Chapter 3 Process Planning and Manufacturing Planning
3.1. Introduction
- Process planning deals with setting up machines while manufacturing planning refers to setting up the production. In this chapter we will study both.
- This chapter is a combination of Chapter 5 (process planning) and Chapter 10 (manufacturing planning) in the textbook and some supplementary materials from the references
- Process planning links the design to manufacturing and is very important part of the manufacturing system. Figure 1 shows the role of process planning in manufacturing systems.
Fig. 1: process planning in manufacturing systems
3.2 Process Planning
(1) Introduction
- Process planning serves as an integrated link between design and manufacturing. It helps to translate design to product
- The questions to be answered:
- What is in a process plan?
- What are basic steps of process planning
- What is computer aided process planning (CAPP)
- CAPP methods (including variant method, generative method and knowledge-based method).
- All manufacturing processes require process plans. However, we will focus on machining process only
(2) The basics of machining processes
- As discussed in Chapter 2, there are many different machining processes. For example
- Turning as illustrated in Figure 2
- Milling as illustrated in Figure 3
- Grinding as illustrated in Figure 4
- Let us study turning (external) in a little more details
- The turning process is illustrated in Figure 5
Fig. 5: illustration of turning (external) process
- The important parameters include:
- Depth of cut, d (mm or inch)
- Rotation speed, N or RPM (rotation per minute)
- Feed, f (mm / rev. or inch / rev.), or feed rate, fr (mm / s or inch / s)
- The length of cut, L (mm or inch)
- Turning is called single point cutting as there is only one point (the cutting edge) engaged in the cut in any given time
- Some basic calculations in turning
- Cutting speed, V (mm / m or inch / m):
V = pDN
- Machining time (minute):
where, La is the allowance distance (mm or inch)
- Material removal rate, MRR, (mm3 / min, or inch3 / min):
MRR = pDNfd
- Tool life (Taylor’s formula), T (min):
VTn = C
where, n is a tool material dependent constants, C is the cutting speed at which the tool life is one minute. In particular, for HSS tools, n » 0.1, for carbide tools, n » 0.2, and for ceramic tools, n » 0.4. The constant C is dependent on the work material and its value can be found from manufacturing handbooks. In particular, for low carbon steels, C » 500.
- the cutting force, F,
F = FsA
where, Fs is the specific energy and A = d·f is the area of cutting. The specific cutting force is a function of many factors such as work material, tool geometry, …. For low carbon steels, Fs » 58.15 MPa.
- You can always check your calculation using the unit.
- An example:
- given
- D = 100 mm
- d = 0.2 mm
- N = 600 rev. / min
- f = 0.1 mm / rev.
- L + La = 200 mm
- n = 0.2 (carbide cutter)
- C = 500 m (low carbon steels)
- The cutting speed
V = pDN = 188485.59 (mm / min) = 188.5 (m / min) = 3.14 (mm / s)
Note that (m / min) is usually used.
- The machining time
= 3.33 min
- The material removal rate
MRR = pDNfd = 37699.1 ((mm) (1/ min) (mm) (mm)]
= 37.699 mm3 / min
= 628 mm3 / sec
- The tool life
=108 mm.
Note that increase the cutting speed will increase the productivity but reduce the tool life. Hence, there is an “optimal” cutting speed as shown in Figure 6.
Fig. 6: illustration of the optimal cutting speed
- The cutting force:
F = FsA = 58.15*2*0.1 (MPa*mm*mm) = 11.629*10*10 (N) = 1162.9 N
(3) Determining the cutting condition
- There are many models available. However, we will discuss the minimum unit cost model only.
