured. FIG. 2 shows a suitable vertical surface 34 of an object 36 in dashed outline. The peripheral edge 26 of the contact element 28 is brought into contact with the vertical surface 34 and the probe 20 is then moved vertically upwards in the direction M. To maintain the required alignment of the perpendicular protrusion direction P of the contact element 28 with the surface normal N, the probe 20 is moved vertically along a direction M that is both parallel to longitudinal axis L of the stylus shaft 30 and also parallel to the plane of the surface 34. The transducer 23 of the scanning probe 20 measures the magnitude of stylus deflection as it is dragged along the surface 34 and these measured deflection values are used to ascertain surface roughness.
[0056] The present inventors have recognised a number of drawbacks associated with known surface finish measurement apparatus of the type described above. In particular, the majority of surface finish styli typically used with profilometers generate too much contact pressure if used with a machine tool scanning probe. This can lead to scratching of the surface. Although the use of the stylus disk geometry described in US2016/0231108 reduces the contact pressure due to the increased effective radius of curvature and elliptical contact dimensions, it still remains high when used with the described measurement probe. This can lead to surface damage of the art being measured and could affect the measurement accuracy. As will be explained below, in one aspect the present invention provides a deformable contact element clamped to a stylus shaft. This allows the contact element to be replaced. Also, deformation of the contact element can ensure it is securely affixed to the stylus shaft and/or that it adopts a required shape.
[0057] FIGS. 3a and 3b illustrate a surface finish stylus 50 of the present invention. The stylus 50 comprises an elongate stylus shaft 52 having a longitudinal axis L. The proximal end of the stylus shaft 52 includes a screw-thread attachment member 54 that enables the stylus to be attached to the stylus holder of a multi-directional scanning probe (not shown). The distal end of the stylus shaft 52 comprises a contact element in the form of a hollow, conically shaped metallic disk 56. The metallic disk 56 is held in a truncated conical recess 58 by a wedge-shaped retaining member 60 that is attached to the stylus shaft by a bolt 62. The recess 58, retaining member 60 and bolt 62 thus form a clamp for retaining the stylus disk. The metallic disk 56, after being deformed by clamping, provides a skirt-like contact element that extends around the circumference of the stylus shaft 52 and protrudes along a direction Q that is angled at 45 relative to the longitudinal axis L.
[0058] In use, the surface finish stylus 50 can be used to measure the surface finish of multiple surfaces having different orientations without having to re-orientate the scanning probe. For example, as shown in FIG. 3a, the surface finish stylus 50 is inclined at an angle of 45 to the horizontal (and vertical). The surface finish stylus 50 may then be moved in the direction M1 in order to measure the surface finish of a horizontal surface 70; i.e. the contact element 56 is pulled along the horizontal surface 70 by horizonal motion M1 whilst the scanning probe retaining the stylus 50 measures stylus deflection. The surface finish stylus 50 may then be moved in the direction M2 in order to measure the surface finish of a vertical surface 72; i.e. a diametrically opposed part of the contact element 56 is pulled along the vertical surface 72 by vertical motion M2 whilst the scanning probe retaining the stylus 50 again measures stylus deflection. For both the horizontal and vertical measurements, the protruding contact element 56 maintains a perpendicular orientation to the surface normal N (i.e. the direction Q along which the contact element protrudes from the elongate shaft is kept aligned to the local surface normal N as the stylus is moved along directions M1 and M2).
[0059] Referring to FIGS. 4a and 4b, a technique for making the surface finish stylus 50 described with reference to FIGS. 3a and 3b will be described. In particular, the method for forming the metallic disk 56 that provides the contact element of the stylus using a clamping action will be described.
[0060] As shown in FIG. 4a, a flat metallic disk 56 is provided. The disk 56 includes a central aperture 80 and an annular region 82 of material that is thinner than the rest of the disk. Referring also now to FIG. 3b, the wedge-shaped retaining member 60 is located on the bolt 62 to engage the flat metallic disk 56 already placed over the threaded bolt; the diameter of the central aperture 80 being slightly larger than the diameter of the bolt thread to provide clearance. The bolt 62 is then screwed into the stylus shaft thereby forcing the flat metallic disk 56into engagement with the truncated conical recess 58. The flat metallic disk 56 is thus sandwiched and clamped between the wedge-shaped retaining member 60 and the truncated conical recess 58 and tightening the bolt 62 acts to deform the flat metallic disk 56 to provide the conically shaped metallic disk 56 shown in FIG. 4b and in FIG. 3b. This ensures the metallic disk 56 is securely attached to the stylus shaft and also enables the metallic disk 56 to be easily replaced (e.g. if it becomes worn or damaged) without having to replace the entire surface finish stylus.
