MechanicalDesignAndFab-GuidelinesAndProcedures

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Guidelines and Procedures for the Design and Fabrication of MechanicalComponents in Senior Design

Students in the EE, CompE and BME programs typically are not familiar with the process ofdesigning & fabricating mechanical components. Recently, I have been seeing drawings ofmechanical assemblies which are so dreadfully incomplete that no machinist could possibly build

the device the student needs from just these drawings / sketches.

This document is primarily to provide guidance to those students who have a significant mechanicalcomponent to their project. Many students have either no mechanical components in their project,or the mechanicals constitute only a minor part of their project, e.g., a simple box to enclose acircuit board. Such students should read through this document, but not get worried because atmost they might have to specify where to drill a few holes in an aluminum box that they havepurchased, and thus, the only mechanical drawing required for their project might be a front view ofa single removable panel.

Use COTS (commercial, off-the-shelf) mechanical components wherever possible

If your project has a mechanical aspect, your design should make use of as many commerciallyavailable mechanical components as possible. This will allow you to reduce your work, reduce thecost of your project, reduce the work load of the SEAS machine shop, and finish your projectsooner.

For example, if all you need is a simple sheet metal box to hold (and electrostatically shield) yourcircuit board, and perhaps, mount a few switches and connectors, there is absolutely no reason tocustom design a box and have it built for you. Simply purchase a COTS (commercial, off-the-shelf)box and, at most, you might need to drill a few holes in it. The savings in time and money foreveryone involved will be enormous a few dollars vs. many hundreds of dollars for a custom box.

Thousands of different enclosures are available from vendors such as Newark Electronics, e.g.:http://www.newark.com/NewarkWebCommerce/newark/en_US/endecaSearch/searchPage2.jsp;jsessionid=3OYQRV2FKUQO4CXDUY2SFFYK2OTCIIV1?comSearch=true&select1=156452&paramSelectCount=7&st=parametricSelection&N=1002250+0&comSearch=true&paramSearch=true&st=parametricSelection&x=32&y=5&showImages=true&viewType=images

Use metal, not plastic, enclosures unless there is some over-riding reason not to use metal. Theadvantage is that it provides electrostatic shielding, instant grounding points, and is easy to work.Some of the more commonly used COTS enclosures are shown below (in order of increasing size):
http://www.newark.com/NewarkWebCommerce/newark/en_US/endecaSearch/searchPage2.jsp;jsessionid=3OYQRV2FKUQO4CXDUY2SFFYK2OTCIIV1?comSearch=true&select1=156452&paramSelectCount=7&st=parametricSelection&N=1002250+0&comSearch=true&paramSearch=true&st=parametricSelection&x=32&y=5&showImages=true&viewType=imageshttp://www.newark.com/NewarkWebCommerce/newark/en_US/endecaSearch/searchPage2.jsp;jsessionid=3OYQRV2FKUQO4CXDUY2SFFYK2OTCIIV1?comSearch=true&select1=156452&paramSelectCount=7&st=parametricSelection&N=1002250+0&comSearch=true&paramSearch=true&st=parametricSelection&x=32&y=5&showImages=true&viewType=imageshttp://www.newark.com/NewarkWebCommerce/newark/en_US/endecaSearch/searchPage2.jsp;jsessionid=3OYQRV2FKUQO4CXDUY2SFFYK2OTCIIV1?comSearch=true&select1=156452&paramSelectCount=7&st=parametricSelection&N=1002250+0&comSearch=true&paramSearch=true&st=parametricSelection&x=32&y=5&showImages=true&viewType=imageshttp://www.newark.com/NewarkWebCommerce/newark/en_US/endecaSearch/searchPage2.jsp;jsessionid=3OYQRV2FKUQO4CXDUY2SFFYK2OTCIIV1?comSearch=true&select1=156452&paramSelectCount=7&st=parametricSelection&N=1002250+0&comSearch=true&paramSearch=true&st=parametricSelection&x=32&y=5&showImages=true&viewType=imageshttp://www.newark.com/NewarkWebCommerce/newark/en_US/endecaSearch/searchPage2.jsp;jsessionid=3OYQRV2FKUQO4CXDUY2SFFYK2OTCIIV1?comSearch=true&select1=156452&paramSelectCount=7&st=parametricSelection&N=1002250+0&comSearch=true&paramSearch=true&st=parametricSelection&x=32&y=5&showImages=true&viewType=imageshttp://www.newark.com/NewarkWebCommerce/newark/en_US/endecaSearch/searchPage2.jsp;jsessionid=3OYQRV2FKUQO4CXDUY2SFFYK2OTCIIV1?comSearch=true&select1=156452&paramSelectCount=7&st=parametricSelection&N=1002250+0&comSearch=true&paramSearch=true&st=parametricSelection&x=32&y=5&showImages=true&viewType=imageshttp://www.newark.com/NewarkWebCommerce/newark/en_US/endecaSearch/searchPage2.jsp;jsessionid=3OYQRV2FKUQO4CXDUY2SFFYK2OTCIIV1?comSearch=true&select1=156452&paramSelectCount=7&st=parametricSelection&N=1002250+0&comSearch=true&paramSearch=true&st=parametricSelection&x=32&y=5&showImages=true&viewType=imageshttp://www.newark.com/NewarkWebCommerce/newark/en_US/endecaSearch/searchPage2.jsp;jsessionid=3OYQRV2FKUQO4CXDUY2SFFYK2OTCIIV1?comSearch=true&select1=156452&paramSelectCount=7&st=parametricSelection&N=1002250+0&comSearch=true&paramSearch=true&st=parametricSelection&x=32&y=5&showImages=true&viewType=imageshttp://www.newark.com/NewarkWebCommerce/newark/en_US/endecaSearch/searchPage2.jsp;jsessionid=3OYQRV2FKUQO4CXDUY2SFFYK2OTCIIV1?comSearch=true&select1=156452&paramSelectCount=7&st=parametricSelection&N=1002250+0&comSearch=true&paramSearch=true&st=parametricSelection&x=32&y=5&showImages=true&viewType=images


