I screw up sometimes too. Twice...

[aametalmaster]

I have a simple project that i have redone twice and tomorrow will be the 3rd time. I am making a chuck backplate with an odd thread. No biggie as i have done backplates before. My first screw up was not using my thread gauge at the start of the thread. I have never been wrong before so why check? Because you are supposed to. Yup my threads were off 6 TPI and it wouldn't fit. My SBL needed a gear changed around to cut the bigger pitch threads and i missed that. So i cut plate #2 got my gear swapped and started threading. Yup checked the start with a thread gauge and it was perfect. About 2/3 of the way thru i saw the threads weren't going to be pointed and i am almost at full depth. Now what did i screw up? Checked all of my measurements and they were spot on. So i went back to the tap drill chart i have already used a zillion times before and yup the same measurement.
http://www.newmantools.com/tapdrill.htm#callink It says on the page to use .500 for 1/2" instead of a fraction so my threads are 2 1/4 so i used 2.250 Boy was that a mistake. If i punch in 2 1/4 6 it comes out 1.84 and if i use 2.250 6 it comes out 2.088 which i used and it was to big of a bore. Now if i had done it old school like i was taught and read my Starrett centering gauge and doubled the 6 tpi depth and subtracted the final OD it sure would have been closer to what the bore should have been. Oh well back to the lathe and welder and the lathe again...Bob