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Sheet Metal Fabrication Tutorials

How to develop a Square to Round

A true understanding of what "True Length" means and how to find and see true lengths is everything in pattern development by means of triangulation. Here we take you step by step in developing a Square to Round, just keep in mind "Practice makes perfect".


Take a look at how we got started, before you can begin to layout your pattern you have to find all your true lengths. The only way you can see a true length line is perpendicular to its plane, the best way to see the true lengths in a square to round fitting is to draw a top view of the fitting as we did above.

You need to develop a true length bar. Using your dividers or trammel points, transfer from your top view points A1, A2, A3, A4, A5, A6, B6 and B1. Making sure to mark each one.

Draw a line from the top of your True length bar (which must be the total height of your fitting) down to each reference mark on the base of your true length bar .

The two illustrations above are all you need to start your pattern.

Take the dimension A to A from your top view and draw a straight line.

Set you dividers to dimension A to B and draw a small arch as a reference point. Once your dividers are set, place one end on A and make a reference mark as we did in Fig.1 (you will need to do this with each side.

  • Set your dividers on points 6,5 in your top view and transfer this dimension to your drawing as we did in Fig.2
  • Set your dividers to the A5 length from your true length bar and make a reference mark that intersects with 6,5 reference mark
  • Reset your dividers to the distance of 5,4 from your top view and transfer this to your drawing.

Continue what you were doing in Fig.2 until you have all A6 through A1 lines transferred.

Does your drawing look like Fig.3?


Determining line B1. We know from our top view that B is half the distance from A to A and we referenced this with an arch. Now we need to transfer line B1 from our true length bar by setting your dividers to that distance and transferring it by placing one end on point 1 and drawing an arch across the arch we labeled B. The intersection of these two arch's becomes point B.

Draw a line from point 1 down to point B and draw another line from point A to point B 2 pieces required to make one fitting.


One last thing to do is to connect the points 1 through 6. We did not add allowance for any seams or overlaps.

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