Variables: Th - part handling time Co (Cg) – operator (grinder’s) cost rate/min Tm – machining time Ch – cost of part handling time Tt – tool change time Cm – cost of machining time np – number of parts cut by Ctc – cost of tool change time tool during tool life Tc – cycle time per part Ct – cost per cutting edge T – tool life Ctp = Ct/np - tool cost per part - Total time per part produced (cycle time):
- Tc = Th + Tm + Tt/np
- where Tt/np is the tool change time per part.
- Consider a turning operation. The machining time is given by
- Tm = D L/(v f)
- The number of parts cut per tool is given by
- np = T/Tm= f C(1/n)/( D L v(1/n -1) )
- Maximum production rate - turning
- Substituting, we get the total cutting time
- Tc = Th + D L/(v f) + Tt[ D L v(1/n -1)/( f C(1/n) )]
- Minimizing cycle time (dTc/dv = 0 ) gives optimum (max) cutting speed and tool life:
- vmax = C/[(1 - n) Tt/n]n
- Tmax = (1 - n) Tt /n
- Cost of part handling time:
- Ch = CoTh
- Cost of machining time:
- Cm = CoTm
- Cost of tool change time:
- Ctc = CoTt /np
- Minimum cost per unit - turning
- Tool cost per part:
- Ctp = Ct /np
- Tooling cost per edge:
- Disposable inserts Ct = Pt /ne ne = num of edges/insert Pt = original cost of tool
- Single point grindable Ct = Pt /ng + Tg Cg “includes purchase price” ng = Num tool lives/tool Tg = time to grind tool
- Minimum cost per unit - turning
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