Spindle power on the 440; what RPM is best?


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    Default Spindle power on the 440; what RPM is best?

    I have a 440; I've had it ever since TechShop went belly up and I couldn't use their 1100 anymore.

    In general, when I take easy cuts in aluminum, it works just fine; I don't seem to have the Z or Y axis chatter that some people complain about, and the finishes are good enough for my needs! Full enclosure with flood coolant and power draw bar makes working with it convenient, too.

    Today, I was slotting in 303 annealed, using a 3/8" (Maritool carbide) end mill in high quality ER20 collet, using a 1.8mm stepdown (that's 70 thou for you still stuck with imperial.) Using flood coolant to clear chips, which works great. However, on the second pass, it bogged down, and stopped, and I'm somewhat annoyed at this. I was spinning at 3000 rpm with the belt on the low-RPM range (so equivalent to 6000 rpm on the high range) and feeding at 13.78 inches per minute.

    Formulas on the interwebs, and GWizard, says that this cut (this MRR in this material) should use about 0.36 horsepower, or 270 Watts. The spindle is rated at 500 Watts, or 2/3 horsepower, so this cut should have been only half of what it's supposed to be able to do. I thought I took it easy, but yet another end mill and yet another piece of stock goes in the trash. I have similar problems with fly cutting; cuts deeper than 10 thou will often just stall up (although the insert and even stock is often salvageable in this case.)

    So, is there really that much difference between 3000/lo RPM compared to the optimal RPM in horsepower? What would the optimal RPM be? Or, (and this is my pet theory) is the PID controller for the VFD for the spindle too slow to react, so it doesn't have time to add more power between one alternation and the next of the BLDC motor, and thus once it stops up a little bit, it's in a death spiral and immediately seizes up? Basically, it has to be ready to dump all power into the windings on the very next commutation if it detects that the spindle is even a little slow; it doesn't feel like it's doing that. (Motor control nerding here :-)

    I understand that a small mill that fits in my laundry area is not going to be a 10 kW monster; I'm more interested in understanding exactly what the limitations really are, and what the optimal choices are for RPM, and whether there perhaps exist patches for the VFD to make it react faster? Any experience/advice would be welcome.

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    Quote Originally Posted by jwatte View Post
    I have a 440; I've had it ever since TechShop went belly up and I couldn't use their 1100 anymore.

    In general, when I take easy cuts in aluminum, it works just fine; I don't seem to have the Z or Y axis chatter that some people complain about, and the finishes are good enough for my needs! Full enclosure with flood coolant and power draw bar makes working with it convenient, too.

    Today, I was slotting in 303 annealed, using a 3/8" (Maritool carbide) end mill in high quality ER20 collet, using a 1.8mm stepdown (that's 70 thou for you still stuck with imperial.) Using flood coolant to clear chips, which works great. However, on the second pass, it bogged down, and stopped, and I'm somewhat annoyed at this. I was spinning at 3000 rpm with the belt on the low-RPM range (so equivalent to 6000 rpm on the high range) and feeding at 13.78 inches per minute.

    Formulas on the interwebs, and GWizard, says that this cut (this MRR in this material) should use about 0.36 horsepower, or 270 Watts. The spindle is rated at 500 Watts, or 2/3 horsepower, so this cut should have been only half of what it's supposed to be able to do. I thought I took it easy, but yet another end mill and yet another piece of stock goes in the trash. I have similar problems with fly cutting; cuts deeper than 10 thou will often just stall up (although the insert and even stock is often salvageable in this case.)

    So, is there really that much difference between 3000/lo RPM compared to the optimal RPM in horsepower? What would the optimal RPM be? Or, (and this is my pet theory) is the PID controller for the VFD for the spindle too slow to react, so it doesn't have time to add more power between one alternation and the next of the BLDC motor, and thus once it stops up a little bit, it's in a death spiral and immediately seizes up? Basically, it has to be ready to dump all power into the windings on the very next commutation if it detects that the spindle is even a little slow; it doesn't feel like it's doing that. (Motor control nerding here :-)

    I understand that a small mill that fits in my laundry area is not going to be a 10 kW monster; I'm more interested in understanding exactly what the limitations really are, and what the optimal choices are for RPM, and whether there perhaps exist patches for the VFD to make it react faster? Any experience/advice would be welcome.
    Don't have a 440, but I would try a smaller end mill and HSM the slot at full depth at 5% of the end mill diameter for step over.



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    Default Re: Spindle power on the 440; what RPM is best?

    Quote Originally Posted by jwatte View Post
    Formulas on the interwebs, and GWizard, says that this cut (this MRR in this material) should use about 0.36 horsepower, or 270 Watts.
    This is the required power to make your cut is at the spindle. It's not the same as the manufacturers rated HP which is often inflated and/or not stated very well.

    Quote Originally Posted by jwatte View Post
    So, is there really that much difference between 3000/lo RPM compared to the optimal RPM in horsepower?
    Yeah, there is: HP = RPM x Torque. So if you half the RPM, well there goes half your power.

    Sound to me like you're just running out of torque. You really need the motor curves & VFD programming information to know how much power is available at a given RPM. It's going to be a lot less than you think & the limiting component will be torque.

    Quote Originally Posted by jwatte View Post
    Or, (and this is my pet theory) is the PID controller for the VFD for the spindle too slow to react, so it doesn't have time to add more power between one alternation and the next of the BLDC motor, and thus once it stops up a little bit, it's in a death spiral and immediately seizes up?
    The VFD is not too slow to react. It's primary function is to operate the motor at a given frequency. The motor can draw as much current as it wants up to the programmed current limit. Then it's going to stall & the VFD should throw an error code.

