Machinability

Question 1

Tool wear affects?

Answer 1

Surface integrity (residual stress, roughness and…
Dimensional accuracy of machined part
Process stability
Increased force and temperature

Question 2

Name four criteria which are normally used to assess the machinability of the
materials? briefly explain the reason.

Answer 2

Tool life
Material removal rate
Cutting force
Surface finish
Chip shape
Environmental impacts

Question 3

How does the flow stress properties (ductility vs. brittleness) affect the chip breakability? How does the tool geometry (rake angle) influence the chip shape?

Answer 3

Flow chips yy_b

Discontinuous chips y>y_b

Question 4

How does the grain size affect the chip shape in turning Alloy 718?

Answer 4

Large grained materials give serrated chips at all cutting speeds.
Large grained materials show more irregular deformation than small grained materials.
Small grained material gives continuous chips at low
cutting speeds (30 and 60 m/min) and
segmented chips at high cutting speeds (120 to 480 m/min)

Question 5

Mention and briefly explain some (at least 2) of the requirements on the tool for machining Titanium.
Or in other words: what features can be introduced on the tool to increase machinability and reduce problems

Answer 5

Positive rake angle to lower cutting force and increase cutting ability
Coating to reduce thermal tool load, low adhesion to titanium
Small edge rounding to stabilize the cutting edge
large clearance angle due to the high deformation due to the low E-module

Question 6

Describe what makes a material difficult to machine
in general! Describe by exemplifying. You can consider what will make tools wear or other things that can be considered under “machinability”.Describe three material “features” (properties or constituents) briefly OR select one and discuss in detail

Answer 6

Strength is maintained at high T 
during machining due to their high temperature properties
✓
Work hardening 
occurs rapidly during machining, 
✓
High abrasive wear owing to the presence of 
hard abrasive carbides.
✓
Poor thermal diffusivity
of nickelbased alloys often 
generates high temperature at the tool tip as well as high thermal 
gradients in the cutting tool

Question 7

Why is chip managing important?

Answer 7

Long unbroken snarls winding
around the tool or workpieces.

Very Short chips,often sticking together, caused by too hard chip breaking
Hard Chip breaking often causes reduced tool life or even insert breakages due to too high chip load on the cutting edge

Question 8

How do you manage chips, too long? too short?

Answer 8

Too long:
increase the feed and depth of cut.
Select an insert geometry with better chip breaking capabilities.
Use a tool with high precision coolant.
Select a smaller nose radius

Too short:
choose a geometry designed for higher feeds or reduce the feed.
Select a larger nose radius.

Question 9

What is BUE and why is it unwanted?

Answer 9

at small cuttting speeds and with certain tool-worpiece combinations adhesion of material particles is appearing on the rake face.
BUE is worsening the surface and provoke toolwear.

Question 10

Kinematic, cutting, additional cutting roughness factors?

Answer 10

Tool motion-R_e
Cutting edge-wear

Chips,BUE,Dead zone- Temp, material,geometry
Alternation of cut surface- corner/flank wear, Cooling, Friction

Vibrations, chips fuckups- random factors

Question 11

If a material is hard to machine, what can we do?

Answer 11

Pre-machining Heat treatment.