Engr 22 lec-21-sp07_fasteners (1)

BMayer@ChabotCollege.edu • ENGR-22_Lec-20__Fasteners-1_Specs.ppt1
Bruce Mayer, PEEngineering 22 – Engineering Design Graphics
Bruce Mayer, PE
Registered Electrical & Mechanical Engineer
BMayer@ChabotCollege.edu
Engineering 22
ThreadedThreaded
FastenersFasteners

Skill-Development GoalsSkill-Development Goals
 Define and Label the Parts of A Screw
Thread
 Identify Various Screw Thread Forms
 Draw Screws in Forms
• Detailed
• Schematic
• Simplified
 Properly Apply Screw Thread Technical
Specifications

Skill-Development Goals cont.Skill-Development Goals cont.
 Identify Different
Fasteners and
Describe Their
Use
 Draw the Various
Types of Screw
Heads

Skill-Development GoalsSkill-Development Goals
 State the Two MOST Important
Threaded Fastener References for
Structural (Mech/Civ/Chem) Design
Engineers
 Use AutoCAD to QUICKLY Draw Most
Threaded Fasteners in Any Size and
Any Representation
 Invoke the AutoCAD “Design Center”
tool to Construct Fastener
Representations

Threaded FastenersThreaded Fasteners
 Helical Threads Have Three Main
Applications
1. Used to HOLD parts together
• The Classic Application
2. Used to ADJUST the Position of parts with
reference to one another
• The “Screw Adjustment” knob
3. Used to TRANSMIT Power
• e.g, screw Jack,
Worm Gear Drive

Screw Thread TerminologyScrew Thread Terminology
 Ref ASME B1.7M-1984 (R2001)
• Nomenclature Definitions, and Letter
Symbols for Screw Threads

Common Tread formsCommon Tread forms
Joining
Joining
Lite-Duty (e.g., Light Bulbs)
Pwr Xmission

Thread PitchThread Pitch
 USA → Pitch is Inverse of Thread Count
 Metric → Pitch in the Thread Designation

Unified (USA) Thread SeriesUnified (USA) Thread Series
 The Unified System (UN) Adopted on
18-Nov-1948 by the USA, UK , &
Canada
• Made fasteners Interchangeable
 The UN Designation (Spec) has Two
Main Elements
• The Major (outside) Diameter
– Specified by Either
A size No. running from 0-12 (0.06-0.216 inches)
 The Major Dia, in Fractional (ugh!) inches (¼-4 inches)
• The Inverse Pitch in Threads per Inch

UN Thread Series – 6 TotalUN Thread Series – 6 Total
1. COARSE Series (UNC or NC)
• For General Use Where Rapid Assembly
is Required
– Threads Engage, or “Start” Easily
2. FINE Series (UNF or NF)
• For Applications Requiring Greater
Strength or Where the Length of
Engagement is Limited
– Used Extensively in Aircraft and
Automobile Manufacturing

UN Thread Series – contUN Thread Series – cont
3. EXTRA FINE Series (UNEF or NEF)
• For Highly Stressed Parts
3. 8N Series (8N)
• A Substitute for Coarse-Thread Series for
Diameters larger Than 1”
– All diameters have
8 threads per inch.
Often Used on bolts
for high pressure
pipe applications.

UN Thread Series – cont.2UN Thread Series – cont.2
5. 12N Series (12 UN or 12N)
• A Continuation of the Fine-Thread Series
for Diameters Larger than 1.5”
– All diameters have 12 thds/in. Used in boiler
work and in Machine Construction.
5. 16N Series (16 UN or 16N)
• A Continuation of the ExtraFine-Thread
Series for Diameters Larger than 2”
– All diameters have 16 thds/in. Used on
adjusting collars and other applications where
thread must have fine adjustment
regardless of diameter.

