SME Technical Paper: Implementing Design for Manufacture Rules:
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nd W n < n a+ 8 Society of Manufacturing Engineers 2002 MSO2-124 Implementing Design for Manufacture Rules author(s) R. DUMITRESCU T. SZECSI Dublin City University Dublin, Ireland abstract This paper shows a new approach in incorporating manufacturing constraints at the design stage into an intelligent system, which analyses the design features from a CAD drawing, relates them to machining feature, and then suggests the available manufacturing processes capable of producing these features. The system also examines whether design for manufacture rules are violated by the features’ characteristics and conclude on their manufacturing possibility. Production type, materials, tolerances, surface finish, feature’s characteristics, and accessibility are taken into consideration. This paper originally appeared in the Proceedings of the 1 lti International Conference on Flexible Automation and Intelligent Manufacturing (FAIM ‘Ol), July 16-I 8, 2001, Dublin, Ireland, and has been republished with permission of the authors and the Dublin City University. terms Design for Manufacture Design & Machining Features Computer Aided Design Society of Manufacturing Engineers One SME Drive l P.O. Box 930 l Dearborn, MI 48121 Phone (313) 271-1500 www.sme.org
Transcript
nd W n < n
a+ 8 Society of
Manufacturing Engineers
2002
MSO2-124
Implementing Design for Manufacture Rules
author(s)
R. DUMITRESCU T. SZECSI Dublin City University Dublin, Ireland
abstract This paper shows a new approach in incorporating manufacturing constraints at the design stage into an intelligent system, which analyses the design features from a CAD drawing, relates them to machining feature, and then suggests the available manufacturing processes capable of producing these features. The system also examines whether design for manufacture rules are violated by the features’ characteristics and conclude on their manufacturing possibility. Production type, materials, tolerances, surface finish, feature’s characteristics, and accessibility are taken into consideration.
This paper originally appeared in the Proceedings of the 1 lti International Conference on Flexible Automation and Intelligent Manufacturing (FAIM ‘Ol), July 16-I 8, 2001, Dublin, Ireland, and has been republished with permission of the authors and the Dublin City University.
terms
Design for Manufacture Design & Machining Features Computer Aided Design
Society of Manufacturing Engineers One SME Drive l P.O. Box 930 l Dearborn, MI 48121
Phone (313) 271-1500 www.sme.org
SME TECHNICAL PAPERS
This Technical Paper may not be reproduced in whole or in partin any form without the express written permission of the Societyof Manufacturing Engineers. By publishing this paper, SMEneither endorses any product, service or information discussedherein, nor offers any technical advice. SME specificallydisclaims any warranty of reliability or safety of any of theinformation contained herein.
IMPLEMENTING DESIGN FOR MANUFACTURE RULES
R. Dumitrescu and T. Szecsi
School of Mechanical and Manufacturing Engineering, Dublin City University, Ireland.
ABSTRACT
This paper shows a new approach in incorporating manufacturing constraints at the design stage into an intelligent system, which analyses the design features from a CAD drawing, relates them to machining features, and then suggests the available manufacturing processes capable of producing these features. The system also examines whether design for manufacture rules are violated by the features’ characteristics and conclude on their manufacturing possibility. Production type? materials, tolerances, surface finish, and feature’s characteristics and accessibility are taken into consideration.
KEYWORDS: Design for Manufacture, Computer Aided Design (CAD), Design & Machining Features
1. INTRODUCTION ,
Classically, the designing environment was based on what we call today the “over-the- wall” system [2], where the interaction between designers and manufacturing engineers was minimal and manufacturing issues were only superficially considered from the beginning of a design. One way of overcoming this problem is to have a team of designers and manufacturing engineers working together at the design stage. These teams use analysing tools, which help them evaluate the design. Design For Manufacture is one of these tools that enhance a number of general rules about the manufacturability of a part (i.e. the relative ease of manufacturing a Par-Q
In the last years, Design For Manufacture (DFM) approach has become a real interest as it was found that the design stage determines most of the cost of the development of a product. As market needs have increased and the competition to remain on the market has become very tight, it is a crucial issue to reduce the time of the product development [ 11. Customers are demanding high quality products at competitive prices and the design is the first step in satisfying their requirements [7]. Thus, it is necessary to apply manufacturing constrains from the very beginning of the design stage, in order to avoid costly changes that may occur later because of the difficulty or impossibility to implement some manufacturing processes. This practice is essential in reducing the cost of a product considerably.
