Guide to Mechanical Dissection

W. Durfee
sep-08

The purpose of dissecting a product (also known as take-aparts or reverse engineering) is to learn what it does and how it was made. Just like learning anatomical dissecting methods, there are ways of doing careful mechanical dissection that will maximize what you learn. Yes, you can take apart a VCR with a chain saw, but you probably won't learn much about how the end-of-tape sensor works. Maximum learning comes from careful observation and from thinking about each next step in the dissection process. You are like an archeologist or a field biologist or a crime scene investigator. You need to learn what is there while disturbing the product as little as possible.

When dissecting, you are looking for three things. First, how the product functions. For complex products, this is not at all clear from the outside, so you dissect to understand how things work. You have to dissect carefully. For example, if you crack open a case and a spring flies out, you will have a hard time understanding what that spring did in vivo.

Second, how the product was assembled. As you open each cavity or take off each part, ask yourself how it was assembled. Was it by hand? By machine? What method was used for joining? (Snap fit, threaded fastener, glue, heat stake, spot weld,...?)

Third, what materials are used. Examine each part. What material was it made from? (Metal, plastic...?) Can you tell what type of metal, or even what alloy of metal? Can you tell which type of plastic? Determine how each part was made. For metal parts, were they stamped? Forged? Cast? Machined? For plastic parts, were they injection molded? Blow molded? If some of these terms are not familiar, consult any introductory manufacturing text, or see the resource list at the end of this guide.

Here are some additional tips to help you with dissections:

  1. Maintain a dissection notebook for recording what you see and what you do using annotated quick sketches. You can use your sketchbook for this purpose.

  2. Keep notes on the takeapart steps. In many cases you will be returning the product back to its original, working state.

  3. For new products, pay attention to the packaging. Record the materials used and the steps (in order) required to package the product.

  4. As you disassemble, keep track of the steps required for assembling the product. Record in reverse assembly order. For each step, think about what type of machine could be (was) used to automate the assembly step.

  5. Try to identify common mechanism and machine components that you find. For example, levers, rack and pinions, shafts, bearings and fasteners. Understand the purpose of the mechanism or component and why the designer chose to include it. Think of other machines that contain the same or similar components.

  6. Maintain a parts list, sorted by category (bearings, housings, buttons, screws, and so forth). By the time you are finished, you should have a complete parts count of the product. More details on the suggested format for this parts list are provided below.

  7. One of the more difficult tasks is deciding what constitutes a "part" on your product. Although your product can be broken down completely into elemental parts, sometimes it makes sense to leave things as a subassembly. For example, a hard drive contains at least two electric motors. One could dissect the motor into its component parts, but on the hard drive BOM it would probably be best to simply list it as a motor with as much detail about the motor as you can determine.

  8. Try to identify the material used for each part. Be as specific as you can. If it is metal, what kind? (aluminum? steel? brass?) If it is steel, what alloy? If plastic, what kind of plastic? (ABS? acrylic? polyethylene?) If you can't identify it, determine the properties (stiffness, hardness, clarity, and so on).

  9. If you can help it, do not disassemble one-time fasteners such as rivets or glued joints. Break or force only if necessary to determine internal workings or parts count of the product. As you do so, think about the implications to maintenance of the assembly choice made by the designer.

  10. Sketch and draw as you proceed. Exploded views, thumbnail sketches of parts. Whatever it takes to record what you did. Although the notes are for you, later you may be asked to create a "disassembly manual" for someone else.

  11. Record all the information you can find that identifies the product: product name, manufacturer, model number, patent number, serial number, where bought, price.

  12. If working in a group and dissecting a complex object, consider splitting into groups to tackle the dissection of subassemblies. Afterwards, the subgroups can report on what they found.

Bill of Materials

When dissecting, keep track of all of the component parts which make up a product through what's commonly called a "Bill of Materials" or BOM for short. This is nothing more than a parts list. Do the final BOM in a word processor or spreadsheet table with one row for each distinct part. The table columns should be labeled "Item Num" (where you enter a reference number for each part, e.g. 001, 002, 003, ...). "Quantity" (where you put how many of that part there are in the product. For example if there are four of the same type of screw, enter "4" under quantity), "Description" (where you name and write a succinct, precise description such as, "Housing, top side," or "Screw, 1/4-20x1, round head, slotted." If you don't know the precise name for a part, write a brief descriptive phrase which describes its function), and "Material" (where you enter as much detail as you know, such as "Steel" or "Alum, 6061-T4" or "Aluminum, cast" or "Plastic, injection molded" or "Polycarbonate"). An example of a partial BOM for an audio cassette appears below.

On-Line Resources

Here are some excellent places to look for more information on design, components, assembly and materials

http://www.machinedesign.com/
Machine Design magazine's "Basics of Design Engineering": an excellent tutorial on all aspects of mechanical engineering design. Highest recommendation. The link from the Machine Design home page to the basics of design engineering (BDE) page keeps changing. Try this link: http://machinedesign.com/channel/basics-of-design-engineering If the link is broken, Google search. From the BDE home page go to Materials then explore the links on plastics.
http://en.wikipedia.org/wiki/Plastic
Plastics, on Wikipedia. See section on Common plastics and uses.
http://www.efunda.com
eFunda has coverage of just about any engineering topic imaginable. Look under Materials > Polymers, and under Design Center > Plastic Design.
http://www.plastics.dupont.com
Dupont is a major polymer manufacturer. Search for their "Top Ten Design Tips" (you'll have to hunt, it's a bit hard to find) which discusses plastic material choice and guidelines for injection molding.

Sample Bill of Materials

Here's part of the BOM which might result from dissecting an ordinary T-90 audio cassette.

Item Number Quantity Description Material
001 1 Storage case, top Acrylic(?), injection molded
002 1 Storage case, bottom Acrylic(?), injection molded
003 1 Outer label Cardboard, stamped
004 5 Screws, self-tapping, Phillips head Steel
005 1 Case, top Polycarbonate(?), injection molded
006 1 Case, bottom Polycarbonate(?), injection molded
007 1 Internal Label Plastic Film
008 1 Tape pressure pad Stamped spring steel, felt
009 1 Tape pad support Steel, stamped and bent
010 2 Spool hub ABS (?), injection molded
...and so on...




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