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Studies

School of Engineering and

Applied ScienceMechanical and IndustrialEngineering Department

Presentation ofprinciple of Dynamo-

meter

Edited by: Tarik NElbusefi

Supervising : Dr. Hussam El-DinEl-Sheikh

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The existence of some physicalvariables like force, temperature etcand its magnitude or strength cannot

be detected or quantified directly butcan be so through their effect(s) only.For example, a force which canneither be seen nor be gripped butcan be detected and also quantifiedrespectively by its effect(s) and theamount of those effects (on some

material) like elastic deflection,

General principle of

measurement.

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These effects, called signals, oftenneed proper conditioning for easy,accurate and reliable detection and

measurement. And one way for measure the

cutting forces are by instrument

called dynamometerwhere wetrying to describe its design andprincipal of his work

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Physical variable(cutting force, temp.

etc.)

Conversion intoanother suitable

variable (deflection,expansion etc)

Amplification,filtration andstabilization

Reading or recording

Transducing stage

(1)

Conditioning stage

(2)

Read out stage

(3)

Signal

Conditionedsignal

General principle ofmeasurement

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Stage 1 : The target physicalvariable (say force) is convertedproportionally into another suitablevariable (say voltage) called signal, byusing appropriate sensor ortransducer.

Stage 2 : The feeble and noisysignal is amplified, filtered, rectified (ifnecessary) and stabilized forconvenience

The measurement process iscomprised of three stages asillustrated in previous slide:

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Different types of transducers usedin dynamometers for measuring

machining forces.Measurement of cutting force(s) is based onthree basic principles :(A) measurement of elastic deflection of abody subjected to the cutting force(B) measurement of elastic deformation, i.e.strain induced by the force

(C) measurement of pressure developed in amedium by the force.The type of the transducer depends upon

how that deflection, strain or pressure is

detected and quantified.

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(A) Measuring deflection causedby the cutting force(s)

Under the action of the cuttingforce, say PZ in turning, the tool ortool holder elastically deflects asindicated in next Fig. Such tooldeflection, is proportional to the

magnitude of the cutting force, PZ,simply as,

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PZ L

where, L = overhang or equivalent projected length of thecantilever

type tool (holder)E = physical property (Youngs modulus of elasticity

of the beam)

I = size (plane moment of inertia) of the beamsection.Since for a given cutting tool and its holder, E and I

are fixed and the equation 1 becomeswhere, K= is a constant of proportionality.

1

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Deflection measurement.

The deflection, , can be measured: mechanically by dial gauge (mechanical

transducer)

electrically by using several transducerslike;

potentiometer; linear or circular

capacitive pickup

inductive pickup LVDT

as schematically shown in next slide

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(a)

(d)

(c

)

(b)

d

R

E

fixedcoilmoving

core

Electrical transducers working based on deflectionmeasurement

(a) linear pot(b) circular pot(c) capacitive pick up (d) LVDT

type Version

Figure(1)

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Design requirementsfor Tool force

DynamometersFor consistently accurate and reliable

measurement, the following

requirements are considered duringdesign and construction of any tool forcedynamometers :

SensitivityRigidity

Cross sensitivity

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Design requirements- Cont

Sensitivity :the dynamometershould be reasonably sensitive for

precision measurementRigidity:the dynamometer need

to be quite rigid to withstand the

forces without causing muchdeflection which may affect themachining condition

Cross sensitivity : the

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Construction and workingprinciple of some common tool

force dynamometers.The dynamometers being commonly used now-

a-days for measuring machining forces desirablyaccurately and precisely (both static anddynamic characteristics) are

either strain gauge type

or piezoelectric type

Strain gauge type dynamometers are

inexpensive but less accurate and consistent,whereas, the piezoelectric type are highlyaccurate, reliable and consistent but veryexpensive for high material cost and stringent

construction.

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Turning Dynamometer(over view)

Turning dynamometers may be straingauge or piezoelectric type and may beof one, two or three dimensionscapable to monitor all of PX, PY and PZ.For ease of manufacture and low cost,strain gauge type turningdynamometers are widely used and

preferably of 2 D (dimension) forsimpler construction, lower cost andability to provide almost all the desired

force values.

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Turning Dynamometer

Design and construction of a strain gaugetype 2 D turning dynamometer are shownschematically in next slide Fig (2) andphotographically in Fig (3,4)Two full bridges

comprising four live strain gauges areprovided for PZ and PX channels which areconnected with the strain measuring bridge fordetection and measurement of strain in termsof voltage which provides the magnitude ofthe cutting forces through calibration. Fig (5)pictorially shows use of 3 D turningdynamometer having piezoelectric transducersinside.

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Figure(2)

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Photographs of a strain gauge type 2 D turningdynamometer and its major components.

Figure

(3)

Figure

(4)

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Use of 3 D piezoelectric type turningdynamometer.

Figure(5)

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Drilling dynamometerPhysical construction of a strain gauge

type 2 D drilling dynamometer formeasuring torque and thrust force istypically shown schematically in next slideFig (6) and pictorially in after next slide

Fig (7,8) Four strain gauges are mountedon the upper and lower surfaces of thetwo opposite ribs for PX channel andfour on the side surfaces of the other two

ribs for the torque channel. Before use,the dynamometer must be calibrated toenable determination of the actual valuesof T and PX from the voltage values orreading taken in SMB or PC.

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Schematic view of construction of a strain gaugetype drilling dynamometer.

Figure(6)

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A strain gauge type drilling dynamometer andits major components.

Figure(7)

Figure(8)

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Milling dynamometer

Since the cutting or loading point is notfixed w.r.t. the job and the dynamometer,the job platform rests on four symmetricallylocated supports in the form of four O-rings.

The forces on each O-ring are monitored andsummed up correspondingly for getting thetotal magnitude of all the three forces in X, Yand Z direction respectively. Fig. (9) shows

schematically the principle of using O-ringfor measuring two forces by mounting straingauges, 4 for radial force and 4 fortransverse force.

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Scheme of strain gauge type 3 D milling

dynamometer

Figure(9)

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Milling dynamometer -Cont

Fig. (10) typically shows configurationof a strain gauge type 3 D millingdynamometer having 4 octagonal rings.

Piezoelectric type 3 D dynamometersare also available and used formeasuring the cutting forces in milling(lain, end and force)

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A typical strain gauge type 3 D millingdynamometer

Figure(10)

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Grinding dynamometer

The construction and application ofa strain gauge type (extended O-ring)grinding surface dynamometer andanother piezoelectric type are typicallyshown in Fig. (11) and Fig. (12)respectively. Unlike strain gauge typedynamometers, the sophisticated

piezoelectric type (KISTLER)dynamometers can be used directlymore accurately and reliably even

without calibration by the user.

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A typical strain gauge type 2 D grindingdynamometer

Figure(11)

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Piezoelectric type grinding dynamometer in

operation.

Figure(12)

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Dont hesitate to ask

any question

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Thank you foryour attention