fitting poles to rocks

the band shell in Eastsound on Orcas Island is pretty great.  Tim Forbes, one of the builders involved briefly described to me how he carved out the base of the vertical timbers to fit the foundation rocks they're sitting on.  it looks like the trees are growing right out of the rock.  I imagine they're also much more stable that way.

but that was a while back and I've completely forgotten how he did it.  anybody know how to do something like that?


(click for larger size)(not my photograph)

I have never done this but one thought comes to mind, 

figure where the center of the "post" is to go,  and drill a hole in the rock, put in a rod and make sure it is vertical,  use a square or a device that is square and will pivot on the rod,
make a number of equal spaced by degrees radiating out from the rod, (say every 15 degrees,  then with the square held at the same height, measure at 3" out 6" out and so on until the entire "post" would be covered,  then do a similar to the "post" but use the reverse number on the spots that they would represent,  then drill that depth into the post/log,  then carve out to the depth of the holes,  one would want some type of pin to keep the post in place any way so you would use a pin/rod  to align the post on the rock with the drilled hole, the base of log would have to be at a right angle to the center of the log to make it set properly.

or one could do the same by making templates of the rock on the radials and make a reverse template to carve the post out to,  the measurements are just a mathematical template,  and the holes would mark the depth to a visual mark.

a chain saw type carving system could make it go faster.

I think you're on the right track, Birdman.  that sounds sort of familiar.  I remember Tim's method striking me as really simple and precise, though probably fairly time consuming.

I asked Tim about that process and I cant remember the details but do remember it was beyond my means in that it required an excavator on standby to raise and lower the log many times.

the poles I'm using are a lot smaller than the trees Tim used in the band shell, so I think a human ought to be able to take the place of the excavator.  but yeah, there was some large equipment involved in that project.

I think I'm going to do roughly what Birdman suggests.  I'll cut a small piece of wood and stick a pencil to the top of it.  then slide it around the rock tracing the contour on the pole as I go.  that will work for the edges at least, and I'll just fudge it gradually with an angle grinder for the center until it fits relatively well.

I contacted Tim and he suggested a one-day lesson.  tempting, but not something I've got the time and cash for, so I'll muddle through with a little help from the internet.

This may be too late, but I think using a log scribe would be easier than making templates. Its a simple tool that looks like a compass...

Sol wrote:
This may be too late, but I think using a log scribe would be easier than making templates. Its a simple tool that looks like a compass...

I'm familiar with scribes, but I don't have one.  the stick + pencil idea was an attempt at a poor man's scribe.

you're not too late, though, as I haven't gotten very far with that project.  I've moved 150 miles closer to it in the meantime, though, so the odds of it getting done have improved since then.

Have you ever seen one of those toys that has all the metal pins, and you can push your hand shape up from the back? Know what I mean? Could make something like that pretty easy and use it over and over.

I have done this several times, and it is an art.  Those logs he used are so huge, you would want to have already practiced on small stuff, and you need that excavator too.  I have done logs at 12 inches by 14 feet with only human power, and only having to adjust a couple of times.  How you say?

Easy, get some duck tape, card board, scissors, pen, etc.  Now your project is to make an exact replica of the rock using these materials.  You can lay the cb on the rock to start.  Next, carve the bottom of the pole so that it matches the cardboard form perfectly.  Then lift it into position and see how it fits.  Use the pen to mark carefully where you need to take away more.  I use chisels, but also a tool called a lancealot- a chainsaw blad on a grinder.

Happy building,