An integrated process planning and robust fixture design system 5

The purpose of a set-up plan is to locate and fix a part in a specific manner on a machine tool so that machining can take place according to design specifications.. Figure 5.1 Self and

Optimal Set-up Planning

5.1 Introduction

Set-up planning is a function of both process planning and fixture design (Ong and Nee, 1994) Its task is to determine the number and sequence of set-ups, the features to

be machined in each up, and the part orientation and locating features of each

set-up The purpose of a set-up plan is to locate and fix a part in a specific manner on a machine tool so that machining can take place according to design specifications

Two factors have to be considered in set-up planning, design specifications and manufacturing resources Design specifications include workpiece geometry, dimension, tolerance, and features which can be both functional and aesthetic Manufacturing resources include production requirements, available machines, cutting tools, and fixtures A set-up plan which considers all these factors optimally can ensure

to deliver the product with not only high quality but also high throughput rate and low cost

From published work, these two factors of set-up planning are treated separately Most

research attempted to satisfy the first factor, i.e., analysis of the design specifications,

including tolerance analysis, precedence constraint satisfaction, geometric data analysis, and tool access direction verification The main objective of these studies is

to reduce the locating error and minimize the number of set-ups The second factor was normally considered at the optimization stage in terms of cost, quality and lead time, and under an assumption of the availability of certain machine tools

Different set-up plans can be generated in a different manufacturing environment Different set-up plans may also lead to different locating methods and manufacturing cost, and different fixture configurations can result in different locating stack-up errors and stability Machining accuracy and the capability of available machine tools would need to be considered simultaneously during set-up planning in order to achieve a higher level of optimization An optimized set-up plan can eliminate unnecessary machining error stack-up, improve product quality and reduce production cost

In this chapter, an optimized set-up planning approach which considers machining error stack-up and the capability of available machine tools simultaneously is addressed It is assumed that a machining environment contains several machine resources which include 3-, 4- or 5-axis machine centers, and can be distributed and located in different places A tolerance cost factor, which will be applied in the case of

a stack-up, has been introduced The strategies are achieved by minimizing a cost model among the distributed machine resources

5.2 Set-up Planning System

5.2.1 Consideration of Design Specifications

Set-up planning should satisfy the design specifications, i.e., geometric, dimensional

and tolerance requirements, precedence constraint satisfaction, and tool approach direction (TAD) verification Product design information in a CAD model would need

to be recognized and extracted before set-up planning

In this research, hole and plane machining features, which commonly exist on a cast part, are considered The heuristics used for reasoning the hole and plane features are shown in Table 3.2 in Chapter 3 The TAD of a feature is determined by searching for any intersection entities in the candidate direction with a ray which has a radius similar

to the cutter If the result is negative, the candidate direction can be considered as a TAD Otherwise, this candidate direction should be discarded For a hole feature, the candidate directions are the two directions of the hole axis For a plane feature, the candidate direction is the direction of the face normal

Tolerances, which represent the characteristics and relationships of features on a part, serve as functional description of the design requirements which should be satisfied during manufacturing processes Tolerances can usually be classified into self-tolerances and relative-tolerances Self-tolerance is the tolerance reflecting the size deviation of a feature It is related to the operations, but not directly related to other features The examples are the straightness for feature B and flatness for feature A in Figure 5.1 Both are typical casting features The form tolerances described in Chapter

4 are self-tolerances Relative-tolerance reflects the position tolerance in relation to

the other features, such as feature C, which is a machining feature having dimension tolerances with A and B respectively, which are shown in Figure 5.1 The dimensional tolerances described in Chapter 4 are relative-tolerances Relative-tolerance can be used to identify the locating datum of a feature For example, in Figure 5.1, to guarantee the dimensions of C, it is logical to use A and B as the locating datum Otherwise, tolerance stack-ups would arise and tolerance compression might happen

Figure 5.1 Self and relative tolerances

Tolerance compression means a feature has to be machined with higher tolerances compared with the blueprint values, and therefore a more accurate machining centre or operation may be needed Tolerance compression can happen between set-ups and within a set-up The tolerance compression between set-ups usually happens due to tolerance stack-up Figure 5.2 shows an example of how the compression of operational tolerances happens In Figure 5.2, dimension 10±0.10 is to be obtained from the previous operation To obtain dimension 5±0.05, if it is machined having B as the base, the tolerance for dimension 10 has to be compressed to less than 0.05 by

considering the tolerance stack-up For a process plan with multiple set-ups, this could happen quite frequently

