jmarsden Wrote:
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> Cool!
>
> The "how many pieces of studding of length L do I
> need to get *this* set of studs with minimal
> waste" question is, I suspect, a variant of the
> bounded knapsack problem (see
> [
en.wikipedia.org] if
> you care about the math), or something
> mathematically related to that. It's probably not
> worth implementing a solver for it in the BOM
> Generator!
interesting. i think it may be overkill to implement at this stage though =)
> Unfortunately, this issue means we should probably
> revert back to seeing each studding length (and
> rod length) in the output, so the user can "solve
> the knapsack problem" in their head. Right now,
> the output of "M8 stud, 7612mm" is precise, but
> somewhat unhelpful, since you don't know whether
> you need 8 1m chunks, or more than that. (Worst
> case, if one piece of studding needed was 97mm and
> the rest were all 501mm, you'd need 15 1m
> chunks!). In practice, based on data from a BOM
> parts list I made on 16 July, I think you need to
> obtain 10 1m chunks of studding to get what we
> need.
yeah, good idea. i'll revert back to showing each length for now.
> FWIW, on July 16th, I costed out the fasteners and
> studding from Amazon at US$73.51 for fasteners,
> plus US$35.90 for M8 studding, plus US$5.40 for M3
> studding, for a total of US$114.81 plus tax and
> shipping, just for a bunch of metric metal ... and
> that doesn't include the rods and other stuff only
> listed as available from McMaster.
one of the things i'd like to try doing is using 5/16" rods/studs instead. they are MUCH cheaper to get in the US, and are only 0.0625mm smaller. hopefully that works out. i'm also looking into finding someone to make us a fastener / rod / stud kit with pre-cut lengths as well as the proper numbers of nuts/bolts. this will have to wait until the darwin design has been through some heavy testing.
on the idea of optimizing the nuts/bolts on quantity, it would be possible to add a quantity field to each supplier and then do some math to find the best combination of products to get the best price. however, this starts to get really tricky and we're probably going to want to switch over to a real database if we want to do stuff like this. its definitely going into v2.0 of the BOM generator.
>
> While I remember... it might be handy if the final
> parts list could also include the set of "how many
> of which subsystems" info that you entered on the
> initial screen (omitting subsystems whose checkbox
> you didn't check, obviously)? I now have a bunch
> of BOM Generator generated printouts and saved
> HTML files (just testing!), and remembering what
> the input data for each one was gets challenging
> :-)
good idea. it shouldnt be hard to add in.