Get a pre preload - Roger Cortesi

1 48 American MachinistDecember 2002machine designhe Overbeck L/T Super Preci-sion grinder developed in apartnership between the MassachusettsInstitute of Technology and OverbeckCorp., Hauppauge, is perhapsthe first production grinder that uses acombination of linear electric motorstogether with aerostatic linear configuration not only provideshigh precision at a low cost, but it alsoensures a great deal of modularity and a result, end users get grinders fully customizedfor their applications in a fraction of the timeneeded for more traditional L/T pairs a PC-based CNC with Axtru-Getapreload of thispresion actuator/bearing technology (so called be-cause it appears as if the axis could be ex-truded). Essentially, air bearings ride on two in-tersecting perpendicular planes for simple andprecise linear motion, while a linear motor at aninclined plane preloads the bearings.

2 These twoplanes represent the most accurate and lowest-cost geometry to guide linear motion. And byplacing the motor at a specific position and an-gle with respect to the air-bearing pads, any de-sired distribution of preload force is grinder takes advantage of the powerfulattractive force of open-faced linear force, which is typically 5 the axial forcewith which the motors actually push an axis,BY PROFESSOR ALEXANDER SLOCUM, MURAT BASARAN, Roger Cortesi , AND PROFESSOR SAMIR NAYFEHEDITED BY PATRICIA L. SMITH, MANAGING EDITORThe authors are with the Massachusetts Institute of Technology in Cambridge, technology ischanging how grinding machines 2002 American Machinist49can so heavily load conventional rolling-element or sliding-contact linear bearings thatthey heat excessively and quickly wear remedy this situation, many other buildersconfigure two open-face motors back to back oruse an ironless core motor.

3 But this arrangementresults in a more complex geometry, which esca-lates the cost of the machine. Instead, the L/Tgrinder uses this magnetic force to hold down thebearings, eliminating the need for an underside keeper rail and, thus, simplifying machine de-sign. In addition, the magnetic force makes theL/T system inherently well preloaded and stableeven in the presence of large externally appliedforces that might induce pitch, yaw, or the motor s attractive force for preload -ing maintains air-bearing stiffness and damping,even when the machine is heavily loaded. TheAxtrusion configuration also eliminates the needfor costly and delicate wraparound preload mech-anisms and lets the entire machine be essentiallymade from simple planar surfaces. Because onecan virtually extrude an axis from simple planarsurfaces to a desired length, ballscrews and linearbearings no longer constrain machine design.

4 In addition to open-faced linear motors, theL/T grinder uses New Way modular porous-carbon air bearings, which, when mounted andThe L/T grinder features a granite surfaceplate base that rests on a steel frame. Upon this are two granite straight edges that providelinear guidance againstwhich the Axtrusionslides Axtrusion concept uses linear motors at an inclinedplane to preload air bearings arranged to provideelastic averaging and high pitch, roll, and yaw trackMotor coreAir-bearing padCarriageBearing rail50 American MachinistDecember 2002pressurized correctly, provide near-critical damping for machine axes. Tomaximize stiffness and damping, sixbearing pads per axis are actually used,where they are suctioned down to theplanes first, and then epoxied into placeto yield a rigid, damped, elastically av-eraged design.

5 Critical damping pre-vents vibration in the machine struc-ture. For further damping, the machinehas a layer of viscoelastic dampingmaterial between its granite surfaceplate base and steel support frame. The final piece of the modular puz-zle is the machine s PC-based CNC,called MachineMate L2. This easy-to-program CNC, which offers both linear and cir-cular interpolation, combines with the Axtrusiondesign to provide plenty of design first production version of the L/T debutedat IMTS 2002. This machine supports up to a1,000-lb load on each axis and accommodates upto a part. Its 20-hp spindle takesfull grinding loads with only submicron deflec-tions in the machine. Overbeck reports that it willcustomize the L/T to handle just about any loadcapacity. In addition, end users can specify theL/T as an internal, external, cylindrical, face, sur-face, center, centerless, or universal grinder.

6 Theycould also request certain wheel spindles, dressertypes, in-process gages, coolant-delivery systems,and many other features. AMOverbeck s L/T grinder went from concept tobuild in just seven months. This aggressive develop-ment plan involved a peer-review evaluation process(PREP), in which Professor Slocum of MIT and Over-beck engineers individually developed machine con-cepts and then reviewed each other s ideas the early concepts for the new grinderwere more complex models with round shaft bear-ings of roller or hydrostatic design. The MIT/Over-beck group also considered a stacked-axis what quickly became apparent was that the bestdesign was the simplest. And the Axtrusion conceptwas the team finalized the design concept, itused an analytical model and spreadsheet to developdimensions and determine system stiffness and thesystem error budget.

7 From there, creating part draw-ings from the solid model was a straightforward task. So that modular elements could rapidly come to-gether, the team members independently precisionmachined components and then aligned them usingfixtures. Members then went to work vacuumingbearings to the granite base, positioning carriages,and injecting epoxy to secure the bearings intoplace. The team also assembled sheetmetal ele-ments to keep grinding fluid away from the granitebase and the bearings. This also insulated the struc-ture for better temperature final step, the electrical system, took less thana week. This was accomplished by implementingOverbeck s new plug-and-play electrical-cabinetdesign method. This system makes it possible toseamlessly integrate all electricals in a short periodof time with a high degree of reliability and and Overbeck designed and built the L/T grinderin seven short months and launched it at IMTS developmentFrom Whitworth s 3-plate scraping method, to Philip skinematic arrangement of air bearings preloaded by alinear motor ( Patent 4,817,930), to replicating inplace an elastically averaged array of air bearings andthen preloading them with the linear electric motor( Patents 5,488,771 and 6,150,740), the Axtrusionis a part of the continual evolution of machine tools.

8 Infact, MIT is no stranger to designing such Slocum and then graduate students NathanKane and Eric Marsh once partnered with Weldon Ma-chine Tool Inc. to build a CNC cylindrical it in an October 1994article entitled Out of the lab and into the shop (p. 60). That same year, the gang at MIT also built an all-ceramic grinding machine with hydrostatic (water) self-compensating bearings with CoorsTek, a ceramic-component manufacturer located in Hillsboro, bearings, combined with mounting the machine tothe floor, give it near-critical damping. This was PaulScagnetti s designs of the pastDecember 2002 American Machinist51 According to Slocum, the half-life of the other preci-sion grinders at CoorsTek is about two years. Afterwhich, ceramic dust basically wears away cast ironstructures.

9 However, MIT s ceramic grinder has beenrunning nonstop, three shifts a day, since Maas, senior engineer at CoorsTek whoworked closely with Scagnetti confirms that thismachine is a production workhorse, seeing about 18 to 20 hours a day of service in rough stock removal. He says the machine often runs unmanned,with operators loading parts at the end of a shift andtaking them off the following morning. Interestingly, Maas was also involved in the peer-review process for the Overbeck L/T grinder. He sug-gested that the first prototype should use a granitesurface plate. But future versions of the machine willlikely offer a lightweight ceramic MIT-designed, all-ceramic grinding machine has been grindingceramic materials at CoorsTek since1994 easily outliving its other Overbeck L/T grinder s linear motor evenlypreloads the air bearings to compensate for thenumerous forces that typically act on amachine want your feedbackArticle is 575 For product or technicalinformation on this 576 or more information on machine simple, modular designof Overbeck s L/T SuperPrecision grinder makescustomizing the machine forspecific applications quickand easy.

10 (Arrows indicategeneralized forces onthe machine carriage)