Added 55 Degree Thread Profile

[DrJPK]

This pull contains the code to add 55 degree Whitworth threads as per https://en.wikipedia.org/wiki/British_Standard_Whitworth

This allows for BSW and BSPP / BSPT threads to be generated.

Demos folder contains 2 screen shots and a FreeCad file showing the new ThreadProfile in use.

Have not added the links to https://amesweb.info/Screws/bspt-thread-chart-calculator.aspx for BSPT and I cannot find a simple calculator for BSW. But this site https://www.britishmetrics.com/images/pdf/technical/bswstd_1.htm has a good lookup table so long as the minor diameter is used for both internal and external threads

If desired I can modify the numbers so that the "Minor DIameter" parameter box would work for the Major diameter for external threads.

[mwganson]

Thanks for the pull request.

I select 1/4 - 20 from the v thread presets, then switch to internal thread type, and then select 55 from the variations property. The minor diameter shows to be 4.98 mm. According to the chart the minor diameter for this thread should be 4.72 mm. I assume the v thread presets won't work for this variation then. That could be very confusing for users. I think the user entering major diameter from the chart into the minor diameter property would be too confusing, too.

The charts I found only show major diameter options for external threads, but the user would need to enter manually the minor diameter in the property field. You could add BSW presets to the v thread preset list. The format is

(line 899) ['1/4 in-20 UNC',25.40.0500,25.40.1887,25.4*0.1959],
['description', pitch, external minor diameter, internal minor diameter], #in mm

Something like ['BSW 1/4 - 20', 25.4/20, external minor goes here, 4.72], but I don't know how to calculate from major diameter listed in the chart to the proper minor diameter. If the user selects the BSW preset, then the variant needs to be also set to 55 for him, I would think, otherwise he would get the standard 60 degree variant.

Here are a few screws and nuts from thingiverse for a couple of these sizes. https://www.thingiverse.com/thing:4395165

These are in stl format, but there is a tool in the mesh workbench to generate a cross section on the xz plane from them, which could be used to compare a cross section of the solid body object created with the similar cross section tool in part workbench.

How did you generate the internal_55 and external_55 data numbers?

Instead of "55" it could be better to use "BSW 55"? Then the user sees it is a different thread type and not just a different angle being used.

[mwganson]

Can you rebase this PR? There have been changes that I think will prevent it being merged currently.