The Home Machinist!

Not the first time I have run into this. I was doing this on the lathe.

I was making a thread in some aluminum 1/2 X 13 TPI according to the chart I should drill the hole to 27/64 .

All good so far, then I get the tap in about half way and OMG is it tight. get it threaded, and try a bolt and it's really tight too.

I have run into this problem with other tap sizes too.



As always look forward to the great input

Sandman

It sounds like you are using fairly dull hardware store grade carbon steel taps.

I believe that a new HSS tap and some tapping fluid would make a huge difference in your results.

Yes it is a hardware store tap, I did use cutting fluid.

Thanks

29/64 for 50% thread in iron and steel.
Any tapping chart worth referring to will tell you what % of thread the drill size will yield, going for 100% in steel isn't going to do your tooling any favours

27/64 is what the chart in the Machinery's Handbook says for a 75% thread. What grade of aluminum were you using and what was the wall thickness around the tapped hole? Aluminum is not known for spring-back but some of the tougher grades present all kinds of issues. WD-40 is a great lube for machining aluminum.
When I was writing programs and specifying tooling for a Cincinnati Milicron T-10 Horizontal mill, one of the things learned was that using a thread cutting tap in aluminum castings almost always broke the tap after a few holes. We switched to a thread forming tap and cruised on to success. Of course the thread forming taps require a bigger tap drill and quite a bit more torque to form the threads.

Switching to a new cutting tap that might be sharper may be your answer as was mentioned.
--earlgo

Are you rotating back to break the chip after every turn? Is this a blind hole?

I favor spiral point taps for through holes and spiral flute taps for blind holes. Both have cutting rake angles that are much more favorable than conventional hand taps. Neither requires backing up to break chips. Actually backing up is undesirable with these taps.