33. Process Planning
• Process planning is the selection of operation and parameters required to manufacture a part.
• A process plan is a list of operations required to manufacture a part.
• This technique is quite inexact, and never perfect.
• Some strategies that can be used to do process planning are,
Most significant feature first
• In general a process plan is put together in pieces, but at all times we will be trying to convert features on the design to operations on the process plan.
• Many of the parameters that effect the process planning decisions are related to a part, and to a process being considered.
• Some parts have features that are so advanced or specialized that they can only be produced with one technology. Consider a ferrous part with a high strength that requires a specific heat treating operation.
• The general approach is to start at the end of the process plan, and work backwards. When the design features left are normal
• Generally we can identify the most significant geometry. Some things to look for are,
the features are all cut from a base piece: cutting
there is a natural parting line: welding/molding/casting
features seem to be stuck-on a base piece.: assembly/molding/casting
• An example of this is an angled block with a hole,
• Identifying significant features can be difficult, but some experience can help.
• Large volumes of metal make parts hard to handle,
• Thin walls can be difficult to manufacture and will collapse under force,
• Small hole diameters can be difficult to produce,
• Tolerances can be difficult to maintain.
• Surface finish can be difficult to obtain, (micro-inches),
• Difficult to produce features have preferred processes,
• Materials tend to dictate suitable processes,
• We can use a comparative graph of surface roughness to pick a process.
• When process planning we may use historical data to select operation parameters.
• Metcapp basically does this, it uses sampled data, and then interpolates to find a suitable speed and feed for a cut.
• A sample of a table of roughness measurements is given below [Krar],
• When we are planning operations we must consider the economy of scale.
• If we are producing large volumes, we may want to produce tooling (expensive), but amortize the costs over thousands of parts (less expensive). Examples of this include,
injection molds for thermoplastics
molds for wax, to make investment casting molds
• Purchased components are typically well designed and inexpensive.
• When you do not have experience designing or manufacturing a certain component, and volumes are low, it may not be possible to produce components at a lower cost than they can be produced.
• These parts must still be considered in the process plans so that they are orders, and arrive in suitable forms.
Problem 33.1 A block of metal 5” by 5” by 5” will be milled to 5” by 5” by 4” and then will have two separate holes drilled 2” deep. The hole will be finished with a reamer.
- setup time = 5 min. per part
- setup time = 2 min. per hole
- 10 teeth with a tooth load of .004” per tooth
- setup time = 1 min. per part
a) Estimate the machining time required to make 50 parts.
b) Estimate the cost of the 50 parts.
c) Estimate the machine horse power required for the drill, reamer and the mill.
Problem 33.2 Calculate the machine tool spindle speeds for the following:
a) Milling with a tungsten carbide tipped face cutter on a stainless steel work piece. C.S. = 65 m/min., cutter dia. = 150mm.
b) Drilling with a High Speed Steel drill in Machine Steel work, with C.S. = 70 ft./min., and a drill diameter of 19/32”
c) Turning on a lathe with a High Speed Steel tool in a mild steel work piece. Surface cutting speed = 100 ft./min., and a workpiece diameter of 2.75”
d) Milling with a High Speed Steel cutter in tool steel work with a cutter speed of 60 ft./min., and a cutter diameter of 3/4”.
Problem 33.3 The part below has three turned diameters, on one end there is a square tang that has been milled. The part is made of aluminum. (Note: R indicates radius)
a) Write a process plan that describes the operations necessary to produce the part.
b) If the milling cutter is a 1/2” diameter end mill with 6 teeth, determine a reasonable feed and speed for cutting the tang.
c) What are reasonable speeds and feeds for turning the part for rough and finish turning? (Note: pick one cut for your calculations)
Problem 33.4 Develop a process plan for the part below. You should include speeds, feeds and times. Hint: The part will be easy to make if a combination of milling, drilling and turning is used.
33.6.1 Case Study Problems
OVERVIEW: You have just been hired as a manufacturing engineer by Sports Wares and Equipment Technology (SWET) Inc., a small company. The engineer you are replacing was working on a new product but had to resign for family reasons. On your first day there are a number of desperate problems that have to be examined, and the company is counting on the quality of the answers you provide. To make your life harder, your boss made it quite clear that your reputation is on the line and small mistakes may be very costly.
YOUR ROLE: You are to follow the day of the manufacturing engineer, and interact with others to get the job done right.
THE FACILITIES: As the manufacturing engineer it is your job to direct the setup of equipment for the new product. At present you know about the following machines, and their hourly costs.
THE NEW PRODUCT: The product you are working on is a mounting bracket for sports equipment. As can be seen in the figure below the bracket involves a rectangular base plate with one hole, one press fit bushing and one threaded shaft. The drawing is not yet final, and the design engineer needs a couple of answers before he can complete the drawing. In addition, the people in production are asking you questions.
9:10am Joseph Blough drops by your office and explains he has just been hired in production and is not sure how to read a micrometer. a) He asks you to draw an example of a simple micrometer reading 0.235”. b) He then asks why a bench micrometer has a larger thimble.
9:20am Ian Specter in Quality Control phones you and explains he is ordering metrology equipment and asks you to recommend inspection techniques for a) the overall height tolerance, b) the locational tolerance between the center of the thread and the bushing, and c) the surface roughness.
9:35am Ian calls back and says that all of your earlier suggestions were too expensive and asks for alternatives.
9:55 am Pete Rout in engineering calls and asks how the surface finish specified on the drawing impacts production and quality control. You mention it is not very specific and a) suggest a more specific symbol. You also check your old MEC015 notes and see a chart of surface roughness for different manufacturing processes. You use this to b) suggest a more appropriate surface roughness value.
10:20am Anne Nuther drops by your office and seems concerned about the dimensions and tolerances of the bushing and hole in the part. You quickly a) provide the dimensions and tolerances for both.
10:40am Wyoming Knott in maintenance left a message requesting instructions for where to place the machines for the new product. You decide that before you can do the layout you need to determine the required production steps. Luckily you found a partially completed process plan to start with. You decide to a) complete the process plan and b) calculate all required machine settings, times, tools and costs.
11:30am Phillip Etup in shipping sends a memo saying that they will be testing the bushings that arrived this morning (it seems that your predecessor ordered ahead). They want to know a) if the n=50, c=1 plan from before will give them an AQL of 1% with a producer’s risk of 5%?
1:15pm Norman Alguy calls and says that the cutting parameters for step 0030 in the process plan look too high for the 10hp mill. If the mill has a 1/4hp idle power and is 95% efficient, a) is it sufficient for the cuts?
2:20pm Anne drops by again to discuss the cutting conditions in your process plan. When they tried the same type of process on the lathe previously, they got a tool life of 1 hour at 500 fpm and 2 hours at 300 fpm. a) How long should the tool last for the proposed process plan?
2:45pm You are looking over the drawings and decide to b) check the capabilities of the drilling process to see if it can locate the 1.245” to 1.255” tolerance. You pull out the old data for a similar bracket that was produced before, and a) construct an X-bar chart to verify the tolerance.
33.1 Ullman, D.G., The Mechanical Design Process, McGraw-Hill, 1997.