Bruce Mayer, PE Registered Electrical & Mechanical Engineer BMayer@ChabotCollege.edu

1. Engineering 22 ThreadedFasteners Bruce Mayer, PE Registered Electrical & Mechanical EngineerBMayer@ChabotCollege.edu

2. 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

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

4. 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 Representations

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

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

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

8. Thread Pitch • Metric → Pitch in the Thread Designation • USA → Pitch is Inverse of Thread Count

9. 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

10. UN Thread Series – 6 Total • COARSE Series (UNC or NC) • For General Use Where Rapid Assembly is Required • Threads Engage, or “Start” Easily • 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

11. UN Thread Series – cont • EXTRA FINE Series (UNEF or NEF) • For Highly Stressed Parts • 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.

12. UN Thread Series – cont.2 • 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. • 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.

13. Screw 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

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

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

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

17. 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

18. 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

19. Best Overall Simplified & Schematic Forms • 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

20. 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

21. Threads in Section • Normal Practice is to NOT Section the Fastener

22. 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 • For Light-Duty applications, can use 1.5

23. Forming Tapped Holes

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

25. Pipe Thread Representations • Taper Should be Shown • Need Not be to Scale

26. 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 • Callout Requires Only the “NPT” Notation, Along with the “Size”

27. 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

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

29. Threaded Inserts • Use With Soft Parent Materials Such as Aluminum, Wood, Plastic, etc.

30. 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

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

32. Mach-HdBk, 23red Ed Page 1325(of 2511)

33. 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

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

35. All Done for Today FastenersMake theWorld GoRound Molded-In Inserts for Plastic Materials Bolt-Failure by Reversing-Load Mechanical-Fatigue→ See ENGR45

36. Engr/Math/Physics 25 Appendix  Time For Live Demo Bruce Mayer, PE Licensed Electrical & Mechanical EngineerBMayer@ChabotCollege.edu

37. Open File New File using Acad.dwg Make Layer “Bolt” Continuous & GRN Save As 38-16_125_shcap.dwg Set Snap to 0.1 Open Fastener Forms file Threaded_Fastener_Forms_0509.dwg Find SH Cap Form and Copy to ClipBd Paste SHcap Form into new dwg Return to Forms and Copy Detailed Thread Rep Paste detailed Thd-Form into new-dwg Chk Major dia at 1” Build-Screw Demo - 1

38. Scale all by Ratio of 1:0.375 Move TopV of Screw Head Rotate Detailed Thd-form by 90° Explode screw Side view Make Xline at base of Scr Hd Offset xline to rt by 1.25 (scr length) 0.5 (unthd’s shank) Erase Simplifed-form lines Stretch shank line to 0.5 xline Make xline on CL of Screw Build-Screw Demo - 2

39. Early Result • Just before trimming out unneeded thds

40. Trim & Erase unused SideV line to Rt of 0.5 xLine Erase Unneeded Thd-details Connect UnThd’s Shank to Thd’d Major Dia. With Fillet Move TopV of Cap Scr to Lt of SideV Delete xLine Detailed Thd-Form Put all objects on Bolt Layer Adjust LtScale to 0.1 Build-Screw Demo - 3

41. 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