3) Tools Required
Here's the rub. To make an accurate metal sextant, you need accurate metalworking tools.
This sextant is designed to be made with a metalworking lathe, mill and rotary table. The actual lathe size can be as small as a watchmaker's lathe, allowing a bar of 25mm diameter to be turned. The milling is designed to be carried out with one 6mm diameter cutter and, optionally, one 3mm cutter. The smallest throat depth you need on the mill is about 25mm, so a very small mill could be used too. A bigger mill, obviously, would be better. You'll need a rotary table of at least 150mm (6") diameter, and a lump of timber/ ply to go under it.
The list of tools you need is:-
1) Lathe + lathe tools
2) Gears for lathe to cut a 1.5mm pitch thread OR M20x1.5 fine die.
3) Milling machine, milling head or adapter + 6mm end/slot mill. A 3mm mill is nice.
4) Rotary table
5) Gear cutter. I've drawn one up where it's used, so don't panic!
6) M3, M4 and M6 coarse taps, dies and tapping drills. HSS is preferred, as they tend to cut better threads.
7) Engraving cutters. I've drawn up two different ones that you can make.
8) 3.9 & 5.9mm dia drills and 4/6mm reamers. I've drawn up home-made reamers that are suitable.
9) A range of files, some needle and some normal sized.
10) Hacksaw & piercing saw.
11) A range of drills. 2, 4, 6 and 7mm being the minimum, a 14mm drill can be used if you've got one handy.
12) Glass cutter.
13) Metal Polish & Lacquer.
14) Range of metalworking layout & marking tools.
15) Blow Lamp/ Blow Torch powerful enough to melt silver solder.
16) Medium or fine straight knurl.
17) 6mm diameter HSS toolbit, or hardened Silver Steel.
18) A face mill would be nice, but the end mill and/or filing will enable the job to be done without one.
What lathe & mill do I use?
A combined mill/lathe with a 300mm swing over the bed and 175mm mill throat capacity. (If you're interested it's a Warco WMT300.) My rotary table is 6" in diameter with an MT2 central taper for which I've made a number of fixtures to help me machine the sextant. To be honest, the lathe is almost too big for this little sextant. Holding a 3mm diameter bar in a 127mm (5") chuck is ridiculous. I have only one milling collet, hence the use of 6mm shanks wherever possible.
If you don't have a lathe or mill, and can't afford one, but do have the time to make a machine shop from scrap materials then I'd suggest that you look at the series of books at http://www.lindsaybks.com/dgjp/djgbk/series/index.html. It wasn't the way I went, but I have made a furnace similar to the one shown in the first book. A nice bit of kit really, and surprisingly easy to use.
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