Saturday, September 22, 2012

How to use Dividers

Dividers were a mystery to me for a while, but they are a low-tech, elegant way to accomplish very accurate work.  I love stuff like that.  Here are some different scenarios.

The fundamental task is to divide a length into an arbitrary number of sub-lengths.  Let's say you have a board about a foot wide that you want to make nine evenly spaced holes in for a coat rack or game or something.  Draw a centerline, and set the dividers for about 1/10th of the distance across the board.  Just eyeball it if you want.  Then start at one end of the line and walk the dividers along.  When you get to the far end, whatever error you made in the setting will mean over or undershooting the end of the board.  Say you're about your thumb's width over.  Narrow the dividers' setting by about 1/10 of your thumb's width, and step off the line again.  It might take two or three tries, but it's pretty easy to get extremely close to perfect, at which point you step down the line again, but this time press as you go to make a clear pinhole mark at each center point.  Note that at no point in this do you need to measure anything.  The line you're dividing can be any length, and at any angle, although it does need to be a straight line.

Once you've done it a few times, adjusting the dividers to close in on your desired value isn't fiddly.  It is trial and error, but you can home in on a very accurate measurement within three or four tries.  So don't get stuck in the trap that dividers are some kind of old-school imprecision thing.  Let go of your fear, Luke

A common use comes up in laying out dovetails.  This can be used for pins or tails (whatever you do first) but here's my tails-first approach.  Make a mark a half-pin width in from each end of the board  Set you dividers to about what you want a pin plus a tail width to be, and step off down the joint line from one of the half pins.  Do a few trials until the last step goes off the board and lands where the far half-pin would end if it were a full pin.  It doesn't need to be super exact.  Then step off that distance from both ends, pushing in the points to mark out the joint.

Another use is finding the center between two points on a line.  This is the simplest case of division (into two).  Set the dividers for what looks like the center by eye.  Take one step from either end.  The mid-point of those two new marks is the center.  Widen or narrow the divider setting by half the distance between those points (by eye) and try again.  When the points land in the same spot, you have the center.

Wednesday, September 19, 2012

Dining Table Design and Top

Well, I got the top glued up, finally.  I sliced the base of my thumb up pretty good while disassembling a handplane, so there was a week of nothing accomplished while that healed.  This is by far the biggest panel glue-up I've ever attempted.  Ten pieces were jointed by hand and glued up one at a time into a 7' x 3' panel.  I used four Rockler 3/4" pipe clamps (with cauls) and a biscuit every 12" to help with alignment.  The biscuits probably could have been further apart but I was cautious.  One mistake I made was gluing the two halves together separately (into 7' x 18" sections) and then trying to glue that joint down the middle last of all.  Hoisting one of the 50 lb halves up to test the joint while I planed it to match was no fun.

Top all glued up. This is the underside - the top has slightly fewer defects.
My wife thinks it looks good, but I think it looks like a bunch of 2x4's made into some kind of fancy picnic table.  I'm planning on inlaying some dutchmen to hide a couple of knots and other ugly bits, and it will have breadboard ends, so I might be happier with it in that context.

There's around 3/16" of cup across one end, and 1/8" at the other.  Cumulative errors of all the jointed surfaces show that I made a systematic error while jointing, since the whole top curves the same way.  Or maybe it was the way I clamped it during glue up.  I can flex it flat pretty easily with a couple of clamps and a piece of lumber, and I know a top this size will move some on it's own even if I planed it perfectly flat now in it's unrestrained state.  I'm not entirely sure how to proceed though.  How flat does it need to be before I put the breadboard ends on and screw stringers underneath to get it the rest of the way?

Here's the plan.  I'm going to plane the bottom side with my #4 smoother and jack.  That won't straighten it fully, just level the joints and smooth the surface.  Then I'll install the breadboard ends and attach the stringers with figure-eight connectors, as it will be when the table is assembled.  Thus, all the physical flattening devices will be on, and I can use my long planes to get the top properly flat so it looks good.  As long as I don't feel like things are super stressed out when I assemble it, I think this will be ok.  And like I keep telling myself - worst case, I blew $40 worth of framing lumber.

The rest of the table plan is still a little fluid, but will look something like the Sketchup rendering below.  I picked up a 5/4 cherry board to make the breadboard ends and butterfly patches for a couple of spots on top, and I'm going to use walnut for a few small bits like the breadboard pins and the wedge that will hold the trestle together.

A fairly standard trestle table design

El Cheapo Dining Table

I'm making us a new dining table, out of Douglas Fir.  I started with 2 x 10 framing lumber from Home Depot...  I found a couple in my outdoor wood pile that had been temporary supports during one of our remodeling projects.  After two years outside and a trip through the planer they looked too nice to cut up for the fascia boards I had intended them for.  So I went to the big orange store and picked out some more decent pieces of green, soaking wet framing lumber, and let them dry outside for awhile, then brought them to the basement for a couple of months.  Then I ran them all through the planer, bringing them down to 1-1/8" thick.  Today I ripped the pithy centers out of them, leaving strips of quartersawn grain ranging from 3" to 5" wide.  Now I get to joint all the edges in preparation for glue-up.  I don't have a powered jointer, so I'll use the tablesaw to even up the worst of it and then fine tune with my new Veritas jointer plane.

Ten planks ready for jointing

Design?  I don't have a design yet.  Well, it will be a trestle table, and the top will be about 7' x 3'.