This invention relates generally to mechanical systems and more specifically to precision mounting systems relative to a reference plane, with specific application to planar optical components.
Many mechanical assemblies require a component to be precisely positioned on a specific plane. Three points define a plane. It is common to clamp a component against exactly three precise reference pads. For example, some data tape drives use tape cartridges having a metal plate. In these drives, the metal plate must be aligned precisely relative to a magnetic head. The metal plate is typically clamped against exactly three reference pads in a drive chassis. As another example, many optical systems have mirrors, prisms or beam splitter components with a flat side that must be precisely positioned relative to other optical components. It is common to clamp a flat side of these components against exactly three reference pads in a support structure.
In general, more than three pads cannot be fabricated to be perfectly co-planar. If a component is clamped against more than three reference pads, the component is typically either rigidly supported on the three highest reference pads or the component warps to conform to the slightly non-planar (and non-predictable) shape defined by multiple reference pads or the pads deform to a non-predictable (and imprecise) position. However, even though three-pad mounts are assumed to be preferable, there may also be problems with three-pad mounts as illustrated below.
FIGS. 1A and 1B illustrate a typical prior art three-pad rigid mount of a long rectangular flat object 100 with mechanical clamping. FIG. 1A depicts a top view, with three flat pads (102, 104, 106). Clamps press at the points indicated by reference numbers 108 and 110. FIG. 1B illustrates a side view with the clamping forces depicted by arrows 112 and 114. FIG. 1C illustrates an end view of pad 102. In general, machining marks on pad 102 will create some high and low points so that pad 102 is never perfectly flat. In addition, object 100 is never perfectly rigid. Object 100 rests on the highest point or highest surface of pad 102. If clamping force 112 is slightly off-center relative to the highest point or highest surface on pad 102, the high point of pad 102 acts as a pivot point and force 112 tends to cause the object 100 to bend or twist slightly. This is illustrated in figure 1C, with off-center force 112 tending to cause object 100 to bend toward the position depicted by the dashed lines 116. This may change the plane of a critical surface of the object 100 or cause a critical surface of object 100 to be non-planar. With mechanical shock, the object 100 may move from an initial high point on pad 102 to a different high point on pad 102. This may directly change the plane and may also change the pivot point resulting in a different bending or twisting. If clamping force 112 is slightly non-vertical, there is a transverse force 118 on the object 100, tending to cause object 100 to twist or if friction is overcome the object may slide. The system might be initially calibrated, and then with mechanical shock sliding may result in different high points, different bending and different twisting, with all these changes being non-predictable. In some mechanical systems, these slight pivoting, bending, twisting and sliding motions are too small to be of importance. However, some systems require a very high precision that cannot be satisfied by the mounting system illustrated in FIGS. 1A, 1B and 1C. Alternatively, in some systems an initial calibration is made that might be unacceptably altered if the plane or bending of the object shifts with later mechanical shock.
Objects may also be mounted by using an adhesive. In general, adhesives eliminate distortion problems due to clamping forces. In a high-volume production environment, however, curing time may be a disadvantage. In addition, adhesives may result in distortion if there is a thermal mismatch between the material of the mounted object and the material of the substrate. Some adhesives may be susceptible to failure during extreme environmental conditions (heat, humidity, mechanical shock). In general, for high volume manufacturing, mechanical clamping is often the preferred method of mounting.
There is a need for a rigid planar mount with simple mechanical clamping with high precision and predictability. There is an additional need for stable high precision even with mechanical shock.
A precision planar mount is provided with the following important attributes:
1. Support points are symmetrical in pairs about the clamping forces, eliminating bending due to mis-aligned clamping forces. If there are transverse clamping forces, the mounted object can move without changing its shape or plane.
2. True points are used for support rather than flat pad surfaces, eliminating shifting to different points after mechanical shock.
3. The support points are fabricated with shallow angles, providing a stiff mount with controlled compressibility. By design, the points compress by about half of the worst case machining tolerance, reducing the overall variation due to machining.
4. Exactly four points are used, providing the symmetry of attribute 1 above and providing a predictable bending shape that is controlled to an acceptable amount.
The support points are formed at the intersection of curved surfaces or at the intersection of multiple planar surfaces. In an example embodiment, the points are formed at the intersection of cylindrical surfaces. The points are fabricated by a milling bit comprising a cylindrical machining cutter with partial-cylindrical grooves formed circumferentially in the bit. The outer surface of the milling bit is used to mill two intersecting planes, and the grooves in the milling bit result in sections of cylindrical surfaces intersecting at the same angle as the intersecting planes. The resulting support points are true singular points, but with limited compressibility because of the shallow angles of the material just below the points.
In a specific embodiment, the object is a glass mirror. The mechanical clamping forces are sufficient to slightly compress the support points and the glass in the mirror so that the mirror presses against all the support points, with differing amounts of compression at each point.