Turning

Question 1

Turning: definition

Answer 1

Machining operation where the piece rotates, while the cutting tool moves with a feed in axial or radial direction

Question 2

principal components of a lathe

Answer 2

1. spindle: it makes the part rotating
2. carriage: motion parallel to spindle axis
3. cross slide: motion orthogonal to spindle axis
4. tool post: it fixes the cutting tool

Question 3

work-holding devices: functions

Answer 3

• support workpiece during cutting
• reference: they define the position of the part
• blocking

Question 4

types of work-holding devices

Answer 4

1. chuck
   - three self-centering jaws
   - a proper force blocks the piece
   - stair profile to have a distance between piece and spindle
   - conical profile for difficult to block pieces
2. two conical elements
   - holes in the piece, to be removed after the process
   - frontal tooth is necessary because friction is not enough
   - live center: both sides connected to the motor
   - dead center: one side is fixed or free to move
3. collet
   - workpiece in the collet hole (conical)
   - conical walls of the collet are forced by pulling it
4. mandrel
   - mounted between centres of the lathe
   - friction makes the piece move
5. rest
   - to hold very long pieces, avoiding their deflection

Question 5

main characteristic angles

Answer 5

1. normal section angles
   - rake angle gamma0
   - solid angle beta0
   - clearance angle Alpha
2. profile angles
   - main cutting edge angle psi
   - secondary cutting edge psi’
   - cutting edges angle eps
   - back rake angle lambda
3. entering angles
   - main cutting edge entering angle k
   - secondary cutting edge entering angle

Question 6

influence of the clearance angle

Answer 6

• during the turning, the tool describes a helix with angle phi, that reduces alpha0
• Alpha = alpha0 - phi
• Alpha must be positive to avoid contact and frinction with the machined surface
• Alpha too high - > resistant section too little
• Alpha too low - > high wear of the flank
• material with high cutting pressure - > lower Alpha
• tool not tough - > lower alpha

Question 7

influence of the rake angle

Answer 7

higher gamma means a sharper tool, so

• lower cutting pressure
• lower cutting force and power
• lower friction on the rake face
• lower resistance of the cutting edge
• higher gamma for ductile materials
• lower and even negative gamma for brittle materials, to have higher resistance

Question 8

influence of cutting edge angles

Answer 8

psi affects the components of the cutting force:
- higher psi gives higher cutting duration, but higher thrust force

• in normal workinng conditions psi > 0
• psi’ determines epsilon and so the robustness of the tool
• psi’ should be selected high, according to k’ which affects the surface finish
• epsilon determines robustness of the tool and surface finish

Question 9

influence of the entering angles

Answer 9

togheter with nose radius and feed determine the roughness of the surface

Question 10

forces

Answer 10

• feed force Ff in the direction of the feed
• thrust force FD perpendicular to vc
• cutting force Fc in the direction of the cutting speed vc

Question 11

cutting power

Answer 11

P = Fcvc + FDvD + Ff*vf

vD = 0
vf < < vc
- > P = Fc*vc

Question 12

cutting time and machining time

Answer 12

• cutting time gives information about tool wear
  tc = lc/vf
  lc length of the cut
  vf = f*n feed velocity
• machining time gives information about total time to produce the piece
  tm = lm/vf
  lm = lc + ein + eout total distance done by the tool

Question 13

verifications in turning

Answer 13

1. cutting depth compatiible with the tool
2. feed admissible for the lathe
3. cutting speed and so rotational speed admissible for the lathe
4. power constraint
5. parameters give desired values of roughness
6. parameters allow the respect for tolerances
7. fixtures can hold the piece preventing bennding

Question 14

fixture verification

Answer 14

• cutting torque
  Tc = Fc * D/2

- resistant torque 
Tr = z * p * A * mu * D0/2
z number of jaws
p pressure 
A area of each jaw
mu friction coefficient
D diameter of the cut
D0 diameter at the fixturing position

Question 15

fixture verification

Answer 15

• cutting torque
  Tc = Fc * D/2

- resistant torque 
Tr = z * p * A * mu * D0/2
z number of jaws
p pressure 
A area of each jaw
mu friction coefficient
D diameter of the cut
D0 diameter at the fixturing position

• > Tr > Tc
• > p > pmin

Question 16

workpiece bending

Answer 16

maximum bending f depends on the fixture

1. three-jaws chuck:
   maximum f at the end
   f = 1/3 * FcL^3/EJ
2. chuck-dead center:
   maximum f at 0.6L
   f = 1/107 * FcL^3/EJ
3. between centers:
   maximum f at 0.5L
   f = 1/48 * FcL^3/EJ