Strength of Beams

Less waste than you might think when you look at the cunning way they are made with modern CNC cutting gear.

. . . and as I said less weld too.

regards Martin

Here's how Fusion 360 predicted three 60mm long Aluminium beams of equal cross-sectional area and length would deflect with a 250N load on the end.

Winner is tube, 13mm od, 11.34mm id, deflects 0.06mm

2nd square section 10x10mm, deflects 0.17mm

3rd round bar, 11.28mm diameter, deflects 0.22mm

Deflection is magnified above reality in the image to emphasie it. The colours indicate degree of deflection, not safety. Red highlights points of maximum deflection, not that the structure is going to break. Another FEM reports shows the structure has a 7x safety factor.

Although tube is stronger weight for weight than box or rod, the fact doesn't help us design a better boring bar because the diameter of the tube is bigger, and boring bars have to fit into small holes! Other structures make good use of tubes.

Dave

Edited By SillyOldDuffer on 12/09/2020 21:27:54

I think castellated beams traditionally had hexagonal holes, because they were hand burned using an oxy acetylene torch. Straight lines are easier to burn by hand than circles. They bend like bananas when cut, we used to tack them together in a jig using metal wedges to straighten the halves back up.

Nice to see some engineering discussion.

How about suggesting what frequency the tool will resonate at and what surface finish will result?

I'm a bit late to this but a couple of points

on "Computers" I highly recommend the Film "Hidden Figures" Even SWMBO liked it.

Boring bars also require torsional stiffness. I beams have poor torsional stiffness.

Elliptical and teardrop tubes are used on aircraft in place or round for aerodynamic reasons not structural

When I worked for a acttual aircraft manufacturer (Norman Aeroplane Company, sheet metal in one end of the factory, aircraft out the other) in the mid 80's they still calculated stress, strain and strength manually (with calculators). No finite element analysis like Nastran. The critical structures like wings were then tested to ultimate (failure). The boss, Desmond Norman, would be very unhappy if the caluulaed and actual ultimate loads were out by more than a few percent.

Robert G8RPI.