ME 330 Manufacturing Processes MECHANICAL ASSEMBLY: Joining Methods using Fasteners

Overview of processes

Principle of the process Design For Manufacturing (DFM) Process modeling Defects Design For Manufacturing (DFM) Process variation

Mechanical Assembly: Definition Use of various methods to mechanically attach two or more parts together. This can be done by Add pic of assembly by no fasteners Or give example Principle of the assembly: create a force constraint on two parts that are assembled.

Two Major Types of Mechanical Assembly allow for disassembly 2. create a permanent joint with no possibility of disassembly Use of the third part component Use of the deformation between two components to be assembled. Adhesion and bonding Why would you ever want to make a part with a permanent joint?? Pass around phone

Pros of Mechanical Assembly Ease of assembly – accomplished with relative ease by unskilled workers The least number of special tools is required. Can be done in a relatively short time. Ease of disassembly – at least for the methods that permit disassembly

Principle of the process Structure and configuration Process modeling Defects Design For Manufacturing (DFM) Process variation

Use of the third party component - fastener for the joining with a possibility of disassembly See for a good overview: https://www.fastenal.com/products/fasteners;jsessionid=jHfOphv8aCkcv5uQ ZhdXWmxK.d36f637b-91a9-3b2b-9fa6- 50af87272845?r=~%7Ccategoryl1:%22600000%20Fasteners%22%7C~ http://www.mcmaster.com/#fastening-joining These companies have a good selection and good online tools to help you find what you need

Fastens: Screws, Bolts, and Nuts Screw - externally threaded fastener generally assembled into a blind threaded hole. Some screws are self-tapping, so there is no need to tap the hole. Bolt - externally threaded fastener inserted into through holes and "screwed" into a nut on the opposite side. Nut - internally threaded fastener having standard threads that match those on bolts of the same diameter, pitch, and thread form. Show example screw threaded Show bolt and screw on fastenal

Question: why is the assembly not loosen after fastening? TF W Question: why is the assembly not loosen after fastening? Answer: self-locking principle. Show example screw threaded Show bolt and screw on fastenal

Self-locking principle: Step 1: after F is gone, the separate force diagram is shown below N f W Show example screw threaded Show bolt and screw on fastenal Step 2: the condition that the block does not slide back: tan (α) < µ, µ is friction coefficient. α<tan-1(µ) Conclusion: the screw must be designed such that tan (α) < µ

Self-locking principle: Increase µ N f W Increase µ Show example screw threaded Show bolt and screw on fastenal tan (α) < µ Assume no consideration of contact force

Self-Tapping Screws thread‑forming, and Designed to form or cut threads in a pre‑existing hole into which it is being turned. For thicker materials, a hole needs to be drilled first, otherwise cracking may result. thread‑forming, and thread‑cutting, also called a tapping screw.

Nut Types Hex nut: plain type. Wing nut: designed for hand tightening. Nylon insert lock nut: has nylon insert to resist loosening. Also called Nylock nut Castle nut: used with a cotter pin to prevent loosening Give example of BC Ferry failure

Some Facts About Screws and Bolts Screws and bolts come in a variety of sizes, threads, and shapes. Much standardization in threaded fasteners, which promotes interchangeability. Canada and US is converting to metric, further reducing variations. Differences between threaded fasteners affect tooling Example: different screw head styles and sizes require different screwdriver designs.

Head Styles on Screws Common head styles available on screws http://mikesenese.com/ Courtesy of www.mikesenese.com

Drive Styles on Screws Common drive styles available on screws In early 1900’s Robertson had licensed the screw design to a maker in England, but the party that he was dealing with intentionally drove the company into bankruptcy and purchased the rights from the trustee, thus circumventing Robertson.[citation needed] He spent a small fortune buying back the rights. Subsequently, he refused to allow anyone to make the screws under license. When Henry Ford tried out the Robertson screws he found they saved considerable time in Model T production, but when Robertson refused to license the screws to Ford, Ford realized that the supply of screws would not be guaranteed and chose to limit their use in production to Ford's Canadian division.[21][22][23] Robertson's refusal to license his screws prevented their widespread adoption in the United States, where the more widely licensed Phillips head has gained acceptance wikipedia Courtesy of www.mikesenese.com

Setscrews Assembly functions such as fastening collars, gears, and pulleys to shafts Force constraint creation between collar and shaft: setscrew touch the surface of the shaft  pressure  no relative motion between the shaft and setscrew  no relative motion between the shaft and collar

Assembly enhancement: Screw Thread Inserts Internally threaded plugs or wire coils designed to be inserted into an unthreaded hole and accept an externally threaded fastener Assemble into weaker materials to provide strong threads Expand when screwed into, in some cases Pass around sample on sheet metal Great for repair on a hole

(a) before insertion, and (b) after insertion into hole and screw is turned into insert

Elastic force Assembly enhancement: Washer Screw or bolt or nut Washer Hardware component often used with threaded fasteners to ensure tightness of a mechanical joint. Simplest form = flat ring of thin sheet metal. Functions: Distribute stresses Provide support for large clearance holes Protect part surfaces and may help seal Increases spring tension Resists inadvertent unfastening Screw or bolt or nut Washer Parts Force closure Elastic force

Washer Types plain (flat) washers spring washers: dampens vibration and resists loosening of bolt/screw Lock washer: designed to resist loosening of bolt/screw split lock washer: designed to resist loosening of bolt/screw (d)

Use of the third part component - fastener for the permeant joining Rivets Unthreaded, headed pins used to join two or more parts by passing the pin through holes in parts and forming a second head in the pin on the opposite side These companies have a good selection and good online tools to help you find what you need

Five Basic Types of Rivets (a) Solid, (b) tubular, (c) semi tubular, (d) bifurcated, and (e) compression

Applications and Advantages of Rivets Used primarily for lap joints. A primary fastening method in aircraft and aerospace industries. Advantages: High production rates Simplicity Dependability Low cost

Tooling and Methods for Rivets Impact - pneumatic hammer delivers a succession of blows to upset rivet. Steady compression - riveting tool applies a continuous squeezing pressure to upset rivet. Combination of impact and compression. Show example in class of riveting tool

Fastening with Eyelets (a) Eyelet, (b) assembly sequence: (1) inserting eyelet through holes and (2) setting operation

Summary Mechanical assembly concept – providing force constraints on two parts mechanically. Assembly can be: permanent and disassembly. Disassembly possibility: bolts-nuts, screw. Permanent: rivets. Rives: cheap, efficient used in aircraft, ship. Show example in class of riveting tool