Engr 22 lec-21-sp07_fasteners (1)

[email protected] • ENGR-22_Lec-20__Fasteners-1_Specs.ppt1

Bruce Mayer, PE Engineering 22 – Engineering Design Graphics

Bruce Mayer, PERegistered Electrical & Mechanical Engineer

[email protected]

Engineering 22

ThreadedThreadedFastenersFasteners



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 EitherA 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 ClassesA. Refers to EXTERNAL Threads

(Bolts & Screws)

B. Refers to INTERNAL Threads (Nuts & “Tapped” holes)

Class Descriptions1. 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 Treads 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



Detailed Thread Representation 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



Simplified & Schematic Forms 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



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 Section Threads 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/

http://www.amazon.com/gp/product/images/0831127007/ref=dp_image_0/104-4698202-6479938?%5Fencoding=UTF8&n=283155&s=books



Mach

-Hd

Bk,

Mach

-Hd

Bk,

23red E

d P

age

23red E

d P

age

1325(of 2511)

1325(of 2511)



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

FastenersMake theWorld Go

Round

Molded-In Inserts for

Plastic Materials

Bolt-Failure by Reversing-Load

Mechanical-Fatigue

→ See ENGR45

http://www.yardleyproducts.com/mold.htm

http://www.boltscience.com/images/boltf1.jpg



Bruce Mayer, PELicensed Electrical & Mechanical Engineer

[email protected]

Engr/Math/Physics 25

AppendixAppendix( ) 6972 23 −+−= xxxxf



Build-Screw Demo - 1 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• 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”



Build-Screw Demo - 2Build-Screw Demo - 2

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



Build-Screw Demo - 3Build-Screw Demo - 3

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