Hi,
depending on the grade of aluminum to a certain extent then the rotational speed must be such that the surface speed of the cutting tool
is 200-500 m/min for aluminum. Each material has an optimum surface speed.
Thus with a small diameter tool you spin it fast to reach the target surface speed and for a larger diameter tool you spin it more slowly.
Having a lower surface speed than recommended won't hurt but going to much higher invites overheating of the tool and Built-Up-Edge (BUE)
wrecking the tool and the job.
Lets do a few sample calculations:
Assume a target surface speed of 250 m/min, suitable for 6061 say.
With a 6mm endmill:
circumference= PI x diameter
= 3.141 X 6
=18.8mm
= 0.0188m
rpm =surface speed / circumference
= 250 /0.0188
=13,300 rpm
With a 20mm endmill:
circumference=3.141 X 20
=0.062m
rpm = 250 / 0.062
=4032 rpm.
With a 1mm endmill:
circumference= 3.141 X 1
=0.0031m
rpm=250 / 0.0031
=80,600 rpm.
You can see that larger tools don't require high rotational speed but when you get to small and very small endmills
then high rpms are desirable. Note that most of us can't afford 80,000 rpm spindles......that's air bearing territory or at least very
expensive ceramic bearings.....way too costly for me.
If you intend on using small (1mm) endmills then by all means get a high speed spindle. For more normal size endmills then really high speed spindles
are a waste of money.
Just as a matter of interest with a coated carbide endmill cutting mild steel you want a surface speed about 100m/min, ie a lot slower than aluminum.
For stainless you need to get down to 50-60 m/min and titanium and superalloys 20-30 m/min. Thus for tough metals like steels and stainless you want
high torque slow speed spindles.
For aluminum, brass, copper higher speed spindles are OK but you don't need to go overboard.
If you want to try H(igh) S(eed) M(achining) toolpaths then you will need 30,000-40,000 rpm but be prepared to pay $200-$300 for each CBN tool.
I would not recommend going there until you have lots of miles under your belt....its a really good way to spend lots of money and achieve very little
otherwise.
I like HSM Advisor:
https://hsmadvisor.com/
You can download a free trial.
Craig