[0061] FIG. 5a shows an alternative embodiment of surface finish stylus. An elongate stylus shaft 100 having a longitudinal axis L retains a metallic disk 102 (i.e. a contact element) at its distal end. The metallic disk 102 is clamped between a first member 104 provided at the distal end of the stylus shaft 100 and a second member 106 attached to the stylus shaft by a bolt (not shown). The metallic disk 102 has a circumferential edge that extends beyond the first and second members and is arranged to contact a surface to be measured. The edge has an effective radius R.sub.e, as shown in the inset to FIG. 5a. FIG. 5b is a photograph of a surface finish stylus made to the design of FIG. 5a. The thickness of the metallic disk 102 in the stylus of FIG. 5b is 100 m with a peripheral edge having an effective radius R.sub.e of 25 m.
[0062] Referring to FIGS. 6a and 6b, the metallic disk 102 of the above described surface finish stylus is illustrated. FIG. 6a shows the disk 102 in its non-deformed state prior to being clamped between the first and second members 104 and 106. The first and second members 104 and 106 are arranged to deform (bend) an inner annular portion of the disk 102 to form the lip 110 shown in FIG. 6b by the clamping action that occurs when the bolt is tightened. The lip 110 acts to ensure the disk 102 is held firmly in place by the first and second members 104 and 106; i.e. lateral motion of the disk 102 is prevented during surface finish measurements.
[0063] Although a flat metallic disk of uniform thickness of the type shown in FIGS. 6a and 6b could be used, it is also possible to provide a disk 114 having an outer edge region 116 formed from a thinner material than the rest of the disk 114 as shown in FIG. 7.
[0064] Referring to FIGS. 8 and 9, it will be described how a theoretical model has been used to predict how accurately the contact element (i.e. the disk having an edge with an effective radius of 25 m, which is also referred to below as an R25 m disk) of the of the surface finish probe illustrated in FIG. 5b can measure the surface roughness (Ra) of a variety of surfaces.
[0065] FIG. 8 shows how a surface profile 120 can be modelled in two-dimensions. The stylus ball radius (i.e. the radius of the circles 122) is set to 25 m. The stylus ball is then discretised by splitting it into an evenly spaced vertical mesh with spacing equal to the roughness spacing dx. The stylus ball geometry is calculated at each mesh point using the equation of a circle. Initially, an approach distance or separation S is adjusted so that a single mesh point on the stylus ball is in contact with a mesh point on the surface profile. A boundary condition is applied to the first separation point So and the value is set to a similar height to the reference profile.
[0066] At time step t.sub.1, the ball position is moved one sample spacing dx, the separation S is adjusted so that a single mesh point is in contact with the surface profile and the separation and contact point location is stored. This process is repeated for all positions to time step t.sub.N. The new surface profile 124 can be calculated as shown in Equation 1.
P=.sub.i+1.sup.nS.sub.i1S.sub.i (1)
[0067] FIG. 9 shows the error in Ra value predicted using the above model that results from measuring a variety of Rubert samples of known Ra (the Ra of the samples having been measured previously using a calibrated profilometer) using the contact element (i.e. the R25 m disk) of the of the surface finish probe illustrated in FIG. 5b. It can be seen that the majority of surfaces can be measured with an accuracy of better than 5% and that even a casting with narrow vertical valley features can be measured to within 10%.
[0068] FIG. 10 shows the experimental results generated when using the surface finish stylus illustrated in FIG. 5b to measure a vertically milled Rubert samples. In particular the overlapping solid lines 140 in FIG. 10 show three measurements taken along the same path on the surface of the Rubert sample. These experimental results were generated with a sample spacing of 1.67 m, a feed of 100 mm/min and a sampling rate of 1000 points per second. It can also be seen from FIG. 10 that the three profiles taken in the same location give consistent values of 13.46, 13.5 and 13.5 Ra. The profiles thus have good agreement with each other with only some minor differences visible. The dashed line 142 shows the surface roughness of the same sample measured using a profilometer with a much smaller stylus radius. Although this gives a lower value of 13.17 Ra with a very similar profile, it should be noted that the Talysurf profile was not taken in the same location as the experimental results and is likely the cause of the difference. It can thus be seen that reliable surface finish measurements can be taken using the surface finish stylus illustrated in FIG. 5b.
[0069] Referring to FIG. 11, the contact pressure applied to a sample as a function of applied stylus force is illustrated. Examples of standard stylus forces applied by a dedicated profilometers are shown as dashed lines 144 and 146, whilst the force applied by an existing machine tool probe is illustrated as dashed line 148. The curves 150, 152 and 154 show the contact pressures for the conically tipped styli of a standard profilometer having radii of 2 m, 5 m and 10 m respectively. The Young's Modulus (E) of such conically tipped styli is assumed to be 614 GPa. Curves 160, 161, 162 and 163 show the contact pressures for steel disks (210 GPa) of effective radius 2 m, 5 m, 10 m and 25 m respectively. Curves 175, 176, 177 and 178 show the contact pressures for 25 m effective radius disks of WC (614 GPa), ruby (435 GPa), Macor (67 GPa) and nitinol (45 GPa) respectively. It can thus be seen that materials with a lower E impart less contact pressure when engaged with a surface with a certain force. Selection of such materials can thus also reduce contact pressure for a given application force
[0070] The skilled person would recognise that the above are merely examples of the invention. Alternative stylus structures could also be provided in accordance with the present invention.
