Formalized procedure for the selection of optimal setup plan for machining the features of prismatic parts.

Hebbal, S.S. ; Mehta, N.K.

Abstract: The present work focuses on the development of a formalized procedure to select an optimal setup plan for machining the features of a given prismatic part. It is based on the concept of grouping of features of prismatic parts into different groups for which the following new concepts have been introduced: primary groups, machine tool based secondary groups and eligible groups.

Key Words: CAPP, Features, Setup Planning.

1. INTRODUCTION

The present paper is part of work on development of a Computer Aided Process Planning System for prismatic parts and the proposed system consists of three subsystems, namely; Subsystem for Feature Recognition (SFR), Subsystem for Machining Planning (SMP), and Subsystem for Setup Planning (SSP).

SFR recognizes the manufacturing features for the given part employing the feature recognition methodology proposed by the authors in an earlier work [Hebbal and Mehta, 2001], whereas SMP identifies feasible operations for each feature and selects the machine and cutting tool and machining parameters with the help of machinability database. The subsystem for set-up planning takes the output of the machining planning subsystem as its major input and automatically generates the set-up plans as well as their order of execution. For this it first identifies the groups of features that can be machined in a single setup and determines a suitable workpiece orientation for each set-up. The present work focuses on this function i.e. issues related to grouping of features of prismatic parts.

[FIGURE 1 OMITTED]

2. GROUPING OF FEATURES

For grouping of the features the following concepts have been introduced by the authors in an earlier work {Hebbal & Mehta,2003]: i)primary groups, ii) machine tool based primary groups, iii)secondary groups and iv)eligible groups. A description of these groups is given in the following sections with the help of an example part (Fig. 1) on which all the three feature F1, F2 and F3 can be machined by the following two operations: End Milling (EM) on vertical milling machine (VM) and Side Milling(SM) on horizontal milling machine (HM).

2.1 Primary Group

In a rectangular block, each rectangular face represents a primary datum. For each primary datum used as a locating base, the features that can be accessed for atleast one of the selected operation are grouped together as a Primary Group(PG). Altogether a maximum of 6 primary groups can be obtained to include all features of a given part. Some features may be included in more than one primary group.

2.2 Machine Tool Based Primary Group

The Machine tool Based Primary Group (MPG) is defined as a group of features which satisfy the following two conditions; i) all the features are accessible with the same primary datum as the locating base and ii) the same machine tool is required for all the machining operations for obtaining different features in the given arrangement. The MPG's are derived from the primary groups by considering each primary group one at a time and identifying the machine tools that can carry out the operations to obtain the features included in the primary group under consideration.

2.3 Secondary Group

For a particular workpiece orientation using a given primary datum as the locating base, the left and rear rectangular faces are used for clamping of the part and are defined as the secondary and tertiary datum respectively. In principle, four secondary groups are possible for each MPG by successively rotating the vertical faces through 90 degrees. However, feasible secondary groups are obtained for each Machine tool Based Primary Group (MPG) by selecting secondary and tertiary datum planes for a MPG such that atleast one feature can be machined after clamping the workpiece in the orientation under consideration.

2.4 Eligible Group

An Eligible Group (EG) represents a group of features that satisfy the following three conditions i) all the features are accessible from a common arrangement of primary, secondary and tertiary datums, ii) the machine tool required is the same for all operations considered for machining these features in the given arrangement, and iii)for each feature the available tolerance for all dimensions is more than the resultant error. Since the secondary groups satisfy the first two conditions automatically, they are considered one after the other in order to check the possibility of converting each of them into an eligible group. For his conversion, every secondary group is evaluated through a tolerance analysis in order to verify for each dimension whether the resultant error is less than the available tolerance or not. The resultant error depends on the design and process tolerance associated with the locating surface when design references are selected for location. However, it becomes necessary to recalculate some dimensions and the available tolerances when the design and locating surfaces do not coincide. On the basis of tolerance analysis, all those features of a secondary group under consideration that satisfy the third condition are retained and others are deleted from the group. The resultant group formed by this process represents an eligible group

3. PROCEDURE FOR FORMULATING SET-UP PLANS

A setup refers to a group of features that can be machined in a certain fixturing configuration. Setup planning is the process of grouping of features into setups, ensuring that all the features can be machined with specified accuracy[Ong & Nee,2001]. A setup plan consists of a combination of setups, which includes all the features of the part. Since each eligible group represents a group of features that can be machined with specified accuracy in a certain orientation, it can be used as a candidate setup in the development of a set-up plan. Therefore, a feasible setup plan can be formed as a combination of eligible groups. However it is possible that a particular feature finds place in more than one eligible group. While forming the set-up plan in this case, the feature under consideration is retained in one eligible group and neglected in the rest.

