The depth of drilled holes, when stated in note form, refers to the depth of the cylindrical portion and not to the point left by the drill. If no other indication is given they are assumed to go through the material. Holes in flanges or bosses are generally positioned around a pitch circle (PCD) and may be spaced on the main centre lines of the component (on centres) or as shown below equally spaced off centres. Holes are usually drilled off centres to provide for maximum access to fixing bolts in the case of valves and pipeline fittings. Special flanges need to have each hole positioned individually and an example is given with three tapped holes (see Fig. 14.25).
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