Screw Thread Identification

Why use Threaded Fasteners? Screws and Bolts are used in engineering to mechanically attach (assemble) two or more parts together. These are non permanent joints which allow for repeated dismantling and assembly

History of Screw Threads 400BC- First used on presses for the extraction of juice from grapes 212 BC- Archimedes developed the screw principle and used it to construct devices to raise water. 1750- Antoine Thiout, equipped a lathe with a screw drive. 1770- Jesse Ramsden made the first screw-cutting lathe. 1860- Joseph Whitworth standardised: The angle the thread flanks should be 55 degrees. The number of threads per inch should be standardised for various diameters

1864 in America, William Sellers independently proposed another standard based upon a 60 degree thread form and various thread pitches for different diameters. This became adopted as the U.S. Standard and subsequently developed into the American Standard Coarse Series (NC) and the Fine Series (NF). The thread form had flat roots and crests that made the screw easier to make than the Whitworth standard that has rounded roots and crests. Around the same time metric thread standards were being adopted in continental Europe with a number of different thread flank angles being adopted. The standard international metric thread eventually evolved from German and French metric standards being based upon a 60 degree flank angle with flat crests and rounded roots.

Threads Today Today there are a large number of threaded fasteners available on the market and it important that the correct fastener is selected for a given application. Selecting an incorrect fastener can affect safety or an increase in cost to the component or fabrication Today most countries have converted to, or are converting to the Metric system. This hasaided standardisation of threads used, however in older machinery other thread types may still be used including: BSW (British Standard Whitworth) BSP (British Standard Pipe) UNF (Unified National Fine) UNC (Unified National Course) BA (British Association) BSF (British Standard Fine) Because of these variations it is important that you can identify the different types, forms and sizes of screw threads

Identifying thread types and sizes Step 1 Visually determine the thread form The Vee form is the most common form. Used for fastening and sealing, they are strong and relatively easy to produce. Production methods include- milling, grinding, rolling, taps and dies, as well as screw cutting on the lathe

Terminology The Major Diameter of the thread is the maximum diameter measured over the tops of the threads. Also known as the nominal diameter, i.e. the diameter by which it is known and specified The Pitch (simple effective) diameter is the diameter at which the pitch of the thread is measured. The Minor Diameter is the diameter of an imaginary cylinder that just touches the roots of an external thread and (or) the crests of an internal thread The Crest is the prominent part of a thread, whether internal or external. The Root is the bottom of the groove between the two flanking surfaces of the thread whether internal or external The pitch of a thread is the distance, measured parallel to its axis, between corresponding points on adjacent surfaces, in the same axial plane The flanks of a thread are the straight sides that connect the crest and the root The angle of a thread is the angle between the flanks, measured in an axial plane section.

Square Form This form is used for power/force transmission i.e. linear jacks, clamps.  The friction is low and there is no radial forces imposed on the mating nuts. The square thread is the most efficient conventional power screw form. It is the most difficult form to machine.   

Buttress Form A strong low friction thread.  However it is designed only to take large loads in one direction.  For a given size this is the strongest of the thread forms. When taking heavy loads on the near vertical thread face this thread is almost as efficient as a square thread form. Used on quick release vices

Acme Form Used for power transmission i.e. lathe lead screws.  Is easier to manufacture compared to a square thread.  It has superior root strength characteristics compared to a square thread.  The acme screw thread has been developed for machine tool drives.  

By far the most common form used in Europe today is the Metric thread form Metric threads are designated by the letter M followed by the nominal major diameter of the thread and the pitch in millimeters. For example M10 x 1.0 indicates that the major diameter of the thread is 10mm and the pitch is 1.0mm. The absence of a pitch value indicates that a coarse thread is specified. For example stating that a thread is M10 indicates a coarse thread series is specified of diameter 10mm (giving the thread a pitch of 1.5mm).

Types Sir Joseph Whitworth proposed this thread in 1841 . This was the first standardised thread form. The principal features of the British Standard Whitworth (BSW) thread form are that the angle between the thread flanks is 55 degrees and the thread has radii at both the roots and the crests of the thread. The thread form is now redundant and has been replaced by Unified and Metric threads. The British Standard Fine (BSF) thread has the same profile as the BSW thread form but was used when a finer pitch was required for a given diameter.

This thread is used for small diameter threads (below 0 This thread is used for small diameter threads (below 0.25 inches diameter). The thread has radiused roots and crests and has a flank angle of 47.5 degrees. The thread size varies from BA number 23 (0.33 mm diameter with a pitch of 0.09 mm) to BA number 0 (6mm diameter with a pitch of 1 mm). The thread form is now redundant and has been replaced by Unified and Metric threads.

Unified thread (UNF,UNC) In November 1948 the Unified thread was agreed upon by the UK, the US and Canada to be used as the single standard for all countries using inch units. These forms are still used in the US hence unified threads are often used in automotive and aerospace industries

Identifying thread types and sizes Step 1 Using a micrometer or vernier determine the diameter and determine if the thread is imperial or Metric. This may be difficult for some sizes, For example 5/16" & 8mm are very close together

Thread Size and Type Step 2 Determine the thread size and type – is it metric or one of the many imperial threads? You will need to determine the threads major diameter and decide if its metric or imperial- remember imperial sizes are normally common fractions – ¼” = 0.250” Next you will need to determine either the pitch (metric) or the amount of threads per inch (T.P.I) for imperial sizes

Identifying thread types and sizes Step 3 Next step is to determine the pitch – if the diameter is metric, this will be a measurement between threads in mm (Pitch=0.75mm) If the diameter is Imperial – you will need to count the number of threads per inch (T.P.I) this will determine the type of thread i.e bsw, unf, bsf etc In both cases Thread pitch gauges can be used, If these are not available Taps, or known existing threads can be used

Identifying thread types and sizes Step 4 Cross reference the Data you have just found with screw thread charts (Zeus Book) If this data matches more than one type you may need to determine the flank angle. It can be very difficult to establish what the thread angle is, but easy to state what it is not. For example if is bigger than 47½° & smaller than 60° it is almost certainly 55° and so on.

Thread classification Metric M10 x 1.5

Thread classification Imperial ¼ B.S.F

Exercise 1. m8 2. m16 3/8 BSW 3/8 BSF