The history of DFM is described in [l] from the early beginning. Eli Whitney is known as the person who introduced the interchangeable parts concept. Still in a growing phase, DFM is believed to become even more important in the next years. Nowadays, the increase of the importance of the DFM concept is closely related to the drastic development and huge progresses
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achieved in computation resources during the last decade, making available high performance hardware and software at affordable prices. Although CAD systems have been available since almost 30 years and they are progressively spreading in almost all fields of today’s engineering, there is still a lot to do in the field of computerisation of DFM [ 13.
The aim of this research is the development of an intelligent system for implementing DFM rules, which has its origins in the idea of helping designers by offering them support with manufacturing constraints information. The purpose is to improve the design quality and to decrease the time-to-market, as rapid product development is becoming a critical factor to a company’s position [2]. It is well known that improperly designed parts can still be produced but with an unjustifiable increase in manufacturing costs and time, and it is the aim of the DFM tools to help users to optimise their designs.
In the following section of this paper, feature concept terminology and usage are briefly reviewed. The third and fourth sections explain our approach and intentions in developing the system. Discussions and examples of hole features are also included in the fourth section. The capabilities and limitations of the approach are concluded in the fifth section, and the further work planned to continue our research and its perspectives are explained. Some examples of machining features are given in the annex at the end of the paper.
2. FEATURE CONCEPT
The integration of CAD systems to CAM and CAE systems could not be achieved without the help of feature concept. Indeed, CAPP has to interpret the part from a CAD data in terms of features [I 11, which most often are manufacturing features. Based on the viewpoint, different types of features can be defined. During the design process, a part is created using design features, which later have to be interpreted into manufacturing, assembly or inspection features via dedicated recognition tools.
2.1 Feature Definition
A feature is a set of faces or regions of one part with distinct topological, geometrical and/or manufacturing information. Features characterise the product and help in analysing the design concurrently using numerical or knowledge-based systems [ 121. If the product is viewed from the designing stand point. then features are called design features and they present only topological and geometrical information; if the product is viewed from the manufacturing stand point, then features are called manufacturing features and they present topological, geometrical and manufacturing Xorrnation.
In [S], a feature is defined as a stereotypical geometric shape associated with some engineering significance. The authors also mention about the predefined feature, which has a fixed topology and has been defined in a library. Another way of defining a feature is given in [12], where a feature represents the geometry of a part or assembly and building blocks for product definition or for geometry reasoning. It has to be mentioned that as the research goes deeper into this area, different types of features are developed systematically.
2.2 Design Features
Design Features are those features used at the design stage defined by the user or from the CAD modeller library and which do not take into consideration any manufacturing, assembly or
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inspection constrains. They might have shapes and/or locations impossible to produce and/or reach with the available technology at a given moment and in a given company. [7] deals with three types of design features: depression, protrusion, and transition features. The authors define the depression feature as an increment to the volume of a shape such as boss, and the protrusion feature as a decrement such as a hole. The transition feature could be either a decrement or an increment, depending on whether its profile is convex or concave.
Examples of design features can be found in [4,6,7] as slot, hole, pocket, rounding, cylinder, block, protrusion, cut, chamfer, user-defined features, etc. The user-defined design features are based on planar profiles swept into three-dimensional shapes by extrusions and revolutions.
2.3 Feature-based Desiw
A system where the designer creates the part by picking the entities from a feature library is called a feature-based design system [4]. Today’s CAD systems have their own pre-defmed feature library. [4,12] show that the main advantages of designing with features are the less-time consuming aspect when re-design of a part is needed. In this case, once a parent-feature is re- positioned it will automatically re-position all its child-features, as features are relatively positioned one against another. Reuse of design is not usually the case in classical CAD systems, where everything had to be reconsidered almost from the beginning.
When the feature library does not satisfy a company, a user-defin.ed one adaptable to different types of products can be created where there is a real need for this, as it is time and money consuming [9]. Still the creation of an adequate feature library is not trivial.
2.4 Manufacturing Features
A manufacturing feature is interpreted in [8] as a continuous volume that can be removed by a single machining operation in a single set-up. It depends not only on the shape and size of the geometric feature, but also on the manufacturing process to be used to produce this feature [9]. The definition of the manufacturing features is general&d in [lo] to the whole engineering approach, as being a function of the part (or some portion of the part) and specific factory resources to be used to produce that (portion of the) part; from here the authors conclude that a manufacturing feature is the function of machine tools, setup, tools and parts.
