ME 330Manufacturing Processes

MECHANICAL ASSEMBLY:Joining Methods using Fasteners

Overview of processes

Principle of the process

Structure and configuration

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

Two Major Types of Mechanical Assembly

1. allow for disassembly

2. create a permanent joint with no possibility of disassembly

Adhesion and bonding

Use of the third part component Use of the deformation between two components to

be assembled.

Pros of Mechanical Assembly

1. Ease of assembly – can be accomplished with relative ease by unskilled workers Minimum of special tooling required Can be done in a relatively short time

2. Ease of disassembly – at least for the methods that permit disassembly Some disassembly is required for most products to

perform maintenance and repair

Principle of the process

Structure and configuration

Process modeling

Defects

Design For Manufacturing (DFM)

Process variation

Use of the third part component - fastener for the joining with a possibility of

disassembly

See for a good overview:

http://www.fastenal.com/web/search/products/_/N-gj4wao&

http://www.mcmaster.com/#fastening-joining

Fastens: Screws, Bolts, and Nuts

1. 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.

2. Bolt - externally threaded fastener inserted into through holes and "screwed" into a nut on the opposite side.

3. Nut - internally threaded fastener having standard threads that match those on bolts of the same diameter, pitch, and thread form.

Self-Tapping Screws

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, or cracking may result.

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

Some Facts About Screws and Bolts

1. Screws and bolts come in a variety of sizes, threads, and shapes.

2. Much standardization in threaded fasteners, which promotes interchangeability.

3. Canada and US is converting to metric, further reducing variations.

4. Differences between threaded fasteners affect tooling

Example: different screw head styles and sizes require different screwdriver designs.

Common head styles available on screws

Head Styles on Screws

Courtesy of www.mikesenese.com

Common drive styles available on screws

Drive Styles on Screws

Courtesy of www.mikesenese.com

Assembly functions such as fastening collars, gears, and pulleys to shafts

Setscrews

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 threadsExpand when screwed into, in some cases

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

Hardware component often used with threaded fasteners

to ensure tightness of a mechanical joint. Simplest form =

flat ring of thin sheet metal.

Functions:

1. Distribute stresses2. Provide support for large clearance holes3. Protect part surfaces and may help seal 4. Increases spring tension 5. Resists inadvertent unfastening

Assembly enhancement: Washer

Washer Types

a) plain (flat) washers

b) spring washers: dampens vibration and resists loosing of bolt/screw

c) Lock washer: designed to resist loosening of bolt/screw

d) 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

Five Basic Types of Rivets

(a) Solid, (b) tubular, (c) sem itubular, (d) bifurcated, and (e) compression

Applications and Advantages of Rivets

1. Used primarily for lap joints.

2. A primary fastening method in aircraft and aerospace industries.

3. Advantages:

High production rates Simplicity Dependability Low cost

Tooling and Methods for Rivets

1. Impact - pneumatic hammer delivers a succession of blows to upset rivet.

2. Steady compression - riveting tool applies a continuous squeezing pressure to upset rivet.

3. Combination of impact and compression.

Fastening with Eyelets

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

Summary

1. Mechanical assembly concept – providing force

constraints on two parts mechanically.

2. Assembly can be: permanent and disassemble.

3. Dissemble possibility: bolts-nuts, screw.

4. Permanent: rivets.

5. Rives: cheap, efficient used in aircraft, ship.