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Before you go down the path ofany custom-made mechanical assembly (but especially chassis andenclosures), thoroughly familiarize yourself with the catalogs of a few of the major suppliers ofmechanical components such as McMaster-Carr (www.mcmaster.com), Grainger(www.grainger.com), etc..

Custom-made mechanical parts or systems

Obviously, not all mechanical components will be available commercially, and you will have todesign and fabricate such items. Below is a summary of the process you should follow in SD forboth completely custom-made parts as well as for COTS components that might need some minormodification:

Step #1 Come up with a tentative design (e.g., hand drawn sketches) and discuss it withProf. Kay.

Step #2 After agreement is reached with Prof. Kay on a preliminary mechanical design,the student and Prof. Kay should discuss it with one of the machinists to see if it can be

further simplified, and can be fabricated with reasonable effort in their shop.

Step #3 After the input and agreement of the machinists is obtained, make up a completeset of mechanical drawings.

Step #4 Have them reviewed by Prof. Kay, or, in his absence, Prof. Manuccia.

Step #5 Take the drawings to one of the machinists to discuss scheduling, which aspects ofthe fabrication you will be required / allowed to participate in, which shop tools you will beallowed to use, etc.

Step #6 Fabricate and check the individual parts, assemble into the complete mechanicalassembly, check the assembly for function. Fix as necessary.

Mechanical Drawings

If your project needs a custom-made mechanical part or system, you will need to produce acomplete set of mechanical drawings for it. If a mechanical component in your project involvesmultiple pieces, you will need an assembly drawing, as well as mechanical drawings of each of theindividual pieces. For information on these, in addition to this document, please refer to the filesaccompanying Prof. Kays lecture on mechanical drawing (link is on the references pages of thecourse website). Another short overview of mechanical drawing can be found here:http://www.bbc.co.uk/schools/gcsebitesize/design/graphics/drawingformalrev1.shtml

The drawings for the individual pieces of your project should be standard orthographic or first-angleprojections (e.g.,http://en.wikipedia.org/wiki/Orthographic_projection ),
http://www.mcmaster.com/http://www.grainger.com/http://www.bbc.co.uk/schools/gcsebitesize/design/graphics/drawingformalrev1.shtmlhttp://en.wikipedia.org/wiki/Orthographic_projectionhttp://en.wikipedia.org/wiki/Orthographic_projectionhttp://www.mcmaster.com/http://www.grainger.com/http://www.bbc.co.uk/schools/gcsebitesize/design/graphics/drawingformalrev1.shtmlhttp://en.wikipedia.org/wiki/Orthographic_projection


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Three orthogonal views (as shown) are generally required , and they must be placed on the page inthe correct relative locations, i.e., the top view is directly above the front view, the right side view isto the right of the front view. If the part is just a thin piece of metal with holes in it (i.e., the front

panel of a box to hold your electronics), only the front view is needed. If it has cylindricalsymmetry, only two views might be necessary, e.g.,

If the part is complicated, the standard orthogonal views should be supplemented by as many

oblique projections, and cross-sectional views as needed to fully depict structures at unusual anglesor internal to the object (http://en.wikipedia.org/wiki/Cross_section_%28geometry%29 ):
http://en.wikipedia.org/wiki/Cross_section_(geometry)http://en.wikipedia.org/wiki/Cross_section_(geometry)


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If the mechanical aspect of your project involves multiple mechanical components, an assemblydrawing keyed to the drawings of the individual components is absolutely essential. The assemblydrawing can be either orthographic ( https://reader009.{domain}/reader009/html5/0410/5acc48a57b647/5):

or an isometric / perspective / exploded assembly drawing such as shown in the followingexamples: (http://en.wikipedia.org/wiki/Exploded_view ) :
http://www.me.metu.edu.tr/me307/drawings/assembly.jpghttp://en.wikipedia.org/wiki/Exploded_viewhttp://www.me.metu.edu.tr/me307/drawings/assembly.jpghttp://en.wikipedia.org/wiki/Exploded_view


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and,


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The individual components should be clearly labeled either as shown in the illustration immediatelyabove, or by means of a keyed table / parts list, as shown below:


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Drawings for each of the individual components shown in the assembly drawing must appear and befully dimensioned. However, assembly drawings generally have only overall dimensions (if any).All fasteners should be included and completely described.

