When only one of two parallel surfaces needs to be ground, try to arrange for the non-machined surface to lie below the other, in order to provide tool clearance.

(k) Effective Tapping

The first suggestion with regard to tapping is: Don't! It is easier, safer and less expensive to use a nut and bolt, when feasible.

Blind tapped holes should not be threaded all the way (a). A less expensive design, which also reduces the probability of tool breakage, is to leave an untapped portion, the length of which should not be less than the O.D. of the thread (b).

In the case of a partially tapped bore, the diameter of the smooth portion should, if possible, be equal to the internal diameter of the threads (c). Design (a) requires either two setups, or a tap, the unsupported length of which is larger than necessary. Design (b) could involve one setup (drilling from the bottom followed by tapping), but the unsupported length of the tap is larger than in (c), in which the machining proceeds from the top. Since tapping is a more critical operation than boring, the tapping requires the most favorable circumstances possible.

In tapping from a square shoulder (a), there is more chance of material breakage at entry than with a slight countersunk surface as in (b). The countersink also provides better bearing conditions for the nut or bolt, which is added during assembly.