Finishing polishing method

Abstract

The invention provides a method of finishing polishing a surface using a polishing element and an abrasive, the method implementing the following steps: (a) prior to said polishing, sticking the polishing element in permanent manner on a support of deformable material; (b) applying pressure on the deformable support once the polishing element is in contact with the surface to be polished so that the polishing element is shaped to match an outline of said surface; and (c) performing finishing polishing using the polishing element shaped in this way.

Description

The present invention relates to a method of performing finishing polishing on a surface using a polishing element and an abrasive.

BACKGROUND OF THE INVENTION

It is already known to perform finishing polishing of a surface with the help of a polishing element, e.g. a felt, and an abrasive, e.g. colloidal silica. The technique that is generally used consists in sticking the felt on a metal tool, optionally with flexible material of the foam type, for example, being interposed between them. That technique presents the drawback of deforming the surface to be polished, and so the results obtained are not as good as they should be.

OBJECTS AND SUMMARY OF THE INVENTION

The present invention provides a method that enables the above-mentioned problem to be remedied, at least in part.

The invention thus provides a method of finishing polishing a surface using a polishing element and an abrasive, the method implementing the following steps:

a) prior to said polishing, sticking the polishing element in permanent manner on a support of deformable material;

b) applying pressure on the deformable support once the polishing element is in contact with the surface to be polished so that the polishing element is shaped to match an outline of said surface; and

c) performing finishing polishing using the polishing element shaped in this way.

Advantageously, said material is deformable in creep and/or is hardenable. By way of example it may be pitch which is heated to a temperature that enables it to creep and which, after b) and before c), is subsequently cooled in order to cause it to harden, or it may be an adhesive which is in the form of a gel and which is hardened, after b) and before c), for example by being polymerized, or by applying radiation, and in particular ultraviolet (UV) radiation. The material may be plaster.

The polishing element may be a felt or any other polishing material suitable for performing finishing polishing that leads to very low levels of roughness (of the order of 1 Angstrom (Å) to a few Å, for example). The polishing material is advantageously carried by a polishing support.

In particular, the finishing polishing may be performed with the help of a machine that imparts relative displacement in translation between the polishing element and the surface to be polished in two perpendicular directions, and possibly also in rotation about an axis parallel to one of said perpendicular directions. Finishing polishing is particularly suitable for use with surfaces of the kind that have a generator line, the surface being disposed in such a manner that the direction of its generator line is parallel to that one of the two above-mentioned directions which is parallel to the axis of rotation.

The method applies in particular to surfaces which, prior to finishing polishing, have previously been subjected to pre-polishing (or “coarse” polishing), and to polishing, proper.

After polishing proper, which can be performed by any known technique, the surface presents roughness lying in the range 3 Å to 15 Å, for example, whereas after finishing polishing, its roughness may lie in the range 1 Å to 5 Å, and more particularly 1 Å to 3 Å for surfaces made of glass, silica, silicon, or ceramic (or even in the range 1 Å to 2 Å for surfaces made of glass or of silica), or in the range 3 Å to 5 Å for a nickel coating on a metal surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention appear better on reading the following description given by way of non-limiting example and with reference to the accompanying drawings, in which:

FIGS. 1A and 1B are section views showing the finishing polishing method of the invention being implemented; and

FIG. 2 shows a machine for performing finishing polishing on a surface in the context of the present invention, while FIGS. 3A and 3B or 4A and 4B show two other embodiments of the machine.

MORE DETAILED DESCRIPTION

The invention relates to finishing polishing for low-roughness surfaces (in particular mirrors) of a shape which is rectangular, for example. The invention relies on using polishing machines having drive systems for imparting movement along two perpendicular linear axes.

The shapes generated on the surfaces may be plane, spherical, or cylindrical in long dimension (meridianal cylinder) or in short dimension (sagittal cylinder), mirrors having a meridianal profile that is elliptical or parabolic, or mirrors that are parabolic or elliptical, representing a portion of a body of revolution.

One of the applications for such surfaces is making mirrors suitable for use in processing X-ray beams emitted by synchrotron type or free electron laser type light sources.

By way of example, the various steps are as follows:

(1) Roughing: this step is conventional for working on materials of the glass or ceramic or silicon type and it consists in shaping the support on which the low-roughness surface is to be made.

This is done using a milling machine or a machining unit fitted with tools that are specific for working with such materials. These tools are conventionally made of brass having fine particles of diamond deposited thereon and fixed to the tool by means of a binder. These operations are performed under a flow of water or other liquid depending on the materials machined. Surface shaping is also performed using this technique.

