Nov 20 Tool life - ME 250 Nov 20, 2007 Machining economics...

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ME 250 Nov 20, 2007 Economics of machining Motivation: maximize profit Maximize profit Æ reduce costs Maximize profit Æ zero defects Maximize profit Æ increase parts / day 1. Maximizing parts / day • Maximizing production rate Æ minimizing cutting time per unit (cycle time) •I n turning , unit cycle time is composed of: 1. Part handling time (per part) = T h 2. Machining time (per part) = T m 3. Tool change time (per part) = where n p = number of pieces cut in one tool life p t n T Production rate Unit cycle time (turning): T c = T h + T m + Due to T m above, we note that cycle time T c is a function of cutting speed p t n T Production rate vs. Cutting speed 2. Reduce costs In turning, unit cost consists of: 1. Cost of part handling time = C o T h , where C o = cost rate for operator and machine 2. Cost of machining time = C o T m 3. Cost of tool change time = C o 4. Tooling cost = where C t = cost per tool p t n C p t n
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Minimizing unit cost Unit cost (for turning): C c = C o T h + C o T m + C o + p t n T Again, unit cost is a function of cutting speed, since T m and C t are functions of v p t n C Unit Cost vs. Cutting Speed Economics of machining Maximizing production rate and reducing costs: Æ cutting parameters Æ cutting tool life Selecting cutting parameters • An important task in process planning • For each operation, decisions must be made about machine tool, cutting tool(s), and parameters – Cutting parameters: depth of cut, feed, speed, and also cutting fluid • These decisions must give due consideration to workpiece machinability, part geometry, surface finish, and so forth Selecting depth of cut – In roughing, depth is made as large as possible to maximize material removal rate, subject to limitations of • Machine tool horsepower • Machine tool and setup rigidity • Strength of cutting tool – In finishing, depth is set to achieve final part dimensions and surface finish Determining feed • Determining feed depends on: – Tooling – harder tool materials require lower feeds – Is the operation roughing or finishing? • Constraints on feed in roughing – Limits imposed by forces, setup rigidity, and sometimes horsepower • Surface finish requirements in finishing – Select feed to produce desired finish
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Optimizing cutting speed • Select speed to achieve a balance between high metal removal rate and long tool life • Determining optimal speed is an non- linear optimization problem • Two alternative objective functions: 1. Maximize production rate 2. Minimize unit cost Cutting Tools Two components: 1. Tool material 2. Tool geometry Tool materials Desirable properties for a tool material: – Toughness - to avoid fracture failure – Hot hardness - to retain hardness at high temperatures – Wear resistance - to resist abrasive wear High speed steel is better Hot Hardness Typical hot hardness relationships for selected tool materials. Plain carbon steel shows
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Nov 20 Tool life - ME 250 Nov 20, 2007 Machining economics...

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