*All* mechanical drawings should have a text box which states aspects of the design common to allthe views shown on the page, e.g., measurement units, surface roughness and finish, tolerances, part

name, project name, engineer name, date, drawing version number, etc.

Mechanical drawings, like all your other representations of design, must be sufficiently complete,accurate and self-standing that if they were given to a typical machinist, that person would be ableto fabricate your parts with no further consultation with you. For the electronic parts of yourproject, the assumption is that the fabricator will read, understand, and refer to your written report.However, for mechanical fabrication, the standard you must follow is that the machinist should beable to construct your device from the mechanical drawings alone, never even seeing a copy of yourwritten report.


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Use of the Machine Shop in Tompkins Hall

The staff of the machine shop in Tompkins Hall are extremely helpful, courteous and well-intentioned. For years, they have gone out of their way to help ECE students. Unfortunately, theyare becoming overwhelmed with work, much of it helping ECE senior design students. Thefollowing comments are designed to help optimize your interactions with them.

1. We expect the vast majority of the work on your senior project to be yours. We expect (and

strongly encourage) you to get general help from as many reputable sources as you can,however no one should do your actual work for you, except where it is unavoidable (e.g.running a machine you are not trained / qualified to use). This is true whether you needhelp in electronics, computer programming, mechanical fabrication tasks, or anything else.

2. The staff of the machine shop are employed by the MAE department, not by SEAS or theECE department. They are *not* a commercial service organization, and do not operate as aservice facility even for their own MAE students. They certainly are not working directly orexclusively for you. They help ECE SD students only as a favor, and because they trulyenjoy helping others.

3. Although the machinists and technicians in the Tompkins Hall shop could undoubtedly learnabout the exact mechanical requirements of each project, they dont have the time to do this,and should not be asked to do so. What this means is that you need to present them with acomplete mechanical design, and not expect them to do the mechanical design part of yourproject for you. This is why steps #1 and #3 (above) exist.

4. On the other hand, the machinists and technicians in the shop have enormous experience,knowledge and skills. They often can suggest improvements or simplifications.Consultation with them is critical. This should come before you lock into a particular

design, material or fabrication approach. They know what can be produced in their shop andwhat can not. This is why step #2 (above) exists and comes between steps #1 and #3.

5. The usage of the machine shop and the demands on the time of the shop staff is alwaysextremely heavy in the last few weeks of each semester, i.e., right at the time you will needto complete your project. If humanly possible, schedule your use of the shop before the lastfew weeks of the semester. In addition, make sure your design doesnt needlessly increasetheir workload. For example, dont insist on a custom-made enclosure for your projectwhen an box you can buy at Radio Shack for $10 will work just as well (see photos above).The custom approach could easily take the shop 10 hours ( ~ $500 in loaded labor costs) tofabricate. Think and get advice before you convince yourself of the need for a design which

will be 50 times more expensive!

6. Under no conditions should you ever expect to be able to hand the staff of the shop a roughsketch of a mechanical concept, and expect them to complete your design, fabricate yourdevice, and hand it back to you working exactly they way you intended. As part of yourtraining you have to actively participate in this process.

For example, suppose you need 20 holes drilled and tapped. You dont know how to dothis, so you ask one of their staff for help. They will explain the procedure to you, and will


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likely drill and tap the first hole while you watch. You then drill and tap the next couple ofholes under their supervision. They will watch and offer suggestions. Once they aresatisfied that you have mastered the technique, it is your responsibility to finish theremaining holes yourself, freeing the machinists to work with other students.

Safety & training considerations:Almost all of the equipment in the machine shop is dangerous, and even something as simple as adrill press can tear your hand to pieces if you dont know how to use it correctly. Carl or Bill, inconjunction with Prof. Kay will give an introductory lesson on machine shop safety and generalprocedures. Until either Carl or Bill have explicitly given you permission to use a particularmachine, you are not allowed to use that machine. You will likely get certified to use only one ortwo of their machines. On the other machines, they will likely offer to do the work for you. Insuch a case, you should offer to remain, observe, and learn both the technical and safety aspects ofthe operation.

What should you do if you barely know what a screwdriver is?If you are a junior, you have several choices: (a) modify your project so that the mechanical aspectsare minimized or even eliminated; (b) get friends / colleagues / classmates / relatives to give you acrash course on the basics of mechanical design and fabrication. The latter will hopefully allow youto fill in any blanks with tightly focused conversations with GWU staff and faculty; or, (c) If youhave an elective slot open, take courses in this subject from the MAE department (e.g., MAE-192).If you are a senior, dont know what a screwdriver is, and just now realize that you have amechanical part of your project that is much more significant than you thought, you need to seeyour ECE SD instructor ASAP.

MechanicalDesignAndFab-GuidelinesAndProcedures

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