The majority of charts quoting drill sizes for tapping holes give just one drill
size and no more. This gives no indication to the user as to the percentage thread
depth that is being achieved or the result of moving up or down a drill size, a move
that have very little effect at coarser pitches but a major effect with finer pitches.
The novice metalworker perhaps may even gain the impression that there is just one
acceptable hole size and this is when a full form thread is being produced.
Typically, for an M6 thread, ignoring metal extrusion, a hole size of 4.77 mm results
in a 100% thread depth. Many lists quote 5mm (82% thread depth) indicating that it
is standard practice not to attempt to produce a full form thread. What then are
the benefits of not producing a full thread, well a much reduced likelihood of a
tap being broken and in the home workshop where hand tapping is invariably employed
this is a very major advantage especially at smaller sizes.
The reader may though be concerned regarding a reduction in the strength of the resulting
assembly. Providing the mating thread is up to size and well made, a situation that
is easily achieved, then even at 70% thread depth the loss of strength is very small
but with an appreciable reduction in the torque required to turn the tap.
With these factors in mind a starting point is to attempt a thread depth of between
60 and 70 %, actual values depending on the drill sizes available. If these figures
come as a surprise then note that Tubal Cain in his book "Model Engineer's Handbook"
quotes an even lower figure of 50 to 60% for most materials. If you would like more
detail on the subject then Tubal Cain goes into greater depth in his book.
Factors that should be born in mind when choosing the drill sizes are.
1 Tend towards the larger thread depths when tapping into thin materials, say less
than three thread pitches.
2 Ere towards a lower thread depth for the smaller threads and visa versa
3 Work at 70% to 75% when tapping into cast iron
4 At finer pitches, usually at the smaller diameters, the percentage thread depth
varies considerably with a small change in hole size and because of this I have chosen
drills in increments of 0.05mm at the smaller diameters.
5 Because of the effect of hole size at small diameters it is important that the
drill being used produces an accurate hole size, to be sure of this, always use
drills as supplied, not re-sharpened.
6 Where strength is vitally important work between 70 and 75% bearing in mind that
any increase in strength beyond that is minimal.
I have attempted to chose drills from the standard drill set sizes, that is in increments
of 0.1mm, choosing intermediate 0.05mm sizes at the smaller thread diameters. The
reader may like to consider using intermediate sizes for some of the larger diameters,
particularly in the case of the model engineers 40 tpi series. In all cases only
purchase the 0.05mm increment drills that you require and retain these for thread
tapping uses only, as I suggest above.
Tubal Cain's book "Model Engineer's Handbook" is published by Special Interest Model
Books, the publisher of the Workshop Practice series.
These quote three drill sizes for each thread listed and have been chosen with the
above factors in mind, typically, reduced thread depths for smaller sizes and visa