Page 207 - Mechanical Engineer's Data Handbook

MANUFACTURING TECHNOLOGY 195

5.7.7 Calculation of machining cost

The ‘total-time cost per workpiece’ is made up of 8
‘machine-time cost’, ‘non-productive-time cost’ and 2i
‘tool cost’. ‘Machining-time cost’ is for actual machin- 1
ing and includes overheads and wages. ‘Non-produc- 8
tive-time cost’ covers ‘setting-up’ and ‘loading- and
unloading-time cost’. ‘Tool cost’ combines ‘tool-
change-time cost’ and actual ‘tool cost’. The former is Cutting speed rnirnin -
the cost of changing the cutting edge, the latter is the
cost of the cutting plus resharpening. When ‘total cost’
is plotted against ‘cutting speed’ an optimum speed for
minimum cost is found.
tR
Let: c =E
“60
C, = machining-time cost per workpiece
( 3:
C, = non-productive-time cost per workpiece c,= t,+J -
C, = tool-change-time cost per workpiece
C, = tool cost per workpiece
t tR
C,==-
Total cost of machining C,,, = C, + C, + C, + C, at.
(Elworkpiece)
C,=- ctt + tshtmR
Total tool cost per workpiece C,, = C, + C, l+n, at,
Let :
t, = machining time per workpiece (min) 5.7.8 Cutting fluids
t, =loading and unloading time per workpiece (min)
t,=setting time per batch (min) It is necessary when machining to use some form of
t, = tool life (min) fluid which acts as a coolant and lubricant, resulting in
t, = tool change time (min) a better finish and longer tool life. The fluid also acts as
t,, = tool sharpening time (min) a rust preventative and assists in swarf removal. The
R =cost rate per hour (E) following table lists various fluids and their advan-
nb = number per batch tages.
n, = number of resharpenings

Group Description Advantages
~ ~~~~~
Soluble oil Oil, emulsifier and 2-10% water Good coolant. Poor lubricant

Clear soluble oil As above, with more emulsifier Good coolant. Poor lubricant

Water based fluids Solution of sodium nitride and Good coolant. Poor lubricant
triethanolamine
EP soluble oils Soluble oils with EP additives, e.g. Fairly good lubricant
sulphur and/or chlorine