For a CNC machine, no chain analysis is needed for the relative and positional relationships between the geometry surfaces in a set-up because they can be programmed accurately If the specified tolerances cannot be obtained, nothing can be done in the sequence unless a machining method or a machine tool with a higher process capacity is adopted or a higher rate of scrap can be accepted For example, assuming the two dimensions in Figure 5.2 have to be achieved in a set-up A more accurate machining method may be used for dimension 10±0.10 comparing with obtaining it in separate set-ups In a multi-axis machine tool environment where multiple operations can be carried out in a single set-up and the design datum cannot always coincide with the set-up datum, tolerance compression would occur quite frequently

Figure 5.2 Tolerance compression

The compression of operational tolerances will lead to an increase of the manufacturing lead time and production costs and should be taken into consideration during set-up planning

5.2.2 Consideration of Real-Time Machine Resources

For set-up planning, the application of set-up datum may vary according to different machining environment, e.g., 3-, 4- or 5-axis machining centre, whether vertical or horizontal The number of set-ups and the selection of the machining features in each set-up depend on the machine tool configuration, that is, the number of axes and the orientation of the axes

In set-up planning, features are grouped into set-ups according to the type of machining centres For a 3-axis machining centre, the machining features are grouped based on their TADs Features with the same TADs are assigned to the same group In this case, the number of set-ups is determined by the number of TADs of the machining features For a 4/5-axis machining centre, the machining features are grouped based on the Tool Orientation Space (TOS) of the machining centre Features with TADs within the machining centre’s TOS are assigned to the same group In this case, the number of set-ups is determined by both the TOSs of the machining centres and the TADs of machining features To determine the locating features for a set-up, the position tolerances for the machining features in a set-up are verified

In this research, it is assumed that a machining environment contains several machining centres which could be distributed and located in different places Each machine has different capabilities (rigidity, power, accuracy, etc.), schedule, tooling, operation cost, with unique machine type, configuration, table size, main axis direction, machine ID code and location For a particular distributed environment, the database also includes the information of the traveling distance between the places where the

machine resources are located Among them, the schedules of machining centres are very important during set-up planning From a technical viewpoint, a set-up plan may appear to be good, but by taking into account the schedules of candidate machining centres, it may not be the most economical

Machine resources are provided in a database in the managing module A user interface is developed to provide a way for the user to configure and update the

machining environment in real-time, i.e., the currently available machining resources

along with their capabilities, attributes and their operating schedules It is integrated with the database and therefore each time the user updates the manufacturing environment using this interface, the database will be updated accordingly The process planning module will read information from this database when performing set-up planning Therefore, the set-up planning is performed with the machining resources with real-time response, which takes into account the production schedule and some unexpected events, such as the machine tool breakdown and an urgent job which needs to be handled immediately

5.2.3 Tolerance Analysis

Depending on the accuracy of the machine tool, features machined in a single set-up can be maintained in accurate relationship with respect to the machine tool coordinate system This position will be lost if the part is dismounted from the machine tool and remounted again in a different fixture The errors in the alignment of the part and fixture on the machine tool can be equal to or even larger than the accuracy requirements of small-tolerance relations As a result, the position accuracy of a feature machined in a previous set-up can be insufficient to realize the required

accuracy in the relation to the features to be machined in the present set-up Even in a single set-up, when the set-up datum is different with the design datum, the required position tolerances of a feature may not be guaranteed It is necessary to check the blueprint tolerances during set-up planning to ensure the set-up to be used is a feasible one

Case 1: Dimension datum coincides with set-up datum

If the set-up datum coincides with a feature’s dimension datum, it is not necessary to check the tolerance for this feature It is based on the assumption that the selected machining process and fixturing method can guarantee the dimensions and tolerances

Case 2: Dimension datum does not coincide with set-up datum

In this case, it is necessary to take into consideration the stack-up error For example,

in the workpiece illustrated in Figure 5.3, the position dimensions clearly state that the centre of the hole (a machining feature) should be at the distance X from face A and Y from face C Consequently, face A and face C must be used as datum to locate the workpiece while drilling this hole This would ensure that the hole is at the specified distances from face A and face C If one uses face B as a stopper, the deviation in length X1 between faces A and B would cause inaccuracies in the position of the hole

