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Tuesday, March 1, 2016

The Footstool mk2


The footstool broke.
Sure. That'll sand out, no problem.
The Accident Investigation Board concluded that when a lateral force in excess of tolerances was applied to the feet of one side, the resulting force upon the first point of resistance (the lower set of dowel pins) was sufficient to overcome the structural integrity of the wood.  Archimedes would be proud.  The proximal cause was operator error; however, the investigation turned up other contributing factors.  The dowel pins were ½" in diameter, and had been inserted into the edge of a 1x10, which is only about ¾" thick; this left, at the narrowest point, only 1/8" of wood to support the stresses.  The manufacturer's excuse was "it's what I had in the shop."  The other contributing factor was that because of the horizontal grain the force wasn't distributed across a wider area but remained focused at the thinnest portion of the wood.  Particularly disturbing, though, was that the manufacturer had encountered a similar structural failure during production and made no design changes.

So I decided I needed to make new end-pieces for the footstool, incorporating the lessons learned from the first go-around.  I cut new pieces off of that 1x10 (it sure seems to go a long way; after this project I still have two feet of the first 1x10 and haven't touched the second yet).  Since I'll be using vertical grain, I cut the boards square so the resulting height of the footstool will be the same as the mark 1 footstool.

Since I wanted a rounded arch to form the feet this time, I marked a centerline on the boards, decided how tall I wanted the arch, and marked the board accordingly.  I loaded up a hole saw and cut the tops of the arches.  I set a bevel gauge to some angle and drew a line tangent to the hole, then mirrored the tangent line on the other side of the hole and repeated on the second board.  What angle did I use?  Dunno -- I picked an arbitrary angle that looked pleasing for the arch and the resulting feet.

Out comes the bandsaw to cut the curves on the sides, to cut those sides of the arches, and to round off the corners of the feet.  Hey, whaddya know?  The drum sander fits nicely in the arches.  Then I used the dado blade to cut the slots for the half-lap joints in the end pieces and in the side-pieces.  Then the holes for the dowel pins and the dowel plugs (recalling that the bottom set of dowels are now purely decorative).

She's a repeat offender.
After dealing with a sorghum thief, I cut the tapers into the ends of the cross-pieces and then I sanded, sanded, sanded until everything was ready for staining.  Since Caryl wanted to try some spray-stain that we discovered, I set the project aside until she was ready to stain it.  You can read her opinion of the product.

A couple of nights later assembled the base.  For reasons beyond the scope of this blog entry, the holes for the dowel pins on one side were deeper than the holes on the other side; these dowels established the reference protrusion beyond the side of the stool, so they went in first.  Then I used a depth gauge to determine how deep the holes on the other side were.  The measured value is unimportant; I used the depth gauge to position a piece piece of wood over the side with the reference dowel pins.  Now I set the depth gauge for the distance from that piece of wood to the end of a reference dowel pin.  This told me how long the dowel pins on the other side needed to be, and I marked the pins accordingly.  I used the same technique to determine the length of the dowel plugs so they'd stick out only as far as the dowel pins.  The bandsaw made quick work of shortening the dowels, and the base was done.

Finally I re-drilled the screw holes to connect the base to the top (the base is narrower now so the original holes no longer line up), and it's done!

Except for the stain that came off the end-pieces and onto my hands.  That spray-stain's going to take a bit longer to fully soak in, I guess.

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