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Machining Economics Notes for GATE Mechanical Engineering

Machining Economics Notes for GATE Mechanical Engineering

Limitations of machining/material removal operations:

  • Wasted material (although may be small)
  • Longer time (vs. forming/shaping): cutting/non-cutting
  • Require more energy (vs. forming/shaping)
  • Adverse effects on surface quality/properties of the product

Importance of machining (despite above):

  • Producing complex workpiece shapes (e.g. internal features)
  • High dimensional accuracy/surface finish

Costs/factors involved with machining:

  • Machine tools, work-holding devices, fixtures and cutting tools
  • Labor and overhead
  • Setting up time (machine for operation)
  • Material handling and movement (e.g. loading blank, unloading machine part)
  • Gaging for dimensional accuracy and surface finish
  • Cutting times and non-cutting time

Minimizing Machining Cost per Piece

  • Important in all manufacturing processes to minimize:

            Machining cost per piece, C

            Machining time per piece, T

  • Various approaches exist (using software)

Total machining cost per piece, Cin turning is, C= C+ Cs + CL + Ct

Cm = machining cost

Cs = cost of setting up for machining-including mounting the cutter, setting up fixtures, and preparing the machine tool for the operation.

CL = cost of loading, unloading, and machine handling

Ct = tooling cost, often only 5% of the total cutting operation. Consequently, using the least expensive tool is not always an effective way of reducing machining costs.

Machining cost per piece Cm is given by Cm = Tm (Lm + Bm)

Tm = machining time per piece

Llabor cost of production personnel per hour

Bmburden rate (also known as overhead charge), including:

  • depreciation
  • maintenance
  • indirect labor etc.

The setup costCis fixed amount (in $) per piece:

  • Loading/unloading, machine-handling cost, Cper piece: CL = TL (Lm + Bm)

 Ttime required to.

  • load/unload part
  • change speeds
  • change feed rates etc.

The tooling cost, Cper piece: 

Nnumber of parts machined per insert

Nf = number of parts that can be produced per insert face

Tc = time required to change the insert

Ti = time required to index the insert

D= depreciation of insert (in $)

Time required to produce one part is: 

Tcalculated for each operation

Example: for turning: 

L = length of cut

f = feed

N = rpm of the workpiece

D = workpiece diameter

V = cutting speed

From Taylor's tool life equation: 

T = time (mins) to reach a certain flank wear (before regrinding/changing insert)

The number of pieces per insert face: 

Number of pieces per insert: 

m = number of faces actually used

  • m is not necessarily number of faces per insert
  • not all faces are used before insert is discarded

Combining T and Tin N

To determine optimum V (Voand T (To)

  • find Vand Tfor minimum cost, Cp 
  • differentiate Cp with respect to V and set it to zero, 

Now find Vo and To for maximum production, i.e. min Tp.

Differentiate Twith respect to V and set it to zero, 

The qualitative plot of Cp and Tper piece:

  • Calso depends on the required surface finish. Better surface finish means higher value of Cp
  • V= V @ Cp,min
  • Vo = V @ Tp,min

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