[QuinnSjoblom]

I've been dreaming about a mill turn spindle for my mill but after looking at options, it's very difficult to build something capable of 2500rpm and also enough holding torque and resolution for precise index milling, even with a huge servo. More realistic to have 2 gearing options which would solve the issue, but another option in considering is to just build a nice 4th axis that can go relatively fast since I don't actually need to do much turning.
How slow can I go if turning a chunk of aluminum, let's say 1 inch? Would I be capable of getting an accurate cut with good finish at only 200rpm? What about 400rpm? Aside from taking forever, is there a lower sfm limit for getting a decent cut?

[mcardoso]

I have found that with a sharp tool you can go pretty slow and still get a decent cut. I use high polish ground inserts for aluminum like the CCGX and get very acceptable running 1/4 to 1/2 of the published SFM. You aren't making anything fast by doing that, but it is ok.

[QuinnSjoblom]

I have found that with a sharp tool you can go pretty slow and still get a decent cut. I use high polish ground inserts for aluminum like the CCGX and get very acceptable running 1/4 to 1/2 of the published SFM. You aren't making anything fast by doing that, but it is ok.

Sounds good. Would you say I'm more likely to get tool deflection at these low speeds? There's one scenario where I would need to run a small boring bar that needs to fit through a 17mm bore reaching in about 50mm and open up the bore behind it to 19mm for 6mm of depth. All the boring in front I'll just do with drills and reamers, but that one little step behind needs the boring bar. Hoping I can stay accurate for that as it's a bore for a bearing and it'll be hard to sneak up on since I don't have a good way to check the progress. Can't use a telescoping gauge since it can't be pulled out after opening. Maybe a dial bore gauge would get in there but not sure. Would probably just need to dial in that cut on a dummy peice without the material in front of it blocking the way for a gauge pin or telescoping gauge. Once the code is dialed in on the dummy, use it on the actual part. But yeah, not sure how accurate I can be with a little boring bar at such low speeds. Just gonna have to try and see how it behaves

[skrubol]

I think as long as you aren't using exotic high-temp coated tools (which I don't think you'd use for aluminum anyway,) you can't really go too slow. Chip breaking and clearing may be impacted, but that's more of an issue milling than turning. I saw a video last night using a form tool in brass where he was getting chatter, so for the last pass he turned off the lathe and turned it by hand (Clickspring on youtube, his latest video.) Granted that was brass, which can be very forgiving, but I can't imagine aluminum would be that much worse.
I don't think tool deflection should be any worse than running at proper speeds.

[Jim Dawson]

I agree with the above, you can cut aluminum really slow with sharp tools.

[ERUS]

You can go as slow as you want as long as the machine has the torque to do it. There really is not a upper or lower limit with aluminum and carbide tools. At the upper end the problem becomes being able to evacuate the chips from the flutes fast enough that the tool doesn't load up, the bottom end is does the machine have enough torque at that low of a speed, which most commercial machines without gearing do not have.

If you think about a 10ft. diameter part spinning at 1 rpm - every second about 6 inches of material is passing through the tool.