"John Fields" wrote in message
...
On Sat, 10 Oct 2009 10:11:16 -0700, Jim Thompson
wrote:

1/4"-20 tap calls for 0.201" tap drill (13/64")

Into a steel 1/8" panel, tap is hard enough to turn that handles flex
:-(

Is that normal, or do I need to go up one or more drill sizes?

---
IME it's not normal.

As someone else suggested, make sure your tap is sharp and use lube.

If you haven't done any tapping in a while, it may be that the tap isn't
being held normal to the panel when it starts biting into the metal,
which can cause it to bind up.

In order to make that go away, get or make a "tapping block" which is
nothing more than a block with a 1/4" hole drilled through it
perpendicular to the face being drilled, hold it against the panel,
insert the tap into the hole and start the tap into the panel that way.


For tapping blind holes there's another trick you can add to your toolbox.

Many years ago a TV science show demonstrated a new invention, plugs of wax
like lubricant (a bit like glue stick) that you cut to length and push into
a blind hole before running the tap in, as the tap runs into the hole it
forces the lubricant out through the flutes - no part of the cutting edge
escapes lubrication and the extruding lubricant pushes out the swarf.

A search on the web failed to find any examples so maybe the invention
didn't catch on, but you can get a similar effect by injecting the blind
hole with grease from a glue applicator syringe.

Probably a bit OTT for mild steel, but could be a life saver on tougher
metals.