As a reference guide, the diagram below "selection/identification of the tap" gives a general overview of tapping. With reference to it, we are going to give a brief review of the main factors involved. / THREADING This part is divided into 3 fundamental areas or sections: • Characteristicsofthethread • Characteristicsofthehole • Characteristicsofthematerialtobeworked 1.50: This is the thread pitch or distance between two adja- cent profiles expressed in mm. For threads of the ANSI (USA) and BS Anglo-Saxon systems, this information is expressed as the number of threads or profiles per inch (16 threads, 25.40/16 =1.5875 mm). This is a highly critical parameter for the precision of the threads and conse- quently it is subjected to severe tolerance demands for both a single thread and accumulative dimension over successive pitches along the threaded length. These tole- rances are fixed in each one of the Standards and gene- 1) Characteristics of the thread As an initial premise, it is essential to clearly identify the basic characteristics of the thread to be machined: thread class, measurement, pitch and tolerance. For explanatory purposes, we are going to take an M 10 x 1.50 6H thread as a reference example. rally the maximum permitted error is approximately M: this information refers to the thread class. In this case it 0.10%. is a metric thread, regulated by the DIN-ISO Standard. There are four main standards throughout the world: • DIN-ISO: the European system predominant in Europe The thread-tap binomial and in constant growth in the rest of the world. • ANSI: USA System predominant in North, Central and 6H: This refers to the diameter tolerance of the internal thread in the DIN-ISO system. In combination with the tolerance of the external thread, it determines the level of fit between them. South America. • JIS:Japanesesystem,predominantinAsia. • BSI: British system, limited to the area of Anglo-Saxon This is the Medium class (6H nut - 6g bolt) of the three standard qualities, Fine, Medium and Coarse, it is unders- tood as the normal standard tolerance unless another quality is expressly indicated. The classification of toleran- ces in three categories - Fine, Medium and Coarse - is a criterion common to all standards. influence. Fundamentals of machine tapping All this results in there being a large number of types of threads whose relationship can be consulted in the DIN 202 standard. Thread direction (right-hand or left-hand): As a standard criterion it is understood that the thread is right handed: screwed in in a clockwise direction, seen from the bolt head. The left-hand option (opposite to right-hand) is of marginal use and is expressly indicated and receives specific treatment. 10: This information refers to the nominal measurement of the thread in mm. Here, in the ANSI (USA) and BSI (British) systems, this information is expressed in fractions or decimals of an inch (for example: 3/8" equivalent to 9.525 mm). In fact, the threads have three diameter measurements: • Major diameter (or nominal diameter): this is measured on the crests of the threads and its criticality/demand levels with respect to accuracy are less than those of the pitch diameter. • Pitchdiameter:thisismeasuredonthevirtuallinewhich Tap longitudinally intersects the midpoint of the thread profile, dividing the pitch exactly in two, therefore creating identical magnitudes between the solid and empty part of the thread profile. This is the determining parameter in the tolerance of the threads and therefore its demand level is the highest of the 3. • Minordiameter:thisismeasuredattherootofthethread and its criticality/demand levels are notably less than the other two parameters. pitch diameter D2 min = Min pitch diameter. / 39 D = Nominal measurement D1 max = Max root diameter D1 min = Min root diameter D1 max = Max D D máx. 2a D máx. D máx. 11 D mín. Minor diameter Pitch diameter Major diameter