Composite Bracket Stiffness

Offending bracket

One of the first observations I made after successfully mounting my desktop was how compliant in torsion it was. This error motion was previously a source of concern — I decided to make fibreglass-reinforced nylon brackets because it turns out the 12-gauge steel framing brackets I was planning on using would have yielded at the corners under full design load. Some preliminary testing with 20 lbs of dead weights on the edge of the desktop showed that the brackets were significantly less stiff than I expected.

Error in Analysis

Going back through my error budget spreadsheet, I realized that I had made the mistake of using material properties forĀ fiberglass instead of the composite material, resulting in my modulus being off by a couple of orders of magnitude. Looking back, this is really a juvenile error — the Markforged printers lay down continuous-fiber reinforcement instead of extruding chopped-fiber filled polymer, so the resulting part stiffness really depends on how much reinforcing fiber you put into your print and where you put them. I should have taken more time to do a sanity check when designing this part.

Remediation Plans

Fortunately, I managed to finish my desk about a week before the demo, so there will be time to fix this. Simply making the same part out of aluminum would give me all the stiffness I need and more. The easiest way to do this would be to reproduce the sandwich structure by stacking two aluminum angle sections to give the two parallel projections. However, I didn’t like the aesthetics of this design, so I decided to machine my replacement brackets out of some scrap aluminum stock I found at the Makerworkshop. This is obviously not the most efficient way to manufacture these parts, but they are going to be highly visible, and this desk is going to be a onesie, so I decided to do it anyway.

The print for this part can be found here.

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