Q O Q o a


Fig. 54 Methods of dimensioning tapped holes.

from the centreline, and will (or should be) so interpreted by the driller whereas (b) is a more positive indication and the driller will take more care so should always be used in preference to (a) where space permits. At (c) the method of dimensioning indicates that the distance of the holes from the edge of the strip is more important than their spacing. Sketch (d) illustrates the common case of a row of holes. Method (i) is acceptable, but there is an implication that must not be forgotten. Unless set out with a jig borer the dimension 'x' may be up to 0.060 inch (or 1-1/2mm) oversize or undersize due to errors in marking out. This may not matter, but if the two end holes must line up with a set running at right angles, then the method shown at (ii) is preferred. Note that the spacing dimension is shown in brackets, as it is now a redundant dimension. Where the position of the holes is important throughout, then the co-ordinate method of (iii) should be used. If this is done the maximum error at 'x' will be very small.

Sketch (e) shows another application of a redundant dimension. It is a principle that where a series of dimensions is given the sum should not be shown as a separate figure. The whole length of the part can be obtained by adding the 1 inch and the 4 inch. However, it is also a rule that the machinist should not have to add things up, and he needs to do so in this case to decide how much stock to saw off. So, the total length in this case is given in brackets. If this overall length was important, then one of the others would be set in brackets and the 5 inch defined. Another application of a redundant dimension is shown at (f). Here the holes are positively located by coordinates. But the fact that they also lie on the arc of a circle is shown in brackets.

Tapped holes must often be dimensioned on the face of the workpiece, and this must be done with some thought, see Fig. 54. Where the depth of neither the thread nor the tapping drill matters (a) is adequate. Further, the thread is fully described by the letters BA. The depth of thread is specified at (b), leaving the driller to determine how deep the drill must go -this depends on the type of tap used. Note that the type of thread is fully defined. Normally 1/4 x 40 can be taken to imply the M.E. thread form, but overseas

Fig. 55 (a) and (b) Examples of unnecessary views and poor dimensioning practice, (c) A redundant view and duplicated dimension. Note also the impossible decimal dimensions on the hole centres.

2 HOLES re ME x 40

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