Breeches Piece

Breeches Piece (TINK) Version 1.05 first created 11/05/06 - last modified 18/10/08 Page Author: Ty Harness

News

Tink has now been released. The full version will be free for any user who has all the first 4 applications: STOR,CONET,SEGB,STOS. You just use the serial codes from those 4 applications to unlock TINK's full features.

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Figure 1 shows a 2 way breeches piece made from 2 oblique cones; traditionally you could have developed this pattern out using the radial line method. Like oblique cones the input and outputs are circular allowing an easy connection to circular pipes. The subcontrary cut would also produce a circular entry/exit but rarely suitable for the sheet metal worker. There's more info on the subcontrary cut on the Conet math page.

Figure 1 - A 2 way breeches piece from oblique cones

Using TINK, you can choose as many branches as you require and they will be equi-spaced and central to the base circle. Again like other Ty Harness Sheet Metal Applications you can choose more or less divisions to triangulate-out the pattern where the greater the number of divisions the less truncation error but at the expense of the processing time. If you want to print the pattern from TINK then set the maximum number of divisions (120) but say you need a lean VRML model then you can maybe get away with 48, and if you have to type in the 2D pattern coordinates into G code then maybe 12 or 24 points is all you need for the job.

Figure 2 a,b - 2 way and 3 way breeches pieces

Figure 3 - Some of the first 3D exports from TINK - which are oblique cones

In general, the breeches piece is not made from oblique cones (so you can not use the radial line method). The conventional method is to slice the branch exit square and perhaps use a segmental bend. From figure 4 you can see what is meant by a square cut where the angle B = 180 - 90 - A. Because we've abandoned all hope of using a pure oblique cone we can force a circular exit at any angle but as a rule a square cut is usually preferred for sheet metal work. A square cut produces less curvature in the pattern curve thus making it easier to tackle with the 'Gilbow' shears.

Figure 4 - Segmental bends are often used in conjunction with a breeches piece.

Figure 5 - Tin Knickers from 22 SWG Mild Steel (45 degree branch with a square cut - not from oblique cones)

You can swap between a parallel and square cut in TINK at any time by selecting the radio button required. You can set the top diameter (TD) to which you'll be connecting up to for example a segmental bend. The angle A from figure 4 is chosen by defining the offset (OX) and the elevation height (H).

Figure 6 - 3 way breeches piece - (Not a cone in any way shape or form)

We can see the branch exit on the plan shows an ellipse when not cut parallel to the base plane.

Pattern Development for the Breeches Piece

Using the Pattern Properties first introduced in STOS 1.04 you can control the pattern start and end points (the joint lines) of the development.

Figure 7 - 4 way Breeches piece using a half pattern

Pattern accuracy is always important and when making half patterns because any error is magnified by the handing process of the pattern. From figure 7, I only had to make the one hand of the pattern which I used to cut all 8 pieces for the job.

Figure 8 - A full pattern Start pos. 0 End Pos 0.

Figure 9 - A full pattern with the joint line at position 24

Figure 10 - Translation and rotation of the joint line 0 - 0

A new feature for version 1.05 of the interface is the ability to translate and rotate the pattern to any desired position. From figure 10 the pattern point 0 on the base curve has been translated to a coordinate of 175,50 and the joint line 0-0 is rotated to 45 degrees anticlockwise from east. The rest of the pattern is then triangulated out in an anticlockwise manner. Originally the other Ty Harness Sheet Metal Applications started the pattern joint at 90 degrees, but the pattern did not fit a landscape or portrait paper concept too well. Using the min. bounding box calculation you can now orientate the pattern either portrait or landscape. So for the specific geometry shown in figure 11, and rotating the joint line to 36 degrees aligns the minimum bounding box with a landscape view.

Figure 11 - The pattern orientated to suit landscape concept.

The full version of TINK allows the export of 3D VRML files which is an excellent way to communicate your ideas over the web. Here's a 2way,3way,4way and 12 way breeches pieces.

VRML: 2 way Breeches Piece
VRML: 3 way Breeches Piece
VRML: 4 way Breeches Piece
VRML: 12 way Breeches Piece

The 12 way is a bit extreme for sheet metal work - might make a good jelly mould.

Limited Version Features

Draw a breeches piece using key parameters to control the geometry.

Stand Alone Windows application. CAD software not required.

Snap measuring feature in the user interface.

Print a simple specification sheet for job cards or quotation

Print a full size or scaled template for pattern making and fabrication purposes.

New user controlled pattern translation and rotation.

Full Version Features

No Nag Screens or Nag Prints

Additional geometric properties calculated like the surface area and internal volume.

2D DXF Export of front elevations and pattern - useful for CAD operators

2D DXF pattern only and 1 continuous closed pline designed for CADCAM applications

3D DXF Export using 3DPOLYFACE objects for immediate rendering.