Cast or Machine

[Kipper]

I prefer casting as it means:- a: Less work b: Less expense (electricity is prohibitive in price here) and buying billets of Al versus getting it for free means it's a no brainer...Yet all the time I see items which are (imho) better suited to casting than machining being produced...From the swarf cut out of these items I could run a new part off So whats the lowdown on this billet buying frenzy folks

[Runner4404spd]

not everyone has a way to cast the aluminum. i plan on machining alot of my brackets out of aluminum because i don't want the hassle of casting this at home. now if i go out to shop to have the casting done, it will be expesive...read more expensive than me machining it.

[NC Cams]

Billets usually have the benefit of having some rolling or forming operations performed on them. The intentionaly yielding of the material tends to work harden it thus adding ductility to the metal. This makes it "tougher" (more area under the stress-strain curve).

Castings are quite suitable for near net shape items - especially, complex shapes that defy traditional machining methods.

Billets tend to resist tensile loadings better than castings and are inherently more robust than castings.

Examples for illustrative purposes: A cast cylinder head is both durable and affordable. Billet heads are made and are FAR stronger than mere castings BUT, they are horribly expensive. They also do NOT have good properties for long term operation due to the fact that they don't have the internal water passages that cast heads do.

On the other hand, forged con rods are constantly subjected to violent swings in compressive followed by tensile loadings. Although Briggs and Stratton made die cast rods for lawn mower engines, aluminum con rods (mostly used in drag racing) are made of either forged billets or bar stock billets - they are simply stronger.

NOTE: both billets and castings start off as molten metal that are poured into a form to make a basic shape/slab of metal to begin with. Stuff that will be wrought into a billet recieves subsequent hot and/or cold forming to work the metal so as to revise the grain stucture so as to better taylor/obtain the desired properties of the metal.

Metal that will be cast is simply molten and the desired alloy mix achieved and then poured into molds. In some cases, the metal is poured into molds so that the metal can be stored as a "virgin" alloy for remelt later.

There's more to consider but this should get you a basic understanding.

[Geof]

Originally Posted by Kipper .... So whats the lowdown on this billet buying frenzy folks

There is a certain mystique to saying something is made out of 'billet aluminum'; it almost has the same ring as 'made out of aircraft alloys'. There are practical reasons for using extruded or forged (i.e. billet) aluminum because then you have defined properties, often the surface finish can be far better and anodizing is easier.

[NC Cams]

Key words in Geof's post "defined properties".

When you check out the propreties of the material, be it billet/wrought material or simple castings, you'll see that there can be HUGE differences between the materials and the properties associated with them in their particular form.

Yes, there is "magic" in the definition but it is also definite differences in material properties.

[Kipper]

I'll be sure to obtain a basic understanding of the whole process someday meanwhile i'll keep on casting :yay: My point was why is the whole world seemingly machining 90% of the Al away to end up with a simple shape! The voting is a good indication as to why...people maybe think casting is beyond them (it's not in any way) or they dont yet have the method to do it...or the drive to try it. As far as "magic" try opening the flask to find a perfect casting in there...theres no better feeling than to make it with your hands :yes:

[Mcgyver]

Originally Posted by Kipper So whats the lowdown on this billet buying frenzy folks

the low down is a pile of crap imo - this is nice crowd here but on some of the sites you'd better duck for cover if you call AL bar stock 'billet'

My pet peeve is that rolled bar stock or plate of AL is NOT billet. Billet isn't a grage or alloy, its a bulk shape and size when AL that comes from the smelter enroute to the extruder. its a pet peeve because the word is so erroneously used, many uninformed thinks something made from billet is special vs if told them it was made from AL bar stock or the particular alloy. "I made this out of 7075" "ah man, you shoulda gone quality used billet". ARRRRG. make it out of 6061 call it billet and charge twice as much, doesn't seem right to me.

iirc billet specifically refers to the round shape that is loaded into extrusion machines, ingot being the more generic term for a bulk shape that may go on to be cast or rolled. in any event, both are cast - a billet is cast AL of in more advanced plants continuous cast AL , you can't get it alloyed so its ready for an extruding machine if the metal isn't molten. if you are going to call it billet, it least apply the stupid tax and double the price vs if it was made of AL

to the original point, I've done AL casting but would really like to try the lost foam AL casting, neat stuff. next up would be iron/steel casting or better yet lost foam iron/steel casting. then again, one must know when to stop!

[Geof]

Originally Posted by Mcgyver the low down is a pile of crap imo - this is nice crowd here but on some of the sites you'd better duck for cover if you call AL bar stock 'billet' ....