From the above discussions it is clear that while developing a set plan some set-ups are created from certain complete eligible groups and others from part of the remaining eligible groups. Therefore, with the objective of involving the effect of every complete eligible group in the analysis, a heuristic procedure for obtaining feasible setup plans has been developed which first starts with any complete eligible group and converts it into a setup. It then selects other possible complete eligible groups with maximum features and finally part of the other eligible groups. In this way with each eligible group one feasible plan is possible.

The procedure for the formation of feasible setup plans which start from one complete eligible group is explained below:

A) the eligible group under consideration is selected as one set-up to machine the features associated with it. For example consider the eligible group EG1[(HM),(F1,F2)].

B) the features which are included in the previous setup are deleted from all other eligible groups. This step eliminates eligible groups EG3.

C) from the modified/remaining eligible groups, the setups for other feature(s) are formed by searching for those groups which have maximum number of features, preferably using the same machine. For machining feature F3, EG2 is selected because it requires a horizontal milling machine which is the same as that in EG1 that has already been finalized.

D) if all the features are included in the set-up plan i.e. set-ups have been identified for all the features then the procedure is terminated, otherwise the procedure is repeated from step 'B' onwards till the set-up plan includes all the features of the given part.

In a similar way, beginning from eligible group EG3 a set-up is formed to machine features F1 and F2. Then eligible group EG4 provides a set-up to machine F3.

Repeating the procedure by selecting each eligible group as the initial set-up one after the other, it is possible to obtain as many set-up plans as the number of eligible groups.

4. SELECTION OF OPTIMAL SETUP PLAN

All the setup plans are evaluated on the basis of total set-up time which includes time for fixing the workpiece in the specified orientation for each set up and machining and tool changing time for each feature associated with the setup plan. If two or more set-up plans are favoured by total setup time then the setup which involves minimum recalculation of dimensions is selected.

The total setup time for a plan is given by,

Setup time = [summation] [N.sub.s] * St +

[summation]Tpr * Nf

Ns = Number of setups in the plan

[S.sub.t] = Average setup time

Nf = number of features

Tpr = unit production time for a feature which includes machining and tool changing cost.

For the example part the optimal setup plan is presented below in Table 2..

5. CONCLUSIONS

In the proposed work a formalized procedure for selecting an optimal setup plan for machining the features of a prismatic part has been presented. It performs the functions of grouping of the features of a given prismatic part into candidate setups, determination of all feasible setup plans, evaluation of setup plans and selection of an optimal setup plan. Further, for the optimal plan it is necessary to determine the order of execution of setups and a sequence of operations within each set up. The work related the sequencing aspect will reported later

6. REFERENCES

Hebbal. S.S & Mehta. N.K (2001) Extraction of depression typefeatures of prismatic parts from their CAD models. Proceedings of 12th DAAAM International symposium on 'Intelligent Manufacturing and Automation' Jena Germany, pp 179-180.

Hebbal. S. S & Mehta.N.K (2003) Grouping of features for the development of setup plans for prismatic parts , Proceedings of 14th DAAAM International symposium on 'Intelligent Manufacturing and Automation' Sarajevo, Bosnia and Herzegovina.

Ong S K & Nee A Y C (2001) Intelligent setup planning systems for parts production in manufacturing systems Computer Integrated manufacturing edited by Courneliu Leondes CRC press, .5-1 to 5-26

Table 1. Eligible groups.

EG M T Datum Features

EG1 HM S1-S3-S6 F1,F2
EG2 HM S1-S4-S5 F3
EG3 VM S1-S3-S6 F1,F2,
EG4 VM S1-S4-S5 F3
EG6 HM S3-S1-S5 F3
EG8 VM S3-S1-S5 F3

Note : MT = Machine Tool

Table.2 Optimal setup plan

 Primary Secondary Features
Setup MT datum datum

Setup1 VM S1 S3-S6 F1, F2
Setup2 VM S! S4-S5 F3