1. Lecture Prepared by: Gp Capt Dr. Hamid Ullah Khan Niazi

2. Learning Objectives After completing this unit you should be able to:
Recognize and state the purpose of six common thread forms.
Set up a lathe to cut inch external unified threads.
Set up an inch lathe to cut metric threads.
Set up a lathe and cut internal threads.
Set up a lathe and cut external Acme threads

3. Introduction-Definition
Thread may be defined as a helical ridge of uniform section formed on the inside and outside of a cylinder or cone.
Threads have been in use for more than 100 years for holding parts together making adjustments and for transmitting power and motion.
Threads are mass produced by taps and dies, thread rolling, thread milling and grinding to exacting standards.
PURPOSES: 1. To fasten devices
2. To provide accurate measurement
3. To transmit motion

4. Thread Terminology

5. Explanation of Different parts of Thread
External Thread: Threads made on the outside of a cylinder or cone.
Internal Threads: Threads made on the inside of a cylinder or cone.
Major Diameter: The largest diameter of an external or internal thread.
Minor Diameter: The smallest diameter of an internal or external thread.
Pitch Diameter : The pitch diameter is equal to the major diameter minus a single depth of thread.
TPI: The number of threads per inch is the number of crests or roots per inch of threaded section.
Pitch: The distance from a point on one thread to a corresponding point on the next thread measured parallel to axis.
Lead: It is the distance a screw thread advances axially in one revolution.
Root: The bottom surface joining the sides of two adjacent threads.
Crest: it is the top surface on a thread
Flank : It is a thread surface which connects the crest with root.
Depth of thread: It is the distance between the crest and the root measured perpendicular to the axis.
Helix Angle: It is the angle which the thread makes with a plane perpendicular to the thread axis.
Right Hand Thread: A helical ridge of uniform cross-section onto which a nut is threaded in a clock wise direction. On a lathe machine, the tool bit advances from right to left.
Left Hand Thread: On this the nut is threaded counterclockwise and the tool bit advances from left to right.

6. Difference Between Left Hand And Right Hand Thread

7. Types of Thread
The American National Standard Thread: It is divided into four main series, all having the same shape and proportions. National Coarse (NC), National Fine (NF), National Special (NS) and National Pipe threads (NPT). This thread has a 60 degree angle with the crest and root truncated to 1/8th the pitch. It is used in fabrication, machine construction and assembly and for components where easy assembly is desired.

8. Types of Thread-Continued
British Standard Whit Worth Thread: It has a 55o –V form with rounded crests and roots. This thread application is the same as for the American National form thread.

9. Unified Thread: The unified thread is a combination of the British standard whit worth and American National form thread. This thread has a 60o angle with a rounded root and the crest may be rounded or flat.
Unified Thread

10. The American National Acme Thread: It is replacing the square thread in many cases. It has a 29o angle and is used for jacks and vises etc.
American National Acme Thread

11. Brown And Sharpe Worm Thread
Brown and Sharpe Worm Thread: It has thread angle of 29o, depth is greater, width of the crest and root are different. The self-locking feature makes it adaptable to winches and steering mechanism.
Brown And Sharpe Worm Thread
Winches

12. Square Thread: It is being frequently replaced by the Acme Thread due to the difficulty of cutting it, particularly with taps and dies. Square threads were often found on vises and jacks.
Square Thread

13. International Metric Thread : It is used as a standardized thread used in Europe. It has thread angle of 60o with a crest and root truncated to 1/8th the depth.
International Metric Thread

14. Thread Fits and Classification
Fit: It is the relationship between two joining parts determined by the amount of clearance or interference.
Allowance: It is the intentional difference in size of the joining parts or the minimum difference between the joining parts.
Tolerance: It is the variation permitted in part size and may be expressed as plus or minus or both. In Unified and National system, the tolerance is plus for external threads and minus for internal threads.
Nominal Size: It is the designation used to identify the size of the part.
Actual Size: It is the measured size of a thread or part.
Basic Size : It is the size from which the tolerance levels are set that is the upper natural tolerance level and lower natural tolerance level.

