I need to tap a 1/4-20 hole roughly 2 inches deep for a slightly odd part. All of my 1/4-20 taps (spiral flute, spiral point, straight, etc.) are all threaded roughly 1.25" long. Where can I buy a tap with extra long flutes? What are they called?
You might want to look at "long reach" taps with undersized shanks --- that is, the threaded part goes in as far as you like because the shank behind it is smaller than the thread itself, so it's limited only by the overall length of the tap. One source I have is Travers, current catalog, page 290 (it's available online. travers.com; I have no connection to them, just happen to have their catalogue.
Another possibility (same catalogue page) is a long reach tap without the undercut if you need a two-inch threaded hole but not necessarily two inches of thread, if you follow me. Drill, say, the top inch of the hole as clearance diameter and just thread the bottom inch. Makes a lot less work if that works in your application and the tap is somewhat stronger. Often called a pully tap, as Jimsehr points out.
I'm thinking I might do an even easier thing and drill out the first inch of the hole with a LTR. F drill so the tap body can pass into the part. This will hold a set screw, so I don't really need the entire length threaded. Save me $30-50 on a specialty tap I won't need again for another 20 years.
But now if I need one, I know where to look. Thanks all!
You want a nut tap. The shank is smaller than the ID if the thread. One could tap a bunch of nuts, letting them accumulate on the shank then stop the machine, remove the tap and dump the nuts into a box. Pulley taps have a shank diameter equal to the OD of the thread.
Many of the catalogs I have seen have the taps incorrectly identified.