• Machine Tool Path Error -form error

• Tool Centre & Height Error-machining pips, radius and form errors

• Tool Tip Waviness & Wear -increased surface roughness, form error
• Component Distortion due to clamping force -form error
• Component & Spindle Balance -form error and or increased surface waviness
• Incorrect Feed Rate & Depth of Cut -increased surface roughness & waviness
• Poor material flow & chip extraction -surface damage

• Small Pad Path Error -form and mid-wavelength errors
• Contaminated Slurry -polish marks on the component (Sleaks)
• Tool Edge Touch Down Error -localised form and midwavelength defects
• Improper Slurry Selection -material removal errors
• Dwell Time Errors -form errors and localised surface damage
• Hardness of Polishing Pad -'Orange peel' or surface ripple effects
• Inconsistent Slurry Flow localised hot spots can be generated,
  inducing surface defects

• Visual appearance
• Paintability
• Formability
• Corrosion resistance
• Fatigue behaviour
• Sealing capacity
• Thermal contact resistance
• Electrical resistance
• Friction / Wear

Artificial hip joints

• Consists of two machine generated
  components
• Ball and the socket

Ball

• Consists of a metal stem or rod, which is
  inserted into the thigh bone
• On the top of the stem a ball is attached at an
  angle to mimic the natural joint
• The ball can be made from either metal
  (CoCr, Ti) or ceramic (Alumina, Zirconia) with
  a super smooth surface finish.

Socket

• Generally made from Ultra High Molecular
  Weight Polyethylene (UHMWPE) or metal
  and more recently ceramics
• The outer surface of these components will
  be rough so that it can be attached to the
  hipbone
• The inner surfaces will have a super smooth
  bearing finish for low friction movement and
  low wear characteristics.

Hip joints transfer very large loads

• The hip joints carry the weight of the upper body
• These pressures can cause the hip joint to suffer from a
  gradual wear process, which will limit its life span
• as the artificial joint wears and third body debris is produced, surface roughness can increase and in turn may promote further abrasive wear

It is therefore critical that the surface roughness is controlled
during manufacturing

• so that the risk of surface breakdown and component
  failure is reduced to ensure a long implant life

Metrology is Key to Controlling Surface Roughness and to Reduce the
Effects of Wear

• The surface finish of an orthopedic component is one of
  the most important contributors to the reliability of an
  implant.
• Smooth surfaces will minimise the abrasive process and
  the amount of material (debris) displaced.

This in turn will reduce the possibility of stem loosening
(a common cause of implant failure)

• In fluid dynamic bearings the bearing function is taken over by a layer of lubricant
• The elimination of metal to metal contact in fluid dynamic bearings eliminates non-repeatable runout due to surface imperfections
• Control of surface finish and form is critical
• In addition there are typically grooves machined in the shaft and rotor which assist in the build up of a stable lubrication layer
• Measurement of these grooves is highly important

• Avoid Etch/Time rate errors-quantify that slots and discreet features are at correct heights

• Check free standing devices are correctly released -Confirm that the sacrificial layers are remove

• Produce good Anchor point connections -improve quality of contact points

• Avoid switching errors or wrong resonant frequency -check devices are correct shape, size or thickness

• Reduce material issues and production problems -reduce general manufacturing errors

• Device fails to move -avoid excessive component stress

• 2D Step Height
  • provides a method of measuring step height for simple geometric shapes such as etched lines or rectangular areas

  • 2D step heightcan also be used for measuring thickness where an edge is exposed
• ISO 5436-1 Step Height
  • when attempting to compare measurements of step height taken on different measurement instruments it is very important to comply to a standard method of measurement
  • ISO 5436-1 provides an internationally recognised method of measuring step height
• 3D Step Height
  • provides the height difference between two planes defined by two areas on a surface
  • The first area is defined as the reference and the software fits a least squares plane through this area
  • The second plane is then measured from this
  • As well as mean step height, maximum height and minimum height, the angle difference can also be measured