If length X1 is oversized by 1mm, the centre of the hole will be at (X+1) mm away from face A If the length X1 is undersized, the hole would shift towards face A and would be nearer than distance X from face A However, if location is on face A, the hole would always be at the same distance from face A irrespective of the variation in length X1 Similarly, the same situation will occur when locating with face D instead

of face C for dimension Y

To satisfy the dimension requirements, sometimes a more accurate process or even a more accurate operation has to be chosen, and it would be more expensive To reflect the additional cost if a higher accuracy machining process/operation is required, a

tolerance cost factor (f) is introduced, which will be applied when calculating the

machining time Each operation can achieve a typical tolerance, and it is always within

a certain range (Figure 5.4) Machining processes operating under normal conditions would produce parts within the tolerances as indicated in Figure 5.4 (a) Figure 5.4 (b) indicates the ANSI B4.1 Standard Tolerances According to blueprint tolerances specified on a workpiece, suitable machining processes will be selected to generate the machining features

Figure 5.3 Tolerance chain

To calculate f, it is first assumed that the operation selection for a machining feature is

according to the lowest tolerance which this operation can achieve For example, if there is a hole with tolerance around 0.25mm, a drilling operation with the lowest tolerance 0.254 as shown in Figure 5.4 is chosen for this The tolerance range is defined as grades, and a grade represents a cell in Figure 5.4 (a) For example, for the drilling process, the tolerance range can be divided into four grades: grade 10 to grade

13 f is calculated based on the grade The initial value f is set to 1 If the tolerance jumps to a new grade, f is increased by the number of grades jumped If the jump is in

between the grades, half a jump is used If a selected machining operation cannot achieve this higher tolerance, a more accurate process will be selected

(a) Tolerance grades

(b) ANSI B4.1 standard tolerances

Figure 5.4 Dimensional tolerance capabilities of operations

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5.2.4 Cost Model

One of the ultimate goals of an enterprise is to be profitable Hence, every company has the mandate to reduce cost and increase profit margin, which can be achieved more effectively at the design planning stage rather than the manufacturing stage In this research, set-up planning is performed based on a cost model and an optimization methodology is formulated to minimize the overall cost of machining all the features

on a workpiece It justifies the machining overhead with machining time, and considers the tolerance requirements simultaneously

Depending on how the features are to be located on the faces of a workpiece, they can

be grouped into different TADs Usually, the smallest number of TAD groups would

be the best as the cost of set-ups will be lower However, that is not always true because the grouping is dependent on the type of tooling used The schedules and the locations of different machine resources will result in different machining times thus the manufacturing costs A planned machine tool with a schedule requiring additional waiting time to start work will cost more considering the wasted waiting time, and a machine tool located elsewhere may cost more than one which is nearby, considering the transport cost In addition, different machine tools have different fixturing methods, leading to different costs For example, for a 5-axis machining centre, fewer set-ups are needed, so the total machining time would be less However, there may be

a trade-off between reduced machining time and a higher overhead on a 5-axis machine In addition, since the fixturing method is likely to be more complex, it will cost more Conducting tolerance analysis of a set-up incurs additional steps and this will move up the manufacturing cost Therefore, the cost model considers all those factors and is a composite of: (1) machine tool overhead; (2) cutter cost due to wear and tear; (3) fixture cost; (4) schedule-based cost per unit time; (5) set-up time cost; (6)

tool change time cost; (7) transport cost Using this cost model as the optimization objective function for the algorithm, feasible set-up plans with the minimum cost can

be found The above seven cost factors are described in detail in the following

Machine tool cost per unit time (MCP) The machine tool cost per unit time is the

summation of the operating cost per unit time and the fixed investment cost amortized over time It is a constant for a machine

Cutter cost per unit time (CCP) Similar to MCP, the cost of a cutter per unit time is the

summation of the operating cost per unit time and fixed investment cost amortized over time It can be considered a constant for a cutter when machining the features on a specific part with a particular material

Fixture cost per unit time (FCP) It is the summation of the operating cost per unit time

and fixed investment cost amortized over time Modular fixtures are considered and used in this research for all types of machine tools

Schedule-based cost per unit time (SCP) Schedule-based cost occurs when a machine

is needed to wait for some time to perform the operations planned It is based on the schedule of a machine tool

