Guidelines and Procedures for the Design and Fabrication of Mechanical
Components in Senior Design
Students in the EE, CompE and BME programs typically are not familiar with the process of
designing & fabricating mechanical components. Recently, I have been seeing drawings of
mechanical 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 mechanical
component 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 a
circuit board. Such students should read through this document, but not get worried because at
most they might have to specify where to drill a few holes in an aluminum box that they have
purchased, and thus, the only mechanical drawing required for their project might be a front view of
a 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 commercially
available mechanical components as possible. This will allow you to reduce your work, reduce the
cost of your project, reduce the work load of the SEAS machine shop, and finish your project
sooner.
For example, if all you need is a simple sheet metal box to hold (and electrostatically shield) your
circuit board, and perhaps, mount a few switches and connectors, there is absolutely no reason to
custom 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 for
everyone 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;jses
sionid=3OYQRV2FKUQO4CXDUY2SFFYK2OTCIIV1?comSearch=true&select1=156452&para
mSelectCount=7&st=parametricSelection&N=1002250+0&comSearch=true&paramSearch=true&s
t=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. The
advantage 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):
Before you go down the path of any custom-made mechanical assembly (but especially chassis and
enclosures), thoroughly familiarize yourself with the catalogs of a few of the major suppliers of
mechanical 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 to
design and fabricate such items. Below is a summary of the process you should follow in SD for
both completely custom-made parts as well as for COTS components that might need some minor
modification:
Step #1 – Come up with a tentative design (e.g., hand drawn sketches) and discuss it with
Prof. 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 complete
set 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 of
the fabrication you will be required / allowed to participate in, which shop tools you will be
allowed to use, etc.
Step #6 – Fabricate and check the individual parts, assemble into the complete mechanical
assembly, 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 a
complete set of mechanical drawings for it. If a mechanical component in your project involves
multiple pieces, you will need an assembly drawing, as well as mechanical drawings of each of the
individual pieces. For information on these, in addition to this document, please refer to the files
accompanying Prof. Kay’s lecture on mechanical drawing (link is on the “references” pages of the
course 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-angle
projections (e.g., http://en.wikipedia.org/wiki/Orthographic_projection ),
Three orthogonal views (as shown) are generally required , and they must be placed on the page in
the correct relative locations, i.e., the top view is directly above the front view, the right side view is
to 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 cylindrical
symmetry, 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 angles
or internal to the object (http://en.wikipedia.org/wiki/Cross_section_%28geometry%29 ):
If the mechanical aspect of your project involves multiple mechanical components, an assembly
drawing keyed to the drawings of the individual components is absolutely essential. The assembly
drawing can be either orthographic ( http://www.me.metu.edu.tr/me307/drawings/assembly.jpg ):
or an isometric / perspective / exploded assembly drawing such as shown in the following examples:
(http://en.wikipedia.org/wiki/Exploded_view ) :
and,
The individual components should be clearly labeled either as shown in the illustration immediately
above, or by means of a keyed table / parts list, as shown below:
Drawings for each of the individual components shown in the assembly drawing must appear and be
fully 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 all
the 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 able
to fabricate your parts with no further consultation with you. For the electronic parts of your project,
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 be
able to construct your device from the mechanical drawings alone, never even seeing a copy of your
written report.
Use of the Machine Shop in Tompkins Hall
The staff of the machine shop in Tompkins Hall are extremely helpful, courteous and wellintentioned. For years, they have gone out of their way to help ECE students. Unfortunately, they
are becoming overwhelmed with work, much of it helping ECE senior design students. The
following 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 need
help 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 the
ECE department. They are *not* a commercial service organization, and do not operate as a
service facility even for their own MAE students. They certainly are not working directly or
exclusively for you. They help ECE SD students only as a favor, and because they truly
enjoy helping others.
3. Although the machinists and technicians in the Tompkins Hall shop could undoubtedly learn
about the exact mechanical requirements of each project, they don’t 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 a
complete mechanical design, and not expect them to do the mechanical design part of your
project 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 and what
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 always
extremely heavy in the last few weeks of each semester, i.e., right at the time you will need
to complete your project. If humanly possible, schedule your use of the shop before the last
few weeks of the semester. In addition, make sure your design doesn’t needlessly increase
their workload. For example, don’t insist on a custom-made enclosure for your project
when 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) to
fabricate. 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 rough
sketch of a mechanical concept, and expect them to complete your design, fabricate your
device, and hand it back to you working exactly they way you intended. As part of your
training you have to actively participate in this process.
For example, suppose you need 20 holes drilled and tapped. You don’t know how to do this,
so you ask one of their staff for help. They will explain the procedure to you, and will likely
drill and tap the first hole while you watch. You then drill and tap the next couple of holes
under their supervision. They will watch and offer suggestions. Once they are satisfied that
you have mastered the technique, it is your responsibility to finish the remaining 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 a
drill press can tear your hand to pieces if you don’t know how to use it correctly. Carl or Bill, in
conjunction with Prof. Kay will give an introductory lesson on machine shop safety and general
procedures. Until either Carl or Bill have explicitly given you permission to use a particular
machine, you are not allowed to use that machine. You will likely get certified to use only one or
two of their machines. On the other machines, they will likely offer to do the work for you. In
such a case, you should offer to remain, observe, and learn both the technical and safety aspects of
the 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 aspects
are minimized or even eliminated; (b) get friends / colleagues / classmates / relatives to give you a
crash course on the basics of mechanical design and fabrication. The latter will hopefully allow you
to fill in any blanks with tightly focused conversations with GWU staff and faculty; or, (c) If you
have an elective slot open, take courses in this subject from the MAE department (e.g., MAE-192).
If you are a senior, don’t know what a screwdriver is, and just now realize that you have a
mechanical part of your project that is much more significant than you thought, you need to see
your ECE SD instructor ASAP.