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  1. #1
    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 . . .
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  2. #2
    I haven't tried milling aluminium yet but plan to very soon. I'll be very interested to see what you discover.

  3. #3
    You could try a sensorless vector VFD that should give full torque at zero speed but people, myself included, seem to have problems getting them to work with high speed spindles.
    AC Inverter Drives (230V) filtered by Input: 230Vac 3ph, Power: 1.5kW (Page 1 of 3)
    Spelling mistakes are not intentional, I only seem to see them some time after I've posted

  4. #4
    Thanks Eddy. I'll look into that but I would need something which I knew definitely worked with a spindle before purchasing. Also if I ended up spending that much I probably go for a 2.2kW spindle + new VFD instead which is not too much more money.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  5. #5
    The 'geared' belt driven idea sounds juicy. I thought you might get a few more replies on this, regarding how others do it successfully. There's not been any definitive answers yet, or at least a bigger sample of a variety of solutions that work for different people. It may be that there are so many parameters that need controlling or considering, and you just have the odd one not in your favour, but I hope you don't have to resort to too much empiricalism to sort it.

  6. #6
    Can't help with torque guides etc but I can tell you that 2.2Kw cuts ali fine with speeds down to 5000rpm thou to be honest I mostly cut between 8-12K depending on cutter size.

    Now if your getting Sticking and melting chips then I suspect your cutting to shallow so getting very thin chips which heat up quickly and melt. Try cutting deeper and you may well be pleasantly surprised what depth you can cut at. Set the VFD to amps and watch the load this will give you an idea of how the spindles coping with the depth.!
    Last edited by JAZZCNC; 12-02-2014 at 10:23 PM.

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  8. #7
    Thanks for that Jazz. Gives me some hope for experimentation with the existing spindle. The machine is still under final construction so when I am set up properly I'll include higher DOC in the runs. I've only done a bit of quick machining to help build the last bits of the bed so used the 0.5mm DOC from the previous much weaker machine.

    I'll also dig out the VFD manual and set up the display for amps. Seem to remember this being an 8A VFD but it was a while back so I'll confirm so I can see how it is coping.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  9. #8
    Hi Charles,

    I think the biggest factor is not buying the 2.2kW spindle to start with! It's not much more than the 1.5kW spindle, takes the much larger ER20 collet, and although I suspect it is a little heavier, I can't see any real world downsides.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  10. #9
    Generally speaking the best speed for aluminum is 8000 rpm. Thats what my tests show. 25 ipm feedrate is good feedrate to start with. No coolant or soap water or sprayed alcohol via airbrush.

    I believe the problem is in the VFD, as with 0.8kw spindle i have no problem at that depth and feedrate / my machine is not strong/.

    Torque boost is for very low speeds. The VFD should be set to Constant Torque.

    Single flute bits work best if speeds are problem.

  11. #10
    Quote Originally Posted by silyavski View Post
    Generally speaking the best speed for aluminum is 8000 rpm. Thats what my tests show. 25 ipm feedrate is good feedrate to start with. No coolant or soap water or sprayed alcohol via airbrush.
    That's far too general and you can't say "Best Speed" because there is no "one" best speed for cutting aluminium there are far to many factors come into play which determine the speed.!! . . . . Material grade, cutter size, No flutes, DOC, Cut type ie: slot or side cutting, step over, coolant type . . . .And thats before you get into Spindle power and machine strength.

    Can tell thou 100% There is no ONE speed does all and you'll fluctuate between 7-15K when cutting ALi with 4-10mm tooling with varying no of flutes and DOC & S/F etc.

    Experiment is the best way to learn speeds & feeds and dont be affraid to cut deep, just be prepared and accept that you will break tools while finding the best cutting conditions for your machine and material.
    Last edited by JAZZCNC; 13-02-2014 at 04:36 PM.

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