Screw Fastener SpecificationScrew Fastener Specification
 Defaults That Need NOT be Part of Callout
• NC or NF implied by Diameter & TPI
• Class → Default is 2A or 2B
• Hand → Default is RIGHT

UN Thread ClassesUN Thread Classes
 Two Types of Classes
A. Refers to EXTERNAL Threads
(Bolts & Screws)
B. Refers to INTERNAL Threads
(Nuts & “Tapped” holes)
 Class Descriptions
1. Provide Liberal Allowance for Ease of
Assembly Even When Threads are Dirty
or Slightly Damaged
• Not Commonly Used

UN Thread Classes contUN Thread Classes cont
 Class Descriptions cont.
2. For Commercially Produced bolts,
screws, nuts, and other threaded
fasteners
• By Far the Most Common
3. Used in Precision Assemblies where a
Close Fit is Required to WithStand Stress
& Vibration
• Typical Use is Aircraft or other Hi-Vibration
Applications

Metric Thread SpecificationMetric Thread Specification
 Note That Pitch is Stated Explicitly
• Compares to Inverse Pitch (TPI) for
the Unified Specification

Left & Right Hand TreadsLeft & Right Hand Treads
 Why LH Threads???
• To Distinguish a Critical Connection
• To Make One End of a Turn-Buckle
• When Needed to Counter Loosening
Tendency of Rotating Machinery

Detailed Thread RepresentationDetailed Thread Representation
 Used when diameter of thread is 1” or
larger on plotted or Hand drawing.
 Use ONLY When It is Important to
Show the Function of the Thread
• Not Typical for Hand Drawings

Simplified & Schematic FormsSimplified & Schematic Forms
Best Overall
 Imply depth of thread with hidden lines for
simplified representation
 Use alternating LONG THIN & SHORT
THICK lines to represent ROOT & CREST
lines in schematic representation
• Spacing is SCHEMATIC; need not Match
Actual Pitch

Representation ComparisonRepresentation Comparison
 Detailed → Very Laborious to Construct
• Very Infrequently Used on
Engineering Drawings
 Simplified → Fast but Potentially
Confusing
• Hidden Lines can be Mistaken for
Object Features
 Schematic → Best Overall
• Fast To Draw, Clearly ID’s the Threads

Threads in SectionThreads in Section
 Normal Practice is to NOT Section
the Fastener

Tapped HolesTapped Holes
 To Avoid the use of a Thru-Hole & Nut,
The “Parent” Material of An object May
be Thread “Tapped”
• That is, Threads are Cut Into the Side of a
Hole Drilled into the Base Material
 Even Thin parts may tapped By Use of
A thru Hole
• Rule of Thumb for the MINIMUM number
of threads = 3
– For Light-Duty applications, can use 1.5

Forming Tapped HolesForming Tapped Holes

Pipe ThreadsPipe Threads
 Pipe Threads have An INTERFERENCE
TAPER that Results in PERMANENT Material
DEFOMATION That (Hopefully) Produces
a Fluid-Tight Seal at the Joint.

Pipe Thread RepresentationsPipe Thread Representations
 Taper Should be Shown
• Need Not be to Scale

Pipe Thread RepresentationsPipe Thread Representations
 Only ONE TPI Spec for Pipe Threads
• ASME B1.20.1-1983 = American National
Standard Taper Pipe Thread
 National Pipe Thread (NPT) for Short
• Callout Requires Only the “NPT” Notation,
Along with the “Size”

Bolts, Screws, and StudsBolts, Screws, and Studs
 Does Anyone Know the Difference?
 A BOLT is Used with A NUT
 A SCREW is Used with A TAPPED hole
• The Hole May be SELF-Tapped
– e.g., Wood Screws
 A STUD is a TREADED ROD that is
Inserted into a TAPPED HOLE to leave
Exposed a Threaded STEM
• A Stud May Also be WELDED to a Surface

Machine Screw Head TypesMachine Screw Head Types
 Slot Head Provides More Driving Torque, But Driver
is Hard to Center → Not good for Power Driving

Threaded InsertsThreaded Inserts
 Use With Soft Parent Materials Such as
Aluminum, Wood, Plastic, etc.

