You are probably thinking about something like this:
http://www.cornerhardware.com/item_2...huck/item.html

If you are drilling at any angle greater than about 20 degrees you need to
make a shallow starter hole straight in so the bit will not wander.

Usually backing plates are not tapped unless it is a blind hole. You drill
straight through and add a lock washer and nut. For a belt and suspenders
approach tap the plate and add a locking nut on the back side. Be aware
though that backing plates are usually aluminum and yow can get some
corrosion that will make removing the bolt very difficult. Use some
anti-sieze like Tef-Gel.

For blind holes or holes with very little back clearance it is best to use 3
taps. Start with a taper tap (most common) and follow with a plug tap to
finish out the last few threads. If it is a completely blind hole finish
with a bottoming tap.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"Chris" wrote in message
oups.com...
I saw at the Miami boat show a salesman demonstrating an adaptor that
when attached to the drill you could adjust the angle of the drive more
than just a 90 degree fitting. (Should have bought it) I need to drill
into the coaming through heavy glass and tap into a metal backing plate
and the bottom holes clearance is less that 11/2" above the deck. Any
suggestions out there??

Thanks, Chris