Chapter 3 Cutting Tool Technology Flashcards
(43 cards)
How does tool wear occur and how does it affect the workpiece?
Tool wear occurs as the machining operations undergo friction force, especially at the face of the tool & chip interface + tool & surface interface -> generating heat, wear and tear.
Affects the dimension accuracy & finishing
Where does tool wear usually occur?
Rake face & Flank Face
What are the regions of tool wear vs cutting time graph?
Rapid-initial wear (break-in)
Steady-state wear region
Failure region (accelerated wear)
What are the parameters affecting tool wear rate? (3)
- Cutting speed (most significant)
- Tool material/design
- Taylor Tool life eqn
What is the purpose of the Taylor’s Tool Life Equation?
It is used to predict the lifespan of cutting tools in machining operation. The equation gives us the relationship between cutting speed and tool life, allowing us to optimise machining operations. (however, only the cutting speed is considered)
What is the purpose of the Extended Taylor’s equation?
(Also known as General Tool Life equation)
Includes additional machining parameters that influence tool life & cutting speed, giving a more detailed/accurate prediction of tool life.
What are the 3 critical attributes of tool materials?
High temperature stability (hot hardness)
Brittle fracture resistance (toughness & strength)
Abrasive wear resistance (hardness)
What are the tool wear mechanisms?
- Abrasion
- Diffusion
- Oxidation
- Fatigue
- Adhesion
How does abrasion affect tool wear?
Abrasion causes flank & crater wear.
How does it happen?
Frictional force occurring at cutting tool & chip interface mainly, and as the chip forms, the tool continues to rub against it, causing abrasion.
How does Diffusion affect tool wear?
Diffusion causes crater wear.
How does it happen?
Due to high temp (around 500 deg) it gives the atoms enough energy to diffuse across the interface (chip and cutting tool)
e.g. cutting tool high in C & Co, workpiece high in Fe. Fe from chips to diffuse to tool, C & Co diffuse to chips, as diffusion occurs form high concentration to low concentration,
How does Oxidation affect tool wear?
Changes in-material properties.
How does it happen?
High temp due to machining, causes metal to naturally react more with air to produce oxides (mainly at workpiece and tool interface + tool and chip interface)
How does Fatigue affect tool wear?
Causes fatigue failure if too extreme, and also a mechanism of wear and tear.
How does it happen?
Occurs due to cyclic loading, which occurs at the chip and tool interface. As chip flows along the cutting, it gives a tensile force on the cutting tool, but as it flows away the tensile force becomes a compression force.
Whats the difference between M03 and H23 ISO Classified Tool Materials?
M03 is a Stainless/Heat-resisting steel while H23 is a Hardened steel
How does Adhesion affect tool wear?
Causes flank wear.
How does it happen?
Deposition of BUE onto cutting tool (including workpiece) occurs when high hardness of work material.
Whats the difference between K023 and K030 ISO Classified Cast Iron Tool Materials?
K023 has a higher hardness but lower toughness than K030.
What are the features of High Speed Steels (HSS) as a tool material?
- High toughness
- 10% Turning, 80% Drilling, 40% Milling
- Max cutting spd = 50m/min @ 600 oC
- Tin coating for high speeds
- Manufactured via hot working & powder metallurgy
What are cemented carbides made of?
Composite made of Tungsten Carbides (WC) + Cobalt (Co) as binder.
Additives such as Titanium Carbide (TiC) and Tantalum Carbide (TaC) to improve certain properties.
How does one change the material properties of cemented carbides?
- Tungsten Carbide (WC) grain size:
+ size, + shock resistance, - hardness - Cobalt (Co) content %:
+ Co%, + toughness, - hardness, - wear resistance
(Note: Useful range is Co @ 5-12%, WC grain size @ 0.5 to 5 µm)
What is the role of additives in Cemented Carbides?
- Motivation: Fe absorbs WC at high temp
- Constituents: Titanium (TiC), Tantalum carbides (TaC)
- Mechanism: Low solubility in Fe, +hot hardness
- Results: Prevent edge deformation, + resistance to crater wear, + thermal stability
Comparing Steel and Cast Iron workpieces, why do we use HIGH binder content cement carbide cutting tools for STEEL and LOW binder content for CAST IRON?
For Cast Iron, high wear resistance is more important than toughness in the cutting tool, hence low Co% will allow a higher WC%, leading to a higher wear resistance.
Vice versa for Steel, higher toughness is required so increase Co%, and TiC + TaC % is increased to help with the reactivity of Fe with WC, and improve hardness (since sharpness is important here)
How does heat affect Cemented Carbides? And how do you solve it?
High temperatures causes the WC in the carbides to react with any Fe workpieces, reducing the WC%, leading to lower hardness at high temps, in the tool.
Can be solved by introducing additives (other carbides) that do not react with iron. Coating can also be introduced to reduce solubility in Fe (increased hot hardness)
What are the features of Coated Carbides?
Extremely low solubility in Fe
Good hot hardness
Enable high cutting speeds
Increase wear resistance
Constituents:
Titanium Carbide (TiC), Titanium Nitride (TiN), Titanium Carbonitride (TiCN), Alumina (Al2O3)
What are the +ve and -ve of ceramic tool material?
+ve:
very high hardness
no affinity with work material
applicable to hardened steel
inert (does not react to oxygen/steel)
-ve:
very brittle
performance:
high speed machining - good efficiency and surface finishing
good wear resistance - stable dimension and longer tool life
Ceramics can also be used to make composites to improve/introduce certain properties, what are examples of such?
Alumina based ceramics - Al2O3 -> improve thermal stability
with ZrO2 -> improves crack resistance by volume expansion
with TiC -> improves hardness
with SiC (w) -> whiskers, that reduce crack propogation
