Outline – Machine Structures• Design Requirements

• Structural Elements– Materials

• Design Considerations– Columns– Columns

– Beds

• Manufacturing TechniquesC I– Cast Iron

– Welded Steel

– Polymer Concrete Casting

– Granite-based Structures

– Carbon Fiber Composites

• Structural Dampingp g

• Finite Element Analysis

• Elimination of Static Deformations

Chapter 5 ME 551 2

Design Requirementsg q

• Structure of the machine houses (and supports theoperation of) all the vital (moving or stationary) elements ofthe machine.

It i th k l t f th hi– It is the skeleton of the machine.

– Without a good structure, the rest of the machine will be ineffective.

• Some design requirements are• Some design requirements are– Symmetrical (and Simple) Design

– Minimum Weightg

– High Static and Dynamic Stiffness

– High Structural Damping

– High Secular and Thermal Stability

– Independent Foundation

Chapter 5 ME 551 3

Structural Elements• Structural elements can be

classified asFlat Bed

classified as– Machine Beds

• T-Bed, Slanted Bed,

• Floor Plates, etc.

– Columns• Open or Enclosed Design

Slanted Bed

• Open or Enclosed Design

– Portals/Bridges• Open or Enclosed Design

Column

• Machine structures can be categorized into three classes:

Column– Open Frame

– Closed Frame

Truss type (Enclosed) Structures– Truss-type (Enclosed) Structures

Chapter 5 ME 551 4

Open Frame Structures1,2p

• Most traditional machinetools employ thisconfiguration.– Also known as C- or G frames

• Provides easy access to theyworkspace.

• Not as stiff as the closedNot as stiff as the closedframes.

• Employs stacked axes• Employs stacked axes.

• Prone to Abbe offset errors.

Chapter 5 ME 551 5

Closed Frame Structures1,2

• Most precision machinetools utilize thistools utilize thisarchitecture:

Commonly referred to as O

Deformed Frame

– Commonly referred to as Oframes

• Accessing the workspace

Axial Force

Tool

• Accessing the workspaceis fairly easy.

S t i l t t i

Structural Loop(“Force Flow”)

• Symmetrical structure isquite rigid.

Workpiece

• Main actuator must belocated on the bridge.

Chapter 5 ME 551 6

Truss-type Structuresyp

• In precision machine design advanced frame geometries• In precision machine design, advanced frame geometries are deployed:– Cubic / CuboidCubic / Cuboid

– Tetrahedron

– Octahedron etc.

• Such (truss-type) geometries yield stable- and strong enclosed structures that are especially suitable for parallel

h i (lik H d l tf )mechanisms (like Hexapod platforms).– High thermal stability

Workspace is relatively small in proportion to the overall– Workspace is relatively small in proportion to the overall dimensions of the frame.

Chapter 5 ME 551 7

Example - Octahedral Hexapodp p• Machine is built by Ingersoll Company

• It employs an octahedral geometry tosupport the hexapod “tool” platform.

• The hexapod (Stewart platformconcept originally developed for flightsimulators) gives six limited degrees ofsimulators) gives six limited degrees offreedom.

– The tool angle is limited to about 20The tool angle is limited to about 20degrees from the vertical.

• Advanced controller architecture andd a ced co o e a c ec u e a dalgorithms make programmingpossible.

Chapter 5 ME 551 8

Structural Materials

Ferrous metals Nonferrous metals Non-metalsCompositesComposites

Cast IronSteel

Al. (Cast 201)Al. (6061-T651)

GraniteZerodur™

InvarSuper Nilvar™Nitralloy™

CopperBrass (Cu Alloy)

Polymer concretePortland concreteCarbon FibersNitralloy™ Carbon Fibers

Chapter 5 ME 551 9

Column Structures1

Without RibsWithout Ribs

With Ribs

Chapter 5 ME 551 10

With Ribs

Stiffness Properties of Columns1p

Chapter 5 ME 551 11

Bed Structures1

Chapter 5 ME 551 12

Bed Structures (Cont’d)( )

Chapter 5 ME 551 13

Properties of Bed Designs1p g

Chapter 5 ME 551 14

Cast Iron Structures2

• Widely used in machine constructionconstruction

• Stable with thermal anneal, aging or vibration stressaging, or vibration stressrelieve

• Provides good damping andProvides good damping and heat transfer

• Low cost for moderate sizes

• Integral features can be cast in place

• Design and manufacturing rules are well-established.

