Chris

I have spent a fair bit of time looking into aluminium milling reading things on-line and some assistance from Eric at stepmores.

Most of the machines can be used for aluminium but to varying standards.

The common basic machines with a 230w spindle (well more of an engraving tool than a true spindle) will engrave aluminium and with a bit of effort to get the right depth of cut and feed speed you might just about cut aluminium but the results will be pretty poor and the spindle is likely to wear out quickly due to the basic bearings. To be honest I would avoid this sort of machine unless you are only working with soft plastics and wood and have modest needs.

The next step up is the 800W type spindle and realistically it would look like this is the minimum realistic option for metal work ideally coupled with ball screw feed system. You will typically get good results for engraving and it will also do some cutting jobs but one issue you have to account for is that when cutting metal you don't need high speed you need torque so that the motor does not stall. As you will be slowing the spindle speed down probably to somewhere around 8000 RPM for aluminium the motor will loose power. The cost of upgrading to a 1.5KW spindle is not huge so is probably worth it rather than having to buy one later.

With this kind of set-up you will get adequate results with aluminium as long as high precision is not essential. The aluminium framework on the machines is sturdy but there will be some vibration, you will probably have to polish surfaces to get them smooth after working.

One of the ways to reduce the vibrations is to have a moving table machine, stepmores have SM-4040 and 6060 where the gantry is securely fixed to the heavy cast base instead of moving on rails. This is perhaps the cheapest way so the best compromise for someone on a tight budget to achieve more accurate results without having to go to the expense of cast iron. If you are going to do a lot of cutting aluminium and accuracy is important then cast iron will essentially eliminate vibration because of the density/strength of the iron. Stepmores offer Iron casting option on those two models.

You can turn your object on the table to cut each side but it does take a bit more effort to do your cutting in a way that makes sure your object can lay flat when you turn it. From what I have seen the 4th axis gadgets that turn the object can easily be turned by hand so I wouldn't imagine they would do very well for metal work unless someone has one and can say otherwise. You can also cut some underhangs using an appropriately shaped tool but you are somewhat limited in what you can do.