(2) Smoothing: this is done using tools of a shape that is plane, or cylindrical, or of a profile complementary to the surface to be polished.

The dimensions of such tools are close to those of the surface to be polished. Such tools can be obtained by machining of the kind that is conventional in engineering. The material used may be aluminum, brass, and in some cases pieces of glass stuck to a mechanical support.

A polishing machine is used having a drive system with two linear axes, serving to impart relative shapes to the tool and to the surface to be polished.

For this purpose, the surface that is to be polished is applied to the tool and relative movement is imparted between the surface and the tool parallel to the major length of the surface together with a movement that is perpendicular for the width.

Either the tool or the surface is held stationary and the complementary piece is moved, being driven by the two movements in translation. Alternatively, the tool is driven in translation while the surface is driven to move in perpendicular translation. In addition, either the tool or the surface is free to turn about an axis parallel to its long direction.

A bedding operation is performed using conventional polishing abrasives and the grain size is reduced until a surface is obtained having a fine polished appearance.

Throughout this stage, the tool is retouched until the final geometrical shape is obtained on the surface to be polished to within a few microns (μm).

(3) Pre-polishing using pitch or coarse polishing felts and a conventional polishing abrasive such as cerium oxide, and using the above-described machine. Movement in the long direction (X direction) is of an amplitude lying in the range 5 millimeters (mm) to 200 mm, for example, and in the width direction (Y direction), it is of an amplitude lying, for example, in the range 1 mm to 100 mm.

(4) Polishing proper with iterative correction of shape until reaching the specifications for departure from the ideal shape. The polishing material is pitch which is cast onto the tool and then shaped with the surface to be polished by being heated.

Pitch is removed locally where bulges have been measured.

The polishing abrasive used is constituted by cerium oxide, for example. Polishing pressures lie in the range 5 grams per square centimeter (g/cm²) to 100 g/cm², for example. This polishing is terminated when the surface to be polished has the right shape and there are no longer any points or defects corresponding to the preceding steps. At this stage, roughness lies in the range 3 Å to 15 Å for square analysis zones having a side of 1 μm to 1000 μm.

(5) Super-polished finishing is performed using an abrasive of the colloidal silica type in association with a felt.

If this operation is performed in conventional manner, it generally deforms the surface that is to be polished.

In the invention, the felt is stuck directly to the pitch and advantage is taken of its ability to creep in order to shape the surface of the felt so that it is complementary to the surface that is to be polished. Shaping is performed by heating the pitch, the tool, or the substrate of the surface to be polished, the mold being the surface that is to be polished itself. Heating is performed to a temperature of about 60° C. at which pitch becomes sufficiently viscous to deform in creep without running. The movements and the pressures that are applied are similar to those used during polishing proper (step 4 above), i.e. they lie in the range 5 g/cm² to 100 g/cm².

In the art as normally known for this stage, the felt is stuck to a metal tool, optionally with a flexible material of the foam type, for example, being interposed between the tool and the felt. The advantage of the method of the invention is that it makes a tool available that is hard (pitch at a temperature of 20° C.) with an outline that matches the shape of the surface to be polished. At this stage, the roughness obtained generally lies in the range 1 Å to 2 Å for materials of the glass or silica type. It generally lies in the range 1 Å to 3 Å for silicon and ceramic, and in the range 3 Å to 5 Å for nickel-coated metal surfaces.

In FIG. 1A, which relates to an optical component 1 for polishing that is in the form of a cylinder or a torus, the polishing element 2 (e.g. of thickness lying in the range 0.1 mm to 2 mm) is stuck on the pitch 3 which is secured to a support 4, e.g. a cylindrical support that acts as a polishing tool. The polishing element 2 may be of a porous material such as a felt, and in particular a microcellular polyurethane foam from the supplier Rhodes (Bierkeek, Belgium) or a porous synthetic material (“Finishing Pad”) from the supplier Rodel, or polishing supports (or “drops”) from the supplier Buehler. After the pitch has been heated so as to give it viscosity enabling it to creep without running (e.g. by heating it to a temperature of about 60° C.), the tool 4 is pressed against the surface to be polished 6 of the optical component 1 at a pressure that is sufficient for creep deformation of the pitch to enable the felt 2 in contact with the surface 6 to adopt locally the shape of the surface 6. The felt 2, which may be constituted by one or more pieces, preferably has a surface 5 of area that is equal to that of the surface 6 plus or minus 20%.

