Question about CNC milling (and also about brass casting)

(Also asked on rec.nodels.engineering)
I'm thinking about how to make some ornate guitar roses, similar to this one:
http://www.maxwellplace.demon.co.uk/pandemonium/rose.jpg . The original is cast brass but I'm wondering whether a CNC mill could be used to mill/carve the basic shape in a dense hard wood so I could then hand-finish the carving and gild it with gold leaf. Has anyone used a CNC mill for anything similar, or are there any CNC experts that could tell me how easy or hard a task this might be?
(Hidden agenda: this would also provide the excuse I've been looking for to buy a small CNC mill ;-) )
Alternatively, is there a sensible way to produce a brass casting to replicate the original?
Dave
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http://www.elektor.com/magazines/2007/january/milling-machine-from-a-kit.58644.lynkx
Any use ? See the free download of the original article on the page. There was recently an updated version of it, I seem to recall.
Arfa
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On 20/06/2011 01:33, Arfa Daily wrote:

http://www.elektor.com/magazines/2007/january/milling-machine-from-a-kit.58644.lynkx
Interesting, I haven't seen Elektor since it launched when I was a kid and I hadn't realised it was still going. The kit looks good at first sight but it's nearly Ł1.5k and is relatively lightweight, but Arc-Euro sell their "proper" KX1 CNC mill for Ł2.6k and this would be more use overall. Maybe the answer is to look for a secondhand CNC mill.
Dave
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You can't afford it. Unless you're phenomenally lucky (hang around tech colleges) there's not much useful to be had S/H. Industrial kit is too big, educational kit is too wimpy and also ridiculously priced. The only S/H stuff worth having is that very narrow range of apprentice training, with machinery bought specifically for apprentices.
CNC mills are not a practical home option. With a couple of Łk, three phase, air supply, forklift access and a concrete floor, then you can have a a nice S/H machining centre with a tool changer (now you're talking!) Add another couple of Łk for tooling.
What most home "CNC mills" are are actually high-speed routers. These have manual tool change in a collet chuck, so there's no end registration. Repeatable depth setting after a tool change is thus a bit hit & miss. Still useful though. They trade low leadscrew forces (and thus allowing cheap steppers turning leadscrews, rather than needing expensive ballscrews and servomotors) for needing a high-speed small-diameter cutter that will work plastics, brass and aluminium. You also start to care about precise free-cutting alloys and distrust chilled castings. Still fun and useful though.
For home-build, the popular option at present is a US-built Taig mill and doing your own CNC conversion. I'm not a fan of these - they're cheap, but you throw too much away. The electrics are the usual hideous American crap and the 240V conversion from taig is just plain unsafe (no earth continuity through the 110V transformer). It's also usual to throw away the Taig spindle (either one, they're both too slow) and to replace it with a clamp bracket and something like a Kress router at 20,000 rpm. Stepper mounts are bootstrapped or bought from someone on the same web forum who already has their mill up & running. Steppers, coupling, stepper drives and a copy of Mach 3 (interprets G code and turns it into stepper movements) all cost money, but not much these days.
For CAD, take a look at CamBam. This is mostly there to import .dxf from AutoCAD (i.e. designs) and make G code (i.e. machining instructions) from it, but it's also usable as a CAD tool directly. Just don't ask me to _like_ the UI. Good web community.
IMHO, home CNC means about three machines as a reasonable level of comfort. Something small for doing jewellery, engraving and PCBs (maybe a Proxxon), something Taig-sized in the middle for making "model engineering" and something with two and a half axes and a 4'x4' flatbed for routing out MDF & plywood. If you have a fast spindle on a small machine, you can afford to have a slower spindle on the mid- range mill and do something resembling milling in the softer steels. Not easy to make the leadscrew work mind you, and that typical model engineer's vertical mill that everyone has just isn't precise enough to convert. An MDF cutter is great fun, easy to make your own frame for, and you can put it into a box with its own dust extract.
Toymaking in MDF gets fun with CNC, as you can make repeats of complicated parts. http://quercus.livejournal.com/300504.html http://jarkman.co.uk/catalog/cnc/batwheels.htm
OTOH, these were done on a Taig machine, took an hour for each big wheel (1/2 hour if I'd optimised the tooling and toolpaths) and made a filthy amount of MDF dust. They'd have been much cleaner on the router machine since.
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This would be a reasonable piece of CNC work. Take it to the usual web forums for more on home-built CNC.
CNC is not a good approach for perfect replication of cast pieces. Casting and patternmaking are additive, CNC is mostly subtractive. A fine intaglio detail in the piece can be simply impossible with really good CNC, because it's smaller than the tool diameter. For a heavily decorative piece like this you might need to go back to a fairly simple version of it and then re-add detail in a way that's more amenable to milling.
You could also machine it additively, with something like a RepRap (as if!), a Makerbot, or a credit card and a trip to Shapeways. If you want it in metal, Shapeways and the like are a good option.