Chapter 5 ME 551 15

Welded Steel Structures2

• Often used for larger structures or small lot sizesstructures or small-lot sizes

• Stable with thermal anneal

Low damping improved with• Low damping, improved with shear dampers

• Low cost• Low cost

• Integral features/parts can be welded in placewelded in place

• Structures can be made from tubes, profiles, and plates.tubes, p o es, a d p ates

Chapter 5 ME 551 16

Polymer Concrete Casting2y g• Polymer concrete (PC) is a relatively new

material used in precision machinematerial used in precision machinedesign.– Special polymers are mixed with specially

prepared/sized aggregate.

– Epoxy-granite-, mineral-, and reactive-resin concrete castings all refer to theresin concrete castings all refer to thesame technique.

• For PC castings, the same rules for draftallowance apply as for metal castings ifthe mold is to be removed.

Unlike metal castings a PC will not– Unlike metal castings, a PC will notdevelop hot spots while curing even inthick, uneven sections.

Chapter 5 ME 551 17

PCC (Cont’d)( )

• Instead of ribs PC structures useInstead of ribs, PC structures useinternal foam cores to maximize theirstiffness-to-weight ratio.

• PC can accommodate cast in placecomponents such as bolt inserts,conduit, bearing rails, hydraulic linesetc.

Hi hl l d d hi b t t• Highly loaded machine substructures(e.g. carriages) are made from castiron or steeliron or steel.

Chapter 5 ME 551 18

PCC (Cont’d)( )

• PC structures can have the stiffness of cast iron structures.– They can have much greater damping.

• PC does not diffuse heat as well as cast iron.

f– Attention must be paid to the isolation of heat sources to prevent the formation of hot spots.

• When bolting or grouting non-PCcomponents to a PC bed, bimaterialeffect must be considered.

Chapter 5 ME 551 19

Granite-Structures• Used exclusively in precision instruments and

CMMs.– Serves as reference planes/surfaces

– Quite costly

Just like concrete the granite is hydrophilic:• Just like concrete, the granite is hydrophilic:– It must be sealed off properly to avoid

absorbtion of water. Otherwise, it will distort!

• This very hard (and brittle) material is verystable:– Density: 2.6 [g/cm3]Density: 2.6 [g/cm ]

– Elasticity modulus: 40 [GPa]

– Tensile strength: 16 [MPa]

Th l ffi i t 7 35 10 6 [1/K]– Thermal exp. coefficient: 7.35×10-6 [1/K]

• Not all grades are suitable for precisionmachine design.g

Chapter 5 ME 551 20

Carbon Fiber Composites3p• Fibre reinforced composites have

hi h l f ifivery high values of a specificmodulus of elasticity and specifict thstrength.– Mechanical properties can be tightly

t ll dcontrolled

– Joining process can be complicated

– Quite expensive

• The application of this new material to this field is still in its early stages.

Chapter 5 ME 551 21

Slanted Bed Designs for Lathe1g

Cast iron guideway plateon cast concrete

Adhesive JointAdhesive Surface

Location of Cast-ribs

Lower Section

Steel Insert for

Mounting for Hydraulics

Lower Section of Bed

Steel Insert for Machine Foot

Transfer Tap Conduit

Oil Chamber

Chapter 5 ME 551 22

Transfer TapTurning Fixture

Structural Dampingp g• Damping is needed to absorb energy from the process:

T t h tt d d t th f– To prevent chatter and damage to the surface

– To absorb energy from structural modes excited by the servosand other sources

• Damping can be obtained by internal means:– Material damping

– Damping by micro-slip at joints

• Damping can be obtained by external means:– Constrained layer dampers (or shear dampers)

– Vibration absorbers

Active dampers– Active dampers• Velocity control loops in servo systems

• Actively controlled masses attached to the structure

Chapter 5 ME 551 23

Combined Damping Effect6p g

• Major part of the damping for a machine system can be generated atthe mating surfaces (i.e. bolted joints, slides) of the variouscomponents via micro-slipcomponents via micro slip.

