Optimization of Process and
Tool Selection in Hard Part
Turning Operations
Doug Evans
Grade Development Specialist
Sandvik Coromant
1
Cutting Tool Materials for HPT
Cemented carbide
Diamond
• Oxidizes at high
temperatures and is
therefore unsuitable
Ceramics
• One alternative for HPT
CBN
2
• Not sufficient at higher
cutting temperatures
• First choice for HPT
production application
Why use CBN materials in cutting
applications?
• Excellent wear resistance and good
toughness
• To be used for highest surface finish
demands
• Semi finishing - super finishing
• To maintain close tolerances
• Continuous and interrupted cuts
• Depth of cut less than 0.5 mm (.020")
3
Choosing the right Grade
CBN microstructure
cBN grains
(dark gray phase)
For continuous cuts in hardened
steels, 55-64 HRC
2d phase
ceramic- binder
SEM picture of a 50%
low cBN content grade
4
Choosing the right Grade
CBN microstructure
cBN grains
(dark gray phase)
For continuous and interrupted
cuts in hardened steels, 55-64
HRC
2d phase
ceramic- binder
SEM picture of a 60-65%
medium cBN content grade
5
Long section with
splines and chamfered
keyway
Choosing the right Grade
CBN microstructure
cBN grains
(dark gray phase)
•First choice for gray cast
irons and powdered
materials
•Alternate choice for heavy
interruptions in hardened
steel 55-64 HRC
2d phase metallic binder
SEM picture of a 80-85%
high cBN content grade
6
Non-chamfered
splines and keyways.
Heavy DOC > .020"
Choosing the right Grade
Solid CBN microstructure
cBN grains
(dark gray phase)
•Primarily targeted for
machining white iron (high
Chrome iron) 550-700 BHN
•Can also be used in high
carbon steel machining.
2d phase
ceramic- binder
SEM picture of a 75%
high Solid cBN content grade
7
Best Practices
Choosing the right Edge Preparations
-
F-land
E-land
Cutting forces
edge strength
T-land
+
8
Sland
Edge Preparations for HPT
F - Land
• Sharp edges are generally not recommended on
CBN/Ceramics.
• Carefully applied, it can bring benefits in special
finishing applications.
• Wears quickly.
E - Land
• Honing helps strengthen the edge, protecting
the edge line from chipping and fracturing.
• Feed rates must be greater then the hone size
to allow actual cutting action and prevent
rubbing.
9
Edge Preparations for HPT
T - Land
• T-land is a common edge preparation on
CBN/Ceramics.
• The larger the T-land, the greater the cutting forces,
temperature, and wear.
S - Land
• S-land, honing added to T-land.
• Provides an even stronger edge then T-land,
prevents chipping.
• Feed rate must be greater then hone size.
10
Edge Preparations for HPT
Guidelines to optimized edges
Chamfer width
Chamfer angle
Edge radius
Low feed
Weak
Micro
chipping
High feed
Strong
Edge strengthening
11
Pre-wear
vibrations
Surface consistency
Tool life extension
Edge Preparations
Considerations
Chamfer width
Chamfer angle
0.05 mm
.002 inch
0.1 mm
.004 inch
0.2 mm
.008 inch
Chamfer width
Chamfer angle
20°
25°
30°
35°
Shape and tolerance
Demands
Process security and tool life
When?
- Weak workpiece
- Stable conditions
- Tool deflecting
- Heavy interruptions
- High component L/D (length/diameter) ratio
12
Best Practices
Geometries
13
Use the nose radius to thin the chip
14
Insert nose radius (rε) guidelines
 Bigger nose radius
– Better surface finish
– Increased edge strength and tool life
 Smaller nose radius
– Better chip breaking
 Wiper geometry
– Best productivity and surface finish
r = 0.2 mm
(.008 inch)
15
r = 0.8 mm
(.031 inch)
r = 1.6 mm
(.063 inch)
Wiper Geometries
Standard radius vs wiper
 Simple guidelines when considering standard radius vs. wiper radius
– Twice the feed rate gives the same surface finish
– The same feed rate gives twice as good a surface finish
Surface roughness (Rz)
Wiper
Standard
Feed, mm/r (fn)
16
Best Practices
Standard radius vs wiper
 Example
– Previous cutting data
•
•
•
•
cBN insert with 0.8 radius
Cutting speed 230 m/min
Feed 0.08 mm/rev
Depth of cut 0.3 mm
– New cutting data
• cBN insert with WH - wiper 0.8 radius
• Feed 0.25 mm/rev
– Result
Gear 58 HRC
17
•
•
•
•
•
•
Tool life increased by 35%
Cycle time halved
Cost per part reduced by 42%
Increased productivity
Reduced cost per part
More time available in machine
Geometries
Lead angle inserts
 Application area
– Should be used for hard part turning (HPT) of hardened
components with features parallel or perpendicular to the
component’s center line.
– 1.8mm (.070 inch) clearance on shoulder.
 The entering angle (r) determines the maximum
depth of cut
 A straight cutting edge gives constant chip
thickness and constant wear.
