2024 Aluminum Carbon Fiber Knee Moulds

Project:
Create 10 pairs of left and right knee moulds measuring 4.5" x 16.5" x 6" in 2024 aluminum for carbon fiber product manufacturer.

Details:
Material: $300 per mould for raw material
Effort: 8 hours machine time + 5 hours manual polishing per mould x 20 moulds (260 hours total)
Accuracy of estimate: Don't get me started

Description:
This was a steep learning curve with lots of valuable lessons since it was our first ever real mould making job. Customer approached and suggested a single piece of 400lb aluminum with an array of moulds top and bottom, and indicated prior quotes showing ~90 hours of machine time. Our machines can't handle that size of mould, so we recommended to customer instead to modularize the moulds by breaking down into smaller 4.5 x 6 x 16.5 pieces to make them more manageable. An MDF prototype was fashioned first to get customer signoff on the shape and tolerances.

Calculations of tool bit loading and CAM simulations of toolpaths (with contraints inferred from customer's existing quote) initially put machine time in around 4 hours per mould. As machining began, horrible, screeching, waking-the-dead chatter emitted from the cutters forcing a reduction in feed speed lengthening the toolpath to 8 hours. This was later determined to be a resonance issue due to the RPM - a slight adjustment, down by 250 RPM mostly eliminated the chatter, but the risk was too great to optimize further.

As each mould was cut, 5 hours of manual sanding with pnuematic tools followed by hand-sanding with water paper, with a final buffing again with pnematic tools put a mirror finish on each mould. (The last image shows before and after conditions.) By the 10th mould, we were expert polishers. Oddly, the quality increased, but the time was constant!

Click on each photo to enlarge.