The Davis quadrant was the most popular marine navigator's instrument in Northern Europe and North America for over 100 years. Invented around 1600 by Captain Davis, it was gradually improved until just before 1670 when tyhe diagonal scale became standard, improving its precision significantly. In fact, it is speculated that the reason there are no remaining pre-1670 backstaffs is because the new versions with diagonal scales were so popular the older ones were all replaced.
I have been fascinated with the backstaff for many years as a very attractive and interesting navigational instrument. I started gathering materials for this project around 20 years ago. Because of and incomplete survey of mostly American instruments I thought Brazilian rosewood was commonly used for the frame and started searching to find some, a process made difficult by its restriction for importing it due to its endangered status in the early 1990's. However I was fortunate to find a plank at our local hardwoods supplier which had been legally imported before the 1990s and stored in musical instrument makers shop. I immediately purchased it, but then found that boxwood was equally difficult to acquire in the US (I even contacted Roy Underhill of "The Woodwrights Shop" on PBS who replied that he was unaware of American sources. I finally got a couple of small boxwood planks when a colleague went on sabbatical in England and bought them for me at a lumber store there.
In the original drawing all frame members have a rectangular cross-section of 16 mm x 18 mm with lengths of up to 645 mm. Since most of my tools and measures are in marked in inches I converted all measurements. As a result I initially cut about six feet of 3/4" square rosewood sticks then planed them to a rectangular cross-section of 0.71" x 0.625 (5/8)". I simultaneously planed the boxwood boards to 16 mm (5/8" = 0.625") to match the rosewood backstaff frame. Contemplating the plans, I concluded that I would have difficulty completing the joinery needed successfully, so I decided to make butt-joints and hold the backstaff together with stainless steel machine screws tapped into the wood with the screw caps counter sunk to be level with the surface in each case.
As an alternative I decided to try using standard machine shop taps to thread holes in scraps both the rosewood and the boxwood. I chose some metric hex drive cap screws recovered from discarded lab instruments. Both woods were easily threaded and the threads hold well, the only difficulty is that, unlike with metal or plastic, drills can drift slightly due to the grain of the wood.