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Diamond turningis a process of mechanicalmachiningof precision elements

usinglathesor derivative machine tools (e.g., turn-mills, rotary transfers)

equipped with natural or syntheticdiamond-tippedtool bits.The term single-

point diamond turning(SPDT) is sometimes applied, although as with other

lathe work, the "single-point" label is sometimes only nominal (radiused toolnoses and contoured form tools being options). The process of diamond turning

is widely used to manufacture high-qualityasphericoptical elements

fromcrystals,metals,acrylic,and other materials. Optical elements produced by

the means of diamond turning are used in optical assemblies intelescopes,video

projectors,missile guidancesystems, lasers, scientific research instruments, and

numerous other systems and devices. Most SPDT today is done withcomputer

numerical control(CNC) machine tools. Diamonds also serve in other machining

processes, such as milling, grinding, and honing

Process[edit]

Diamond turning is a multi-stage process. Initial stages of machining are carried

out using a series of CNC lathes of increasing accuracy. A diamond-tipped lathe

tool is used in the final stages of the manufacturing process to achieve sub-

nanometerlevel surface finishes and sub-micrometerform accuracies. The

surface finish quality is measured as the peak-to-valley distance of the grooves

left by the lathe. The form accuracy is measured as amean deviationfrom the

ideal target form. Quality of surface finish and form accuracy is monitored

throughout the manufacturing process using such equipment as contact and

laserprofilometers,laserinterferometers,opticalandelectron microscopes.

Diamond turning is most often used for making infrared optics, because at longer

wavelengths optical performance is less sensitive to surface finish quality, and

because many of the materials used are difficult to polish with traditional methods.

Temperature control is crucial, because the surface must be accurate on

distance scales shorter than the wavelength of light. Temperature changes of a

few degrees during machining can alter the form of the surface enough to have

an effect. The main spindle may be cooled with a liquid coolant to prevent

temperature deviations.

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The diamonds that are used in the process are incredibly strong in the vertical

downward direction but very weak in the upward and sideways directions.

The machine tool[edit]

For best possible quality natural diamonds are used as single-point cutting

elements during the final stages of the machining process. A CNC SPDT lathe

rests atop a high-qualitygranitebase with micrometer surface finish quality. The

granite base is placed on air suspension on a solid foundation, keeping its

working surface strictly horizontal. The machine tool components are placed on

top of the granite base and can be moved with high degree of accuracy using a

high-pressure air cushion or hydraulic suspension. The machined element is

attached to an airchuckusing negative air pressure and is usually centered

manually using a micrometer. The chuck itself is separated from the electric

motor that spins it by another air suspension.

The cutting tool is moved with nanometer precision by a combination of electric

motors andpiezoelectricactuators. As with other CNC machines, the motion of

the tool is controlled by a list of coordinates generated by a computer. Typically,

the part to be created is first described using aCADmodel, then converted toG-

codeusing aCAMprogram, and the G-code is then executed by the machine

control computer to move the cutting tool. [citation needed]The final surface is achieved

with a series of cutting passes of decreasing depth.

Alternative methods of diamond machining in practice also include diamond fly

cutting and diamond milling. Diamond fly cutting can be used to

generatediffraction gratingsand other linear patterns with appropriately

contoured diamond shapes. Diamond milling can be used to generate aspheric

lens arrays by annulus cutting methods with a spherical diamond tool.

Materials[edit]

Diamond turning is specifically useful when cutting materials that are viable as

infrared optical components and certain non-linear optical components such

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asKDP. KDPis a perfect material in application for diamond turning, because

the material is very desirable for itsoptical modulatingproperties, yet it is

impossible to make optics from this material using conventional methods. KDP is

water soluble, so conventional grinding andpolishingtechniques are not effectivein producing optics. Diamond turning works well to produce optics from KDP.

Generally, diamond turning is restricted to certain materials. Materials that are

readily machinable include:[1]

Plastics

Acetal

Acrylic

Nylon

Polycarbonate Polypropylene

Polystyrene

Zeonex

Metals

Aluminumandaluminium alloys

Brass

Copper

Gold

Electroless nickel platingon other materials

Silver

Tin

Zinc

Infrared crystals

Cadmium sulfide

Cadmium telluride

Calcium fluoride

Cesium iodide Gallium arsenide

Germanium

Lithium niobate

Potassium bromide

Potassium dihydrogen phosphate(KDP)

Silicon

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Sodium chloride

Tellurium dioxide

Zinc selenide

Zinc sulfide

The most often requested materials that are not readily machinable are:[1]

Silicon-based glasses and ceramics

Ferrous materials (steel,iron)

Beryllium

Titanium

Molybdenum

Nickel(except for electroless nickel plating)

Ferrous materials are not readily machinable because the carbon in the diamond

tool chemically reacts with the substrate, leading to tool damage and dulling after

short cut lengths. Several techniques have been investigated to prevent this

reaction, but few have been successful for long diamond machining processes at

mass production scales.

Quality control[edit]

Despite all the automation involved in the diamond turning process, the human

operator still plays the main role in achieving the final result. Quality control is amajor part of the diamond turning process and is required after each stage of

machining, sometimes after each pass of the cutting tool. If it is not detected

immediately, even a minute error during any of the cutting stages results in a

defective part. The extremely high requirements for quality of diamond-turned

optics leave virtually no room for error.

The SPDT manufacturing process produces a relatively high percentage of

defective parts, which must be discarded. As a result, the manufacturing costsare high compared to conventional polishing methods. Even with the relatively

high volume of optical components manufactured using the SPDT process, this

process cannot be classified as mass production, especially when compared with

production of polished optics. Each diamond-turned optical element is

manufactured on an individual basis with extensive manual labor

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