The Arte & Misterie of Coopering: Making a Mary Rose tankard, Part Two

This being the part where we turn math into a drinking vessel…

You may remember this bit of algebra from last week. Contrary to what I’ve been seeing on an alarming number of YouTube videos and woodworking forums, it’s the way to find the correct angle to cut your bevels on the staves of your bucket/tankard/butter churn, or whathaveyou.

n = the number of staves you want in your vessel.

And it doesn’t change no matter how big or small your vessel gets.

There seems to be a lot of confusion about that out in the Interwebs. The angle is the same whether you have a diameter of ten feet or ten inches or two inches. For an octogon (or an eight-piece circle) the angle for the cut is 67.5 degrees.

Geometry is awesome like that.

The mystery of the bevel solved, it only falls to us to decide how to cut that angle accurately. As I said in the lead-in to this project, every source I have just says “the cooper eyeballed it.” And the tool they used for that eyeballed angle was a big jointer plane mounted upside down on the floor of their shop.

You can see one being used in this engraving from my muse and tormentor Jost Amman. That’s one seriously large piece of equipment. I checked around and those things are expensive even if you make your own (mostly the cost of getting the blade made).

Image source: Wikimedia Commons
Cleaned-up digitally by yours truly.

You can see it in use in this video I posted the other day of Ramona Vogel, journeyman cooper at the Colonial Williamsburg living history village: http://vimeo.com/11313428 I think that beyond some basic geometry, the real “art and mystery” here is how to finagle the right tools to make it all work.

Thankfully, the woodworking blog I got that video from (The Village Carpenter) came to my rescue once again with this post about a friend of hers who makes buckets.

He doesn’t use a huge floor-mounted jointer; he uses a hand plane mounted upside down and it seems to work just fine. He also uses a jig, but I wanted to try my hand at this “eyeballing” thing that everyone seems so impressed by, so I grabbed my largest bench plane and a handful of clamps and my eyeballs this is what I ended up with…

I’ve already leveled the sole of the plane, now I’m making some fussy adjustments on the blade.
Taking the first few swipes across the blade.

Note that I’m wearing a glove on my pushing hand and keeping my fingers well back. I want to keep my fingertips on my hand and not on the ground where they’d get dirty and I’d have to hire someone to sew them back on…

Thankfully, the upside down plane worked a treat and I didn’t so much as trim a fingernail on that razor-sharp blade that made such short work of that dense oak stave.

Take another look at the Mary Rose tankard we’re imitating here and you’ll note that the top is narrower than the bottom. Sort of an inverted pint glass shape. So I have to not only trim a bevel on either side of each stave, I needed to make each stave slightly smaller at the top than at the bottom. 
Forgot to mention that I also made it thinner when I was shaping them on the shaving horse. The intent being to give a more substantial foot to this thing since it’s going to be quite tall and on the rolling deck of a ship no one wants to spill their beer.

The tool I’m using to check the angle in the photos below is called a sliding bevel.  I pre-set it to the correct angle and locked it off. I’ve only to hold the piece up to the light and slide the blade of the tool along the wood, watching for gaps.

  
I tried to find out how they did this in the 16th century. A lot of tools like dividers and the like predate the period, but I can’t find anything concrete on the topic of the sliding bevel. If anyone has anything on this, I’d love to hear it.
The results are eight staves, evenly shaped to form a circle which will, under compression from its hoops, swell to become water-tight.
It’s almost but not really discouraging to note that if you have a sliding compound miter saw, a couple of router bits, and a table saw, this project really would take you an afternoon. But it wouldn’t be nearly as cool as this one.
Or so I keep telling myself…

Still a lot of work to do with the scrapers on the inside and quite a lot of fiddly work aligning the staves and making everything work out just right.
Then I can make a handle and a lid.
Note: Don’t worry, it only looks like I’m behind. I’m also learning to knit, making a leather bottel, and getting started on hornwork.  More updates on all that stuff later this week… I hope. 
~ Scott

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