- Reflecting a machining operation, the unit cost, Cu, is:
Cu = non-production cost per unit (loading, unloading, setup, …)
+ machining cost per unit (machining time x constant)
+ tool change cost per unit
+ tooling cost per unit
- Thus, the minimum unit cost model:
where, C0 – machining cost with overhead ($ / min)
t1 – non-production time (min)
tc – machining time (min)
tc – tool change time (min)
(tc / T) – percentage of tool life used per unit
Ct – tool cost (per edge) ($)
- Note that
- the machining time is:
- the tool life is:
- Substitute the machining time and the tool life equation to the above equation, it follows that:
- The solution of this model can be found by partial differentiating the equation, equating to zero and solving; and the result is as follows (fortunately, somebody has solved it for us):
and the resulting machining time is:
- The other commonly used model is production rate model, which will be discussed in the tutorial together with the manufacturing lead time
(4) An example
- Given the following data:
- The parts: 50 units, L = 300 mm, D = 60 mm, f = 0.2 mm / rev., d = 1 mm
- The tool: tool life coefficient: n = 0.2, C = 200
- Machining cost: machining labor cost $10 / hr, overhead = 50%
- Tooling cost: tool cost $30.96, 6 edges
- Tool change time td = 0.5 min
- Calculation:
- The machining cost: C0 = (10 + 0.5*10) = $15 hr. / 60 = $0.25 / min
- The tooling cost per edge: Ct = 30.96 / 6 = $5.16
- The optimal machining time:
= 84.56
- The optimal cutting speed:
= 82.337 m / min
(5) Discussions:
- How to deal with the constraints on speed (i.e., v £ vmax)?
- What is the optimal feed (together with the speed)?
- How to deal with the uncertainty on the cost factors (e.g., C0) and / or time factors (e.g., td)?
- To solve these problems, we have to use linear programming (and nonlinear programming) method
(6) The procedure for machining processes planning
- Step 1: analysis of part requirement (size, tolerance, surface finish, …)
- Step 2: selection of raw material
- there are over 2,000 different materials available
- we have learnt how to design part for better material utilization in Chapter 2
- Step 3: selection of machining sequence
- Step 4: selection machine tool (we have learnt that in Chapter 2)
- Step 5: selection of cutters (there are over 10,000 different cutters available)
- Step 6: selection or design of fixtures and inspection equipment (optional)
- Step 7: determining cutting conditions
- this includes V and f (and d for multiple passes machining)
- we will focus on how to determine the cutting condition
(7) An example: Designing a machining plan for the order given below:
- the work order (lot size): 200 pieces
- the work design
Fig. 7: illustration of the work design
- the work material: mild steel
- the time and cost factors:
- machining cost with overhead (C0 in the optimization model): $60 / h = $1 / min
- tool cost per edge (Ct in the optimization model): $60 / cutter = $10 / edge
- loading and unloading time (t1 in the optimization model): 1 min / piece
- tool change time (td in the optimization model): 3 min
Solution:
Step 1: Analysis of part requirement (size, tolerance, surface finish, …)
- the internal hole requires extra tight manufacturing tolerance and surface finish
Step 2: Selection of raw material:
- material is given (given mild steel, C = 500 in the Taylor’s tool life equation)
- since the work design is simple, we can just order a batch of steel bars with 102 mm diameter
Step 3: Selection of machining sequence: the machining sequence:
- Facing
- External turning
- Drilling
- Boring (because of the tight manufacturing tolerance and surface finish)
- Parting
Step 4: Selection machine tool
- It is noted that the manufacturing tolerance and surface finish is not very high and the lot size is low
- However, several different tools are needed to complete the work
- Hence, we can use a turret lathe
Step 5: Selection of cutters
- use carbide tools for all operations, which is the most popular choice
- n = 0.2 in the Taylor’s tool life equation
Step 6: selection of fixtures and inspection equipment (optional)
- use standard hydraulic clamp of the lathe
Step 7: Determining cutting conditions and calculating the machining time and cost
(a) facing
- it is noted that the cutting speed changes from a maximum value, Vmax, to zero. Hence, there is no “optimal” cutting speed. Based on experience, we choose:
- N = 800 rpm
- f = 0.1 mm (in radian direction)
- d = 0.5 mm (in axial direction), to ensure the surface is machined
- At this time, we have:
- Cutting speed:
Vmax = pDN = (3.14)(102)(800) = 256224 mm / min = 256.224 m / min.
