Machining and Finishing of Metallic Implants

Question 1

How are metallic implants usually fabricated?

Answer 1

Casting
Forging
Sintering

Question 2

How does surface finish effect implants?

Answer 2

o	Wear resistance
o	Friction & Lubrication
o	Crack initiation and fatigue life 
o	Biomedical:
- Cleaning and sterilisation 
- Adhesion of bioactive coatings

Question 3

What is the difference between a Twist drill and an Indexable drill?

Answer 3

Ordinary twist drills removed large amount of material quickly whereas indexable are slower but have more precision

Question 4

What is broaching?

Answer 4

Multi-point process – removed material by axial cutting with several transverse cutting edges

Question 5

What is broaching used for?

Answer 5

Used to create surface features, holes, slots, cut-outs
Usually where other forms of machining are not practical – classic example is keyways
Rotary (wobble) broaching on a lathe to create Torx/Hex insets

Question 6

What is laser cutting and what is it used for?

Answer 6

o Laser cuts through material by melting or ablating it
Thermal gradients and re-solidification
Thin sheets of material (up to 1mm)

Question 7

What is water jet cutting and what is it used for?

Answer 7

o Jet contains high velocity water and abrasive power
No thermal gradients
Thicker materials (1mm up to 50mm)

Question 8

What is plasma jet cutting and what is it used for?

Answer 8

o Cutter is a jet of ionised gas that cuts through material by melting it and blowing away the molten metal
Electrically conductive materials only
Accuracy is thickness dependent

Question 9

What are the advantages of electrochemical machining?

Answer 9

• Advantages
  Produces stress free burr free surfaces with no burning or thermal damage to work piece surfaces. Better corrosion resistance than with mechanical finishing. SS surfaces Ra 0.1 to 0.4 µm
• Disadvantages
  Low machining accuracy, problems with clear cuts and sharp corners

Question 10

What is the process of shot peening?

Answer 10

Puts compressive stress into surfaces by cold work, achieved by directing a stream of shot at the metal at high velocity

Question 11

What are the effects of shot peening?

Answer 11

• Increases fatigue strength, relieves tensile stresses and cleans surfaces
• Kinetic energy impinging on surface causes compressive stress below surface, may cause crystalline to amorphous conversion in Ti allows (improved friction and corrosion resistance

Question 12

What media is used for shot peening?

Answer 12

• Media used for peen: iron or glass shot (cleaning SS and Ti alloys) or cut steel or stainless wire