Fixturing And Alignment Of Free-Form Optics For Diamond Turning

Alex Sohn∗

North Carolina State UniversityCampus Box 7918, Raleigh, NC 27695

The fabrication of free-form optics presents unique challenges not only to thediamond turning process, but also to the process by which the whorkpiece islocated with respect to the machine axes. While rotationally symmetric parts onlyrequire alignment in, at most, three directions, free-form optics require alignmentin up to six directions. Distinct from alignment in an optical system, fixturealignment for diamond turning requires a separate set of techniques toaccommodate the forces encountered in machining. Some general principles,alignment techniques, and an application are discussed. While the techniquesdiscussed here focus on diamond turning, many of the considerations apply toother fabrication techniques as well.

Background

Why is alignment important in fabrication? One important thing to remember about partalignment in diamond turning is that it has no effect on the figure error of the part. Figurealignment is mainly impacted by the geometric errors of the machine and tool alignment errors.Hence, it can be argued part placement has no impact on the final optical surface.

While in many cases part location is not critical to the precision of a diamond turned part,for many parts the location of the optical surface with respect to other part surfaces is critical.This is increasingly becoming the case as optical assemblies become more complex and moredifficult to align. Many systems now employ off-axis elements, aspheric surfaces and,increasingly, free-form surfaces. Their presence eliminates many of the characteristic opticalsignatures that were once used to dial in the correct location of spherical and flat surfaces. Thisis why fiducial surfaces and marks are now a requirement for many systems to be aligned withreasonable effort. In fact, optical alignment would be impossible without built-in alignmentaides for some systems. When this is the case, the optical quality of a surface is irrelevant whenit cannot be located properly within the optical system.

Adjustment MechanismAligning optics in an optical system is usually performed with a special alignment fixture

that allows adjustment via micrometer screws. Unfortunately, these fixtures are inherentlycompliant and therefore not suitable for ultraprecision machining operations. Kinematic or quasi-kinematic mounting are also commonly employed for optical mounts. Again, these mounts tendtoward low stiffness due to the limited amount of contact with the workpiece, risking vibrationand static deflection during machining.

The mainstay of optical workpiece fixturing in diamond turning is the vacuum chuck.Vacuum chucking allows rigid mounting of the workpiece without the need for fasteners that

∗ alex_sohn@ncsu.edu

could distort the part. It also allows translation of the workpiece on a flat vacuum chuck and tipand tilt adjustment on a spherical vacuum chuck. The adjustment mechanism is usually atapping device such as a small hammer. While this may seem crude in comparison to amicrometer screw, sub-µm adjustments can be made.

Geometric ConsiderationsWhile the techniques in optical alignment and fabrication alignment are closely related in

purpose, they differ somewhat in their geometric constraints (see Table 1). Oftentimes,rotationally symmetric parts require little or no fabrication alignment because their high level ofsymmetry makes the optical alignment less challenging. With free-form optics, however, opticalalignment in six degrees of freedom quickly becomes daunting, if not impossible without somebuilt-in fiducials with a known relationship to the optical surface. This point becomes ever moretrue as the number of components in a given optical system increases.

The rotational nature of a diamond turning machine (DTM) can work to tremendousadvantage when fabricating rotationally symmetric parts. In the case of diamond turning,alignment for fixturing rotationally symmetric parts is generally limited to two directions: runoutand depth. The rotational axis on which the part is mounted allows the consolidation of at leasttwo and up to four degrees of alignment freedom. For depth, the fixture (commonly a vacuum

Figure 1. Optical alignment fixtures arenot suitable for fixturing parts for

fabrication.Figure 2. The six degrees of freedom in

positioning an optical part

Table 1. Critical degrees of freedom in aligning various surfaces for optical alignment anddiamond turning

Surface D.O.F. (optical) D.O.F. (DT)Flat 3 (Tip, Tilt) 1 (Z)Sphere 3 (X, Y, Focus) 2 (Z, Runout)Asphere 5 (Tip, Tilt, X, Y, Focus) 2 (Z, Runout)Free-form 6 (Tip, Tilt, Rot., X, Y, Focus) 6 (Tip, Tilt, Rot., X, Y, Z)

chuck) is usually located with the diamond tool, providing a reference to the mounting surface ofthe part to be machined relative to the final optical surface.

Due to the lower level of symmetry inherent in free-form optical surfaces, accurateplacement of these surfaces is more difficult than for conventional surfaces. Take, for examplethe case of placing a plane in an optical system versus aligning it to a DTM. The diamondturning machine normally has either a vacuum chuck or collet for mounting the workpiece. Inthe case of the vacuum chuck, the mounting surface has been produced by the machine axesduring the facing operation. This provides a base reference for the mounting surface of the flat.The only optically significant degree of freedom is now the focus or axial translation of the flat.Translation in the radial direction, as well as rotation about the focal axis, have no opticalimpact, while pitch and yaw have been constrained by the vacuum chuck.

As we progress to spheres and aspheres, radial runout becomes critical as well. In thecase of two-surface parts, relative alignment between surfaces is optically significant. While thisadds a level of operations to fixturing, the most effective method for aligning dual surface optics,such as a biconvex lens, is to machine a contoured vacuum chuck. This will parlay thealignment problem into another runout alignment, in this case adding the axial direction.

Moving on to free-form surfaces eliminates all the alignment advantages that therotational symmetry of the DTM grants. The method of alignment for each direction will dependupon the accuracy requirements, the metrology methods available, and the physical constraints ofthe parts. Ideally, it is at this point that the fabricator is included in the design process, so thatmechanical constraints encountered in fabrication can be respected in the optical system design.Usually, a combination of fiducial surfaces and marks will allow accurate location in six degreesof freedom.

