Volume 7, Number 6 November/December 1999

Transcription

1 Volume 7, Number 6 November/December 1999 A Tutorial on Deterministic Microgrinding By Jeff Ruckman Center for Optics Manufacturing, University of Rochester The perfect glass fabrication process would instantly and exactly remove all excess material from a glass blank. Since this theoretically ideal process has not yet been invented, real world fabrication processes are a compromise between speed and surface accuracy. The result is a multi-step process that uses high removal rate processes (grinding/lapping) to near net shape the part, followed by high accuracy processes (polishing) to achieve the final shape and finishes. The higher forces and larger abrasives used to achieve high removal rates during grinding result in figure errors, subsurface damage (SSD) and rough finishes which are later eliminated or corrected using an iterative, slow polishing process. Traditional glass fabrication processes require the polishing skills of opticians to finalize the part. Unfortunately, the surface quality results achievable with traditional grinding and lapping processes are variable, and the optician must continually adjust the finishing process to the changing states of the surface. As a result, traditional finishing processes are not deterministic and it takes significant skill and time to achieve needed surface specifications. These issues drove COM to develop deterministic microgrinding (DMG), a grinding process that takes advantage of advances in process knowledge and machine and tool design to provide a repeatable, precisely ground surface. Better quality control of a ground surface minimizes the time required to finish the surface. In this article, we review the highlights of these advancements. DMG is a deterministic, computer-controlled grinding process that: Precisely shapes the optical surface to within 1 wave p-v in 5-10 minutes. Minimizes subsurface damage. Produces a microground surface finish as smooth as Å rms, allowing metrology for finishing without further need to gray out the surface. Produces surfaces that meet specifications in many IR applications without polishing. Creates any shape that can be described to a computer-controlled machine. Inside: APOMA Workshop... 5 APOMA Travel Grants... 6 Job Postings... 7 Optical Standards... 7 Industry Calendar continued on page 2 Copyright University of Rochester CENTER FOR OPTICS MANUFACTURING 1

2 continued from page 1 New Process Knowledge One advantage of the DMG process is that it uses grinding tools made from abrasives bound in a metal, resin or vitrified matrix. This advantage was documented in the September 1997 Convergence lead article by John Lambropoulos. Professor Lambropoulos found that the DMG process achieved higher removal rates and produced superior surfaces (reduced surface roughness and subsurface damage, lower residual stress) than loose abrasive grinding (lapping) processes using the same abrasive size (see Figure 1). Figure 1: Lapping vs. DMG; DMG consistently produces better figure, radius, subsurface damage and finish results. Faster grinding rates and better surfaces result in substantially faster polishing, dramatically reducing the amount of time needed to fabricate a surface (see Figure 2). The advantage is in grinding performance results from differences in the way the abrasive acts on the glass surface. In loose abrasive lapping, the abrasive is rolling between a lap and the glass surface under constant pressure. When using the DMG process, the abrasive is bound in a wheel and the rotating wheel is advanced into the glass at a controlled infeed rate. 5X grinding rate + 4X better surface = 4X faster polishing! Nominal abrasive size Surface removal rate (µm/min.) RMS surface roughness (µm) Polishing (hours) Deterministic Microgrinding Deterministic Microgrinding Loose Abrasive Grinding Deterministic microgrinding vs loose abrasive grinding Conventional Lapping Figure 2: Impact on Affordability. Better surfaces dramatically improve affordability. 2 continued on page 3

