Some next-generation thinking fromacademia improves thedamping and way systemsof grinding machines.

Diane L. Hallum, assoc. editor

echnical innovations areplaying an important rolein updating a CNC cylin-drical grinder built by Wel-don Machine Tool Inc. The

firm partnered with Prof. AlexanderSlocum, Massachusetts Institute ofTechnology, to incorporate his novelviscous-shear damper and hydro-static bearing ways into an CNC cy-lindrical grinder originally intro-duced in the late 1970s.

The damper controls vibration byshear of a viscoelastic layer withinthe composite base construction.Longitudinal hollow sections, inter-nal tubes, damping tape, and epoxydeliver amplitude-to-resonance val-

perime-ter, such aslow-cost square shapes, and ideallytake up most of the interior of thestructure. Tubes, with diameters 3to 5 mm smaller than the holes, sus-pend inside the longitudinal holes.Before installing, high-loss dampingpolymer tape called ScotchDamp iswrapped around the tubes. Anepoxy is then injected around thesuspended tubes to fill voids be-tween the viscoelastic film of thetape and the machine structure.The epoxy is VibraDamp, a 100%solids two-component polymerspecifically designed for sheardampers. After hardening of theepoxy, the structure is ready for use.

Shear damps any frequencyCommon approaches to machine

damping have drawbacks. Con-strained layer dampers have exte-rior plates that can delaminate un-der loads, diminishing theirdamping ability. These can also in-terfere with placement of externalcomponents on the base. A tuned

ues betterthan that of cast-ironor concrete-filled bases.Coolant, delivered through thetubes, can also prevent temperaturedifferentials from warping the bedand ways.

Similarly clever are the hydro-static bearings. Design considera-tions let the machine use water andeven grinding fluid as a lubricant.The guides also do not need expen-sive machining, yet they provideprecise movement while handlinghigh-speed direction changes.

Leaving concrete behindPolymer concrete and granite

bases have poor modulus and heat-transfer characteristics. Thermalvariations within a cast-iron, con-crete-filled, or polymer-concretebase can cause bowing of the waysurfaces, producing inaccuracies inmachined workpieces. Researchingalternative base designs that mini-mize thermal effects, Dr. Slocumand Weldon Machine Tools devel-oped the viscous-shear system.

The base is a composite structure.The main structure requiresroughly formed longitudinal holes.Holes can be of any shape that max-imize the interior cross-sectional

AMERICAN MACHINIST OCTOBER 199400

G R I N D I N G

Out of the lab and into the shop

T

A Weldon Machine Toolemployee tightens

down the wheelhead rails to the viscous-

shear-damping base of a CNC cylindrical

grinder.

mass damper, also typical in ma-chine bases, is tuned to a single res-onant frequency so it cannot dampthe entire frequency spectrum.

The viscous-shear damper,in contrast, is frequency

insensitive. Its struc-tural base, encapsu-

lating epoxy, andtubes constrainthe viscoelasticlayer of the poly-mer tape to pre-vent its movementexcept by internalshear, which dissi-

pates vibration en-ergy. The poly-

mer hash i g h

impact resistance and high compressive strength anddelivers excellent damping and re-sistance to vibration and shock.

Damping by shear is quite effec-tive. Traditional cast-iron struc-tures have amplification-at-reso-nance factors, known as Q values, of100 to 200. The viscous dampertakes this Q value down to 20, whichis near the ultimate in damping. Bycontrast, polymer concrete struc-tures and concrete structures typi-cally have Q values of 35 to 50. Thelow Q value and high-dynamic stiff-ness of the shear system deliversmachining precision and fine sur-face finishes and tolerates aggres-sive grinding when high productionrates are preferred.

The composite base offers manybenefits. It can be constructed of anymaterial providing the main struc-ture contains longitudinal holes.Additionally, the internal tubes canact as conduits for pumping coolantto the machine base. The coolingminimizes machining errors fromtemperature differentials.

To be truly effective, the damperrequires careful impedance match-ing during the design phase. Everyinstallation must be properly engi-neered for optimum damping per-formance. After impedance match-ing, vibration abatement of the baseis not affected by changes in mass ormotion of machine components.