You must have been working late 7 posts down. Notice how I discreetly skated around the subject.

Kipper; to find out a bit about what my compatriot is ranting about do a search using the word 'hermeneutics' and read through the thread you will get.

[Mcgyver]

Originally Posted by Geof You must have been working late 7 posts down. Notice how I discreetly skated around the subject. .

Geof you crack me up, needed that. see, that is why you could get elected while I'm still whinin' about the government! hehe but hey I did put the smiley in - even though i'm crusty at the end of the week I try not to take myself toooo seriously, after all no one else does.

[CharCad]

Originally Posted by Kipper My point was why is the whole world seemingly machining 90% of the Al away to end up with a simple shape! The voting is a good indication as to why...people maybe think casting is beyond them (it's not in any way) or they dont yet have the method to do it...or the drive to try it. As far as "magic" try opening the flask to find a perfect casting in there...theres no better feeling than to make it with your hands :yes:

Kipper,

You summed it up. Never underestimate the combined power of ignorance and laziness. It exercises more force than the biggest lathes or mills imaginable.

I can't imagine a better time to be entering the small foundry area, either individually or for small enterprises as part of manufacturing operations. The combination of low cost CNC pattern making and lost styrofoam casting reduces start up and operations costs by unbelievable amounts.

Mark

[handlewanker]

Hi Kipp, back in '75 I did some alluminium casting using the varied and dubious quality of old washing machine bodies, piles of extruded ally swarf from turnings, and any other metal that looked silvery, light and scratched easily with a nail. I kid thee not!
To melt the mix we made an enclosure with a few firebricks laid out in a square, and used a large steel ladle, formally used for melting lead.
This was heated with a Sievert propane torch and gave us about a litre of melt material.
The casting was done by placing steel blocks together to form a cavity 75mm X 75mm and approx 100mm deep.
The metal was simply poured into this up to the brim and on cooling the steel blocks just fell away and we had "billets" of ally.
We cast many billets this way. The only thing you have to observe is that if a specific length is required then you just increase it by a half to allow for the shrinkage and the impurities which will be at the top.
The machining qualities were very good despite the varied mix.
We never used any cast alluminium, as was available in car gearboxes, due to the rough and porous nature of the metal.
I'm not an expert in alluminium but I think the alluminium castings as used in gearboxes and the like is silicon alluminium.
This material machines well, but only if you're into production, and want to prevent the long stringy swarf that extruded and sheet alluminium produces.
When you turn the stuff, the swarf breaks up the same as you get when cutting freecutting or leaded steel, but it is also prone to blow holes and rough inclusions, really crap stuff.
The texture of the cast gearbox ally is rough and brittle, with little black bits in the metal, and does not have any ductile qualities at all.
If you strike the casting with a hammer it will yield and break up into bits.
We also added a bit of Mazac or zink castings to our mix, in the form of carburettor bodies and this had the effect of making the alluminium harder to machine and produced a bright shiny allmost glazed appearance when machined.
My last excursion into melting was with a wood fired kiln type of furnace and gave two litres of mix per firing, but the furnace was hard to use and required constant charging and cleaning.
For a few simple castings, such as square or round stock for turning etc then steel moulds will give good results, making sure that allowance is made for the shrinkage and impurities rising to the top.
One of the "molds" we made was from steel, for an alluminium hammer for use on the milling machine when tapping down the job in the vice.
The mold was split, held together with hose clips and stood upright.
Then the metal was poured down the handle and on cooling had shrunk 50mm.
No special preperation was applied to the steel and no problems were encountered in the final casting.
Simple and cost little.
My next "have a go" will be electric, for alluminium, using 2KW elements wound in a spiral inside a fire brick enclosure to give about 1 litre of metal for a few projects I've got in mind. I'll probably use a steel crucible coated inside with a Plumbago wash.
Ian.

[CharCad]

Ian,

>>but it is also prone to blow holes and rough inclusions, really crap stuff.<<

Mostly that's a sign of hydrogen porosity, which almost always arises from moisture. Preheating the scrap, furnace and crucible to 400 - separately - will get rid of most of this. In aluminum founding with locally built small and likely inefficient foundries, the charge pre-heat will also get you a running start. Pre-heat can be as simple as a rebar grate and a scrap pallet wood fire.

>>the alluminium castings as used in gearboxes and the like is silicon alluminium.<<

Alloy 356 or similar is usually used for these sorts of castings. They run around 7% silicon.

Mark