THREAD CUTTING IN THE LATHE

THREAD CUTTING IN THE LATHE

THREADS SIZES CHART

THREADS SIZES CHART

Useful British Thread Sizes
Size	Thread	TPI	Dia	Drill
BSC = British Standard Cycle Thread  -  Form 60°
1/8	BSC	40	0.125	0.098
5/32	BSC	32	0.156	0.123
3/16	BSC	32	0.188	0.154
7/32	BSC	26	0.219	0.178
1/4	BSC	26	0.250	0.209
9/32	BSC	26	0.281	0.240
5/16	BSC	26	0.313	0.272
3/8	BSC	26	0.375	0.334
7/16	BSC	26	0.438	0.397
1/2	BSC	26	0.500	0.459
9/16	BSC	26	0.563	0.522
5/8	BSC	26	0.625	0.584
11/16	BSC	26	0.688	0.647
3/4	BSC	26	0.750	0.709
7/16	BSC	20	0.438	0.384
1/2	BSC	20	0.500	0.447
9/16	BSC	20	0.563	0.509
5/8	BSC	20	0.625	0.572
11/16	BSC	20	0.688	0.634
3/4	BSC	20	0.750	0.697
BSF = British Standard Fine  -  Form 55°
3/16	BSF	32	0.188	0.148
7/32	BSF	28	0.219	0.173
1/4	BSF	26	0.250	0.201
5/16	BSF	22	0.313	0.254
3/8	BSF	20	0.375	0.311
7/16	BSF	18	0.438	0.366
1/2	BSF	16	0.500	0.420
9/16	BSF	16	0.563	0.483
5/8	BSF	14	0.625	0.534
3/4	BSF	12	0.750	0.643
7/8	BSF	11	0.875	0.759
SPARK = Spark Plug Thread
1/4	SPARK	24	0.250	0.196
3/8	SPARK	24	0.375	0.321
7/8	SPARK	18	0.875	0.803
UNF = Unified Fine  -  Form 60°
1/4	UNF	28	0.250	0.206
5/16	UNF	24	0.313	0.261
3/8	UNF	24	0.375	0.324
7/16	UNF	20	0.438	0.376
1/2	UNF	20	0.500	0.439
9/16	UNF	18	0.563	0.494
5/8	UNF	18	0.625	0.557
3/4	UNF	16	0.750	0.673
7/8	UNF	14	0.875	0.787

Nomenclature
Technical Terms:- Threads Per Inch, Pitch, Diameter, Form
Threads Per Inch (TPI), is the number of threads (Peaks) per inch length of thread. If you place a rule on a bolt and count the number of peaks (think of a thread as a mountain top or the top of a triangle) and you have the Threads per inch. The pitch is the distance between two of these peaks.
Coarse and Fine. Basically people invented threads to suit their manufacture needs. Bicycles and watch makers needed fine threads. Pipe fitters and Bridge makers needed coarse threads. You can therefore have a 1" diameter (25mm) bolt with a fine thread with 12  (TPI) or a coarse thread with 8 TPI. The coarse thread allows quicker assembly and will carry more pressure than the fine thread.
The other variable is the angle of the thread (Form).
There are numerous thread forms, named after by their inventors. BA (British Association), BSF (British Standard Fine), BSC (British Standard Cycle Thread), CEI (Cycle Engineers Institute), ISO (International Standards Organisation) Metric Coarse, Fine, and Special Pitch, UNF (Unified Fine Thread), UNC (Unified Coarse Thread), and Whitworth Threads.
You do not need to worry about it too much, unless you are a designer who has to determine the best thread to suit his need, or you are measuring a thread to find out what it is.

Thread Standards

Thread Standards:

The most frequently used thread standard is the metric ISO Thread. However, there are other thread standards as well. These are either based on foreign standards or are used for special applications. E.G. in the medical technology, in aeronautical engineering or astronautics. The most common threads are:
• ISO Thread (metric)
• Whitworth Thread
• Pipe Thread
• Trapezoidal Thread
• Knuckle Thread
• Buttress Thread

The thread abbreviation includes the thread code letter and the nominal thread diameter or the thread size. Additional values for pitch or TPI, tolerance, multi-lead, taper and left-handedness are added. Often threads that are according to DIN Standard have the major DIN number put in front of the thread abbreviation.

Talking of screws the major diameter for screws is determined by the thread tips.

The minor diameter by the groove of the thread.

The pitch diameter is the distance of two opposite flanks or the distance of the center line of the profile.

ISO Metric System

ISO Metric  System

Although there are two systems in use (Unified Thread Standard and ISO Metric) the thread itself is basically the same (60°). Difference is in the major and minor diameters and pitch.

The organizations concerned with these standards are for UTS ASME/ANSI and for the ISO Metric system the International Organization for Standardization.

Identifying

UTS type thread hardware is identified by its nominal diameter and by the pitch in tpi (threads per inch) and sometimes followed by the letter combination UNC, UNF or UNEF for the coarse, fine and extra fine pitch series.

ISO Metric hardware is coded the same way (diameter x pitch) but leaded by the letter M. For example: M5x10 which has by default a coarse pitch.

