Study of Magnetorheological Fluid Based Flexible Work Holding Fixture

7/30/2019 Study of Magnetorheological Fluid Based Flexible Work Holding Fixture

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Study of Magnetorheological Fluid Based Flexible WorkHolding Fixture

B Gangadhara Shetty, P S S Prasad

Abstract

Surface roughness measurement of a complex geometry has been one of the challenging tasks of a precision componentmanufacturing industry. Holding such a complex and irregular geometry of components in conventional methods is difficultand sometimes impossible too. In this context, flexible work holding fixtures may come to the rescue of users to hold thecomponent in a desired position and orientation with respect to the measuring device. In the present work, a flexible work

holding fixture using Magnetorheological (MR) fluid has been proposed. This is capable of holding all regular, irregular andcomplex shaped components of any material and constrains all degrees of freedom (DOF). A mathematical model has beendeveloped to evaluate the holding strength of the MR fluid, based on the various forces acting on the component. Theproposed device/method was demonstrated to compare with conventional one.

Keywords: Flexible fixture, Surface roughness, Magnetorheological fluid, Complex geometry

1 IntroductionMagnetorheological (MR) fluids belong to the classify-

cation of smart materials which consist of suspension,carrier liquid and additive. Suspensions are of micron- sizedmagnetizable particles dispersed in carrier liquid. When anexternal magnetic field is applied, the suspended particlespresent in the MR fluid get polarized with north and south

and align themselves as a chain, in the direction of appliedmagnetic field [1]. These magnetic particle chains restrictthe movement of MR fluid, thereby influence the rheolo-gical properties. This is called as magnetorheological effectwhich increases the yield stress of the MR fluid. Thisincrease is rapid, reversible, controllable and proportional to

applied magnetic field. Carrier liquid, like mineral oil, sili-con oil, synthetic oil etc., are another constituent of MR

fluid provides a medium for magnetically active particulatesto remain suspended during the absence of magnetic fieldand to facilitate realignment once magnetic field is applied.Additive help to decrease sedimentation, prevent agglome-ration and oxidation, enhance lubricity, modify viscosity,and inhibit wear. In the absence of an applied magneticfield, MR fluid behaves as a Newtonian fluid and the par-

ticles disperse randomly in a carrier fluid. Then, shear stressof the MR fluid can be described asEq. 1. When the mag-netic field is applied, the MR fluids behave like Binghamplastics with a field dependent yield stress [2] expressed asEq. 2.

= (1) += BMR (2)

= shear stress of MR fluids (N/m2), = viscosity of MR

fluid without magnetic field (pa-s), = shear rate (/sec),

MR = Total shear stress of the MR fluids (N/m2), B = yield

stress induced by the applied magnetic field (B) and itsvalues are dependent on magnetic induction field B (N/m

2).

Relation between yield stress and applied magnetic fieldof a Honge oil based MR fluid for different volume per-centages of suspensions (samples), obtained experimentally(Fig. 1). As can be seen, yield stress of MR fluid increases

with increase in either magnetic field or volume percentage

of suspensions or both. A highest yield stress of 13kPa wasobtained with 40% by volume as suspensions and 0.3816 Tas magnetic field. The most exciting applications of ma-gnetically controllable fluids are MR polishing [3], MR

damper [4], MR brake [5], MR Abrasive Flow Finishing

(MRAFF) process [6] etc.Surface roughness, geometrical dimensions, and profiles

are few important quality measures of a manufacturedcomponent. With increasing demands for higher quality,there has been great interest among manufactures in itsaccurate measurement. These quality characteristics are

measured with different instruments like Stylus Profilo-meter, Stylus Profilometer step measurement, Contact Pro-filometer and coordinate Measuring Machine. These usesdifferent measuring tips like Styluses, Rumania probes etc.which exert a force during its contact with the component.Hence the component has to be held firmly on the table to

overcome these measuring forces. However, regular shaped

components are held firmly on the table with vices, flatblocks, v-blocks, flexible fixture etc. But, holding of tinyand irregular shape of components are difficult task e.g.watch component, automobile parts like axle of speedome-ter needle, turbine blades etc.