It can be found in [5,6,1 l] that there are different manufacturing features, namely: hole, pocket, open pocket, face, boss, step, open step, slot, notch, grove, knurl, thread, fillet, chamfer, etc.
2.5 Design by Manufacturing Features
Design by Manufacturing Features would be the most promising in the evolution of CAD systems, where the designer would have to use manufacturing features and to think in process planning terms, i.e. manufacturing techniques and cost issues [9]. This approach assumes that the designer creates the part in terms of manufacturing operations. It would be the best to use in practice, but it is still unnatural for the designers.
2.6 Manufacturability Analysis
Unfortunately, it is still impossible to completely replace the human decision factor with an automatic manufacturing analysis system because the relation between design details and
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processing is often very complex and not easily reduced to formulas or simple relationships. Specifying a manufacturing process for a feature is a difficult decision since for some particular situations one well-known expensive operation can be cheaper than two cheaper operations and an automatic selection of the manufacturing sequence may ignore the expensive manufacturing processes for the two cheaper ones. The main aim of this research is to develop a system which helps designers in making better designs in less time.
3. OVERVIEW OF THE APPROACH
As it was mentioned in the first section, consideration of the manufacturing aspects at the design stage is very important as experience has shown that up to 70 per cent of the product’s cost [2,7] is directly generated at the design stage. The final cost of the product may be significantly increased because of an inefficient design of the product, an improper selection of the material, production type or surface finish. Therefore, the design of one product is the most suitable stage where changes and interventions with regard to the manufacturing aspects should take place for an optimal manufacture. We present here an approach of an intelligent system, which interactively assists the designer during his or her work with manufacturing issues.
Nowadays CAD systems, when using, ask for precise geometric data, which is more than the design aspect of a part and the designer should care only about the functional requirements of the part. This fact could push the designer to think about all the product’s specification. Yet, the designer has no clear idea about the manufacturing processes’ capability of designed part.
producing the
Fig. 1. Information Transfer
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Our intention is to implement a DFM system that will be localised in the cycle of product development as shown in Fig. 1. The DFM system has two modules: firstly, the feature recogniser module transforms the CAD data (design features) into manufacturing features and secondly, the manufacturability analysis module uses a series of design for manufacture rules to evaluate the ease of manufacture of the product. The later, based on the information provided by the feature recogniser module, evaluates which manufacturing processes would be capable of producing one feature or another, and, in the same time, whether there are any manufacturing constraints violated by the design concept or not. If there is any manufacturing constraint violated, the system will be capable to provide the designer (user) with information about the geometrical parameters that are out of the manufacturing capabilities.
It has to be mentioned that the DFM system will not restrict the design process, but will give practical information about the manufacturing constraints which may occur during the product manufacture. The designer can also chose whatever materials he or she wants for manufacturing the parts. At the end, the user would be aware of the producibility of the product with regard to the choice of material, production type, and feature’s characteristics.
4. CURRENT STATUS
In this paper, we deal only with some isolated machining features. An isolated machining feature represents a feature which does not interact with any other feature and which is produced in one manufacturing sequence with a single set-up. We found that machining features, basically, group together into six categories: HOLE, POCKET, FACE, STEP, SIUI and BOSS. The particular features are defined as follows [ 111:
l Hole = an arbitrary contour area machined into a work piece. l Pocket = a closed removal area (depression) on a surface. l Slot = a closed pocket with a constant width. l Step = a 2 or 3 side open pocket. l Face = an all-side open surface with an arbitrary contour. l Boss = a closed remainder area (protrusion) on a surface.
Each of these classes has its own group of subclasses. We found that slots, steps and pockets subclassify into open and blind, all of them with different shapes. Bosses can enclose cylindrical, rectangular or freeform contours. The representations of some of these machining features can be found in Annex A.
The example given in this paper refers to machining hole features, which are briefly ‘represented in Fig. 2.
For-example, we classify holes after the existence or non-existence of the rotational axis or based on their contour. On these bases, some types of the machining hole features are presented in the table below:
Table 1: Feature Classification
Feature
Hole Rotational Axis
Yes
No Through & Blind
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If we refer only to the rotational and non-rotational types of holes, the manufacturing processes for producing them may significantly differ and therefore, a manufacturing analysis has to be done to decide upon the appropriate process to be used.
