I see that several other people answered the question you asked,
however as a suggestion. You may want to take a look at guiding
the tap off the shank and not off the threads. Several
commercial tap guides are available using this approach, and this
avoids problems with tap wear, limits [i.e.H2 v h3, etc.]

On 25 Jun 2005 06:11:03 -0700, wrote:

I am building a jig to drill and tap a series of holes. In operation,
the jig will have slip fit style drill bushings. I will drill the
pilot hole, then run a tap thru the hole. I was planning to use a
drill bushing that fits the thread cutting end of the tap, but don't
have all the taps that I am going to need yet, so I do not have a way
to check what size of drill bushing will fit the tap properly. The
sizes I will be using a
5x40 & 6x40.

Does anyone know of a formula, or real world answer to this question?
TIA, ron