2 HOLES TO 8Un " DOWELS
machinists may not realise this; so, the letters ME are included. Similarly at (c). In this case the depth is stated in terms of the number of fully formed threads required in the hole whereas (d) takes matters rather further. The designer requires a definite depth of thread engagement and has stated the tapping drill size. (This is good practice anyway as the driller should not be expected to look up his tables every time.) the maximum depth of drill is defined - for example to prevent fouling another hole -
and the depth is given again in terms of the number of full threads (which will be less than the number of turns on the tap-wrench if the tap is not a bottoming type). Finally, at (e) we have a through hole, and the depth of tapping is given as a minimum required to ensure adequate design strength.
Of these methods, (a) is very common, but (b) or (c) are to be preferred. On drawings made for my own use I always show the tapping drill, to save time in the shop, and generally state depth in terms of number of threads. This saves walking across the shop to look at the chart and it is easier to count turns than to lay a rule on the tap.
Finally, Fig. 55 shows three examples taken from published drawings that will bear examination. On the left of (a) is the drawing as supplied, though I have added a line that was missing. There is little to fault on the dimensioning - as I have redrawn it the rules are followed. However, I show on the right the preferred method; this saves a view, the centre distance that relates to two views lies between them and it has taken the draughtsman less time to make. I would accept a criticism of my alternative, in that the note to the two tapped holes could be better placed - as it was on the original, in fact. My fault - that is the proper place, but I did not leave enough room on the paper.
Example (b) is the classic case of the redundant view. The inverted plan tells us nothing except that the object is round and this is clear from the use of the symbol for diameter. The sketch on the right shows the proper way. Note that there is no 0 on the 1/4 inch ream dimension; unnecessary as you cannot ream anything but a diameter.
At (c) we have not only a redundant view, but also a duplicated dimension. The top right - hand view gives us no information which is not provided by the other views. You may notice some other slips, but I leave you to find them for yourself. (Holes B are drilled.)
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