Fig. 1Relationship between yield stress and magnetic fieldfor different samples of MR fluid

Presently, these are held using fixtures, templates, mag-netizable v-blocks, universal vice, China clay, adjustablefixture, modular fixture etc. All of these have some limi-tations. A fixture and template may hold only one particularshape and size of the component and hence a separate fix-ture and template is to be designed for each shape and size.Whenever a variety of shapes of component are involved,

consumes more time and needs more cost which alsoincreases the inventory due to the storage of large numberof fixture elements for each shape of component whereas,magnetizable v blocks are suitable only to hold ferrouscomponents. However, universal vice, which have a provi-


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sion to tilt for holding inclined face, but again have thelimitation of shape of component which can held. Next,china clay which holds any shape of the component, butspirit level is to be used for leveling a component each timeand clay is to be changed after every two or three uses.

Adjustable fixture, one type of flexible fixtures, design-ned for family of parts and through adjustment of the

positions of one or more fixture elements (locator and/orclamps), a certain degree of flexibility can be expected [7].

A modular fixture, another type of flexible fixture, develop-ped for small batch production to reduce the fixturing cost[8]. In both these cases, flexibility was obtained from a lar-ge number of different fixture configurations of the fixture

elements which may be bolted to a base plate. But both ofthese fixtures too are not suitable for irregular shaped com-

ponents such as turbine blades. Further, any of the abovedevices could not hold a brittle material like glass, becauseit breaks due to holding force. In addition, other drawbackof vices, adjustable and modular fixtures is concentratedclamping force; acting on surface of the component at con-tact may lead to undesired deformation due to overstressing,especially in precision machined components. It createsmicro-cracks on the surface of the component and subse-

quently leads to fatigue failure.Along with holding a component as discussed, locating

the component in the desired position is also important.Deterministic location is one of the locating schemes beingable to hold the component in the desired positions. Thisfixture model was extended and then formulated under theassumptions of deterministic locations, to study at under

and over locations to hold the components in the desiredposition [9, 10]. Another procedure of locating a componentwas proposed, by determining the DOFs that have to beconstrained. It was also supported by practical constrainedDOFs in a real locating scheme by using the location model[11].

Hence these problems faced by the precision manufactu-ring industries, are to be resolved carefully by an alternate

universal method to hold all regular, irregular, small andtiny components made of any materials (ferrous, non fer-rous) without overstressing during measurement. In the pre-sent study, MR fluids application has been extended to useto hold all those components with a method called MRfluid based flexible work holding fixture. But to design thisfor a given specifications, a relation is to be established bet-ween strength developed by the MR fluid due to applied

magnetic field, applied magnetic field and the parameters ofthe structure. Hence a fundamental design method to deve-lop this fixture was investigated theoretically and then eva-luated experimentally. Bingham model was used to charac-terize the behavior of the MR fluid subjected to an externalmagnetic field strength. An expression for the holdingstrength was derived which provides the theoretical founda-

tions in the design of the fixture.

2 Modeling of MR Fluid Based FlexibleWork Holding FixtureThe main task in the design of MR fluid based flexible

work holding fixture is to establish the relationship between

the forces which destabilize the component while it is im-mersed in the MR fluid and yield stress developed by thefluid under applied magnetic field strength. When the com-ponent is immersed in the MR fluid, the fluid exerts abuoyancy force Bf which acts upwards and is equal to the

weight of the liquid displaced by the immersed portion ofthe body.

Bf= MR* g*v (N) (3)

where MR is the density of MR fluid (kg/m3), v is the

volume of liquid displaced by the immersed portion of thecomponent (m

3)

When the component is immersed in the MR fluid, avertical force due to self weight Wf of the component isacting downwards.

Wf= b * g* A * H (N) (4)

where b is the density of the component (kg/m3), A is the

cross sectional area of the base of the component (m2), H is

the height of the whole component (m)Measuring probe of surface roughness tester exerts a

force which acts downwards on the component.Force exerted by the measuring probe= Fp (N) (5)

For the stability of the component, by applying New-tons second law, from eq. (3), (4) and (5), the net force Fneton the component is,

Fnet= Bf-Wf- Fp (N) (6)When the external magnetic field is applied, yield

strength developed by the MR fluid is proportional to it. Itacts on all the lateral surfaces of the immersed portion ofthe component. Hence resistive force developed by the MR

fluid, FMR isFMR= MR* a immersed (N) (7)

where a immersed =cross sectional area of the lateral surface ofthe immersed portion of the component (m2)

For component to hold firmly in the MR fluid, effectiveforce FMRdeveloped must be more than net force developed(Eq. 8) (Fig.2).

