Strange marking out tool.

[Ben Trout]

I have spent this evening making some three legged stools for a school production. I marked out the seats to drill holes and thought the angles looked a bit off. I dug out a protractor and it turns out that my 60/30 degree set square is actually 55/35 degree.

I inherited the tool from my granddad. I believe he made it and a 45/45 whilst he was at BAE making Hawk cockpit canopies. They are clear plastic, possibly polycarbonate. He apprenticed as a shipwright with a Yacht manufacturer in Portsmouth and went into the Navy from there. BAE was his last job before retiring.

I don't believe that the 55/35 is a mistake. Does anyone know of a task that would require those angles?

 

[Nomad]

If it's for drafting mechanical stuff, it might have been used for drawing some types of imperial threads. Whitworth threads are cut at an angle of 55°, whereas metric is cut at 60°. I have a little angle gauge for checking things like threading tools used on a metalwork lathe, and it has V-shapes at 55 and 60° - you stick the point of the cutter into the vee to see if it's a 55 or 60° cutter (and then put it on the lathe and cut screw threads).

The only thing that makes me think twice about this idea is the fact that it's a set square - a right-angled triangle. I'd have thought a thread drafting thingy would be an isosceles triangle (in this case, 55 and two corners at 62.5).

 

[Gary Elson]

Hi Ben
still use 30 / 60 squares for teaching draughting and tech drawing -I suspect your square is for drawing Isometric projections 3D type views , 30/60 squares are used for convenience whereas the ellipses used for the same projection is something like 33.5 degrees
Hope this helps

 

[ozzy1977]

I guess that it was made for a specific job, I have marking squares made to set angles for marking dovetails etc

 

[Ben Trout]

Thank you for your suggestions.

Whitworth thread drawing seems favourite to me. I think granddad was still a maker rather than a designer or draughtsman at the end of his career, but maybe he was doing some drawing.

Site spell check doesn't like Whitworth, grrrr.