Chapter 5 ME 551 24

Shear Dampers4p• Steel structures are known to

h littl i t l d ihave little internal damping.

• One alternative method toincrease the damping of theincrease the damping of thestructure is to employ sheardampers.dampers.

• Visco-elastic layer dampsmotion between structure andconstraining layer (frombending or torsion) bydi i ti ki ti i tdissipating kinetic energy intoheat.

Chapter 5 ME 551 25

Application - Shear Dampers4pp p

• For this case the structural damping of a round tube is• For this case, the structural damping of a round tube isconsidered.– Inner tube serves as constraining layer.Inner tube serves as constraining layer.

– Constraining layer is wrapped with damping material.

– Coated inner tube is inserted and gap filled with epoxy.

Chapter 5 ME 551 26

Application (Cont’d)pp ( )

Chapter 5 ME 551 27

Example - Precision Grinder7pT-bed of the precision grinder includes4 (visco elastic material covered) steel4 (visco-elastic material covered) steelinserts. Square inserts also allow thecirculation of cooling fluid.

C ti f th hCross-section of the sheardamped test beam

Chapter 7 ME 551 28

Vibration Damping Tapesp g p• Vibration damp(en)ing tape/foil is a

“band-aid” option to increase structuralband aid option to increase structural damping.

• Commercial products like 3MCommercial products like 3M 434/435/436™ constitute a visco-elastic polymer coated on a soft Aluminum constraining layer.

• Very useful in dampening the vibrations f t l l t d it lof metal plates and composite panels.

• Somewhat sensitive to high temperatures:temperatures:– Nominal operating temperature range (for

the tape) is -60 to 20oC.p )

Chapter 5 ME 551 29

Vibration Absorbers1ag

nifi

catio

nfu

nctio

nM

a

Chapter 5 ME 551 30

Absorber Designs2g

TMD

Constrained-Layer Beam

Adjustable Position

St t

Anchor Mass

Chapter 5 ME 551 31

Structure

Finite Element Method• FEM is an indispensible engineering analysis tool

to find approximate solutions to technicalto find approximate solutions to technicalproblems defined by partial differential equations.

• FEA Packages (ANSYS™, MARC/Mentat™,Nastran/Patran™, Abaqus™, etc.) are routinelyutilized to design/analyze/optimize structuralmembersmembers.

• Large number of engineering analysis can beconducted by FEA packages:y p g– Stress/Strain (in elastic- or plastic region)

– Heat Transfer

– Mechanical Vibrations

– Electromagnetic Fields (Maxwell™)

Fluid Dynamics (Fluent™)– Fluid Dynamics (Fluent™)

Chapter 5 ME 551 32

FEM Analysis1yCAD Model of the Machine “Specs” of the Simulation

Determination of thesimulation objectives

■

Abstraction for theFE-model Creation

Abstraction of Guides and Drives as Springs

simulation objectives(deformations, stresses, natural modes)Determination of modelingstrategy (volume- or shell model)

■

model)Choice of element order (linear, parabolic)

■

Presentation of the Results

Definition of boundary conditions(force, temperature) and constraints

■

List of basic stiffness

Program’s Internal Processes

■

Presentation of the ResultsCompilation of the Overall Model

matricesStructure the global stiffness matrixConsideration of the boundary conditions

■

■boundary conditionsSolution of the resulting linear systemDerivation of stress fromdeformation values

■

■

Chapter 5 ME 551 33

FEM Analysis - Illustration1y

Chapter 5 ME 551 34

Example – Portal Frame1

Chapter 5 ME 551 35

Vibration Analysis - Illustration1y

Chapter 5 ME 551 36

Natural Frequencies & Modes1qNatural Frequency: 42.4 Hz Natural Frequency: 73.4 Hz Natural Frequency: 102 Hz

Chapter 5 ME 551 37

Vibration Analysis1 (Cont’d)y ( )

Chapter 5 ME 551 38

Frequency [Hz]

Elimination of Static Deformations5

There are three methods• There are three methods to compensate the elastic deformations of thedeformations of the machine structure under the action of quasi staticthe action of quasi-static loads:

C tia. Compensating curve

b. Preloaded support

C t i htc. Counter-weight

Chapter 5 ME 551 39

Compensating Curve1p g

• The bearing rails are finished (grounded) so that they deform to the desired shape when the machine axes move.– Grinding process is expensive but it saves structural

costs.

