I was recently asked: The ThreadTech Software program says that
the pitch diameter tolerance for a #10-32 UNF-3B STI Screw Thread
Plug Gage is 0.0001" but the STI Screw Thread Plug Gages we
own were certified at 0.0003". Why is this?
This is an interesting question because it brings to light a disparity
between the STANDARDS and the REAL (screw thread gage) WORLD.
The STI screw thread plug gage tolerance was established quite
some time back by Emhart Teknologies for their Helicoil screw thread
insert product line. They decided to use Class W for the STI screw
thread plug gages, but keep the gage information proprietary so
that they could be the sole source for STI screw thread plug gages.
The thing about the STI screw thread plug gages is that they are
not all that unique, and any screw thread gage maker could make
them if he knew the screw thread pitch diameters.
So what happened was that as screw thread gage makers discovered
the screw thread pitch diameters for the screw thread insert requirements
they began making the STI screw thread plug gages using the Class
X tolerance. They did this because that is what ALL screw thread
working plug gages use as a tolerance. Eventually the proprietary
Helicoil screw thread insert requirements became known and published.
The official requirement for the STI screw thread plug gages was
set at Class W. In conflict to the requirements of the standard
the screw thread gage makers continued to make the STI screw thread
plug gages using Class X tolerance. I can only guess at the thought
process. Maybe the the screw thread gage makers realized that they
had been making the STI screw thread plug gages as Class X for years
and no one complained. They saw that the STI screw thread plug gages
seemed to work just fine in application. Alternately the screw thread
gage makers may have considered the cost of making the STI screw
thread plug gages with the extra tight tolerance and rejected the
concept immediately. I know of only one screw thread gage maker
who made the gages to the standard, but to remain competitive in
the STI screw thread plug gage marketplace they recently discontinued
the practice. Today there is no STI screw thread plug gage maker
that I know of that is making STI screw thread plug gages to the
Class W tolerance as standard product. If you are willing to pay
the extra to get the Class W tolerance, the STI screw thread plug
gages are available as a special make item.
This has been going on for many years and it appears that there
is no significant issue that has resulted from the change from Class
W to Class X. The primary result of the change is lower cost STI
screw thread plug gages. A possible secondary benefit is that, depending
on where the STI screw thread gage falls in the Class X tolerance
spectrum, it may have longer life before reaching the point of being
worn-out. I do not expect a change in the standard anytime soon.
These changes take years to incorporate into the official standards
and usually require some compelling pressure to make them happen.
I do not see anyone applying that pressure to make this change official,
so it will really take a long time of occur.
There is only one small dilemma; this question may arise from
time to time when someone takes the time to actually read and understand
the standard, or when they use the ThreadTech and notice the discrepancy.
Know that the ThreadTech software will not change until the standard
changes, because the ThreadTech software must report the thread
data as specified by the standard.
Initial publication date: 7/15/09
This data is accurate to the best of my knowledge as of 9/2/09.
Send comments or corrections to Wayne at:email@example.com