Childs, PRN

Simons, RW

English

Spiral - Imperial College Digital Repository

Small Priced Tools and Equipment Case Study  Childs, P.R.N., Simons, R.W.* School of Engineering, University of Sussex. *Visiting Professor in Principles of Engineering Design.       Abstract A new half module has been introduced into the first year of engineering at the University of Sussex with the aim of exposing undergraduates to the holistic design process at an early stage. The syllabus includes consideration of the interrelated phases of design from specification, concept generation and embodiment to manufacture and the effect on a business as well as an introduction to the technology base. To provide a framework for the module and to illustrate the interrelationships and iterative nature of design the module content is based around one of a number of tailored case studies. One of these is focussed on the design of a cordless screwdriver that conveniently requires the interdisciplinary skills of mechanical, electrical, electronic, materials and product engineering combined with business acumen. This paper describes the case study, identifying the educational benefits and student experience.   1 INTRODUCTION Design is a holistic activity and its practice a rewarding but demanding activity. In order to expose students to the holistic design process at an early stage in their education a new half module has been introduced into the first year of engineering at the University of Sussex. The syllabus content includes consideration of the interrelated phases of design from specification, concept generation and embodiment to manufacture and marketing as well as an introduction to the extensive technology base available.  In order to provide a framework for the module and to illustrate the interrelationships and iterative nature of design the module content is based around one of a number of tailored case studies. One of these considers the design of a cordless screwdriver. This conveniently requires the interdisciplinary skills of mechanical, electrical, electronic, materials and product engineering combined with business acumen. The module begins with an introduction to design and the technology bank available to us. This is accompanied during the first week by a contract tendering session, where the director of a business called SPRITE (Small PRIced Tools and Equipment Ltd), a visiting professor in our case, discusses the requirement for a new item from a commercial viewpoint. Topics covered include: the perceived need for the new product, how this need was identified, evidence of existing competitive similar products, the commercial constraints on the design, development, manufacture, costs of all activities and profitability, selection of contractors, assembly times and the selling of the product.  The half module and associated project are timed over a five week period with 20 hours of contact time split between lectures, seminars and problem workshops. The requirement of the design project includes submission of a specification and conceptual solutions after two weeks. A further submission is required after the fifth week including a general arrangement, transmission design and assessment of the viability of the product from a business perspective to show that commercial aspects have been considered and understood. The configuration of the design project requires some tailoring according to resources and time available. For this short course, in order to minimise the task, aspects such as the costs of tooling have been eliminated and details of an electric motor, battery and switchgear are specified.  2 DESIGN EXERCISE Recognising that this is an educational learning experience the process of learning has itself been designed to maximise the learning experience within the practical constraints of resource and time. To these ends the case study kicks off with the presentation of the design need or market brief as outlined by a stand-in managing director of the firm SPRITE Ltd. The tasks undertaken by the students are then performed concurrently with a short series of 13 lectures and five problem solving workshops covering the aspects of the total design process and introducing a number of machine elements. The lectures and exercises serve the purpose of providing the background information base and skills to undertake practical problems.  2.1 Information given In order to meet the constraints of time and resource mentioned, a body of information is presented to the students to enhance the skills-base of the students and to make a practical case study exercise that retains interest throughout the time available. This includes company background for SPRITE Ltd defining its mission and history, a market brief and some specifications on costs, torque capacity and features as listed below. • Results of the marketing case have been found to be acceptable and design will proceed. • The selling price is required to be set at £8.00, excluding VAT, the charger and tools in order to satisfy the business case. • It will be single speed, fixed torque, but with a reversing capability. • It will be sold by mail order, reducing selling costs, and ensuring that the purchaser pays the postage. • Overall it must be more attractive than the competition.  General course notes are made available as outlined below. • The case for a new product in terms of its business viability. • The identified need or market niche, resulting from a market survey. • Costs: Estimation of direct costs (time and materials), indirect costs or overheads (the costs of running the business) and general advice on cost estimation. The calculation of a typical works cost price. A spreadsheet, with a built in macro, is presented to the students allowing them to trial sales numbers and identify the level of profits. • The processes of design management, the need for design records, and the difficulties of project management.  2.2 Student tasks The product chosen, a cordless screwdriver, is an electromechanical device and its design requires interdisciplinary skills. The ability of a body of students to tackle such a problem depends on their background and attitude. The curriculum at the University of Sussex was designed to place this half module at the end of the first year so that students already have some engineering science skills available to them, namely: materials, mechanics, circuits and electronics. This provides them with some building blocks of knowledge with which to work. The specific tasks demanded of the students could be tailored to match the interests and skills to be cultivated. In this case the cohort of students concerned were mechanical major based and the detailed design aspects of the case study were focussed on the enclosure, transmission and bit holder. Details of a possible electric motor drive and switchgear were identified for use. For other majors the emphasis could be placed on the motor, switchgear and charger according to interest and time available.  