Tuesday, April 19, 2011

Tech Tip Tuesday

Ok, having a contractor who is a very good trim carpenter as a student can make for an interesting class. He sees things from a different point of view - which is good because it challenges some of our assumptions about how we do things and can allow some "higher order" methods to be tried. I know he reads this blog, so I'm hoping that he has a good sense of humor about this.

As we've discussed before (here and here...) , we make the canoe's bottom as symmetrical panels and cut the center-line of each panel to fit when the panels are off the hull. The major downside to this is that the panel "relaxes" when removed from the hull. One way to get around this is to mark the centerline and to make a several kerfs using a saw while each panel is still on the forms, keeping the teeth vertical. This gives you a series of guide marks to plane to. It works, but still takes a bit of skill and patience.

With guys like this student, it can be one of those, "life seldom separates the men from the toys" experiences. Witness the laser he chose to mark the center-line with:

Makes a nice line - although it is almost a 1/16" wide (.062"). Meh. Quite frankly, it is much wider than I'm comfortable with as a mark. (Particularly when you consider the fact that our Japanese saw blades are only about .020" thick.)

Here is where things went off track a bit. There were two engineers (who are also woodworkers) present with this student. The three of us started to posit how we could do this using a panel or circular saw to cut the centerline without removing the panel from the boat. The basic idea was to create a guide assembly for a saw that would be a consistent distance from the centerline and a consistent height at each form. This is the result of our discussion:

What you can't see is the vertical plywood elements that are screwed to the forms. These vertical elements support the plywood deck. There is a plywood fence screwed to the deck. It is set up so that the distance from the fence to the outer edge of the saw's blade is located at the center-line of the hull. As a test, the first cut was made a bit offset from the centerline (to the right in the image) as can be seen in the picture below:

Once we established that a good cut could be made, the fence was moved over to the center-line and the final cut was made. We have yet to make and cut the second panel, so we'll have to see how things go. So far, the downsides are that the cut passes into the forms (i.e. we'll need to re-tape the forms before building the second panel), it takes a fair amount of time and skill to set-up and you need to be very careful at the ends not to cut into the stems

The verdict is still out, but so far it has been an interesting exercise.

I guess this student gets the , "No guts, not glory" award!


Cat said...

Wow. This is a very interesting way of doing the centerline. In fact, the whole idea of removing the bottom panel(s) to cut the centerline is something I had not come across before. I had always planked across the centerline, cut the centerline on one side with the planks still on the forms, then fit the other side's strips individually: this sounds both faster and neater, though with the risk of spoiling a whole side if one cuts too deeply. Though I suppose even then, one could cut back a little farther and install a center strip.

I am very glad you posted about this: thank you very much!

Canoez said...

This is an interesting method, but I don't think it is for the "beginner". We've been cutting the panels off the hull for as long as I've been teaching the class and it works well. To be honest, I think it is a lot easier than trying to fit each strip to fit the centerline.

We've never had to add an extra strip. We HAVE occasionally had to put a little dookie-shmutz into a small cap on the inside from bad bevel angles, but that's about it. As I noted, the panels relax slightly after they come off the hull, so that can make it a bit challenging. This method has the advantage that the hull panel remains in the shape that it was made in - you just need to make sure you've removed all the fasteners in the path of your saw blade!

Cat said...

But aren't you going to have to remove the cut panel anyway to be able to plank past the centerline on the other side? So won't it still relax while the other side is being planked and cut?

Which might actually make fitting the two more challenging, if one side relaxes while it's off the forms and the other side never comes off the forms and thus doesn't relax.

I suppose you could always solve that by taking the second side off the forms for a while--I imagine overnight would do it--and hoping they both relax about the same amount.

It's a pity there isn't a way to take the first side off the boat without taking it off its forms. Removable quarter-forms with their own baby box-beam, perhaps. But that would take longer to set up and I bet it would be heavy.

Canoez said...

Yes, we will remove the panel from the boat and it will relax. The trick here is that we've cut it before it relaxes while it is still boat-shaped.

The next panel will be assembled and cut while on the hull - again, while still boat-shaped.

Once the parts are put back together on the forms, the inner tubes we use to clamp the panels back in place will make it mate properly as it will be forced back in shape while the glue holding the panels in place cures.

If you were to remove "quadrants" of the form, you'd need to be able to articulate them in such a way as to mate them properly. I think this would be very difficult in execution.