Machining Notes for Turning process GATE Mechanical Engineering

GATE Mechanical Engineering exam covers turning process as an important machining process from the subject of manufacturing engineering. In these notes we will look at the turning process as short notes which can be used to prepare for GATE mechanical exam. 

Metal cutting removes material (chips) by using a sharp cutting tool by turning, milling, drilling etc. The major components of this process are:

  • Work piece
  • Work piece holder
  • Cutting tool
  • Tool holder
  • Machine tool

Important characteristics of machining processes:

  • Variety of materials can be machined
  • Variety of part geometries can be produced
  • The process provides good dimensional accuracy and surface finish

Cutting tool classification

Single point tool

  • One dominant cutting edge
  • Typically rounded to form nose radius
  • Turning process uses single point tool

single point cutting tool-gate mechanical notes

Multi point tool

  • More than one cutting edge
  • Motion relative to work achieved by rotating 
  • Drilling, milling use multi point tools

multi point cutting tool gate mechanical notes

Turning process

  • This process is performed on a machine tool called lathe
  • Single point cutting tool removes material from a rotating work piece to generate a cylindrical shape

turning process gate maechanical notes

turning process for gate mechanical engineering

For round shapes, following are the lathe operations.

  1. Straight turning
  2. Taper turning
  3. Profiling
  4. Turning and external grooving
  5. Facing
  6. Face grooving
  7. Cutting with form tool
  8. Boring and internal grooving
  9. Drilling
  10. Cutting off
  11. Threading
  12. Knurling

Important parts of an engine lathe

parts of engine lathe

engine lathe parts-2

engine lathe parts

Two dimensional cutting

Positive rake angle

Positive rake angle

negative rake angle

Negative rake angle

Forces acting on a chip

orthogonal cutting

 = Rake angle in degrees

 = Shear angle in degrees

 = Area of shear plane

 = Thrust force

 = Cutting force

 = Resultant force

 = Friction force

 = Normal friction force

 = Shear force

 = Normal shear force

 = Friction angle

Important formulas:

 , 

Merchant: 

Lee: 

Mechanics of chip formation

Independent variable:

  • Type of cutting tool
  • Tool geometry and sharpness
  • Work piece material
  • Cutting parameters (feed, speed, depth of cut)
  • Cutting fluid
  • Tool and work piece holding devices

Dependent variable:

  • Type of chip produced
  • Force required
  • Energy dissipated
  • Work piece, chip, tool temperature rise
  • Tool wear
  • Surface finish

Let:

V = Cutting speed in m/min

N = RPM

f = feed in mm/rev

 = feed rate in mm/min

D = cutter diameter

L = length of cut

 = Offset length

w = work piece width

 = Original part diameter

 = Final part diameter

 = Average diameter

d = depth of cut

 = uncut chip thickness

 = chip thickness after cut

 = Cutting ratio

 = chip velocity

 = Shear velocity

Important relatins for feed and speed:

Material removal rate (MRR) =  

Example:

In a turning operation, spindle speed is set to provide a cutting speed of 1.8 m/s. The feed and depth of cut are 0.30 mm and 2.6 mm respectively. The tool rake angle is 8 degrees. After the cut, the deformed chip thickness is measured to be 0.49 mm. Determine (a) shear plane angle, (b) shear strain, and (c) material removal rate.

Solution:

Given:  = 0.30 mm,  = 0.49 mm,  = 8 degrees, f = 0.30 mm, d = 2.6 mm, V = 1.8 m/s = 1.8 x 103 mm/s

Shear plane angle () = 33.5 degrees

Shear strain () =  = 1.987

MRR = V.f.d =  

Hot hardness of some commonly used tool materials

Below is the grpah which shows variation of hot hardness of the below mentioned tool materials:

  • Plain carbon steels
  • High speed steels
  • Cast cobalt alloys
  • Cemented carbides
  • Ceramics

hot hardness of cutting tools