Precision Eng. Handbook of Human-centered Design Japanese , Section 7. Slocum, A. Ito, T. Schaefer ed.
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Slocum Closed frame structures Moderately easy access to work zone. Moderately strong structural loop like a micrometer! Easier to obtain common centers of mass, stiffness, friction.
Slocum Tetrahedral structures Composed of six legs joined at spherical nodes. Work zone in center of tetrahedron. Bearing ways bolted to legs. High thermal stability. High stiffness. Viscous shear damping mechanisms built into the legs. Damping obtained at the leg joints by means of sliding bearing material applied to the self centering spherical joint. Inherently stable shape. Concept developed by the late Dr. Kevin Lindsey at NPL patented. Hollow sphere node L 2 6L 9 L Air bearing spindle Carbon fiber drive shaft with kinematic coupling to spindle and motor not shown Tubular strut Damping beams High viscosity oil interface Spindle block Solid film bearing interface 6L 9 3L 6 Vee slideway with kinematically supported carriage by Alexander H.
As the work volume increases, the structure grows less fast than a tetrahedron. The hexapod Stewart platform concept originally developed for flight simulators1 gives six limited degrees of freedom. The tool angle is limited to about 20 degrees from the vertical. Advanced controller architecture and algorithms make programming possible. As the work volume increases, the structure grows less fast than a tetrahedron or octahedron. The structure is damped with internal ShearDampers2 to minimize vibration in the supporting frame.
This hexapod has an additional two axes on the platform supported by the six struts. Damping is discussed in Topic Slocum Cast iron structures Widely used. Stable with thermal anneal, aging, or vibration stress relieve. Good damping and heat transfer.
Modest cost for modest sizes. Integral ways can be cast in place. Design rules are well established see text. Slocum Structure that can use replicated-in-place internal viscous dampers: Linear guide rolling element bearing Integral way for sliding or hydrostatic bearing Inner regions for replicated internal viscous dampers shaped extruded aluminum.
Round inner tube is structurally the most efficient section for resisting torsional loads. Slocum Welded steel structures Often used for larger structures or small-lot sizes. Stable with thermal anneal. Low damping, improved with shear dampers.
Modest cost. Integral ways can be welded in place. Slocum Welded structure that can use replicated-in-place internal viscous dampers: Linear guide rolling element bearing Integral way for sliding or hydrostatic bearing Inner regions for replicated internal viscous dampers. Plates welded to inner tube through 50 mm x 10 mm access ports located every mm along the length of the column.
Slocum The Tubemill Designed for precision milling of grooves for linear guides in long tubes that serve as precision machine structural components3. Welded steel structure. ShearDamped as needed. During the concept phase, every reasonable plate configuration was considered, and FEA was used to study stiffness and natural frequency. Manufacturing was consulted what can they make.
Marketing was consulted how accurate must it be, and how heavy a cut does the customer really want to make. This is part of Eberhard Bambergs Ph. Slocums lab at MIT. Slocum At the start of the detailed design phase, many different detailed structural configurations should be systematically analyzed: by Alexander H. Slocum Epoxy granite structures Can be cast with intricate passages and inserts.
Cast iron or steel weldments can be cast in place, but beware of differential thermal expansion effects. Epoxy granites lower modulus, and use of foam cores means that local plate modes require special care when designing inserts to which other structures are bolted! Sliding contact bearing surfaces can be replicated onto the epoxy granite. Slocum Foam cores reduce weight: For some large one-of-a-kind machines: A mold is made from thin welded steel plate that remains an integral part of the machine after the material is cast.
Remember to use symmetry to avoid thermal warping. Consider the effects of differential thermal expansion when designing the steel shell.
The steel shell should be fully annealed after it is welded together. Slocum Instead of ribs, polymer concrete structures usually use internal foam cores to maximize the stiffness to weight ratio.
They can have much greater damping. Highly loaded machine substructures e. Polymer concrete does not diffuse heat as well as cast iron. Attention must be paid to the isolation of heat sources to prevent the formation of hot spots. When bolting or grouting non-epoxy granite components to an epoxy granite bed, consider the bimaterial effect.
Precision Machine Design