FMR> FnetFMR> Bf-Wf- FpMR* a immersed > (MR *g * v) - (b *g* A* H) - Fp

MR> [(MR *g * v) - (b *g* A* H)- Fp] /a immersed (8)

Fig. 2Forces acting on the immersed component

3 Operational principles

MR fluid based flexible work holding fixture consists ofMR fluid, magnetic coils, container, and locator. In thebeginning, a container with MR fluid in liquid state wasplaced on the table with magnetic coils on either side of it.Then component was inserted at the centre of the containerby exposing the marked points to the measuring probe.Locator was used to locate and ensure the measuring sur-face of the component parallel to the direction of probe mo-

vement. Then magnetic field was applied perpendicular tothe direction in which the component was inserted whichchanges liquid phase of MR fluid to solid like phase andholds the component firmly. Now the locator was taken outand the component is ready for measurement. After the

measurement the magnetic field was removed and compo-nent was taken out of the container.

With the application of magnetic field, the carbonyl ironparticles present in the MR fluid forms a chain within mil-

liseconds in the direction of the field (Fig.3). The liquid


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during surface roughness measurement and there was nodisplacement of the components.

Table 1Components, their surface, dimension and positionduring testing

Table 2 Comparison of surface roughness and maximumheight of the profile

Table 3Two sample assuming unequal variances (Surfaceroughness)

Table 4Two sample assuming unequal variances (Maxi-mumheight of the profile)

6 Comparisons of measuring timeIn this a comparison of time consumed for the surface

roughness measurement of the components using conven-tional and MR fluid based flexible work holding fixturewere studied. These times were determined by conducting

series of repetitive experiments by a skilled surface roug-hness measurer. The results are tabulated (Table 5 and 6).

Statistical t test were used to compare the mean time. Table5 and 6 gives the measuring time of an irregular shapedcomponent using conventional and MR fluid based flexiblework holding fixture respectively.

The total measuring time of a surface roughness usingconventional method was 108.46sec. This excludes manu-

facturing time of each fixture. Whereas MR fluid basedflexible work holding fixture needed 83.81sec.which was23% less as compared to conventional method (Table 6 &Fig. 7). Also the complexity of design and fabrication offixtures for every shape and size of the component wastotally eliminated which reduces the total cost at large.

Table 5 Measuring time using conventional method for

components

Table 6Measuring time for regular/irregular shapedcomponent using MR fluid flexible work holding fixture


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Fig. 7Summary of measuring time

7 ConclusionsA method for holding irregularly shaped components is

developed using a MR fluid based flexible work holdingfixture. This fixture is also capable of holding componentsof various sizes, shapes and materials quickly to accom-odate for surface roughness measurement test. A simple

locator was used to locate components parallel to the di-ection of movement of the probe of the tester. Both surfaceroughness values and measuring time were determinedusing conventional methods for comparison. Surface roug-

ness and maximum height of the profile obtained using boththe methods are very well matched with developed method.Surface roughness measuring time using MR fluid basedflexible work holding fixture is about 23% less than that of

conventional methods. This method is quick, flexible andwell suited to shapes which are impossible to hold. Brittle

material like glass, soft material like sponge and foam maybe held easily. The cost of making fixtures for each andevery shape and sizes of the components and its inventoryare all eliminated. Gripping force on the component may beincreased with increase in exposed area to the MR fluid.This principle may be extended to hold irregular shape

components for machining for future study.

B Gangadhara ShettyP S S Prasad

Department of Mechanical Engineering,PSG College of Technology,

Coimbatore-641004,Tamilnadu, India

Telephone: 91- 0422-2572177, 2572477,Fax: 91-0422-2573833,

corresponding author: [email protected]

References

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[3] KORDONSKI W, and GOLINI D., Magnetorheologicalsuspension- based high precision finishing technology(MRF), J ournal of Intelligent Material Systems andStructures, 65065. 1998.

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[7] TRAPPEY C, and LIU C R., A literature survey offixture-design automation, International J ournal ofAdvanced Manufacturing Technology, p240255.1990.

[8] RONG Y., LI S, and BAI Y., Development of flexiblefixturing technique in manufacturing industry. In: 5thInternational Symposium on Robotics and Manufac-turing, Maui, HI, 1517 August 1994.

[9] XIONG C H., DING H, and XIONG Y L., Funda-mentals of Robotic Grasping and Fixturing (WorldScientific Publishing Co. Pte. Ltd) 2007.

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Study of Magnetorheological Fluid Based Flexible Work Holding Fixture