The pressure P applied during finishing polishing is of the order of 5 g/cm² to 100 g/cm².

FIG. 1B shows the method for a plane surface or a mirror having a generator line G in the plane of the section.

FIG. 2 shows a machine suitable for use in implementing the finishing polishing method, and also for the earlier steps of smoothing, pre-polishing, and polishing proper.

It presents two parallel sliding bars 11 and 12 along which two sliders 14 and 15 can move in translation in a first direction X, the sliders being interconnected by a coupling bar 16. Each of the sliders carries a respective arm 17, 18, e.g. extending in a direction Y that is perpendicular to the direction X, with height adjacent means 17′, 18′ in a direction Z that is perpendicular both to X and to Y.

Each of the arms presents a housing 19, 20 for receiving a rotary shaft 21, 22 of a part carrier 23 that carries the component 1 whose surface 6 is to be subjected finishing polishing. The tool 4 is placed on a support 15 secured to a table 27 presenting means for moving the tool 4 and thus also the felt 2 in translation in the direction Y perpendicular to the direction X. These means for moving in translation are constituted by slideways 26, for example.

Polishing speeds in each of the directions X and Y can be selected to lie in the range 0.05 meters per second (m/s) to 0.5 m/s, for example.

FIGS. 3A & 3B and FIGS. 4A & 4B show two implementations of finishing polishing (after the support of deformable material, e.g. pitch or adhesive, has hardened). Displacement in the X direction (e.g. in the range 5 mm to 200 mm) performs polishing parallel to the direction of the generator lines of the surface, whereas movement in translation in the Y direction (e.g. in the range 1 mm to 100 mm) and rotation about the axis of (21, 22) enables the surface 6 to polish and follow the polishing element 2 by turning as it is displaced along the Y direction. Thus, the combination of these three movements (translation in the X direction, translation in the Y direction, and rotation) serves to polish the surface over its outline and over its entire length.

In FIGS. 3A & 3B the component 1 is of cylindrical or plane section and is carried by the part carrier 23 and the tool 4 is secured to the support 25. In FIGS. 4A & 4B, the component 1 is cylindrical or plane and is secured to the support 25, while it is the tool 4 that is carried by the part carrier 23.

The method also applies to surfaces that do not have a generator line. Shaping the element 2 improves performance relative to the surface that has the closest generator line.

Claims

1. A method of finishing polishing a surface using a polishing element and an abrasive, the method implementing the following steps:

(a) prior to said polishing, sticking the polishing element in permanent manner on a support of deformable material;

(b) applying pressure on the deformable support once the polishing element is in contact with the surface to be polished so that the polishing element is shaped to match an outline of said surface; and

(c) performing finishing polishing using the polishing element shaped in this way.

2. A method according to claim 1, wherein said material is deformable in creep and/or is hardenable.

3. A method according to claim 2, wherein said material is pitch which is heated to a temperature enabling it to creep and which, once the polishing element has been shaped, is sufficiently cooled in order to harden it.

4. A method according to claim 2, wherein said material is an adhesive of the gel type which, once the polishing element has been shaped, is hardened, e.g. by being polymerized or by applying radiation, in particular ultraviolet radiation.

5. A method according to claim 2, wherein the material is plaster.

6. A method according to claim 1, wherein the polishing element is a felt.

7. A method according to claim 1, wherein said deformable material is carried by a polishing support.

8. A method according to claim 1, wherein the finishing polishing operation is performed using a machine comprising a displacement device for imparting relative movement between the polishing element and the surface to be polished along two mutually perpendicular directions.

9. A method according to claim 8, wherein the displacement device is arranged also to produce relative rotary movement about an axis parallel to one of said two perpendicular directions.

10. A method according to claim 8, wherein the surface presents a generator line of direction parallel to one of said two perpendicular directions.

11. A method according to claim 1, wherein the surface on which the finishing polishing is performed has previously been subjected to:

pre-polishing or coarse polishing; and

polishing.

12. A method according to claim 11, wherein, after polishing, the surface presents roughness lying in the range 3 Å to 15 Å, and after finishing polishing it presents roughness in the range 1 Å to 5 Å.

13. A method according to claim 12, wherein said roughness after finishing polishing lies in the range 1 Å to 3 Å for surfaces of glass, silica, silicon, or ceramic, and in the range 3 Å to 5 Å for a nickel coating on a metal surface.