Mechanical Design Principles

Table of Contents

Design for Manufacturability 

It is very important to consider how a part will be machined when designing it, even if you will be sending it out to be manufactured by somebody else. The design of a part has a direct influence on what machine tools and tooling is used to manufacture it. Poorly designed parts can be expensive to produce or sometimes impossible.

Mill

• Design the part to be machined in the fewest number of clampings possible 
• Radii should usually be greater than 1/8". A 1/8" end mill is a common small end mill size
• Features requiring an end mill longer than 4 times its diameter are usually difficult to machine
• Fillet internal corners
• A boring bar can be used on the student shop manual mills to bore large diameter holes
• External radii are usually difficult to machine accurately on a manual mill but you can alway use a file to add cosmetic fillets 
• Steel is more difficult to machine than aluminum 
• Tight tolerances will likely increase cost and manufacturing time

Lathe

• A wide variety of threads can be machined onto external diameters fairly easily. Even in the student shop
• Deep bores can be difficult to make because of tool deflection 

Water Jet / Laser Cutter

• Very quick and inexpensive way to make parts with simple 2D geometry
• Holes with important diameters should be undersized and drilled to size on the drill press
• For tapped holes can be tapped after cutting
• There is usually some draft on the cut edges
• The surface finish of the cut edges is usually quite rough with water jet parts but can be easily cleaned up with files
• Water jet pats usually need to be submerged during cutting
• Flexure mechanism can easily be cut on a water jet

3D Printing

• The most practical application for 3D printing is to prototype injection moulded parts
• MS doesn't do any injection moulding but 3D printing can still be of use to us where a plastics might be used on a real car or where complex geometry is required but strength is not important
• Try not to design with geometry that can only be manufactured using 3D printing (bad practice)
• FDM 3D prints have orthotropic tensile strength
• You can use heat set inserts or helicoils to put tapped holes into plastic parts 

Welding

• Welds should be easily accessible: welding out of position is difficult and welding in tight corner is sometimes impossible.  
• Aluminum is far more difficult to weld than steel 
• Welding can distort the geometry of your part

Design for Assembly and Installation

Consider how the final product will be assembled:

• If you have bolts, make sure there is room to fit a wrench or socket on the bolt head and nut.
• Think about how all of your parts are connected together. A good way to do this is write a brief "installation manual" on how everything would fit together, and the order that they would be installed
• Think about the final placement of the part. Does it need to be attached to the ground? If so, do you have an attachment point (ie. floor anchors)? How will it be moved to it's final location?

Design for Repair and Maintenance

• Consider the consumables and limited life components. Try to place these in areas that are easily accessible for anyone who has to repair your part in the future. If this isn't possible (for example, some grease points), try to find another suitable part (oil impregnated bearings) or alternative method (grease lines or automatic greaser)

Design for Shipping

During shipping, parts will experience extreme temperatures and unexpected loads. Also consider the size of your part, and what can be assembled at the final destination.

Fits

Press Fits

Press fits are typically defined using ISO-standard fits when designing metric parts. Press fits for components under 6mm in diameter should use an M7 fit, while components over 6mm in diameter should use an N7 fit.