Self-lubricating is the keyThe Slocum design is different

from previous hydrostatic bearings.Using analytical spreadsheet mod-eling, Prof. Slocum developed abearing that delivers lubricatingfluid to machine ways as needed,thus being self-compensating. Asthe load is applied over a compensa-tion pad, the pad directs fluid fromthe supply pump to a hydrostaticpocket to oppose the load. Bearing-

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(1) Epoxy. Injected around the tubes to create a tight bond between the square tubes and base. It transmits shock loads and vibrations at all frequencies from the base structure to the viscoelastic polymer tape.

(2) A high-loss damping viscoelastic polymer tape wrapped around the internal tubes. Relative motion between the tubes and the base shears the viscoelastic layer of the tape to dissipate vibration energy to the submicron level.

(3) Internal tubes. Can act as conduits for temperature-controlled fluid to minimize machinery temperature differentials.

(4) Base structure. The longitudinal hole of this structure must maximize the interior perimeter for greater contact with the damping materials.

(5) Neutral axis of the tubes(6) Neutral axis of the base structure(7) Plates. Welded to the interior tubes to displace the tubes’ neutral axes as far away from that of

the base as possible. This results in relative motion between the base and the tubes that shears the viscous layer of the polymer tape.

The hydrostatic bearing design incorporates fullyconstrained ways rather than conventional, expensivelymachined V and flat ways. Since fully constrained waysare preloaded in all directions, they can withstand highaccelerations and direction changes while stillmaintaining their precise motion resolution.

disadvantages of earlier hydrostaticbearings such as hand-tuning eachpocket and sensitivity to contamina-tion, which restricted the bearing tohigh-viscous oils, and extensive ma-chining required for their high-tol-erance construction.

Bearing-load characteristics areattractive. The hydrostatic bearingboasts high bidirectional load capac-ity and stiffness when preloaded inan opposed pad configuration. Theresult is a design that is insensitiveto manufacturing tolerances be-cause bearing gap variations onlyaffect the fluid flow rate, not theload capacity or the shape of thestiffness-response curve of the sys-tem. Port size, typically 3 mm, is in-sensitive to dirt and, because of theconstant flow of fluid, the bearing isself cleaning. Also beneficial, the useof low viscosity water-based fluidspermits high travel speeds for theguides. A state of zero static frictionon the hydrostatic bearing results invery high repeatability, unlimitedmotion resolution, and no wear ofthe guides. This translates to a po-tentially infinite bearing life. �

fluid flow into thebearing pocket isproportional to thebearing load thepocket must re-sist. Large clear-ances at fluidports decrease thepotential for clog-ging. This allowsthe use of water orgrinding fluid, af-ter being speciallyfiltered, to act asthe hydrostaticfluid. These inno-vations overcome

A Ph.D. from MIT brings newtechnology to grinding

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Flow paths of self-compensating hydrostatic bearings (1) Gear pump. Delivers bearing fluid to ports in the self-compensating pads of the machine ways. The gear pump avoids the vibration that hydraulic pumps induce.

(2) Self-compensation pad. Lubricating fluid enters the pad through ports at the edge of the pad and flows to collector grooves that act as feedback sources for the bearing. The fluid collector at the center of the pad feeds and controls the amount of fluid that goes to an opposing bearing pad.

(3) Bearing pad. As the load is applied over the pad, it opposes the load by directing fluid from the supply pump to a pocket or sump.

(4) Two travel paths for lubricating fluid. Fluid from the lower port flows to the upper load-bearing pad of the guide, while the upper port delivers hydrostatic fluid to the lower load-bearing pad.

The bearing design eliminates the hand-tuning of each fluid pocket, large clearances, and tight construction tolerances of earlierhydrostatic ways. The design allows the use of water-based fluids.

Vibration damping byviscous-shear (A) is10 times moreeffective than that ofhollow basesconstructed of steel(C), and four timesmore effective thanthat of concrete-filled(B) bases.

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Vibration amplitudes over time for a number of bases