3 continued from page 2 In both loose abrasive grinding and DMG processes, 2-3 grinding iterations are made using successively smaller abrasives. The reason for this is the compromise between speed and accuracy. The finish of a ground surface and the maximum grinding rate are related to the size of the abrasive. The use of smaller abrasives results in better finishes and lower SSD, but smaller maximum removal rates. The cost-effective solution is to remove most of the material with a fast-removing, large abrasive and then to finish with a slower-removal, small abrasive. DMG is a carefully designed and controlled, multi-step grinding process that is possible because of recent advancements in grinding machine design and control µm tool at Grind Start µm tool at Grind Finish µm tool µm tool diamonds diamonds ~ 5 µm Total material removed ~ 90 µm ~ 10 µm mean p-v surface roughness (σ is approx 3 µm) 2-4 µm tool at Grind Start 2-4 µm tool at Grind Finish ~ 2-4 µm tool ~ 2-4 µm tool diamonds diamonds Total material removed ~ 20 µm Figure 3: Interaction between glass and bound abrasive tooling. Figure 3: Interaction between glass and bound abrasive tooling. Figure 3 shows the interaction of the abrasives at the start and end of the medium and fine DMG cycle. In the top left image, the surface left behind by the rough grind is shown in profile. A small section of the grinding wheel with µm diamond abrasives is shown, at the top, making contact with the glass. DMG removes the material by brittle failure of the surface of hard materials. The cracks or subsurface damage (SSD) in the surface profile result from the brittle removal mechanism. These cracks extend some distance into the surface and are related to the material hardness, fracture toughness and the grinding process parameters. In this case, the SSD depth averages 30 µm with one sigma variation of 20 µm (typical for BK7 with a 65 µm rough tool grinding in a ring tool configuration). In order to be confident that we have ground through the damage left from the rough grind, we set continued on page 4 3

4 continued from page 3 the depth of cut for the µm wheel to be the mean damage depth +3σ (90 µm). The top right picture shows the results of completing this grinding pass. The surface roughness was improved and now the SSD depth averages 10 µm with one sigma of variation equal to 3 µm. We repeat the process with a fine tool (2-4 µm diamond abrasives) and the resulting surface is shown in the bottom right image. Depending on the material, the final surface is as smooth as Å rms, has figure accuracy less than 1 wave p-v and will have less than 2 µm SSD to be removed by the polishing process. An additional benefit occurs when the process is designed properly: the grinding wheels are continually dressed by the relatively rough glass surface, making the process very stable. Machine and Tool Design The machine tool design feature that enables the DMG process is the ability of the very accurate, computer numerically controlled (CNC) grinders to deliver the bound abrasive to exactly the right spot on the surface of the glass. In order to do this, DMG machines have less than 0.1 µm or better programmable resolution, precise control of infeed at slow rates (5 µm/min typical), accurate, stiff and well-damped linear and rotary position control, repeatable tool changers, and high precision speed controlled work and tool spindles. CNC machines have enormous flexibility because they are easily programmed to handle a wide variety of tasks. For example, the tool changer allows one machine to rough grind, fine grind, bevel and center, all in one set-up, replacing three or four machine set-ups. In addition, other unique operations like grinding a mounting flat or drilling a hole are easily programmed. Tool design for DMG also offers advantages over traditional processes. If lapping is used, the lap must be uniquely prepared for each lens radius, requiring substantial investments in both time and dollars for the specialized tooling needed for each lens made. In the DMG process, no dedicated tooling is required. Grinding spherical optics using ring tools requires only 4 sets of tools (one tool set = rough tool, medium tool, fine tool) to be able to grind lenses with a range of diameters from mm, convex or concave with an R# (ratio of radius to the diameter) greater than 0.7 (see Figure 4). One tool set can cut a range of different radius and diameter parts. Tool φ 65 mm Tool φ 65 mm 50 mm 100 mm Figure 4: Deterministic microgrinding (DMG) eliminates the need for dedicated tooling. Deterministic microgrinding is a proven, cost-effective fabrication system that is currently being used by many precision optics fabricators. If you have questions about DMG, please contact Jeff Ruckman (phone , fax , or JRUC-COM@LLE.ROCHESTER.EDU). 4

5 Workshop attendees observing a demonstration on the OptiPro SX50. The inaugural 1999 APOMA Industry Workshop, held at the Center for Optics Manufacturing (Rochester, NY), was a resounding success! The attendees hailed from 34 different companies, for a total of 55 students. Representatives attended from: Advanced Glass Industries, Applied Physics Specialties Ltd., Brent America, Cosmo Optics, Eagle Picher, Eastman Kodak Company, Edmund Scientific Co., ELCAN Optical, Esco Products Inc., Fisba Optik, Glass Fab, Gould Precision Optics Inc., GSI Lumonics Inc., Harold Johnson Optical Lab, II-VI Inc., JML Optical Industries, Lapmaster International, Lightwave Products, McCarter Machine, Mindrum Precision, Model Optics Inc., Optics for Research, Optimax, PFG Precision Optics, QED Technologies, Saint Gobain, Schott Glass Technologies, Spectrum Thin Films, Stefan Sydor Optics, Tower Optical, U.S. Precision Lens, United Workshop attendees observing a demonstration on the OptiPro SX150. Kathleen Richardson (UCF/CREOL) and Alex Marker (Schott Glass Technologies) in the magnetorheological finishing laboratory at COM. Nanotech 500FG, on display at the Center for Optics Manufacturing. 5