 A standard radius insert has a varying chip
thickness, it is largest at the depth of cut, reducing
to zero. The greatest wear occurs at the depth of
cut – maximum chip thickness (hex).
18
9°
Lead Angle Geometry
Favorable wear pattern
Wear after 34min
Radius
19
Lead angle
Best Practices
Machining strategies
20
Best Practices
External process parameters
Machine setup
Machine
Tool clamping
Stability
Vibrations
Insert shape
Component Tolerances
Grades
Edge preparations
Material
Cutting data
21
External process parameters
Machine parameters for form and position tolerances
 Basic machine design (main spindle – opposing spindle – tailstock)
 Stability and dampening effects
 Software resolution
 Precision and performance of guides and moving parts
 Play in axes
 Balance of the spindle, maximum RPM
22
Beat Practices
Component design
Prepare the part now!
 Chamfers on components
– Soften the exit and entry
• Longer tool life
 Chamfer holes and key-ways
 Tighter pre-machining tolerances
– Longer tool life
Problem
 Hardness variation in batch
– Inconsistent hardening process
– Inconsistent tool life
 Action
– Recommend to improve the hardening process
23
Best Practices
Clamping of component
 Clamping area in chuck affects roundness
 Clamping forces should be distributed over a larger area
Regular jaws: “point clamping”
24
Wide jaws: “surface clamping”
Best Practices
Tool stability
 Maximum stability is important
 Maximum overhang 1.5 x the shank
size
 For best tool and insert clamping
–
–
–
–
25
CoroTurn® RC
CoroTurn® TR
Coromant Capto®
Optimum Coromant Capto® turret
Best Practices
Round shank clamping
Best choice
Acceptable
• maximum overhang
 steel bar 2 x D
carbide bar 5 x D
Not recommended
Not acceptable
26
Best Practices
Coolant
 General recommendation: no
coolant
– Eliminates the cost of coolant
– Longest tool life
– Especially for interrupted cuts
 Coolant in case of
– Better chip disposal
– Thermal stability of the workpiece
 High Pressure Coolant
– Chip control
– Improve surface quality
27
Best Practice
Turning a square shoulder
 Avoid facing up a shoulder.
 Normal stock variation can be
outside maximal depth of cut
when facing out.
 Recommended process:
– Face down the shoulder
– Turn diameter and if necessary skim
up the face of the shoulder.
ap max. = 0.12mm (.0045")
28
X
5
ap
le=1.4 mm
(.055")
Two-cut strategy
To keep tight dimensional tolerances
 First cut, roughing cut (stable
conditions)
– R = 1.2 - 1.6 mm (.047 - .063 inch) for
feed up to 0.35 mm/rev (.014 inch/rev)
for external machining
– R = 0.8 mm (.031 inch) for internal
machining
– Edge preparation S01030 or S02035 as
optimizer
 Second cut, finishing cut
– ap= 0.05 - 0.07 mm (.0020 - .0028 inch)
– Use wiper insert and high feed
– R = 0.8 mm (.031 inch)
29
Achievable Production Tolerances
When all the stars align...
 +/- 0.0002 on diameter
 8 RMS (approximately 7 Ra)
finish on straight turns
 12 RMS (approximately 11 Ra)
finish on arcs and tapers
 0.0001 inch roundness
30
Best Practices
Cutting Data Considerations
31
vc
– Short contact times
– Abnormal crater wear
increase
decrease
fn
– Low surface quality demand
– High surface quality demand
– Tool vibrations
increase
decrease
decrease
ap
– Below 0.05 mm (0.002")
– Above 0.5 mm (0.02")
increase
decrease
Best Practices
Grooving operations
 Generally two grooving strategies are used:
– One hit/full slot grooving
• The groove-width is around 0.02 mm wider than the insert
width
• The width tolerance of our inserts is
±0.05 mm
– Two passes grooving
• Step one: full slot grooving
• Step two: open up the width to the right tolerance
– Please note: ae must be greater than corner radius
of the insert to avoid deflecting the blade
32
Best Practices
Full nose radius
•
•
•
•
Wraparound occurs in corners
Creates high cutting pressures
Feed reduced
Vibration may occur
Note: Use dwelling (micro stops) to form short
chips and to avoid vibrations if the same insert
radius must be used
To reduce this problem, the insert
diameter should be as small as
possible compared to the radius that is
generated.
33
Threading
Hard Material
The highest axial cutting forces
of the threading operation are
generated during
Entry and exit of cutting tool
34
Hard Part Threading Insert type
V - profile
 Advantages
– Flexibility – one insert can be used for
several pitches.
– Reduce or eliminate vibrations due to
reduction in cutting pressure
– Minimum tool inventory
 Benefits
– Increase cutting speed
– High stability and reliability
– For internal and external threads
35
Best Practices
Use the modified flank infeed
 Most CNC machines have a
programmed cycle using this infeed
 Chip is similar to that in conventional
turning - easier to form and guide
 Chip is thicker, but has contact with
only one side of the insert
 Less heat is transferred to the insert
 First choice for most Hard Part
Threading operations
36
Shims
The diameter and pitch influence the inclination angles
Lead (Pitch) mm
Workpiece
diameter
37
Threads/inch
38
39