wrote in news:1113334475.276037.242700
@z14g2000cwz.googlegroups.com:

I have been pulling my hair out trying to tap holes to get decent
threads. Instead they are always sloppy and oversized.

I use good technique.....tap absolutely square to the hole;
I use a good cutting oil; and I back the tap out to clear
the chips; etc.

But the fact remains that a 1/4-20 tap measures .255 diameter
while a 1/4-20 bolt measures .244 diameter (threaded portion)

That is .011 difference!!! I have checked my other taps and they are
all oversized.

How can this be and is there any solution?

..244" is within the major diameter tolerance for a 1/4-20 thread. Your tap
appears to be in tolerance as well. As others have pointed out it's the
pitch diameter that counts. Bolts are always going to be somewhat on the
low end of the allowable tolerance for the major diameter. The reason is
that the bolts are thread rolled. Rolling a full form greatly reduces
rolling die life and doesn't really increase the strength of the thread. As
far as your threaded connection goes the same thing applies. It's unlikely
that having a greater percentage of engagement will increase the strength
of the connection enough to warrant the extra headaches that come with
making tight fitting threads. Here is a link:
http://www.precisiontwistdrill.com/t..._of_drills.pdf
If you are using the thread for adjustment rather than fastening and want a
tight fit, you would be better off using an external thread that you cut
yourself. If you are trying to use the thread for locating as well as
fastening, you would be better off designing your part with a pin or key
for locating.

--

Dan