6 The American Precision Optics Manufacturers Association (APOMA) is pleased to announce that it will be awarding up to 5 travel grants to students attending optical fabrication or optical material science-related conferences during the 2000 calendar year. The $500 travel grants are intended to encourage student participation at conferences that broaden their academic education, while providing first hand exposure to optical industry technical and manufacturing issues. The grants may be used to defer costs associated with transportation, registration or housing. Guidelines and eligibility are given below. Deadline for application is February 15th, with awards to be announced mid-march (awards for meetings held earlier in 2000 than the application/award dates will be considered and not penalized, and will be made, where appropriate, following the decision to the respective awardees). Eligible topics closely related to the goals of APOMA and APOMA-member companies will be considered most favorably. Congratulations to 1999 Travel Award Winners! Yi Li, University of Rochester, Department of Mechanical Engineering, presented Mechanism of chatter in deterministic microgrinding of brittle materials at the SPIE Annual Meeting, Denver CO (July 1999). David Fischer, University of Rochester, Institute of Optics, presented, Design and Manufacture of a Gradient-Index Axicon at the OSA Annual Meeting, Santa Clara CA (September 1999). Award Guidelines Students must be enrolled full time in an undergraduate or graduate program. Signature of the advisor verifying this fact is required. Preference will be given to those students presenting an oral or poster presentation of their work. A copy of the submitted abstract is recommended, but not required, for consideration. One award per meeting will be made, unless there are insufficient applicants to warrant such policy. Award winners agree to have their presentation title and abstract listed on the APOMA web site if chosen. Application materials to be mailed to the APOMA student award committee include: Student name, address and phone number, University, major, advisor s name, graduate or undergraduate standing, title of presentation and abstract, validation of full time status by advisor/supervisor, conference the student is attending, dates of meeting, location of meeting. Incomplete applications will not be considered. Please mail applications to: APOMA Student Travel Grants, Dr. Kathleen Richardson, UCF, CREOL/School of Optics, 4000 Central FL Blvd., Orlando, FL APOMA SWAP SHEET For Sale: 29.5 O.D. Hindle reference sphere, R = C.C. with a sling type mount. For information, please contact Bob Goff at Astronomically Xenogenic Enterprises (phone , fax , goffaxe@azstamet.com). WELCOME NEW APOMA MEMBERS! Spectrum Thin Films 100 E. Knickerbocker Avenue Bohemia, NY Rep: Anthony Pirera Phone: Fax: McCarter Machine Co. Inc. P.O. Box 520 Deer Park, TX Rep: Douglas McCarter Phone: Fax: ENJ Medical, Inc East Slauson Avenue Santa Fe Springs, CA Rep: Elias Kim Phone: Fax:

7 COMPANY: GSI Lumonics Inc., Optics 39 Auriga Drive Nepean, Ontario, K2E 7Y8, CANADA Attn: Jean Runyon Fax: Title: Thin Film Coating Technician Description: Responsible for applying thin film coatings to finished optical components. Duties will include the set up, troubleshooting, and maintenance of vacuum systems for multi-cavity optical thin films. Opportunities to work with our engineering group to provide technical input and creativity to designs are available. Experience: Minimum of 5 years hands-on experience in the operation of vacuum systems, ion-assisted deposition, multi-cavity filters fabrication, testing of coating elements and cleaning procedures for optical components, is required. In addition, the proven ability to work successfully within a team environment, as well as independently, is essential. Familiarity with thin film design is an asset. Please send resume to address above. COMPANY: Plummer Precision Optics 601 Montgomery Avenue Pennsburg, PA Attn: Human Resources Department Fax: Title: Senior Optical Fabricator (Conventional Polishing) Description: Assist the Conventional Polishing Manager with all facets of production. Perform complex set ups, troubleshoot production problems, and assist other operators in diagnosing and resolving polishing production problems. Experience: Candidates should have a Bachelor s degree or 7-10 years experience in an optical manufacturing facility, or equivalent combination of education and experience. Knowledge of optical fabricating processes including, but not limited to, the following is necessary: blocking, generating, grinding, polishing, coring, sawing, and beveling. JOB POSTINGS Title: Senior Optical Fabricator (High Speed Polishing) Description: Assist the High Speed Polishing Manager with all facets of production. Set up and operate Optotech CNC equipment in a production environment, including tooling requirements, editing CNC programs to achieve proper machine operation to meet blueprint specifications, troubleshooting production problems and assisting other operators in diagnosing and resolving high speed polishing production problems. Experience: Candidates should have a Bachelor s degree or 5-7 years experience in an optical manufacturing facility, or equivalent combination of education and experience. Title: Senior Optical Coating Technician Description: Set up/operate vacuum coating chambers to perform complex thin-film coatings. Assist the Coating Department Manager with training of new optics coaters and developing new coating designs. Use of appropriate test equipment to verify results against specifications; use of computer software to debug errant coating runs; apply knowledge of operating principles when required; perform routine maintenance and troubleshooting as needed. Experience: BS in Physics or Chemistry with a minimum of 4 years experience in manufacturing. Please send resume to address above. COMPANY: Speedring Systems, Inc Waterview Drive Rochester Hills, MI Attn: Barb Mobey Fax: barbm@speedringsys.com Title: Senior Diamond Machinist Description: To program and turn aspheric profiles in aluminum, nickel, copper and other optical metals. Must be able to use precision optical measuring equipment; read/interpret complex blueprints; perform setups. Some training of entry-level machine operators. Experience: Minimum 5 years experience as a diamond machinist in the optics industry. Please send resume to address above....a New U.S. Optical Glass Standard... OEOSC/OP, the ANSI accredited standards committee, is developing a new U.S. standard for optical glass. This standard will replace the MIL-G-174, Optical Glass, specification that was placed on the inactive list in August MIL-G-174 can no longer be used for new DOD contracts. The draft of the new standard will be publicly reviewed at a session in San Jose, California on Monday, January 24, 2000, from 2:00 p.m. - 5:00 p.m. The location has not yet been established, but will be associated with the Photonics West 2000 conference. Anyone interested in reviewing this draft standard should attend this session; advance copies of the current draft can be obtained from Michele Richard at the Center for Optics Manfacturing (phone , fax , MRIC_COM@LLE.ROCHESTER.EDU). Please contact Gene Kohlenberg, Executive Director of the Optics and Electro-Optics Standards Council, to register your intent to attend and to provide your address, phone, fax and so that the meeting location can be sent to you when it becomes available. Mr. Kohlenberg can be reached at: OEOSC P.O. Box Rochester, NY Phone/Fax: gene.kohlenberg@prodigy.net 7

8 1999/2000 INDUSTRY EVENTS Representatives from COM will be attending the following optics industry events. Stop and see us at: Defense Manufacturing Conference, Miami Beach, FL November 29 - December 2, 1999 Photonics West 2000, San Jose, CA January 22-28, 2000 AeroSense 2000, Orlando, FL April 24-28, 2000 CLEO 2000, San Francisco, CA May 9-11, 2000 OSA Optical Fabrication & Testing 2000, Quebec City, CANADA June 18-22, 2000 NEWSLETTER MAILING LIST UPDATE Please let us know of any address corrections, additions, or deletions by completing this form. Mail or fax to Michele Richard at the Center, fax Name: Title: Company: Address: City, State, Zip: Phone: Fax: 8 Stop by and visit COM on the web! CENTER FOR OPTICS MANUFACTURING University of Rochester 240 East River Road Rochester, NY We welcome your submissions! Fax or your news and job postings to Michele Richard at COM fax: MRIC_COM@LLE.ROCHESTER.EDU