Hundredths of an inch in wood and metal
One of the luxuries of being a student is that you have the time to get things right to the degree that you want, at least within reason. There are legendary stories at school about the $8,000 tiller for instance. Some time ago, a student took about 3 weeks to make a tiller. He fiddled and fussed with it until he was Told that it was done. At a standard shop rate of $65 / hr, it would have run the owner $7,800 for that particular stick of oak… actually in real life, it just would have set the shop that far behind and probably gotten the guy fired.
I’m proud to say that I haven’t taken 3 weeks to fuss over the nibbed scarfs on the covering board. We started last Thursday and 4 1/2 days later we were done with all the fitting. We could have probably trimmed a day off of that it we hadn’t worked so hard to get these joints just right, but they’re some of the most visible joints on the boat, and we wanted them just right.
The covering boards have 3 different joints. Up at the bow they meet in a simple miter joint (like the angled joint at the corners of a picture frame) where the boards butt up to each other at an angle. Along the length of the boat, they connect to each other with a nibbed scarf, and I showed some photos of those going together in the last post. At the stern, they meet in a stopped half-lap miter joint. Sounds fancy, but it’s really just a miter joint with some tricky stuff underneath the visible joint that helps to hold the joint together when it may want to open up with changes in temperature and humidity.
So, bragging time to start. I’m very happy with the first scarf joint.
It came together quite quickly and accurately.
Even close up, the seam is almost perfect. There is the tiniest opening at the ends,
but those will close up as the wood swells and will fill with the first coat of varnish as well. So, from my perspective, it’s perfect.
Remember, though, as Voltaire said, The perfect is the enemy of the good. Get one right, and it’s hard to settle for less.
Case in point, the second scarf.
Here you can see the line opens up to about a half pencil-width by the clamp. It too will fill with the first coat of varnish, and will swell completely tight, but it’s not as perfect as the first one. La Dee Frickin’ Dah.. If that’s the biggest problem I have this week then I’m a lucky man indeed.
The last joint is the trickiest because there’s no wiggle room. With each of these joints so far, if something was a little off, you could plane and adjust things to get it right. Of course, doing that meant that you lost some board length as you adjusted forward to fill in the space you just planed off. Sure, we’re talking about 64ths of an inch here, but you can see that when the tolerances are at the level of hundredths of inch, a couple of swipes of a plane could make a big difference if you have a joint at the other end of your board.
So, the stern joint. The side covering boards meet up with another covering board at more or less right angles at the stern.
The starboard covering board is on top of the stern covering board in this photo. The clamp is holding the stern board down because it has a little camber in it, and wants to spring up. The stern board hasn’t been trimmed down to the right width yet, so it looks extra wide. We’re going to join these two along a line that stretches from where they cross inboard to the corner of the transom. That’s the miter part of the joint. It’s complicated a little by the fact that the the transom is angled up (called a raking transom), so the point where the transom meets the bottom of the covering board is inboard a little of the point where corner exists at the top of the covering board.
Looking at it from the side in cross section:
It’s not a huge deal, but you have to pay attention to it.
So, looking down at the stern covering board, you can see I’ve marked the inner corner where the two boards will intersect. The starboard covering board is just at the bottom of the photo. The older wood here is where we routed out a slight rabbet for these covering boards to sit in.
I should probably mention at this point that Mike has finished the joint at the port side. That means that the stern covering board can’t move now. If I move it at all, it will open up Mike’s joint. I can’t move my starboard covering board either, because I’ve finished the scarf joint at the other end of it.
In other words, I have to get this joint perfect, or very close to it, on the first shot. That’s why I’m taking so much time to measure and lay out things just right. I’ll have room to move things around by maybe a 32nd of an inch when I’ve made my cuts.
Yes, I was very nervous about this.
But at some point, you have to just say that you’ve measured and laid things out as well as you can and get to sawing. And that’s exactly what I did.
This is the end of the starboard covering board. My first cut is close to my layout line. I want to be able to sneak up on the fit using a few strokes of my plane. I’ve only cut halfway through here, because it’s a half-lap miter.
