Poor finish, aluminium

Yes there are gassing compounds you can use to help, and Mix of diff metal that helps, but most of that is more than just the normal backyard foundry person like me is doing.
I am now trying to stay with things that have been machined, and cast for my AL sources. And like SAVARIN said use plenty of WD40 or whatever to help keep it from sticking to the tools. From what I have done, once it starts sticking it takes a lot to get it cleaned up and not sticking again.
 
As a few have mentioned above, there are aluminum specific inserts. I have gone to these over the last year and haven't looked back. I use primarily the CCGX insert (cousin of the CCMT, fits the same holders), but I've also started using the TCGX triangular inserts, and aluminum specific GTN-2 grooving inserts (although admittedly I like the HSS blade cutoff tools better). The inserts are electropolished on the top surface and ground to a razor edge on all sides. They are usually pressed with an aggressive positive rake, and have a chipbreaker which starts to break chips once you cut deeper than the nose radius.

Couple of thoughts... On a given size of insert, you will usually have your choice of nose radii. The larger radius will be stronger and allow you to feed more aggressively, the finish will also be better for a given feedrate. The larger radius requires a larger depth of cut to engage the chipbreaker, usually 1-1.5x the radius in depth of cut. This can be a problem for small lathes. The larger radius increases tool pressure causing more deflection. This can make it hard to dial in very tight tolerances (talking tenths here guys). The *minimum* depth of cut is also increased as the tool will rub rather than cut.

A small nose radius tool will make a cut even when your DOC is .002 or less, however it is more delicate and will require a much slower feedrate to achieve a desirable surface finish. I typically use a large nose radius tool when roughing or when finishing where tolerances below .001 aren't required. The smaller radius tools come out for precision finishing and boring when trying to hit .0005 or less.

I often cut dry with these tools and experience little to no built up edge in 6000 and 7000 series aluminum. If you are machining a 1000 or 2000 series (think about where you got your scrap) then all bets are off as these are MUCH more ductile for the purposes of bending sheet metal. I'd try to get your hands on some practice non-cast stock (look at buymetal.com, they're pretty inexpensive) and a few aluminum specific inserts (try shars.com, not as cheap as they used to be but still only $5-6 an insert).
 
Not exactly "mirror" finish, but the inner bore of this part was cut on the lathe. Bored out using a 3/4" CCMT boring bar (with CCGX insert). On size within .0008" in a single finish pass at .010 depth of cut. No polishing was done afterwards.

1.jpg6.jpg8.jpg
 
Nice finish on those parts.
I read on the web that the 2000 series aluminium has the best properties for machining. This series has some copper in it.
Some guy on youtube states that high temps are not needed to melt the coppert since liquid aluminium helps to melt the copper at a lower temp than the melting point for copper.
 
I suspect the chemical composition of the Al scrap pieces you melted is affecting the machining capabilities.

As Emtor indicated, some grades of Al are easier to machine...& give a better surface finish than others.
 
The issue with casting aluminum at home is, that even if you use use known material of all the same grade, and de-gas the melt, you are still going to end up with sub par results.

The reason for this, is because as cast, you have no temper/heat treatment. take raw 6061, and then compare it to some of its common tempers.

Hardness Brinell
  1. 6061 : 30
  2. 6061-T4 : 65
  3. 6061-T6 : 95

Yield Strength in ksi
  1. 6061 : 8
  2. 6061-T4 : 16
  3. 6061-T6 : 39

6061-T6, is probably the most common aluminum on the market, but 2024-T3, and 7075-T6 are better to work with.
 
emtor said:
Nice finish on those parts.
I read on the web that the 2000 series aluminium has the best properties for machining. This series has some copper in it.
Some guy on youtube states that high temps are not needed to melt the coppert since liquid aluminium helps to melt the copper at a lower temp than the melting point for copper.
Click to expand...

1000 & 2000 series are softer grades of aluminum typically used for drawn or formed sheet products. The 2000 series has a decently high strength for use in aerospace and is considered machinable. 6000 series is a general purpose machinable grade and is quite a bit harder than the aforementioned grades. 7000 series is a high strength aluminum alloy and is harder than the 6000 series. Generally the harder the alloy, the easier it is to achieve a quality finish (when using the right tooling). An exception is the 4000 series silicon alloy aluminum which is very hard and abrasive. 3000 Series are used less often, and are good for higher temperatures. 5000 series aluminum is considered a structural high strength grade is is not often machined.

The 4 digit aluminum numbers are extruded, rolled, or wrought. This typically increases their strength and forms a fine grain structure. 3 digit numbers are cast, and often have lower strength and a large grain structure. They machine very easily, but don't necessarily give a good finish as the aluminum grains can pull out at the cut interface, leaving micro voids which appear a a dull appearance.
 
Someone above mentioned it... it looks like a "steel" insert. Looks like TiAlN, to be exact, which does stick to aluminum. Uncoated or TiN might help with getting that finish right (for the record, I prefer HSS).
 
Dan_S said:
The issue with casting aluminum at home is, that even if you use use known material of all the same grade, and de-gas the melt, you are still going to end up with sub par results.

The reason for this, is because as cast, you have no temper/heat treatment. take raw 6061, and then compare it to some of its common tempers.

Hardness Brinell
  1. 6061 : 30
  2. 6061-T4 : 65
  3. 6061-T6 : 95

Yield Strength in ksi
  1. 6061 : 8
  2. 6061-T4 : 16
  3. 6061-T6 : 39

6061-T6, is probably the most common aluminum on the market, but 2024-T3, and 7075-T6 are better to work with.
Click to expand...

Well, being a knife maker I'm building a programmable heat treating oven. Knowing the temps and time durations to heat treat aluminium would make it possible to do just that. I found the methode for 6061-T6: After smelting, let the aluminium cool to room temp. Then re-heat to 980 deg. F.
Hold for one hour and quench in water. Heat to 400 deg. one hour or 350 deg. five hours or 325 deg. eight hours.
The only problem is that the aluminium will deform quite a lot, but this is no problem in my case since I will be casting round stock, not parts.
 
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