15. BASIC HOLE SYSTEM
The basic hole system is a system of fits in which the design of the hole is the system basic size and the allowance applies to the shaft. When specifying the tolerances for a hole and cylinder and determining their dimensions, you should begin calculating by assuming either the minimum (smallest) hole or the maximum (largest) shaft size if they are to fit together well. Figure 1 illustrates the basic hole system

16. In the illustration, the minimum hole size is the basic size
In the illustration, the minimum hole size is the basic size. To calculate the maximum diameter of the shaft, assume an allowance of .003 inch and subtract that from the basic hole size. Arbitrarily selecting a tolerance of .002 inch, apply the tolerance to both the hole and the shaft.    This give a maximum hole (1.502 inches) and minimum shaft  (1.495  inches).  The  minimum  clearance  fit  is  the  difference  between the smallest hole (1.500 inches) and the largest shaft (1.497 inches) or .003 inch.   The maximum clearance fit is the difference between the largest hole (1.502 inches) and the smallest shaft (1.495 inches) or .007 inch. Determine the maximum shaft size of an interference fit by adding the allowance (.003 inch) to the basic hole size (1.500 inches) or inches. To convert basic hole size to basic shaft size, subtract the allowance for a clearance fit or add it for an interference fit.

17. Isometric Tolerances and Allowances
Tolerance Grade: A medium tolerance, used on a general-purpose thread is indicated by the number 6. Any number above 6 indicates thread with greater tolerance and any number below 6 indicates fine tolerance.
Allowance: Symbols are used to indicate the allowance for external threads and internal threads. Capital letters are used for internal threads and small letters are used for external threads such as shown below.

18. Example
Metric threads are represented in the following example

19. Unified Threads
Unified threads have been classified into three categories and the
applications of each have been defined by the Screw Thread
Committee. External threads are classified as 1A,2A,3A and Internal
threads are classified as 1B,2B,3B.
Classes 1A,1B: They are those threads for work which must be readily
assembled. They have loosest fit, with no possibility of interference
between the joining external and internal threads when the threads are
dirty or bruised.
Classes 2A,2B: They are used for most of commercial fasteners. These
threads provide a medium or free fit and permit power wrenching with
minimum galling and seizure.
Classes 3A,3B: They are used where most accurate fit and lead are
required. No allowance is provided and the tolerances are 75% of those
used for Class 2A and 2B fits.

20. Thread Calculations
To cut a correct thread on a lathe its important to calculate all the thread dimensions properly. See example.

21. Cutting Threads with Quick Change Gear Box

22. Quick Change Gear Box

23. Thread Chasing Dial
A thread Chasing Dial is used on Modern lathes for accurate “picking up” of the thread.
It also helps in establishing the relationship between the cutting and the work piece during thread cutting operation.
It also indicates when to engage the split nut or the half nut with the lead screw in order for the cutting tool to follow the previously cut groove.

24. Thread Cutting Using Dial Indicator

25. Measuring Threads
Threads may be measured by a variety of methods, the most common are :
A thread ring gage
A thread plug gage
A thread snap gage
A thread comparator micrometer
An optical comparator
The three-wire method

26. Multiple Threads
Multiple threads are used when it is necessary to obtain an increase in lead and a deep, coarse thread cannot be cut.
Multiple threads may be double, triple or quadruple, depending on the number of starts around the periphery of the work piece.

27. Difference Between Multiple start threads and single start threads
In case of a single start thread there is a movement of only one thread but in case of multiple threads let say a three start thread, a thread advances three times as far as if it was a single start thread.
The independent threads are called STARTS.

28. Assignment Solve problems on Thread Calculations and submit next week.
Start working on your final project of IE and discuss your ideas.

29. Summary
Thread cutting is a skill that all engineers should possess because it is still necessary to cut threads on an engine lathe, especially if a special size or form of thread is required.

30. Questions and Answers?