Tolerances are + 0.1 mm
Material... Most softwoods are suitable.
Standard Bottom Bars
The straight simple type shown at the top is known as a 'B1' bottom bar in many manufacturer's catalogues. The one illustrated below it is known as a Manley (or B2) bottom bar and both B1 and B2 types are used in pairs. The third type illustrated was more common in the early 1900s than more recent times, 16 mm or 5/8" were also common dimensions for it's width. The 28 wide single bar is known as a 'solid Manley' bottom bar.
The Solid Bottom Bar illustrated below is intended for dummy frames, frame feeders and other odd uses Like cell bar frames.
Special Bottom Bars... there will be a drawing here later, showing half and one third width bottom bars, but they follow the same principles in all except length. Versions of all of the single types can be found with central groove on the upper surface that is 3 mm wide and 3 mm deep.