Tools of the Trade: Rule and Compass Part 2
To continue from yesterday’s post, we’ll discuss how we applied these same geometrical concepts to lay out our new water wheel. One of the beauties of compass geometry is its scalability; so long as you’re able to make a compass large enough, the techniques remain the same.
To lay out our wheel, we needed a flat, preferably level, surface. Normally this would be the floor of the shop.
However, our mill’s floor warps significantly with the seasons. We had to come up with a different method and, as usual, history offered us a solution.
This is a photo of the water wheel at Mabry Mill (Part of the Blue Ridge Parkway National Park) being rebuilt in 1942. The millwrights built a level surface with posts driven into the ground around the circumference of their wheel. This allowed the workers to both lay out and build the wheel at a comfortable working height and on a level surface.
We recreated this in our shop with a series of 8 stands built by both staff and volunteers.
Once these were completed, we laid out sheets of ⅛” plywood in a rough octagon. We later realized this was too flimsy for the kind of work we’d be doing, so we replaced it with 1 ½”thick Douglas fir panels.
To lay out the wheel we created a large beam compass, with one section pinned to a center post.
With this we scribed the outer and inner circumferences. Finally we had a 15 ½’ wheel to work with.
Using a chalk line, we struck the centerline for the wheel. Then using the beam compass, we followed the same process we did in the last post, continually dividing a section in half until we had 8 equal parts.
Finally, to lay out our bucket boards, we stepped out 7 divisions of one of the 8 sections. We used a smaller beam compass to do this, approximated the distance we needed, and stepped out across the outer circumference of the wheel section.
We then adjusted slightly until we had exactly seven divisions across the section.
Next we walked out across the whole wheel, which gave us our bucket locations.
Now we just have to finish building it!
Again, I would normally invite everyone to come visit us in the Millwright Shop, but it’s currently closed due to COVID-19. We will be continuing to post here on the blog, and we’ll be doing more Facebook Live videos, so be sure to follow us on Facebook. Stay safe, and check back here next week!
If you’d like to take the next step and get involved in the shop once this has all calmed, please contact us at:
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 Greenbank Mill waterwheel construction 1998-1999, image courtesy of Tony Shahan.
 Blue Ridge Parkway National Park, “Challenges,” Mabry Mill - Milepost 176, Burks Fork, Virginia.
Blue Ridge Parkway National Park, “Challenges,” Mabry Mill - Milepost 176, Burks Fork, Virginia.