Tolerance Class

The UTS system uses a classification for interchangeability of fabricated nuts and bolts. Class 1 is loose fit and 3 is close tolerance. Class 2 is most commonly used. The class number is followed by an A for external or a B for internal threads.

Under load

When a bolt and nut are put under load they stretch a little and the distribution of the applied load will not be equal on allthreads. In fact, for a infinitely long bolt the first thread takes about 30%, the first three threads 75% and the first sixthreads take all the load.

So its not necessary to have more than six threads on a nut, which saves weight and money.

Preloading

Preloading or torquing a bolt and nut makes sure that the bolt does not fail from fatigue. Preloading is normally done by a torque wrench calibrated in inch/lbs or newton/meter but an experienced A&P or Lame can torque a bolt by feel. More info about torquing bolts and nuts.

NPT

For pipes and fittings mainly used in fuel and oil lines the National Pipe Thread, NPT, is used. With NPT, the thread (also 60°) is tapered under an angle of about 1.7899°.

By using a tapered thread a good seal is obtained where a straight thread is only able to hold the parts together. The taper makes sure that the flanks are also compressed together. For a leak free seal a teflon thread seal tape or sealant compound (Loctite®) is used, this also prevents corrosion and facilitates future removal should that be required.

Whit worth

British Standard Whitworth (BSW) has a thread angle of 55°, so these won't fit ISO metric or UTS ASME/ANSI hardware. Used mainly in the United Kingdom and in Australia, but ISO Metric is becoming more popular than the old Imperial units. ISO 7 also still defines BSW for British Standard Pipe Thread.

Pipe Threads

Pipe Threads There are several variations of pipe threads. Thread forms include the Dryseal American Standard Taper Pipe Thread (NPTF), the National Pipe Straight Threads for Mechanical Joints (NPSM) and the dryseal American Fuel Internal Straight Pipe Thread (NPSF). Each series is normally identified by the abbreviated letter designation. The NPTF thread is recommended for hydraulic service. This thread series is used on both male and female ends and forms a seal by the interference fit at the root and crest of the mating threads. The NPTF male also has an internal chamfer of 30° which allows it to seal at the seat with female NPSM pipe swivels. The NPSM is a straight pipe thread and sealing is obtained by the mating of the 30° seats of the male and female ends. 45° Flared Fittings (SAE): SAE flared fittings seal in the same manner as described for JIC, however the mating surfaces are machined at 45°. These couplings are usually used for low pressure applications such as refrigerant and fuel lines in conjunction with copper tubing which flares easily to 45°. O-Ring Boss | Straight Thread: The O-ring Boss fitting is a modification of the male JIC where the beveled 37° nose has been removed and a groove has been machined between the threads and the hex in which an O-ring is seated. The female is a port which has been tapped to the proper straight thread size and chamfered at the port face to provide an O-ring seat. The seal is made when the O-ring is trapped between port chamfer, thread under-cut, and the male. O-Ring Face Seal (OFS): The O-ring Face Seal uses an O-ring (SAE J515) in a groove on the male face side. This is mated to flat face without an O-ring on the female swivel side, forming a seal without metal deformation much the same as an SAE four bolt split flange connection without the bolts. The O-ring face seal connection has been tested to SAE J343 tests and procedures and meets or exceeds SAE J1453 specifications. This connector has proven its superiority in leak resistance, ease of installation, high operating pressure, increased torque tolerance and reusability with regard to its ability to be connected and disconnected frequently. SAE Threads 37° Flared Fittings (JIC): JIC (Joint Industry Conference) seals are obtained by the mating of two beveled metal surfaces. The function of the threads is simply to draw these two surfaces together. This style is used particularly in high pressure systems. As illustrated below, both the bevel on the male and the seat in the female are machined to 37°. NPTF 37° Flare 45° Flare O-Ring Boss O-Ring Face Seal Dash Fraction Pipe 37 Flare 45 Flare O-Ring Boss O-Ring Face Sear -2 1/8" 1/8 - 27 45428 45428 45428 * -3 3/16" * 3/8 - 24 3/8 - 24 3/8 - 24 * -4 1/4" 1/4 - 18 44028 44028 44032 43359 -5 5/16" * 1/2 - 20 1/2 - 20 1/2 - 20 * -6 3/8" 3/8 - 18 43359 43228 43359 11/16 - 16 -7 * * * 11/16 - 16 * * -8 1/2" 1/2 - 14 3/4 - 16 3/4 - 16 3/4 - 16 13/16 - 16 -10 5/8" * 41828 41828 41828 38000 -12 3/4" 3/4 - 14 11/16 - 12 11/16 - 14 11/16 - 12 13/16 - 12 -14 7/8" * 13/16 - 12 11/4 - 12 13/16 - 12 * -16 1" 1 - 111/2 15/16 - 12 13/8 - 12 15/16 - 12 17/16 - 12 -20 11/4" 11/4 - 111/2 15/8 - 12 * 15/8 - 12 111/16 - 12 -24 11/2" 11/2 - 111/2 17/8 - 12 * 17/8 - 12 38029 -32 2" 2 - 111/2 21/2 - 12 * 2 / - 12