Insert AdvantagesInsert Advantages
 Greater clamping pressure in softer materials -
provides higher strength, better seals.
 Better user of high-tensile-strength fasteners.
 Resistance to vibration in assembly.
 High-quality standard threads without tapping.
 Permanent, wear-resistant threads.
 Protective of expensive castings and moldings.
 Reduced performance loss from cold flow of plastic.
 Reduce risks in molding cycle.
 Easy to install in a drilled or molded hole.
 Suitable for automatic installation.
 Wide variety of types to fit virtually
any application

Fastener Design ReferencesFastener Design References
 If you design with Threaded Fasteners;
you MUST get access to These Books
 ISBN: 0-8311- 2711-2  http://www.mcmaster.com/

Mach-HdBk,Mach-HdBk,
23redEdPage23redEdPage
1325(of2511)1325(of2511)

Wheel ReInventionWheel ReInvention
 It has been estimated that CAD-Using
Engineers Spend up to 20 Hrs/Month
REDRAWING Part for their Designs
 Avoid Reinventing the Wheel by
• Calling Part Maker and asking for CAD file
• Consult OnLine Parts DataBases
• Consult your Company’s CAD-Block Library
• Ask your Colleagues
• Check AutoCAD Tools

Building a FastenerBuilding a Fastener
 When I left Watkins-
Johnson Co. in 2000
We had 775 items in
our CAD Library
 Let’s Build a A Bolt
from a “Forms” File
 The Bolt/Screw
Spec
3/8-16 x 1.25 UNC
SH Cap Screw

All Done for TodayAll Done for Today
Fasteners
Make the
World Go
Round
Molded-In
Inserts for
Plastic Materials
Bolt-Failure by
Reversing-Load
Mechanical-
Fatigue
→ See ENGR45

Bruce Mayer, PE
Licensed Electrical & Mechanical Engineer
BMayer@ChabotCollege.edu
Engr/Math/Physics 25
AppendixAppendix
( ) 6972 23
−+−= xxxxf 

Build-Screw Demo - 1Build-Screw Demo - 1
1. Open File New File
using Acad.dwg
2. Make Layer “Bolt”
Continuous & GRN
3. Save As 38-
16_125_shcap.dwg
4. Set Snap to 0.1
5. Open Fastener
Forms file
• Threaded_Fastener
_Forms_0509.dwg
6. Find SH Cap Form
and Copy to ClipBd
7. Paste SHcap Form
into new dwg
8. Return to Forms
and Copy Detailed
Thread Rep
9. Paste detailed Thd-
Form into new-dwg
10.Chk Major dia at 1”

11.Scale all by Ratio
of 1:0.375
12.Move TopV of
Screw Head
13.Rotate Detailed
Thd-form by 90°
14.Explode screw
Side view
15.Make Xline at base
of Scr Hd
16.Offset xline to rt by
• 1.25 (scr length)
• 0.5 (unthd’s shank)
16.Erase Simplifed-
form lines
17.Stretch shank line
to 0.5 xline
18.Make xline on CL
of Screw

Early ResultEarly Result
 Just before trimming out unneeded thds

20.Trim & Erase
unused SideV line
to Rt of 0.5 xLine
21.Erase Unneeded
Thd-details
22.Connect UnThd’s
Shank to Thd’d
Major Dia. With
Fillet
23.Move TopV of Cap
Scr to Lt of SideV
24.Delete
• xLine
• Detailed Thd-Form
23.Put all objects on
Bolt Layer
24.Adjust LtScale to
0.1

Final ResultFinal Result
 Caveat: Screw HEAD may NOT be to-
Scale for 3/8 Socket Head as we scaled
down from the 1” Version
• If Clearance is an issue, then Chk Hd
Diameter and Height against Machinery’s
HandBook

Engr 22 lec-21-sp07_fasteners (1)

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