I've been machining more aluminium recently and noticed the limitations and problems associated with using a high speed spindle at lower rpms. It's a typical 1.5kW water cooled chinese spindle.

The spindle is rated up to 24000 rpm and ploughs through wood with no problems at these speeds. But if you ever try running at this speed for aluminium using a 2 flute carbide bit it just heats up the cutter and the aluminium sticks to the tool, often meaning pausing/rewinding the job to clear it.

The guide figures for cutting aluminium with a 6mm cutter shows 4000rpm, but dropping the speed this low just stalls the motor even with a 0.5mm DOC and feeds of 300-600mm/min. So I'm forced to run at 10,000 - 12,000rpm to avoid stalling. Coolant helps somewhat but there is still the occasional aluminium buildup on the tool, most likely due to the excessive rpm.

I'm getting by for now but it feels like I'm on the edge of successful machining.

So I've started to research other options such as:
1. Reading the VFD manual for torque boost (need to check it has this and suspect it will heat the spindle)

2. Making a seperate pulley driven slave spindle (e.g. ER20 collet and straight shaft) geared down to run at 3:1 or better from the existing spindle motor, therefore giving loads of torque but lower speed. Also would mean I could fit a wider range of cutters (ER11 currently), possibly even fly cut if I could reduce the ratio still further.

3. Upgrading to a 2.2kW spindle (more money!). Anyone have a 2.2kW spindle torque vs speed graph compared to 1.5kW? I notice lots of people successfully use the 2.2kW spindle for aluminium - are you able to run at the correct rpms for aluminium or are you also slightly over speeding?

Any comments on the above welcome . . .