Re: So far OT it's over the horizon: Anyone into 3D modelling?

Guy Fawkes wrote:

>
> Catman wrote:

<snip>
>
> and for a tenner a pop???
>
> you have LOTS of problems here.......

Yes, but can we stick to the topic? ;)

> 1/ x + y + z axes of _any_ precisions use something known as ball
> screws, 2.5 metre ball screws have a certain minimum cost, even if you
> cut out ALL the middle men you'll be looking at IRO 500 quid per axis
> per rail, and you'll get a precision of the order of a thou for
> that....

*more* than sufficient.

> yes, you could use precision threaded bar and a couple of nuts to
> eliminate backlash, but it's gonna wear quickly, take more power to
> drive, and "finesse-ing" it so it works well is going to put the cost
> beyond a cheap ball screw
>
> 2/ while the "screw" bit provides movement, that movement needs to
> slide along something, so you'll need some sort of rails, again you can
> use the pukka stuff and spend about 500 quid an axis and not destroy
> your precision, or you can try and DIY from stock sections and spend
> the same money getting it to work and destroy your precision...

TBH precicsion ~1mm would probably be sufficient....

> any machine with axes of the order of 2.5 metres is going to have
> issues with thermal expansion affecting precision
>
> 3/ motive power, cheapest option here is stepper motor and driver
> boards, your machine is fairly beefy so these need to be fairly beefy
> too, say 100 quid a stepper and up to the same again for the driver
> circuitry
>
> only gotcha here is stepper is no feedback, eg "move x amount"
> instruction is sent out, there better be enough servo power to overcome
> any loads induced or that movement won't happen, but the machine will
> simply carry on as if it has.

Oh aye, unless you stick an optical encoder (for example) on it

> you can get around this with feedback loops and servos for a closes
> loop system, but lots more money an not necessary as long as you don't
> try to "drive" it too hard
>
> 4/ control, this is the easiest bit, any old PC and a copy of EMC which
> is linux with real time extentions designed especially to control CNC
> machinery

Cool

> 5/ "work" material and "tool" is difficult, say your material is
> aluminium of some grade, a one horespower tool driving motor driving
> say a 6mm end mill is gonna take FOREVER to chew through a 2 metre a
> size block or work, you want faster production then before you know it
> you have a 40 bhp 3 phase motor, and your gantry just had to get 3
> orders of magnitude heavier to accomodate it, and so on and so forth...
>
> I can put you in touch with people who have made 2 metre by 2 metre by
> not that much machines to do things like router wooden doors and
> panneling, which will get you part way there, but....
>
> off hand the only materials I can see you getting and man handling in 6
> cubic metre blocks are "man made" foams like polystyrene, the "oasis"
> stuff flower shops use, and stuff like that

That would be ideal. And it was a typo 2x1x1 would be fine

> right away your precision is down to something like 1mm

That's convenient, then

> the size makes me thing you're trying to model something like
> motorcycles, life size?

No. Nothing like. Much *much* simpler.

> will you want to paint them when done? if so this affects material
> choice too...

No, raw state is fine Doesn't even need to be particularly smooth, though
it would be nice.

> certainly a tenner a pop means you aren't going to be using anything
> more structural than something like a ploystyrene compound

Yep.

> you could go the "add" instead of "remove" material route with a nozzle
> instead of a router but that is going to slow you down even more and
> make even 1mm precision real hard ("hard" is another way of saying
> "expensive and / or slow") to do

Heh. We were wondering if it might be possible to create a mould then fill
with polyurethan foam, or similar. That woul suit our purposes.

> let's assume you are wanting to make motorcycles, accurate
> dimensionally to 1mm, to be finished by painting...
>
> a/ I'd say you'd be right to be very pleased with yourself to build the
> CNC machine for 5000 pounds excluding work head, whether this be router
> or extrusion or hot wire or whatever

Well we could probably go to 5-10 times that amount, although we'd rather
not

> (you prolly have 200 hours work assembling such a machine and setting
> up too)

Not really an issue

> b/ I'd say you'll be looking at making your own blanks on site from
> some sort of foaming plastic, so blanks are prolly gonna cost 30 quid a
> pop

OK, that we can probably accept

> c/ I'd say you make about 5 models before you figure out how to make
> one properly, just pray this doesn't entail a fundamental machine
> redesign, I mean here mebbe cut two female half moulds (if the model
> has appropriate symmetry) and vaccum from a thermoplastic in there

Sounds good

> d/ I'd say your finished models are still going to come out nearer 100
> quid a pop in materials than ten, and they will still need extensive
> hand finishing, and labour ain't free
>
True

> e/ I'd say you might get one model in 24 hours of machine time (you
> aren't going to get any "rapids" with axis movements when you've
> mastered "roughing" and so on and so forth, so that's 24 hours of
> machine attendant watching it.

Not acceptable

> f/ NOWHERE in this have we gone anywhere near ancilliaries, such a
> compressed air of vacuum for chip removal, lubrication, which all
> requires significant amounts of power, thermal management, which can be
> a bitch and very expensive, and most overlooked, power consumed by the
> machine itself and machine maintenance in between runs, which could
> scare the crap out of you

Nah. I'm not funding it :)

> ------------------------------------------
>
> Don't let me convince you what you want cannot be done, it can be, but
> commercially it is vastly expensive, so if you want to cut the cost
> down you'd better be prepared for backbreaking work and ingenuity and
> sacrificing a lot of functionality here and there, and approaching a
> compromise solution, if you do not accept this them you will fail and
> waste a lot of time and money doing it, and be turned off the whole
> thing CNC wise, which would be a shame.

Well, the thing is that the technology we want to use does seem to exist,
but can I damn well find it?

>
> HTH etc

Very much, ta
--
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www.cuore-sportivo.co.uk
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