Biconic Mirror Fixturing and AlignmentIRMOS, the InfraRed MultiObject Spectrometer, incorporates several off-axis elements

and one free-form surface [1,2]. This surface, called M4, is a biconic ellipsoid with no rotationalaxis of symmetry. M4 thus needed to be located accurately in six degrees of freedom to within±25 µm for translation and 15 arcsec for rotation. In order to achieve this goal, two fiducialsurfaces and eight crosshairs weremachined into the part blank asshown in Figure 3. The twosurfaces, one nominally parallelwith the biconic surface, the otherorthogonal, were diamond flycutinto the Aluminum blank. Theback surface would be mated to thevacuum chuck to account for threedegrees of freedom: tip, tilt, andfocus. The four quasi-kinematicmounts could not be used since thethread locking system was onlyintended for a single use. Thesemounts were also too compliant toallow mounting the part off-axis.Accelerations at the machining Figure 3. Diamond flycut back surface of M4 showing

the four of the alignment crosshairs and three partmounts

speed of 120 rpm wouldhave caused significantdeflection. Translation in Xand Y as well as rotationwould be aligned using twoof the fiducial crosshairs.These crosshairs weremachined using a miniatureba l l endmil l in aconventional CNC millingmachine. The samefiducials used in aligning the parts for fabrication would later be used to align the parts in theoptical system.

While fabricating fiducials presented somewhat of a challenge, the greater question washow they would be used to align parts on the machine. For mounting the parts in their properoff-axis location on the diamond turning machine, a special fixture somewhat reminiscent of apropeller was fabricated to mount two parts at one time (Figure 4). The fixture, which boltsdirectly to the DTM spindle, also. has an integrated vacuum chuck at its center for holding analignment center plug. The off-axis location was necessary to minimize the excursion of the fasttool servo used to machine the non-rotationally symmetric component of the surface. Eachmirror rested on an angled pedestal which had been diamond flycut on two surfaces to within 4arcsec of the correct tilt angle. This pedestal was then mounted on a beam that had been facedoff on the DTM, guaranteeing correct angular placement of the rear surface of the parts. Thebeam had been designed with a section large enough to prevent significant bending duringmachining.

The crosshairs, now located against the vacuum chuck surface of the pedestals, wereviewed through several holes bored through the rear of the chuck. As Shown in Figure 5, acamera with a crosshair generator was placed on the DTM’s X-axis to look at the crosshairs.

While relative alignment of the crosshairs to the camera could now be achieved, thecamera view had to be related to the machine coordinates. Vertical alignment was made bylooking at the top surface of the diamond tool, which, in turn, had been aligned to the spindleaxis via a centering operation. The rotational position of the spindle, which the fast tool servouses to contour the part in that direction, could now be related to the crosshairs on the back of thepart. An important limitation in positioning resolution was, in this case, the spindle encoderresolution. With 10,000 counts per revolution, the positional resolution at the radial position of270 mm was only 160 µm. Fortunately, sub-count interpolation allowed a positional accuracycloser to the 25 µm goal, though this arrangement was hardly ideal. No such limitation existedfor alignment in the radial direction. Since the round-nosed diamond tool does not provide adistinct feature on which to align the camera in the radial position, a needle was mounted on thetoolpost. The needle was then touched on the rotating center plug to produce a small circularscribe that could be measured in a microscope. The needle was then moved using the machineaxes by the intended radial position of the crosshairs minus the radius of the circular scribe.With the camera crosshairs then aligned radially on the needle, the intended location of the partcrosshairs was established (Figure 5).

Figure 4. Alignment Fixture Showing Center Plug and twovacuum pedestals, with mirror blank mounted on left

ConclusionThe result of the machining operation revealed an important point to be considered in part

alignment. While static alignment was successful and accurate within specifications, a dynamiccomponent arose in the rotational direction of the spindle that was missed. Early analysisdeemed the spindle speed to be well within the dynamic limits of the fast tool servo. Theexcursions occurred at a rate of approximately 12 Hz, while the servo bandwidth is around 300Hz. Unfortunately, even with a small phase shift of less than 1°, at a radial distance of 270 mm,a displacement error of more than 3 mm was encountered.

In conclusion, alignment and fixturing for fabrication presents unique challenges distinctfrom optical alignment, particularly in the case of free-form optics. The example presented hasbrought to light a component that is usually not considered: The dynamics of the machiningsystem. Consideration of all alignment issues, including dynamic components, is critical inallowing the implementation of free-form optics.

References1. K. Garrard, A. Sohn, R. G. Ohl, R. Mink, V. J. Chambers.”Off-Axis Biconic Mirror

Fabrication.” Proceedings from the EUSPEN 2002 Annual Meeting (2002).2. Raymond G. Ohl, Werner Preuss, Alex Sohn, Shelly Conkey, Kenneth P. Garrard, John

G. Hagopian, Joseph M. Howard, Jason E. Hylan, Sandra M. Irish, J. Eric Mentzell,Mechthild Schroeder, Leroy M. Sparr, Robert S. Winsor, S. Wahid Zewari, Mathew A.Greenhouse, and John W. MacKenty, “Design and fabrication of diamond machined,aspheric mirrors for ground-based, near-IR astronomy,” Proc. SPIE Vol. 4841, p. 677-688, Instrument Design and Performance for Optical/Infrared Ground-based Telescopes(2003)

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Figure 5. Machining setup showing acrylic test parts, cameraarrangement, and centerplug.