Help reading Thread and drill charts
Reply with QuoteSimple answer: Yes.Originally Posted by cotang
Long answer, not that you need for a drawing, but at least you will know why the hole is bigger than the minor diameter.
The major diameter of a thread is the OD to to top of the crest of the thread; presumably you are dealing with standard unified thread from with a 60 degree thread angle. The flanks of the thread do not come to a sharp point they are blended into a top radius, the crest of the thread; similarly the bottom does not go to a sharp vee it also has the radius, the root of the thread. The size of these radii depend on the pitch and you can probably find it in Machinery's Handbook; for 18 tpi it is only going to be a few thousands of an inch.
The minor diameter is what you get when you subtract two times the height of the thread; that is the vertical distance from the crest to the root.
The tapdrill size is bigger that the root diameter so yes some of the thread is missing when you bore a hole this size and use a tap for the thread. You could bore a hole the exact size and use a full profile internal threading tool and then you would have a full thread to the minor diameter of 1.1193".
An 'oversize' hole is generally used for tapping because the driving torque needed for the tap gets very high if the tap is cutting from the true root diameter. In your example the tap would probably be strong enough to do it but with smaller size threads the tap will likely break. So the hole is made bigger than the minor diameter to reduce the load on the tap.
This does weaken the thread but only very slightly; a thread tapped into a hole that results in only 75% of the full thread height being present will have more than 90% of the strength of a full height thread and a 65% thread resulting from an even larger hole still has over 75% of full height strength. But the torque needed to drive the tap through the 75% hole is possibly less than half that need to tap a full height thread and the 65% may be down a half again. It is largely a matter of playing off the reduction in tapping torque with the reduction in thread strength, and to compensate for reduced thread height and strength it is always possible to increase the numbers of thread in engagement.
