Rectangular mast with Radius Corners:
The Designer pecified the mast to have outside dimension of 7.250" x 4.750". It is to be fabricated from 14 gauge (.072") thick cold rolled steel. The inside radius corners will be .500". It will be fabricated in two pieces with a center al divider .072" thick.
With this information a flat layout can be calculated for forming in a press-brake using a 1.000" round top die. The cut size to form a half section is 11.310" x length. The distance from the edge of the material to the center of the bend is 2.184". The layout process is fully described in my book 'Applied Metal Boatbuilding Methods - Sheetmetal Pattern Development'. Available at Amazon.
Below is a round top punch used in a press-brake. Here it is being used to form a large radius fabrication. In the case of forming the .500" radius a single press stroke would be used to form said corner.
Octagon Mast Section:
Below is a steel mast design by me for my personal boat. The Mast is 8.000" x 5.500". It is also constructed of 14 gauge cold rolled steel.
From the dimensioned half layout above (Left) the cut-size for the flat layout, below (Left) can be calculated for forming in a press-brake. A 0.500" vee top punch and a 0.625" open bottom die will be used for the 45 degrees bends of this mast section, below (Right).
The layout process is fully described in my book 'Applied Metal Boatbuilding Methods - Sheetmetal Pattern Development'. Available at Amazon.
Round Mast Section:
The cut size for round mast section is determined by the following formulas. With the outside diameter and material thickness determined substract a material thickness from the outside diameter - Than multiply by 3.1417 to obtain the cutsize. For example the cut size for an 8.000" outside diameter mast from .072 thick cold rolled steel would be: (8.000" - .072") = 7.928" Then 7.928" x 3.1417 = 24.907"
With the mast being fabricated in two pieces divide the proceeding by two. 24.907" / 2 = 12.453". This is the cut size for a mast half section.
The mast section will be formed by bump forming, see figure below. To calculate the number of hits of the press-brake, pick a suitable bend angle and divide that number into 180. If you picked 5 degrees the number of press-brakes strokes would be 36. If you divide 12.453" by 36 = .346" the result is the distance between press-brake strokes. If you do not like this bend angle, for some reason, pick another and repeat the above.
Elliptcal Mast Section:
Elliptcal mast section are Bump formed just like round sections, but with a twist. While a bend angle can be chosen the spacing between the press-brake strokes varies with the curvature of the section. The tighter the curvature the closer the strokes.
The flat layout (below - Right) shows five degree press-break strokes. Note the varying distance been the press-brake strokes. The closer together the more curvature in that area.
While I would like to detail how the flat layout and bend line location were calculate it is beyond the scope on this web page.
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