On Wed, January 20, 2010 11:36 am, John LaTorre wrote:
> Conal O'hAirt wrote:
>> I'm going to be drilling a larger hole ( 1 1/4" )through the width of a
>> 6" piece of yellow
>> pine. see attached file. this piece is going to be glued up along the
>> same plane as the hole and the hole will be centered on the glue joint.
> You didn't say how tight the tolerances have to be, but I'm guessing
> that you have a little wiggle room. The only thing I can think of is to use
> a Forstner bit instead of a spade bit and take it slow. If you really,
> really trust the trueness of your drill table and your marking-out skills,
> you could bore halfway through on one side, flip the work over, and drill
> the rest of the way from the other side. I've done that when boring center
> hubs for spoke tents. (But in that case, I didn't want a tight fit, and if
> it came off a little wrong, a bit of work with a rasp would make the
> offset go away.)
Johann's idea here sounds good to me, along with his warning about careful
layout. I'd add only that if the hole were made undersize to begin with,
from both sides as Johann suggests, then the rasp work could correct any
centering errors without taking the hole over size. Once the hole has
been correctly adjusted to the desired centerline, enlarge it to final
size with a big twist drill or reamer. If you're worried about rough
breakout at the exit wound, do your reaming from both sides as well.
Another possibility just occurred to me, though I haven't tried it.
(Which means you should try it on some scrap first!):
What if you laid out your centerline on each of the mating faces first,
and then scored it with a saw kerf all along each line? That should keep
the center point tracking properly through the assembled block.