Technical Guidance The Basics of Turning, Turning Edition
■ Calculating Cutting Speed
■ Calculating Power Requirements
f
(1) Calculating rotation speed from cutting speed
n : Spindle Speed (min -1 ) v c : Cutting speed (m/min) D m : Inner/outer diameter of workpiece (mm) X : π ≈ 3.14
P c : Power requirements (kW) v c : Cutting speed (m/min) f : Feed Rate (mm/rev) a p : Depth of cut (mm) k c : Specific cutting force (MPa) H : Required horsepower (HP) V : Machine efficiency (0.70 to 0.85)
1,000 × v c X × D m
n =
v c × f × a p × k c 60 × 10 3 × V
P c =
Example: v c =150m/min, D m =100mm n = 1,000 × 150 = 478 (min -1 ) 3.14 × 100
P c 0.75
N
H =
Feed Direction
· n : Spindle speed (min -1 ) · v c : Cutting speed (m/min) · f
(2) Calculating cutting speed from rotational speed
● Rough value of kc Aluminum: 800MPa
: Feed rate per revolution (mm/rev)
X × D m × n 1,000
· a p : Depth of cut (mm) · D m : Diameter of workpiece (mm)
v c =
Refer to the above table
General steel: 2,500 to 3,000MPa Cast iron: 1,500MPa
■ Three cutting force components
■ Relation between cutting speed and cutting force
■ Relation between feed rate and specific cutting force (for carbon steel)
Rake Angle: -10°
8,000
1,600
Traverse Rupture Strength
800MPa 600MPa 400MPa
F f
800
Rake Angle: 0°
6,000
0
F p
0 80 160
240
Cutting Speed (m/min)
F c : Principal force F f : Feed force F p : Back force
F c
■ Relation between rake angle and cutting force
4,000
● Calculating cutting force
P : Cutting force (kN) k c : Specific cutting force (MPa) q : Chip area (mm 2 )
2,800
k c × q 1,000
P =
2,000
2,400
k c × a p × f 1,000
a p : Depth of Cut (mm) f : Feed Rate (mm/rev)
=
2,000
1,600
- 10 - 20
20 10 0
0 0.1 0.04
0.2
0.4 Feed Rate f (mm/rev)
Rake angle (degrees)
When feed rate decreases, specific cutting force increases.
■ Relation between nose radius and 3-part force
● Theoretical (geometrical) surface roughness ■ Machined Surface Roughness
● Ways to improve surface finish roughness (1) Use an insert with a larger nose radius (2) Optimise the cutting speed and feed rate so that built-up edges do not occur (3) Select an appropriate insert grade (4) Use wiper insert
4000
Principal Force
3500
h : Theoretical surface roughness (μm) f : Feed rate per revolution (mm/rev) RE: Nose radius (mm)
f 2
× 10 3
h =
8 × RE
Feed Force
1500
1000
Back Force
500
.0157 .0315 .0472 .0630 Nose Radius (inch) Large nose radius increases back force.
f
RE
Part Material: Alloy Steel (38HS) Insert: TNGA43 ○○ Holder: PTGNR2525-43 Cutting Conditions: v c =330 SFM, a p = .157", f = 0.018 IPR
■ Actual surface roughness Steel: Theoretical surface roughness × 1.5 to 3 Cast iron: Theoretical roughness × 3 to 5
N36
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