4- 3 6/13/2014 The Engineering Process Stock Material Processes Finished part Design specifications Process planning Process capability Inspection Need to understand the process capabilities.
4- 4 6/13/2014 PROCESS CAPABILITIES Process: certain way an operation is carried out, e.g. turning, drilling, milling. Tool: physical object which is used to carrying out a process, e.g. twist drill, spade drill, gun drill. Machine tool: machine on which process is carried out, e.g. lathe, drill press, milling machine, machining center. Process capability: The geometry and tolerance a manufacturing process can produce, and its limitations,. i.e. shape and size, dimensional and geometric tolerances, material removal rate, relative cost, other cutting constraints.
4- 5 6/13/2014 LEVELS OF PROCESS CAPABILITIES Universal level: Handbook and textbook level data. Aggregate characterization of what can be expected. General measures of the process capability such as shape and size. What the process can accomplish in an average shop on a typical machine tool. Shop level: Specific to a particular manufacturing system. What is the best attainable capability in one specific shop, e.g. the turning capability of the student machine shop is far worse than that in the shop of a precision spindle manufacturer. Machine level: Specific to a machine. Machines in the same shop has very different capability. A table top lathe can machine a small part, yet a large slant bed lathe may be able to handle a 20"x 10' part.
4- 6 6/13/2014 PROCESS KNOWLEDGE COLLECTION Few scientific data available or published. Most process knowledge are gained during actual manufacturing practice. Practical manufacturing knowledge is still an art instead of a science. Certain information can be found in the textbooks, handbooks, machining data handbook, etc. Tolerance capability may be obtained from control charts, inspection reports, and on-line sensor data.
4- 7 6/13/2014 EXPERIENCE-BASED PLANNING Relay on one's experience. Most frequently this is the way industry operates. Problems: a. Experience requires a significant period of time to accumulate. b. Experience represents only approximate, not exact knowledge. c. Experience is not directly applicable to new processes or new systems. Need to automate.
4- 8 6/13/2014 MACHINIST HANDBOOKS Universal or shop level knowledge. e.g. Surface-finish chart - limiting extremes of process 8 : in - use grinding, polishing, lapping Usually not with milling, however, finish milling may achieve the specification. The information is general. It does not mean every machine or shop can achieve that accuracy. Turning limit (6.3 - 0.4 :m or 250 - 16 : inch) Diamond turning at Lawrence Livermore Lab (12.5 nm or 0.47 : inch)
4- 11 6/13/2014 Dimensional accuracies for Process Planning
4- 12 6/13/2014 HOLE MAKING KNOWLEDGE Following data is taken from a manufacturer's process planner's handbook. I. Dia < 0.5" A. True position > 0.010" 1. Tolerance > 0.010" Drill the hole. 2. Tolerance < 0.010" Drill and ream the hole. B. True position < 0.010 1. Tolerance < 0.010" Drill, then finish bore the hole. 2. Tolerance < 0.002" Drill, semi-finish bore, then finish bore the hole. II. 0.05" < dia < 1.00"
4- 13 6/13/2014 DECISION TABLES To computerize the decision making, one simple way is to use decision tables. If the conditions set in an entry are satisfied, the actions in the entry are executed. The stub contains the condition or action statements. Entries mark which conditions or actions are applicable. Each entry contain one rule. Conditions Actions Stub Entries
4- 14 6/13/2014 EXAMPLE DECISION TABLE Dia < 0.5 0.5 < Dia < 1.0 T.P < 0.010 Tol > 0.010 0.002 < Tol < 0.010 Tol < 0.002 Drill Ream Semi-finish bore Finish bore XXXX XX XXX X X X X X X XXXX X X XX
4- 15 6/13/2014 DECISION TREES Node Branch To computerize the decision making, one simple way is to use decision trees. Decision tree is a graph with a single root and branches emanating from the root. Each branch has a condition statement associate with it. Actions are written at the terminal. Probabilities may be assigned to the branches. In this case, the tree represents probabilistic state transitions. Root terminal The node may be "AND" nodes or "OR" nodes.
4- 16 6/13/2014 EXAMPLE DECISION TREE Dia < 0.5 0.5 < Dia < 1.0 T.P < 0.010 Tol > 0.010 Tol < 0.010 0.002 < Tol < 0.010 Tol < 0.002 Drill Drill, then ream Drill, then finish bore Drill, semifinish bore, then finish bore
4- 26 6/13/2014 BASIC MACHINING CALCULATIONS Machining time Total amount of time to finish a workpiece. For drilling, one pass turning, and milling: : clearance or overhang distance. For multipass turning integer round up For milling
4- 27 6/13/2014 BASIC MACHINING CALCULATIONS Machine control parameters are: f, V, ap. a. Feed and feedrate turning or drilling milling N: #ofteechinmilling 1indrilling n:rpm V f =fnN V f =fn V f :inch/min V f
4- 28 6/13/2014 BASIC MACHINING CALCULATIONS Cutting speed D: Diameter Depth of cut surface speed Vinsfpm D i D 0 V= Dn 12
4- 29 6/13/2014 BASIC MACHINING CALCULATIONS Metal removal rate Drilling Turning Milling MRR= D 2 4 v f =3D f V MRR= (D 2 o –D 2 i ) 4 v f =6(D o –D i )f V v f ( D 2 o –D 2 i ) 4 MRR=a p wv f = 12a p wn D fV D 2 4 v f
4- 30 6/13/2014 BASIC MACHINING CALCULATIONS Machining time Total amount of time to finish a workpiece. For drilling, one pass turning, and milling: : clearance or overhang distance. For multipass turning integer round up For milling