Almost down to the line. Next, we make a few more cuts that will act as witness marks for cutting half the thickness off.
They go just to to the depth line that I’ve marked on the side of the board.
Next, start chiseling off the waste wood.
The cuts act as guides to tell you when you’ve reached the right depth.
Almost there.
Finish and flatten with a rabbeting skew plane.
There, now we’ve got the miter and the half lap cut. Next, we cut the miter on the mating piece. In this case, that’s the stern covering board. Here I’ve cut the miter and laid it up on top of the starboard board to check the fit.
It’s as good a fit as it gets.
Now, the stern board is sitting on top of that half lap we cut a minute ago. We need to cut a recess into the stern board to receive that half lap. However, if we do a standard half lap, we’ll expose end grain on the starboard piece as it sticks out of the back end of the boat. That’s a bad thing. End grain soaks up water, and we want as little end grain exposed in a boat as possible. So, the solution is to find a way to hide it. The way to do that in this joint is to do a stopped half lap. Here’s what I’m talking about.
Take the half lap we just cut, and mark it about an inch in, parallel to the back edge of the stern covering board.
Then cut that off.
Flip your boards over, line up the miters, and mark the outline of the half lap on the underside of the stern board. Now cut a recess into the stern board that’s the same depth as the half lap. In this case, I routed most of it out because it was quick and easy.
Starting out.
Almost there. Just clean up the edges with a chisel and the underside of the stern board is done.
Like that. (actually that’s Mike’s end, but mine looks the same)
And yep, I could have routed out the half lap too. Just wanted to practice my hand skills on that one.
Now when you lay the stern covering board on top of the starboard board, the only thing that will show is the straight miter seam. The half lap is completely buried in the joint.
The advantage to this joint is that when you sink a fastener though the half lap, you pin the joint together and it can’t open up later. It’s a lot of work for a cosmetic thing.
When I put it up on the boat, it was very close to exactly right, but I had to do just a hair more fitting before it was right. Unfortunately I got so caught up in working on it that I forgot to take photos!
D’oH! Film tomorrow. I’m happy with it.
Before and after school I’ve been working on projects for the Aurora. I’ve mentioned making long bolts from bronze rod stock, and I thought I’d show you what that looks like.
It’s another one of those things that has you operating in very tiny measurements. Luckily, with a lathe it’s all marked out with dials, so the precision is far easier to control.
Here’s our nifty old Jet lathe that allows us to turn screws. It was donated not too long ago and it in excellent condition. I tell ya, we are so spoiled…
Chuck up your stock…
Take off the roughness at the end (not necessary, but it makes it look nicer)
Do this by running the side of the cutter into the end of the stock.
In case you don’t know how a lathe works, the stock moves and the cutter stays stationary. You can move the cutter in and out by very precise increments using a series of levers and wheels.
Next, figure out where the zero mark is… the place where your cuter just barely touches the stock.
Once I know where zero is, I can use that to know how far in I’m cutting with each pass. I usually take off about 15 thousandths / pass. That’s pretty conservative, but the stock can vibrate if you take much more off, and that leads to a lot of of chatter in the cut.
This lathe has controls that coordinate the turning of the stock with the cutter moving right to left along the stock. The result is that the cutter traces a very precise spiral down the stock as it goes. That spiral is what makes a screw… a screw.
Each pass, you increase the depth a little more until the valleys that you’ve cut meet at points in the middle.
I tend to get close to the right depth and then finish the work by taking the stock out of the lathe and cutting the final few thousandths of thread using a die.
This tool cuts the threads at exactly the right depth
and it cleans up any chatter or imperfections from my lathe work. So, I’m mainly using the lathe to do most of the die’s work for me.
I also use the lathe to turn down the diameter of the rod just a hair so that it will fit in some bushings I have.
That’s what makes the rod shiny on the right side. The bushings will only ride in the center of the rod, so I don’t need to turn down the entire length.
One of the payoffs of doing work on the lathe is that you get very nice looking metal shavings.
Hey, beauty’s where you find it.
Tags: coveringboards, miter, stopped half lap, madcap