redrilling holes on the same axis

[ferret30]

When I drill a hole through the transom (or any cored material) and undercut, epoxy, re-drill, I'm concerned that unless I am lucky, the new hole won't be on the same axis and I'll end up elongating the hole on the other side, *and* potentially drilling out the side of the epoxy plug.

How do people ensure that when they're re-drilling an epoxied hole that the second drilling will match? I've come up with a few ideas, but I'm definitely seeking better ones.

1) Drill a hole through a block of something (i.e. 2x4), then hold this up to the surface when drilling the first and second time. This is tricky since it's hard to tell if the block is flat on the transom.

2) Same trick but drill with the block resting against the bit (takes care of one axis), and draw guide lines on the block (other axis)

3) Redrill with a smaller bit, then work up to the final size, making angle corrections each time

4) Eyeball it, but only drill 3/4 of the way through from each side.

For context, I partially installed my swim step, but didn't do the lower bolts yet, and I'm going to go all the way through instead of using lag bolts since the tanks are out.

 

[nimrod]

I tend toward the eyeball technique, trying to remind myself not to overthink the problem. I do center punch the repaired hole to help get the bit going in the right place.

I would think that through-drilling the lower holes for the swim step is unnecessary being that most of the work is being done by the upper bolts, i.e. the stress there is away from the hull. All the stress on the lower bracket is *against* the hull.

jd

 

[Sunbeam]

I eyeball it, and use my eye/feel to keep the drill/bit perpendicular to the surface. I can tell I've gotten used to my drill because I borrowed someone else's the other day when the battery in mine died at an inopportune time, and I found that I had to make adjustments in order to keep it perpendicular. I guess the drill's handle is at a slightly different angle to the body.

A couple of additional notes:

1) I oftentimes don't make the top hole any larger than it was, but instead come from below with a hole saw (pilot bit goes up through the original hole). Then when I re-drill I just come back through the same top hole, and try to line up the drill bit perpendicular to the hole (it can help to have someone else eyeball the second axis if it's critical).

2) If I'm not drilling up from below with a larger saw, then I will still often "back cut" the hole quite a bit larger than the drilled hole (in other words, I "pull" out extra core so the epoxy disc has a larger diameter than the hole in the fiberglass. This way I have a large epoxy annulus to work with, and little worry about popping out through the side into core.

Sunbeam

PS: I thought the same thing about the swim platform, but then also wondered if there could be occasional "up force" when underway. I'm putting two of the lower lag bolts back in.

 

[hardee]

Use a block of hard wood. Cut a notch into it, making it a 90 degree notch, so that when the block is flat on the transom, the notch can be held so that each side goes up 45 degrees, and then the drill bit can lay into the bottom of that notch taking care of both axis. Like a big, maybe slightly widened "V". How's that for simple and easy.

You're welcome.

Harvey
SleepyC :moon

 

[thataway]

At one time I had a device like a drill press which attached to a hand drill, and would give "perpendicular holes". Now I just use a smaller bit, and by eye. My biggest challenge was to do a shaft log boring 16" long with a 2" hole saw. I used a 1/4 Jobber bit to start with and it was true--then used that pilot hole for the hole saw and all of the extensions--took about a day, but it was true. I used a laser for the final alignment of the shaft log.

 

[ferret30]

Sounds like a plan. And this will be done sitting on a milk crate in a foot of water behind the boat hich ill be pulled to the shallow end of the dock. No $250 haul out for this guy! Cordless tools only while in the water. Safety first!

 

[Sunbeam]

My biggest challenge was to do a shaft log boring 16" long with a 2" hole saw.

:shock:

I just got done using a ~12" long 1/4" bit to drill holes for my Permatrim, and I though that was bad....but it was nothing compared to what you describe!

The Permatrim holes themselves were very shallow, of course, but the long bit was just because the motor leg interfered with the drill body otherwise.... still, there was such a "nice" chance to introduce inaccuracy with such a long bit!. Funny, but I can drill holes in my boat all day long without much worry (as I'm used to it), but drilling into that pristine AV plate on the Yamaha.... scary!