ENGR-22_Lec-21_Fasteners

Engineering 22

Threaded

Fasteners

Bruce Mayer, PE

Registered Electrical & Mechanical Engineer

BMayer@ChabotCollege.edu

Engineering 22 – Engineering Design Graphics

1

Bruce Mayer, PE

BMayer@ChabotCollege.edu • ENGR-22_Lec-20__Fasteners-1_Specs.ppt

Skill-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


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Bruce Mayer, PE


Skill-Development Goals cont.

 Identify Different

Fasteners and

Describe Their

Use

 Draw the Various

Types of Screw

Heads


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Bruce Mayer, PE


Skill-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

4

Representations

Engineering 22 – Engineering Design Graphics Bruce Mayer, PE


Threaded 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


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Bruce Mayer, PE


Screw Thread Terminology

 Ref ASME B1.7M-1984 (R2001)

• Nomenclature Definitions, and Letter

Symbols for Screw Threads


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Bruce Mayer, PE


Common Tread forms

Joining

Joining

Pwr Xmission

Lite-Duty (e.g., Light Bulbs)


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Bruce Mayer, PE


Thread Pitch

 Metric → Pitch in the Thread Designation

 USA → Pitch is Inverse of Thread Count


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Bruce Mayer, PE


Unified (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


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Bruce Mayer, PE


UN 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


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Bruce Mayer, PE


UN Thread Series – cont

3. EXTRA FINE Series (UNEF or NEF)

• For Highly Stressed Parts

4. 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.


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Bruce Mayer, PE


UN 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.

6. 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.


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Bruce Mayer, PE


Screw Fastener Specification

 Defaults That Need NOT be Part of Callout

• NC or NF implied by Diameter & TPI

• Class → Default is 2A or 2B

13

• Hand → Default is RIGHT



UN 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


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Bruce Mayer, PE


UN 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


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Bruce Mayer, PE


Metric Thread Specification

 Note That Pitch is Stated Explicitly

16

• Compares to Inverse Pitch (TPI) for the Unified Specification



Left & 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


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Bruce Mayer, PE


Detailed 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


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Bruce Mayer, PE


Simplified & 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


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Bruce Mayer, PE


Representation 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


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Bruce Mayer, PE


Threads in Section

 Normal Practice is to NOT Section the Fastener


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Bruce Mayer, PE


Tapped 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

22

– For Light-Duty applications, can use 1.5



Forming Tapped Holes


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Bruce Mayer, PE


Pipe Threads

 Pipe Threads have An INTERFERENCE

TAPER that Results in PERMANENT Material

DEFOMATION That (Hopefully) Produces

24 a Fluid-Tight Seal at the Joint.



Pipe Thread Representations

 Taper Should be Shown

25

• Need Not be to Scale



Pipe 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

26

• Callout Requires Only the “NPT” Notation,

Along with the “Size”



Bolts, 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


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Bruce Mayer, PE


Machine Screw Head Types

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


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Bruce Mayer, PE


Threaded Inserts

 Use With Soft Parent Materials Such as

Aluminum, Wood, Plastic, etc.


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Bruce Mayer, PE


Insert 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


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Bruce Mayer, PE


Fastener Design References

 If you design with Threaded Fasteners; you MUST get access to These Books

 ISBN: 0-8311- 2711-2


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 http://www.mcmaster.com/

Bruce Mayer, PE



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Bruce Mayer, PE


Wheel 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


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Bruce Mayer, PE


Building 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


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Bruce Mayer, PE


All Done for Today

Fasteners

Make the

World Go

Round


35

Molded-In

Inserts for

Plastic Materials

Bolt-Failure by

Reversing-Load

Mechanical-

Fatigue

→ See ENGR45

Bruce Mayer, PE


Engr/Math/Physics 25

Appendix



2 x

3 

7 x

2  x



6 f

Bruce Mayer, PE

Licensed Electrical & Mechanical Engineer

BMayer@ChabotCollege.edu


36

Bruce Mayer, PE


Build-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

37

• Threaded_Fastener

_Forms_0509.dwg


6. Find SH Cap Form

10.

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

Chk Major dia at 1”

Bruce Mayer, PE



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)

17. Erase Simplifedform lines

18. Stretch shank line to 0.5 xline

19. Make xline on CL of Screw


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Bruce Mayer, PE


Early Result

 Just before trimming out unneeded thds


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Bruce Mayer, PE



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

25. Put all objects on

Bolt Layer

26. Adjust LtScale to

0.1


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Bruce Mayer, PE


Final 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


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Bruce Mayer, PE


ENGR-22_Lec-21_Fasteners

Engineering 22

Threaded

Fasteners

Skill-Development Goals

Skill-Development Goals cont.

Skill-Development Goals

Threaded Fasteners

Screw Thread Terminology

Common Tread forms

Thread Pitch

Unified (USA) Thread Series

UN Thread Series – 6 Total

UN Thread Series – cont

UN Thread Series – cont.2

Screw Fastener Specification

UN Thread Classes

UN Thread Classes cont

Metric Thread Specification

Left & Right Hand Treads

Detailed Thread Representation

Simplified & Schematic Forms

Representation Comparison

Threads in Section

Tapped Holes

Forming Tapped Holes

Pipe Threads

Pipe Thread Representations

Pipe Thread Representations

Bolts, Screws, and Studs

Machine Screw Head Types

Threaded Inserts

Insert Advantages

Fastener Design References

Wheel ReInvention

Building a Fastener

Engr/Math/Physics 25

Appendix

Build-Screw Demo - 1

Build-Screw Demo - 2

Early Result

Build-Screw Demo - 3

Final Result

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