[trialanderror]

I was wondering if its a common practice to use premade casted blanks for high production machining operations. Still new to everything but lets say I have a part that uses about 60% of a 6" by 6" by 6" aluminum block. To save time and money of machining 40% of the material off it everytime, would I just be able to have some blanks made, maybe a few mm bigger than the end product and then machine them down from there to have the machining precision and look?

Is this a common practice? Is there any quality difference?

[HorridHenry]

That depends on the sandcasting process and what grade of Aluminium is used.

the best grade to sandcast is one that has already been cast from a good product.

[trialanderror]

I need to use 6061 aluminum. Doesn't necessary have to be sand casted but I am thinking blanks closer to the shape of the finished product makes more sense. Is this a general practice or do companies really mill up to 50% of a block of aluminum to get their finished product?

[Christian Knüll]

Casting is quite usual - for example the bearing shields of many stepper motors are sand cast aluminium with machined functional surfaces.
Forging is also used - for example pistons are usually forged out of a round blank and then machined to final dimensions.

Whether it is economical or not depends on many factors - your 6061 aluminium may e.g need heat treating after casting.

[Geof]

I was wondering if its a common practice to use premade casted blanks for high production machining operations.......Is there any quality difference?

Aluminum casting alloys have a different composition to extruded alloys. They generally have a lower ductility and a higher silicon content and need a two stage heat treating process to give them maximum strength and machinability.

The higher silicon content means they are more abrasive so tooling does not last as long and the lower ductility can result in less strength in tapped threads.

Casting a part just slightly oversize and then fully machining it is not likely to save money. The combined cost of pattern/mould making, casting and heat treatment along with the reduced machinability, would probably be more than the cost saving on the reduced metal removal.

If a part has a complex shape that can be created by casting so that very little machining is required, in other words just the surface of mating parts or spot facing for bolts, then it may be more economical to cast rather than machine from solid. However, the cost of making patterns/moulds has to be considered so casting is not practical for small numbers of parts, small being less than a few thousand up to tens of thousands depending on the complexity of the art.

[holbieone]

I need to use 6061 aluminum. Doesn't necessary have to be sand casted but I am thinking blanks closer to the shape of the finished product makes more sense. Is this a general practice or do companies really mill up to 50% of a block of aluminum to get their finished product?

some times you can use a casting , others you will need a forging , then there's using a block of material

it all depends on what your making , strength , condition and grain

a few years ago i milled a 7lb. finish part out of 85 pounds of Titanium alloy ( had to have the grain running in a correct direction )

it was cheaper to do it this way then to have forging dies made , only for a few parts

i don't know what your parts look like ,but some times you can get two parts out of one block with a fancy saw cut or wire edm

[MrWild]

Aluminum casting alloys have a different composition to extruded alloys. SNIP Excellent Post SNIP depending on the complexity of the art.

Also needs to be mentioned, your Pattern/form needs to be scaled appropriately. Aluminum shrinks and your form needs to be corrected for this. Different alloys shrink in different ways. A pattern for one alloy, might need adjustments for another.

[packrat]

With the number of stock sizes available, why not order a size that is close to the finished part size?

[trialanderror]

So different none block stock size exist? Whats a good source to order these?

If I was to have the stocks forged, would I have to heat treat them at all or will they hold the same quality as the original 6061 aluminum?

I guess the saw cut method could work as well. EDM is a bit slow no?

[Geof]

Extruded 6061 is normally heat treated and in this state it cannot be forged. It is necessary to first anneal it, then forge it and re-heat treat it. It is very unlikely that the cost of doing this will be less than the amount that is saved by reducing machining time and material removal.

On way to reduce the amount of material removed is to get a custom extrusion with a profile that matches the profile of the part. This is probably the simplest approach but even this is only worthwhile if many parts are being made to amortize the cost of the extrusion die.

[zool]

I have a friend who used cast/machined aluminum parts from a outside supplier. The part is cast [it is an oval shape], then the interior and the faces on three sides were machined so other parts can be mounted on the two ends, and this entire assembly was mounted to major sub-assembly at the bottom of the cast/machined piece.

The exterior of the cast/machined piece that has not been machined has ribs cast in to provide increased cooling surface and strength.

The mounting on the bottom has to be close tolerances and this area has given them fits. The entire assemble is subject to high heat and varying amounts of air-pressure. The torque levels on the machine bolts to join the cast/machined piece to the other assembly are not high, though there are an extraordinary number of bolts joining the two parts given the overall total length being joined. The gasket between the two pieces is made of many thin pieces of copper.

It appears that the amount of shrinkage in the casting process is difficult to control and predict. Though the company that supplies the cast/machined part has been in business for years, the improvement of the process has eluded them, and suggestions from my friend's engineers and machinists have not been well received, even though my friend's company is noted for their expertise in this type of engineering and products.

My friend's company is in the process of switching suppliers to a company that will machine the part from a solid billet. Though each individual part will be more expensive [a lot], the total cost of the part in place, that is with all the other parts attached and this part attached to the major sub-assembly, is expected to decline somewhat, though the main reasons of buying the more expensive piece made from the solid billet is to have consistency in the final product, remove the cause of engineering and machinist time diversion, lower rework machining requirements because of wider than expected variances in the cast/machined piece, maintain over-all quality, lower returns for remove and repair work, and maintain their reputation.

The cost of having diverted time of management, engineers, machinists and the assembly personnel, plus diverting the assembly personnel to remove and repair work, far exceeded the cost differential between the cast/machine part and the part machined from a solid billet. [There was hard accounting data to support these findings.]

Also taken into consideration was the opportunity cost of the diversion of all the human resources into not profitable work when those resources could have been used for profitable work. [In compiling these amounts there were a number of assumptions made, though they were conservative.]

In the end my friend [who is neither an engineer or machinist, but a really smart and clever fellow], the founder and owner of the company, made his decision based on this: The old way is too much of a pain in the ... butt. [Though he was a bit more pointed about the portion of the anatomy being in distress.]

So you can judge the above: I machine wood and have built two CNC routers. My education is accounting, finance, economics and math at both undergraduate and graduate levels, and I did financial consulting for 25-years before taking up woodworking. For the above matter: I completed the higher level data collection and analysis, plus compiled the data and completed the calculations for the opportunity costs estimations. To be succinct: I am the bean counter!

[packrat]

So different none block stock size exist? Whats a good source to order these?

Not knowing exactly where your located can't give a source, but if you Google aluminum suppliers in you area you will get hits.

Heres an example - Ryerson Stocklist Aluminum Bars