Set-up change cost per unit time (SCCP) Set-up change is required when a machine

tool change is needed which will also require a new fixture

Cutter change cost per time (CCCP) Cutter change is required when two adjacent

operations performed on the same machine tool use different cutters In addition,

machine tool change may also result in cutter change The cutter change cost incurred between any two operations

Transport Cost per unit distance (TCP) Transport is required when operations on the

same workpiece have to be performed on different machine tools which are located in different places

For different manufacturing environments, the values of MCP, CCP, FCP, SCP, SCCP, CCCP and TCP may be different Therefore, when performing set-up planning, different database of the machine resources would need to be used

A set-up plan usually contains several set-ups Each set-up contains resources which include a machine tool, a fixture, and different cutters to complete the machining processes in this set-up The cost for the tooling is calculated using the machining times of machining the features in this set-up The set-up change cost and the cutter change cost is based on the change times The transport cost depends on the distance of the current machine to the next machine Therefore, the cost model can be formulated

as in the following:

(5.1)

Where,

k Machining ij IJ

ij

ij Machining

TCP D

N CCCP N

SCCP T

SCP

T CCP T

FCP

MCP

I

i mn c

s I

i

Waiting

i

Machining k K

k k I

i

Machining i i i

i

T machining time for th

i machine tool, a summation of operations’ machining time having on this machine

Machining

k

T machining time for th

k cutter, a summation of operations’ machining time having on this cutter

T waiting time for th

i machine tool, determined by the schedule

of this machine tool

Machining

kj

T machining time for th

j operation performed on k thcutter

f tolerance cost factor for th

j operation performed on k thcutter

I number of machines selected in the current set-up plan

J number of operations selected in the current set-up plan

K number of cutters selected in the current set-up plan

For a specific machine, the machining time, which considers the tolerance cost factor f,

is computed by calculating and summing the individual operation times for all the operations performed on a particular machine in a set-up plan The individual

operation time is estimated by computing the volume of material removed in that operation divided by the material removal rate The tool approach time and other travelling time from feature to feature where no materials are removed are not considered The volume of material removed in an operation can be obtained from the machining feature geometry, while the material removal rate can be computed from tool geometry and processing parameters

In this study, it is assumed that a machining feature can be generated in an operation with a specific cutter In this way, the number of set-ups can be obtained after the completion of the set-up plan, and the total number of cutter changes is the summation

of the cutter changes in each set-up

The distances between the locations of the machine tools and their schedules can be obtained from the machine resource database Cost factors can be used either individually or collectively as a compound cost factor based on the actual requirement and the data availability of the machine resources in a machining environment

5.3 System Implementation

As set-up planning is a NP-complete problem, optimization techniques are commonly employed to achieve an optimal or near-optimal set-up plan ACO is an efficient approach and it has been applied successfully to solve NP-complete problems The ACO meta-heuristic frame work describes the scheduling of several processes and is presented in Figure 5.5

Figure 5.5 The ACO meta-heuristic framework

Construct solution: This process is responsible for the construction of new solutions

This is achieved using probabilistic stepwise solution construction The probability of

a particular solution component being added to a growing solution is based on a combination of problem specific (heuristic) information and learned (pheromone) information of how well this component is used in the past solutions The exact combination of this information and the greediness of the selection mechanism are important implementation specific details

Pheromone trail update and decay: Once solutions have been evaluated, they can

influence the pheromone matrix through a pheromone update process To allow the replacement of old information with new information, a pheromone decay process is also employed that removes the influence of past solutions over multiple successive algorithm cycles

In this study, the feasibility of using the ACO algorithm is studied to address the set-up planning problem

Update Pheromone Pheromone

Matrix

Decay Pheromone

5.3.1 System Structure

Set-up planning starts with extracting workpiece information from the raw and final CAD parts A file, recording the extracted information inclusive of machining features, tolerances, datum, etc., is generated for subsequent searching use The extracted information can be displayed in an interface through which users can check and modify, and can also add other necessary information, such as form tolerances, to certain features Figure 5.6 shows the overall flowchart of this developed system

Figure 5.6 Overall system flowchart

An interface which links with the machine resource database is provided Through this, users can check, modify and update the machining environment In this way, the machine resources used in the search is able to reflect the current resource status and make the set-up planning more reliable During the ACO optimizing process, tolerance analysis is conducted, and the cost is evaluated for each solution based on the cost model Finally, an optimal or near-optimal result can be obtained

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