The aim of the study is to develop appreciation of the total design process and the tasks set to the students reflect this. Following the initial presentation of the design need and background educational information given to the students, they are asked to develop the product design specification, develop conceptual solutions and produce a general arrangement sketch for their preferred solution. This is submitted and assessed prior to a second stage submission including detailed design work on the enclosure, transmission system and bit holder, cost estimates and an assessment of the overall solution. This last aspect is critical in the development of an engineer. Is what you are proposing any good? Can you make a value judgement? As an introduction to this subject and to assist with this process evaluation matrices are presented. Specific instructions as given to the students are listed below. i. From the market brief, develop a product design specification. ii. Develop conceptual solutions. iii. Submission 1. After week two, hand in your specification and concept designs and a preliminary general arrangement sketch.  iv. Submission 2. After week five hand in your Design Report (including specification and general arrangement sketches, tool holder design, transmission and gearbox design, cost estimate for the product assuming that tooling and design costs are amortised elsewhere, decide if the product as you have designed it is likely to be profitable, when will break even occur and an assessment of your final solution). v. Present any calculations and justify your choices.  2.3 Solutions There are a number of conceptually similar devices on the market serving the cordless screwdriver or electric assist screwdriver market as illustrated in Figure 1.    Figure 1: Commercial cordless screwdriver examples.      Figure 2: Gear train details for the Bosch PSR 2.4V cordless screwdriver.  The constraints listed in the specification if unchallenged could result in students producing similar solutions. Typical inexpensive small dc electric motors run at relatively high speeds of the order of 5000rpm. Useable torques for manual use are of the order of 1Nm resulting in a maximum running speed of about 80rpm. This reduction ratio between a motor and tool bit requires some form of rotary transmission. If the designer chooses to constrain diameter then a two stage epicyclic gear train as illustrated in Figure 2 might be suitable. Calculations for this can be undertaken based on the simple Lewis equation bending stress formula [1], [2]. A solution based on standard gears available from HPC Gears Ltd is outlined below and is not substantially different to that used in the Bosch PSR 2.4V product.  Stage 1: module 0.5, Nsun=9 (Tufnol), Nplanets=27 (Tufnol, three planets), Nring=63 (Tufnol). Stage 2: module 0.5, Nsun=9 (817M40), Nplanets=27 (Tufnol, three planets), Nring=63 (Tufnol).  Other solutions are possible. Some students may chose to challenge the specification and suggest a multi-function device, as illustrated by the use of variable speed drills to undertake screwdriver functions. Alternatively the diameter constraint may be challenged to enable consideration of a wide palm gripped device.  3 ASSESSMENT The assessment of the case studies is undertaken in two stages. This is compatible with a design review and steering the design activity before it takes a company in a direction incompatible with management plans and hopefully vision. Here this was achieved by submission and rapid feedback to the students of their specification, conceptual solutions and general arrangement for their preferred solution after two weeks’ work. This review serves to encourage work to deadlines, progress on the case study and to identify areas of strength and weakness in the design direction undertaken by the students.  A second stage submission follows three weeks later to include specific detailed design work on the enclosure, possibly a plastic moulding, the transmission system, possibly an epicyclic gear train, and the bit holder and thrust collar. This detailed design work serves to identify specific skills developed in the area of machine elements and enclosure design. In addition an estimation of the cost of the product is requested and an assessment of the overall design. These aspects are critical to the design process. Costing is the endless driver for much and some might argue all of our commercial activity and the ability to assess or identify strengths and weaknesses in our own work a valuable skill. The marks given to the two submissions currently total 5% and 15% respectively of the overall half module marks.  4 LEARNING OBJECTIVES Identifiable learning achievement is the end goal of the teaching profession. Here the aims of this half module were to develop an appreciation of the holistic or total design process and to enable the development of specific detailed design skills in a number of machine elements. The use of an electromechanical case study enables practical experience of: • the holistic design process • interdisciplinary design skills • specification • concept generation • practical detailed design • product exposure • cost estimation • appreciation of business constraints.  5 CONCLUSIONS A new manageable half module has been developed to introduce the holistic design process to engineering undergraduates at an early stage in their education. The course has been designed around a case study involving an electromechanical product. The case study serves as a focus for the topics introduced and enables students to practice the skills being taught in an identifiable and interesting manner. Other case studies have been developed to limit the scope of inappropriate cross-year communication and encourage the learning process including a chain saw, belt sander, router and cordless drill. The similarities of the products limit the demand on the lecturer in terms of generation of teaching material.   References [1] Childs, P.R.N., Mechanical design, 1998, Arnold. [2] Vogwell, J. in Hurst, K. (Editor), Rotary power transmission design, 1994, McGraw Hill. 

Small priced tools and equipment case study

Abstract

Similar works

Full text

Available Versions

Spiral - Imperial College Digital Repository