After the feature mapping process (i.e. mapping of the design features into manufacturing features), the manufacturability analysis module identifies, if any, the machining processes capable of producing the required feature of the part, based on the:
l production type, l material type, 0 feature’s characteristics, l tolerances and surface finish, o feature’s accessibility and position,
virhich the designer has to select and provide to the DFM system.
At the moment we decided that the manufacturing analysis process will take into consideration only the most common materials, the normal values for tolerances, surface finish and some specific feature’s characteristics. In order to keep the product’s cost as low as possible, the system will not take into consideration the closest values for tolerances and surface finish. The same rule applies for production type and particular features characteristics.
In the example below (Table 2), the feature considered is a cylindrical through hole. Its characteristics are the hole diameter, the hole depth and the depth-to-diameter ratio of the hole. From the design stage, the geometrical and topological characteristics are known. Further, the designer has to provide the DFM system with the information concerning the production type he or she wants to be used when manufacturing the part, the material of the part, and the values of tolerances and surface finish to be achieved. After processing all this information, the most suitable manufacturing process will be selected for the specific feature. In our example, the drilling process was selected from the library. Further, the constraints of the drilling process are applied to the hole and warn the designer about the limitations of the this process. One of these refers to the diameter size (concerning the standard till sizes), which cannot be less than 1.5 mm orgreaterthan38mm [3].
Table 2: Manufacturability Analysis
Feature Production Material Surface Depth-to- Tolerances Manufacturing Type Finish diameter [=a Processes
[run1 ratio Cylindrical Mass Carbon 1.6-3.2 3:l *(0.05-0.25) Drilling Through Production Steel
Hole
Another limitation of the drilling process from the economical point of view is the maximum value of the depth-to-diameter ratio, which should not exceed 3: 1. Although the applicability of
S
this machining process may exceed these limits, this will significantly increase the product’s cost and it is contrary to the aim of our Design For Manufacturing approach.
The system will also analyse the accessibility of the feature and its relative position on the part. For example, if the hole is unreachable for the drill or bushing, or its position is not perpendicular to the entry face, the designer will also be warned to ensure that the hole is in a proper location and position on the part to avoid manufacturing difficulties.
In this work, we will use volume decomposition methods when recognising the manufacturing features from the design features. In the example below (Fig. 3), the work piece from which the part is to be machined is considered to be a cylinder. From the CAD data, we know that this part was designed using cylinder, cone and box design features, but we have to interpret and map them into machining features.
Drilled Hole I, , , I
Counterbored Through Hole
-.
aI Drilled Hole
Part designed with design features ) Machining features to be removed
Fig. 3. Volume decomposition example
The interpretation and mapping of the design features into machining features will be done using volume decomposition methods by identifying the removal volumes from the initial work piece and ascribe them to manufacturing features.
5. CONCLUSION AND FURTHER WORK
The DFM system will be embedded in AutoCAD and be implemented on a WindowsNT platform with AutoLISP as programming language. AutoCAD has been chosen as one of the
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most widely used CAD systems. It is intended especially for product designers but other users may find it useful, too.
The system we wish to implement will manage DFM rules for different types of features and different manufacturing processes. At the moment, the main aspects of the approach have been decided and part of them were investigated and developed. Still at the beginning of our research, further work has to be done in the area of identifying the manuhacturing constrains for other groups of machining features. The second part of the research will be focused on developing the algorithm for the feature recogniser module. The research carried out in implementation of the Design For Manufacturing rules will effectively conclude after implementing the system.
One disadvantage of this approach is that it does not take into consideration the interacting features. Further work will be carried out to deal with interacting features.
Annex A - Some machining features representation
Cylindrical Rectangular Boss Boss
Open Step Wedge Blind Step. Sector
Boss Features Step Features
Through Slot Features
Round V-shaped Slot Slot
Rectangular Slot
Dovetail Slot
FreeForm Slot
T-shaped Slot
Slot Features
Blind Pocket Open Pocket
Blind Freeform Pocket
Open Freefarm Pocket
Pocket Features
References
Blind Slot Features
Rectangular Slot
FreeForm Slot
[l] James G. Bralla, Handbook of Product Design for Manufacturing - A Practical Guide to Low-Cost Production, McGraw-Hill Book Company, 1999.
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