It may be that a Herse pattern- even a forged one- wouldn't fly in today's litigious environment, but sketchy anecdotal evidence such as "they [CNC machined cranks] all broke" cannot really be trusted to tell the whole story and could mislead. Was real failure analysis done? The problem could well have been design, Material QC problems like microvoids in the billet or heat treatment issues rather than the fact the parts weren't forged. F1 crankshafts- steel I know- for years were only machined from forged blanks because it was believed that forged blanks had better "grain aligment". As metallurgy became more sophisticated it was learnt that the asserted advantages of forged vs. billet blanks were largely old gearhead mythology. Modren F1 and other racing crankshafts are today machined from very expensive 300M steel billets then nitrided. Another more on-point example from F1 is Al alloy wheel wheel uprights which are both safety and mass critical. Machined pieces performed well here, though they've since been supplanted by Ti and Al matrix MMCs. Granted these pieces are routinely replaced after x-number of fatigue cycles, but so would forged Al pieces and if forging was significantly advantageous here, it surely would have been used.
Billet isn't billet of course. There are many sorts of alloys, treatments and grades as well as the aforementioned Al-matrix MMCs which resist crack propagation. The stories of crank failures- if true in a statistically significant sense- in those machined parts could easily have been down to simply using the wrong material for the job. People have been known to spec 6061-T6 because its relatively cheap and heck Boeing builds airplanes out of it, it must be high tech! There are also post machining processes like hot isostatic pressing and peening that can be applied.
I don't think it would be fair in any event to make a blanket statement that Al alloy (of any sort) pieces machined from billet (of any sort) are any more likely to fail in use than pieces machined from near net shape forged blanks (of any sort) or that cranks of Herse pattern of adequate strength and durability categorically cannot be made using current technology without looking a little deeper into the matter.
If those Herse cranks were anything like reliable then, they could probably be dimensionally identically made significantly moreso now taking advantage of the strides in metallurgy and manufacturing processes made since then. Were Herse cranks noted for frequently breaking in use?
Kurt Sperry Bellingham WA USA
On 7/16/07, Jan Heine <firstname.lastname@example.org> wrote:
> At 12:58 PM -0700 7/16/07, Kurt Sperry wrote:
> >I think Sugino is probably too invested in the forged blank production
> >methodology that makes it cost prohibitive to make small batches on
> spec. I
> >think they'd have to invest in super expensive dies, so they'd need to
> >pre-sell a ton of them for the project to be economically viable. The
> >CAD/CAM route means small batches can be relatively easily done without
> >per-unit cost being astronomical.
> Many early mountain bike cranks were CAD/CAM machined. They all
> broke. The few companies that remain in business either have their
> cranks forged or make super-sturdy pieces with so much material that
> failure is unlikely.
> Considering that Herse parts were optimized for light weight - from
> what I remember, an (on-topic) Herse crank weighs less than a
> (off-topic) Campy Record carbon - I don't think you can machine from
> billet without beefing up the part in a way that makes it a
> caricature of the original.
> Jan Heine
> Bicycle Quarterly
> 140 Lakeside Ave #C
> Seattle WA 98122