Hardware Components of CMM

• Structural elements• Bearing supports• Drive systems• Displacement measurement systems• Probe heads• Control systems

Structural Elements

• M/C base• Table to support W/P• M/C columns• Slide ways• Probe shaft

Material – Granite, Steel & Aluminium

Ideal Properties of Structural Elements

• Dimensional stability – Stress relieving techniques

• Infinite stiffness – FEA & Hollow structures• Weightlessness• High damping capacity - Servo control system• Low coefficient of thermal expansion – Proper

selection of material – easy workability & lower cost

• High thermal conductivity

Bearing SystemsWhat are bearing systems?

Part of structure & affect measuring accuracyTypes of bearing systems• Non-contact air bearing systems• Hard contact bearingsDesign considerations• Dynamic stiffness• Load carrying capability• Damping & frictional effects

Air Bearings • Air bearings utilize a thin film of

pressurized air to provide a ‘zero friction' load bearing interface between surfaces

• Being non-contact, air bearings avoid the traditional bearing-related problems of friction, wear, and lubricant handling

• Distinct advantages in precision positioning and high speed applications

Working of air

bearings

• The fluid film of the bearing is achieved by supplying a flow of air through the bearing itself to the bearing surface (gap of 5mm)

• The design of the air bearing is such that, although the air constantly dissipates from the bearing site, the continual flow of pressurized air through the bearing is sufficient to support the working loads

• Air bearings are a natural choice for applications in coordinate measuring machines

Hard contact bearings

• Used for higher loads (mainly foundries)

• Precision roller bearings• Ball bearings• Sliding guide ways

Drive systems

• Used only to move the probe• Natural frequency higher than that of machine

structureTypes of Drive systems• Rack & Pinion• Belt • Friction• Lead screw• Linear motor

Rack & Pinion

• Pinion-driver• Rack-driven• Preffered for Lengthy

travel (Gantry)• Form errors and Backlash

limit accuracy• Cost effective• Used for rough

positioning

Belt Drive• It consists of– Belt – fibre-reinforced notched or a

metallic flat tape– Multi-stage speed reducer– Servo motor

• Offers a quiet transmission of power to the moving axes

• Acts as a low pass filter preventing motor oscillations

• Axes driven at high speed & accelerations

• Disadv-Elasticity-cannot be used for high-precision scanning operations

Friction Drive• Also called as capstan

or traction drive• It consists of– Driving wheel - driven

directly by a motor or speed reducer

– A flat or round bar– A supporting back-up

roller

• Constant preload is applied

• Adv of friction drive– Simplicity in design– Low friction force–Minimum backlash–High positioning accuracy

• Disadv of friction drive– Low drive force capability– Low stiffness & damping–Minimum transmission gain

Lead screw drive• Most common type – Ball

screws• Transforms rotary motion to

linear translational movement• Adv – Provide mechanical

advantage – Good dynamic stiffness

• Disadv– Inability to disengage and re-

engage– Noisy– Subject to wear

Linear motor drive• Direct drive • It offer unusually high

&virtually infinite stiffness

Adv• High acceleration

(Gantry)Disadv• No gear reduction

mechanism• Heat generation• Cost is more