Brass sucks, because zinc is awkward (and unpleasant) to deal with. You're better using a bronze or bell metal alloy, made from copper, allowing maybe a smidgen of brass & lead solder, and 10-15% tin (old pewter mugs). This can be much more fluid to work with. Leaded bronzes can be handy if you want a nice clean engraving or CNC metal afterwards.
Most of my bronze casting is done as lost foam patterns in Petrobond sand (try MUTR for small quantities). Sometimes I use wooden patterns, if it's a batch run. Wood or foam will both CNC quite easily for patternmaking - usually MDF is best. This would work nicely as an MDF flat-back pattern. I'd then consider hand engraving it to add detail.
3D scanning is getting easier, but for this I wouldn't worry too much. A flat lattice, then engrave it, would do for starters.
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On 20/06/2011 02:05, Andy Dingley wrote:

Andy, this is a great reply with some very useful advice - but it led to me spending the past two hours reading about 3D printing and now I want to make a 3D printer. What have you done! :-)
I don't have any experience of casting and I think I'd get someone else to do it for me if I took that route. It sounds like you're suggesting that it wouldn't be possible to cast the level of detail shown in that picture, so how did they do it in the 18th century?
Dave
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Don't bother - they're slow, they're crap, the finished results are lumpy and half the machines out there are vapourware.
3D printing is great - but you do it by uploading files to Shapeways, who then do it for you properly, out of better materials.

Practice.
I can't do lost wax casting. Keep trying, never works right. The worst part is that I know some barefooted bloke in Memphis was knocking this stuff out thousands of years ago, and all he had was a lump of copper and some bees. I just can't get a good enough burn out to remove all the wax, and I get blow-holes. I've a new 1000C kiln now, maybe that will help.
Bronze and pewter casting is great fun - other sorts of casting aren't, because they're either too difficult, too hazardous or (for silver) usually too expensive.
You need a foundry to do it in. You really do need a foundry - a reasonably large fireproof shed where you can do serious foundry stuff without causing trouble. It needs space, it needs space to spill molten metal and still leave room to jump. It needs to be fireproof against gas burners and hot metal. It mustn't have a lawnmower in there too, either petrol or a set of flammable plastic garden chairs. You also need good ventilation. You also need a lean-to shed alongside your foundry shed, to keep the propane cylinders in. It mustn't also be used for woodworking, so that every corner is full of flammable shavings. So it's a serious investment in space and clear space, used only for dirty hot metal and nothing else
If you have such a space, go to it. Lots of amateur books around on how to do it, lots of jewellery-scale stuff, then get some 1950s engineering books on how to do it right (runner and riser design needs you to read the books first).
You need a hot crucible. This is a real crucible (a tenner), a bunch of firebricks in a welded steel cage to make a heating furnace, or else a small gas cylinder filleted and lined with kiln insulation. Your burner is a big gas burner, which you buy from a charming evangelical Christian guy in the USA. They're made cheaply from standard plumbing parts (someone's web site design) which everyone uses, but if you can't just go down to Walmart and buy a #2 nozzle, it's easier to buy a ready-made from the Christian chap - he puts little tracts in with the shipping package. Bless.
Your casting is done in a bed of Petrobond sand, which you keep in a closed box or sandpit when not in use (too expensive to do a whole floor). Mine's in a big Quality Street tin, which is plenty big enough. Don't mess with greensand, it's too difficult. Lost-foam casting is an interesting process (yellow insulation foam, cut with scalpel & sandpaper) but it needs ventilation. With the Petrobond, I can also use re-usable patterns, often CNC-ed from MDF or multiple MDF pieces. MDF patterns need to be shellaced, to seal their edges against moisture and to smooth them.
Safety kit is important, but fairly cheap & simple (in comparison to the heating rig). Every part of you needs to be covered with either thick leather, Kevlar or polycarbonate. You also need to be as waterproof as a roof, i.e. any splashes will fall off, not in. A welder's apron and a welder's jacket are cheap. Rigger boots are fairly splash proof, so long as they're leather (not vinyl) and they're inside your trousers, not outside.
You also need some tools to handle a hot crucible. Usually a one or two person ring carrier for carriage and pouring, and a set of long top-grab tongs for lifting it from the furnace to the carrier. Also a stand for the carrier, when you're loading it. This is easy stuff for a bit of welding and some basic smithing.
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RevK's blog currently has some images of a mis-shapen pink Dalek and burnt out stepper motor controller.
http://revk.www.me.uk / http://revk.www.me.uk/2011/06/extrudiate.html
--
Alan J. Wylie http://www.wylie.me.uk /

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