Router / Spindle specs for a newby

You don't seem to know what direction you actually want to go. Unless you want to spend more than a new car, you're not going get a 5 axis stainless milling machine.

What material?
What thickneses?
What's your work area?

Your router/spindle doesn't matter if your machine can't do it.

 

I was in the exact same position about 5 years ago. Long story short: not gonna happen. There is a pretty exciting voyage of discovery along the way if you want to dive in, though.

You're gonna have to put together two machines for the different purposes. The limiting factor isn't the spindle- rotational energy is rotational energy- but the machine structure itself.

Wood is soft, machined in large quantities at a time, doesn't require too much rigidity, etc. Something along the lines of an OX might be good, depending on what you're trying to do. The rigidity of aluminum can help you there if you're trying to add more z-height to start rotating parts. Laguna Tools have some decent machines you could base a build on.

Getting into metals, you have a lot more considerations going on. Aluminum can often be machined with woodworking tools, but the results may be variable. Stainless is a beast to machine; you've got to be careful not to work-harden it to the point where it can't cut and you scrap the part, which requires the force to push through it and never allowing the tool to rub, essentially. This means LOTS of power in your drives. NEMA 34s, ex-industrial servomotors, etc. There are metallurgical concerns with it too, like avoiding precipitating the alloy ingredients out, decarburising it, etc due to heat. That's a rabbit hole to look down separately though.

Generally the main consideration for metals is surface finish, which is simplistically a product of machine mass. The heavier the machine, the more rigid and damping it is, the less everything vibrates, the smoother your finish. The runout of the spindle also matters there- cheap inaccurate spindles (ie. woodworking routers) are no use. The build of the spindle also matters- you'll rapidly destroy small bearings if you're constantly forcing them into hard materials. You've got to be aware of the dynamic loads coming out of your system. The mass of the machine is also kind of proportional to the size of the work being done, because your unsupported axes (sometimes the Y will actually be on the column in 5-axis machines) have a very limited travel before the beams start flexing, the tiny inaccuracies of the linear rail get amplified out by trigonometry, and the harmonic frequency of the whole assembly changes. If you want to get into metals, prepare for a lot of reads like this one: Six keys to more-precise linear motion. If you want to build really big metal parts in one shot, you'll have to build a REALLY big machine. A normal size machine, you might be limited to maybe a couple inches cubed or so.

If you're really looking at >3 axis on steels on any kind of budget, I would go traditional head over high speed spindle; find a used knee mill for around $3k or so, and work on adding a trunnion and rotary table to take it to 4 and 5 axes. You could get a new, small, cheap benchtop mill to convert for maybe $1100-1500, but when you start limiting machine mass (even if you fill the base/column/whatever with concrete) and z-height before even adding extra axis hardware, you're gonna be limited to fairly small parts. The alternative is learning how to build a machining center from scratch from bulk composites and stock steel welded up (the rigidity of aluminum is actually problem when you move into machine tools), for which you could use a spindle. Either way, it's well beyond an OpenBuilds hardware project, and the software & electronics is the least of your concerns (I did cover a little of that right here anyway: 4th Axis DIY Mill?)