• When the primary weight is that of the machine axis (not the workpiece!), this method can be very effective.

Chapter 5 ME 551 40

Counter-weight Systemsg y

• For vertical axis, the servo-motorsystem needs to support the dead-weight of that axis.

M d t h t d– May need to choose overrated servo-system.

– Energy wasted due to IR losses ofgymachine.

• Dead weight can be supported by an(external system (i.e. counter weights,

hydraulic systems, floats, etc).

F d i ti t + b ll• For dynamic motions, motor + ball-screw inertia usually dominates.

Chapter 5 ME 551 41

Counter-weight (Cont’d)g ( )• Counterweight system contributes to the

dynamics of a precision machinerydynamics of a precision machinery– Cogging effect, elastic effect etc.

• Cable braided bands and chains are frequently• Cable, braided bands, and chains are frequentlyutilized to carry the counterweight.

In chain mechanisms the sprocket's pitch diameter varies slightly– In chain mechanisms, the sprocket s pitch diameter varies slightlyas it rotates producing a small cogging effect.

• A cable and a smooth running pulley will give theA cable and a smooth running pulley will give theleast variation in force.– Cables are elastic compared to chains and hence they shouldp y

only be used for quasi-statically (i.e slow) moving elements.

– Pulley friction and friction in the counterweight's bearings shouldbe scrutinized for precision machinesbe scrutinized for precision machines.

Chapter 5 ME 551 42

Some Design Rules2g

• When sizing components keep the proportions of theWhen sizing components, keep the proportions of thegolden rectangle (Height/Width = 1.618) in mind.

• Utilize symmetry wherever possible.Utilize symmetry wherever possible.

• Minimize the structural loop and use closed sectionswhenever possible.p

• Large plate sections should be stiffened with ribs and

other means to keep them vibrating like drumheads.o e ea s o eep e b a g e d u eads– When needed, use active damping systems.

• Maximize thermal diffusivity of the machine andyminimize heat input.

Chapter 5 ME 551 43

Design Rules (Cont’d)g ( )• Locate the work volume at the center of mass and in plane

of supportof support.

• Start at the tool tip (or workpiece) with estimates oncutting forces and accelerationcutting forces and acceleration – Then work backward through the structural system.

– Use guesstimates for sensor, bearing, and actuator limitations to help size structural components.

• Try to make the natural frequencies of the variousvibration modes (e g translational and rotational) closevibration modes (e.g. translational and rotational) closetogether.– This will minimize cross coupling between modes.This will minimize cross coupling between modes.

• Use as many design tools as possible in design stage.– Especially, Solid Geometric Modeling and FEM Packages.p y, g g

Chapter 5 ME 551 44

References1. M. Weck, C. Brecher, Werkzeugmaschinen (Band 2), Springer-

Verlag 2005Verlag, 2005.

2. A. H. Slocum, Precision Machine Design, SME Press, 1992.

• A. H. Slocum, ME 2.075 Course Notes, MIT, 2001.

3. L.N. L. deLacalle, A. Lamikiz, Machine Tools for High PerformanceMachining, Springer-Verlag, 2009.

4 E Bamberg ME EN 7960 Course Notes University of Utah 20064. E. Bamberg, ME EN 7960 Course Notes, University of Utah, 2006.

5. Cranfield Unit for Precision Engineering (CUPE), PrecisionEngineering Course Notes, Cranfield Institute of Technology (UK),1998.

6. Löwenfeld, K., “Zweites Forschungs und KonstruktionskolloquiumWerkzeugmaschinen,” p. 117, Vogel-Verlag, Coburg, 1955.Werkzeugmaschinen, p. 117, Vogel Verlag, Coburg, 1955.

7. E. R. Marsh, A. H. Slocum, “An integrated Approach to StructuralDamping,” Precision Engineering, vol. 18, pp. 103-109, 1996.

Chapter 5 ME 551 45