Diamond Point Turning / Machining (DPT / DPM) Considerations

Diamond Turned Optical Elements

Areas such as the type required for alignment and collimation of the optical elements in their final subassembly or assembly is greatly reduced by use of integral precision reference surfaces that defines the optical axis and a face normal to the optical axis that can be built into the unit. A further advantage of precision machined or diamond turned optical surfaces is that they can be incorporated as a part of the primary structure that would normally be used to support the optics in use. A considerable savings can be realized in the requirement for fewer parts and also the characteristic of automatic collimation by construction.

When designing substrates for diamond turned optics it is essential that the designer and the manufacturer work closely together. The mechanical design of the optical element has a very strong influence on practical fabrication techniques for the element and on its subsequent performance when mounted in an assembly. It must be remembered that the components being produced, even though they are being cut by a machine tool, whether it be milling machine, lathe, or generator type machine, are still to be used as optical elements. The figure of the surface produced, therefore, in order to be of value is in most cases measured in microinch units. Microinch deflections cannot be controlled by accident or fortunate circumstance but must be part of the design concept. In providing a design for micromachining, a mating interface between the fixture and part must be provided that will cause no part deflection when the component is secured in place for machining. Ideally the fixture surface will duplicate the point of attachment of the part in its assembly and will be free of redundant fits. This can be accomplished by use of localized mounting surfaces in the area of retaining hardware as opposed to full surface contact between the part and fixture. Additionally, the part design should decouple mechanical stresses induced in the reflecting surface by assembly mounting surfaces that it may not be possible to machine to the same tolerance as the process fixture.

Diamond turning is a secondary operation, in that the amount of material removed is very small in comparison to normal machine tool function, therefore, in tolerancing the substrates, it must be kept in mind, that the thickness variation of the blanks have to accommodate this minimal stock removal. (.001 to .002 inches is a reasonable tolerance range.)

One of the great advantages of diamond machining is that very precise relationships can be held between the surfaces that are diamond turned and important mounting or reference surfaces. In scanning mirrors, a tolerance of significance is the parallelism of the centerline of the support shaft to the surface of the optic. If this tolerance is not controlled as the optic is scanned about the mechanical axis, the line of sight will be deviated to sweep a conical trace rather than a plane. This will create an azimuthal error.

An expansion of reference surface control by fixturing is illustrated in the fabrication of a beam expander assembly. This is an outstanding example of the secondary advantages offered by the diamond turning manufacturing process. In this example, the expander is an off-axis concave parabola. The substrate has been constructed with precision mounting diameters and a locating pinhole precisely spaced from the back end of the optic. A mounting hole from the center of the fixture is provided which duplicates the offset distance of the optical axis in the system. When the part is mounted in its assembly the precision locating pin will be used to establish part focus and orientation. The bearing diameters establish the pointing direction and the axial location of the optic.

The fixture is a two-piece construction that has been diamond turned on all flat surfaces. The parts are bolted together with bores to hold the parts and holes to locate the locking pins. These are then precision machined so that the blanks will be located as precisely as they will be in the final assembly.

Vibration during the cutting operation needs thoughtful consideration in order to minimize its influence. It is not always easy to determine which parts will vibrate objectionably when subjected to the extremely small forces of diamond machining. There are several theories about part reaction to the tool impact of this kind of machining operation. These range from part vibration either independent of the fixture, or fixture vibration that carries the part, or acoustical waves generated in the surface of the part. Poor surface finish will result if improper fixturing allows the part to vibrate. This subject is always carefully considered when designing workholding setups.

Substrate Heat Treatment

Another step in the process of producing acceptable diamond turned components is heat treatment of the substrate. The purpose of the heat treatment is to stress stabilize the finished part. Our specialized heat treat series are applied after all machining, excepting the final diamond turning, has been completed. The most commonly used materials are aluminum, copper, brass and electro-less nickel.

Worthy of special note is the difference in the two 6061-T6 series aluminum heat treats. The Compression Stabilization heat treat (Series #7) is substituted for the Stress Stabilizing heat treat (Series #3) when the part has a very unfavorable thickness to length ratio and has a relatively thin cross-section. The part must also be stackable and fillers may be used. When this procedure is employed the parts are stacked between appropriately proportioned end plates. Tie bolts are used to compress the stack and flatten the parts. The entire stack is then placed in the temperature chamber and cycled. The object is to relieve the stress in the parts while they are clamped flat so that when they are released there will be no residual forces with enough energy to deflect the plates.

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