7.6) Item 6, Shade Assembly

Seven shades are necessary. They are as below:-

The shade is made from 2 parts. The first being a glass shade. This has to be cut from whatever source you can find. Ideally you'd want a No. 3 welding shade for gas welding, or No. 2, but it's highly unlikely that you'll find a No. 2 shade. Anyway, I sourced mine from 50mm circular welding goggle lenses, No. 3 Shade. With 4 shades in series it's totally dark even when looking directly into the sun – a feature that's safe when first making the sight.

The shade glass (Item 23) is as below:-

The shade body (item 22) is as below, made from 2 to 2.5mm thick brass:-

I recommend that you ream the 4mm hole to reduce the probability of refraction errors due to the shade not being perpendicular to the light transmission direction.

The shade assembly is glued together with a suitable glue to bond glass to brass (Item 78).

When I designed the sextant I measured up my No. 10 welding lens, and found it was 1.75mm thick - the pocket depth on item 22. This seemed fine, until I found that the No. 3 welding shade I ordered came to 2.5mm thick. This is causing me some problems, as I don't have the equipment to grind glass back by 0.75mm. This means that I would have to bond the shade glass in so that it stands proud of the frame by 0.75mm. This extra width to the shade means that I would have to reduce the number of horizon shades from 3 to 2 and the number of index shades from 4 to 3, as there is no room for the number designed. It's beginning to sound like time for a small redesign.

On further reflection, I will change the design and use the thicker brass that is used for the sextant frame. This is 3.2515mm thick, but the change means that I'll have to change the detail around the shades. This will affect the frame, shade body, shade pivot bars and shade stops (items 15, 20-22 and 58). There is sufficient spare material in a 12" square piece of brass used for the sextant frame to be able to do this without any extra cost. Which means I have one piece of 6" square x 12 swg brass spare now. If you want a copy of the revised drawing, contact me at the e-mail address specified in section 11. I'm leaving the original design up until I finish the sextant, but will continue to update the text with comments if I have them.