- Machining time:
= (100 + 10) / (2)(0.1)(800) = 0.69 min. (Da is the clearness)
- Tool life
= [(500)(2) / (256.224)]5 = 902 min = 15 hours
- Unit cost (with the setup cost)
Cfacing =
= (1)(1) + (1)(0.69) + (1)(3)(0.69 / 902) + (10)(0.69 / 902) = $1.70
(b) external turning
- the depth of cut is:
d = 2 mm (the raw material diameter – the required diameter)
- the feed:
f = 0.2 mm (to ensure the surface finish of Ra = 20 mm)
- the optimal cutting speed (note that it is independent of d and f):
= 226.9 m / min
- the machining time:
tc = = (3.14)(150+10)(100) / (1000)(0.2)(226.9) = 0.35 min
- the (optimal) tool life:
= (4)(13) = 52 min
- the unit cost (no setup cost is necessary):
Cturning =
= (1)(0.35) + (1)(3)(0.35 / 52) + (10)(0.35 / 52) = $0.44
(c) drilling
- this can be done in a turret lathe by rotating the work
- the depth (width) of cut:
d = 14 mm (to get a clearness for boring)
- the cutting speed:
- the cutting speed varies from Vmax to 0 (at the center of the drill),
- we choose N = 600 rmp (drilling is a severe machining operation involving large material removal)
- Hence,
Vmax = pdN = (3.14)(14)(600) = 26389.78 mm / min = 263 m / min
- The machining time:
= (150 + 10) / (0.1)(800) = 2 min.
- tool life:
= [500 / 263]5 = 24 min
- the unit cost (no setup cost):
Cdrilling =
= (1)(2) + (1)(3)(2 / 52) + (10)(2 / 52) = $2.5
(d) boring
- boring is necessary to ensure the manufacturing tolerance and surface finish (the problems with drilling include eccentricity error and rough surface)
- the calculation is the same as external turning
- the depth of cut is:
d = 1.5 mm (the designed hole diameter – the drill diameter)
- the feed:
f = 0.1 mm (to ensure the surface finish of Ra = 10 mm)
- the optimal cutting speed:
= 226.9 m / min
- the machining time:
tc = = (3.14)(150+10)(100) / (1000)(0.1)(226.9) = 0.7 min
- the tool life:
= (4)(13) = 52 min
- the unit cost (no setup cost is necessary):
Cboring =
= (1)(0.7) + (1)(3)(0.7 / 52) + (10)(0.7 / 52) = $0.88
(e) parting
- the calculation is similar to that of facing
- N = 800 rpm
- f = 0.1 mm (in radian direction)
- d = 1.5 mm (in axial direction), which is also the width of the cutter
- At this time, we have:
- Cutting speed:
Vmax = pDN = (3.14)(102)(800) = 256224 mm / min = 256.224 m / min.
- Machining time:
= (100 – 15.5 + 10) / (2)(0.1)(800) = 0.62 min. (a hole is there)
- Tool life
= [(500)(2) / (256.224)]5 = 902 min = 15 hours
- Unit cost (no setup cost)
Cparting =
= (1)(0.62) + (1)(3)(0.62 / 902) + (10)(0.62 / 902) = $0.63
(f) Total manufacturing time and cost
- Total manufacturing time
tmfg = 200(tsetup + tfacing + tturning + tdrilling + tboring + tparting) = 1072 min = 17.8 hours.
Note that
- the tool change may not be necessary for some operations such a small batch (a controversy)
- we can calculate the machining time each individual tool and hence, predict the number of tool changes
- Total machining cost
Cmfg = 200(Cfacing + Cturning + Cdrilling + Cboring + Cparting) = (200)($6.15) = $1230
Note that
- The setup cost is included in facing
- The cost can be used to calculate the profit.
3.3 Computer Aided Process Planning (CAPP)
(1) Computer Aided Design changes the world of design. Can we use computer to conduct process planning? The result is Computer Aided Process Planning (CAPP)
(2) There are three types of approaches of CAPP
- variant approach
- generative approach
- knowledge-based approach
In fact, they all have some sort of intelligence (knowledge) involved because process planning is a creative work.
- Many CAPP systems have been developed and most of them use a combination of variant approach and generative approach. Also, most of them are still in the stage of research and development.