Drilling (over-size) when tapping

[Cadillac STS]

Depending on what it is for you can drill a portion over size of the thread overall and just tap part of it.

for example instead of tapping 1 inch thread all the way through it may be enough thread to drill open enough to just need to tap 1/2 inch of it.

 

[mattthemuppet2]

I keep a copy of the same chart from Little Machine Shop to find recommended drill sizes. I just tried tapping my first threads and broke the tap - it was a #1-72 tap. I looked at the chart and I realized I used the drill size for 70% instead of 50%. That was my first mistake. My second mistake was only buying one tap (from McMaster-Carr) instead of two. I should have figured I might break it being my first attempt. Grrr.
Now I have to buy another tap, but this time I'll buy an extra for a backup. Ah well.
I envy you guys who don't even need charts.

bummer, sorry. There's not much difference between "this is a little tight" and "snap!" with really small taps. Hope it wasn't a pain to get out.

I've gone the other way before, drilled oversize by accident then had to step up to the next size (eg. 4-40 to 5-40). That chart is very useful though. I have the metric version on the other side, stuck together and laminated. I use it all the time.

 

[pontiac428]

We used to have a series of out-of-print books in our library called the Workshop Practice Series on pdf. Unfortunately, one member who seems to be an independent iPhone developer (and possibly not a very successful one by the copyright rage he lives with) did not agree with the Google paper trail that shows that this book series is all over the web in pdf and out of print, meaning even the used paper copies that exist deprive the author of revenue, meaning in his opinion a used paper copy was a pirate copy too. I had to say it. So, if you have a copy of Screw cutting in the Lathe by Martin Cleeve (#3 in series), that's another resource for learning how to calculate and cut the right fit factors into your threads.

Edit-
You know what, screw that guy. I've taken a copy, watermarked by a free site, and uploaded it to dropbox for anyone who wants it.
clicky linky for dropbox