Tuesday, March 6, 2012

Calculating the Bend

I have been asked to make a steel table leg style which, if the look is right and the price is right might be used in a table project by an artist who creates custom barnwood furniture.

The issue came up as the result of a conversation in which I described some experiments I was doing in an attempt to define the styles of table support frames which appealed to me and which I may adopt as my preferred styles.

In short we agreed to use 1” square bar stock. Each leg would be a pair of these bars, riveted together, rising to a 90º bend point which when pulled over a 2.5” radius mandrel would support the bottom table top 31” inches above the floor. The terminal eight inches of the top end would be forged into a plate with two bolt holes and there would be a 45º twist which would bring each plate into a parallel line with it’s corresponding side.

This is simple enough in concept but a bit more difficult in the details. At what point on the bar does the bend begin? How much bar length is required? More importantly, to what length should the bar be cut for the most convenient working?

For me, this is one of those “trust but verify” things. It is easy enough to make a full scale drawing which will provide most of those answers but I have been using SketchUp lately and decided to try it here.

I first drew the bending mandrel - a 2.5” circle. Then I used the offset tool to illustrate the 1” stock bending around the mandrel. Finally I extended the bar to the floor and about 8” under the table. Although the physical result of my bending may be slightly different from the calculation, I’m going to say I need to allow the 4 21/32” for the bend, 8” for the table bolt plate limb and 27” for the vertical limb.

I drew the neutral zone of distortion of the bend in red and that should be the actual length required. The area between the red line and the mandrel should compress and the mass in the zone outside the neutral zone should stretch.

I have left the dimensional measurements which show the “off by a 32” or so” notations to illustrate how some error can accumulate when drawing lines by hand rather than typing an exact number in the measurement box.

This works for me and I didn’t have to find the roll of freezer paper an a flat surface to make a full size drawing.


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