The drill guide is developed using 1/32" intervals. The decimal sizes and fractional sizes are listed.
The Threading guide is for tap drill sizes so the name is a bit misleading. Letter and Number drills have been listed as well as fractional.
Note that either of these guides may be used to guide any tap as long as it is a nice loose but not too loose fit in the hole. Don't worry about
the numbers just find a hole that is a close fit. The same goes for drilling so they can work for metric also.
After drilling the correct size tap drill hole in the material you are working on you can then use any of the
many available tapping guides to guide the tap straight into the straight hole you just drilled.
It is also possible to use either of the guides shown here to find a hole in one of the guides that will be quite close
to the OD of the tap to use it for the tapping guide. Just use with care so the holes in these guides are not enlarged.
I have made mine from aluminum but any handy metal or even plastic such UHMW should be fine.
A V-groove could be added to the bottom of either guide so it can be used on round stock but I left mine flat.
One other note. These are not a substitute for the tap/drill selection charts that come with your tap and die sets or that nice Starrett pocket card with all the tapping info on it.