    I think the 440 is just under-powered for anything but AL & plastic. When you lower the RPM's to cut steel well it's the old HP vs torque debate.

    Quote Originally Posted by jwatte View Post
    What RPM is best?
    If you want maximum power it's usually around the upper RPM limit. If you need maximum torque you'll need to develop some motor curves to know what RPM this happens at. Hopefully you can cut at that speed otherwise it's a game of compromise.



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    Default Re: Spindle power on the 440; what RPM is best?

    HP = RPM x Torque. So if you half the RPM, well there goes half your power.
    While that's true, it's also true that torque goes down approximately linearly with RPM in an electric motor, because of EMF. This is why electric motors have a maximum "no load" RPM. Because of this, the torque curve generally looks more like an "inverted U" shape than a up-and-to-the-right shape. Hence why 3000 rpm out of 4500 rpm seemed, if not perfect, then at least reasonable. (When I asked Tormach, they said they didn't have the curves available, but that the torque should be available "through most of the range.")

    The motor can draw as much current as it wants up to the programmed current limit
    That's not how BLDC motor controllers (and thus this VFD) works. (It's also not how AC motors work.) Because the rotation is generated through the alternating of the phases (not by the motor itself,) the motor will attempt to draw all the current, all the time. If the VFD lets it, not only will it run very warm, but it will also cog noticeably (this causes vibration, even at high RPMs.)
    Thus, the VFD for these motors has to adjust now much current it provides in real time, based on load, to make the motor spin smoothly. It does this by modulating the duty cycle of the PWM signal to the motor windings, which happens at a rate faster than the pure commutation between the phases that causes rotation. In advanced ("sinusoidal") controllers, the PWM is so much faster than the commutation switching, that the magnetic field can be turned much smoother than just the poles of the motor would let you do normally. But, againt, the VFD needs to know where the motor is, and apply the appropriate torque to make it move at the desired speed towards the next commutation, and if the motor runs slower than desired, it needs to increase PWM duty cycle, and if the motor runs faster than desired, it needs to decrease the PWM duty cycle. This change in duty cycle is driven by a PID control loop. That PID control loop, if it's not fast enough to react, may cause the "stall of doom" negative feedback cycle when the tool suddenly meets resistance.

    When you lower the RPM's to cut steel well
    I'm only cutting at a surface speed of 100m/m (330 sfm) which should be plenty fine for this carbide in 303.

    If you want maximum power it's usually around the upper RPM limit
    For a gas-powered motor, this is absolutely true. For an old-school brushed motor, this is kind-of true, but not exactly. For AC / BLDC motors, this is not true at all, because the back EMF ends up counteracting your torque at higher speeds.

    Meanwhile, I'll use stubby cutters and thin layers... but, as you say, I'd sure love to have the actual torque curve available!

    I would try a smaller end mill and HSM the slot at full depth
    That's an interesting suggestion! The only reason I'm slotting is actually that I'm cutting out a 1.5" hole in the middle of the piece, so I'm ending up with a .75" center waste bit on a small tab when done. Maybe I could try a 0.25" or even 0.1875" and take it a bit deeper and at some slot size that allows to tool to move but it doesn't have to be 0.375".
    303 doesn't like being skim cut, though. While it's certainly better than 316, it does work harden.



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    Default Re: Spindle power on the 440; what RPM is best?

    Sorry, I did not know Tormach went with a BLDC on the 440. Otherwise, I stand by what I said about a real VFD & 3-phase induction motor operating open-loop as Tormach does on their other machines. I do not consider a VFD & BLDC the same. The VFD operates a 3-phase induction motor whose speed is dictated by frequency & the number poles the motor has - not the same as BLDC.

    Again, the machining calculators relate everything in terms of RPM & required HP. Regardless of any particular motor technology this means you need X torque. So taking your hints of 3000 RPM & .36HP you're gonna need T=.036*5252/3000 (5252 for Ft lbs) or 0.63 ft lbs (121 in oz) of torque. The calculators don't really know if your machine can deliver that even if the you're under the HP rating of your machine. This issue is exacerbated when cutting steels especially on low power machines.

    So obviously you are not constrained by RPM's and if you think you're operating in the peak torque range and your stalling out, well, you just don't have enough torque available.

    I guess what I'm trying to impart it that it's not a valid assumption to assume the 440 with a rating of .75 HP is going to supply enough torque for your cutting parameters even though the HP requirement is less than its rating. And again, the manufactures always inflate these numbers, hopefully not as much as vacuum cleaners manage to get away with ...



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    Default Re: Spindle power on the 440; what RPM is best?

    operating open-loop as Tormach does on their other machines
    Open loop? Really? That's... exceptionally ****ty. Maybe the run the BLDC in sensorless/back-EMF mode then. But then, I would presumably feel 600 Watts of power heating from the spindle even when idling, and notice cogging, and I don't think I do (but now I'm all thinking I've missed it ...)

    That being said, it seems there's less difference between a delta- or y-wound AC motor controller and BLDC motor controller than one might think; they both drive three phases in the same commutation pattern. If you run open loop without Hall sensors, it seems like you can run either kind of motor on either kind of controller. (An AC motor "controller" always tries to emulate the sinusoidal pattern of three-phase mains AC; this is a "special feature" of BLDC controllers, but a BLDC really is a Y-wound AC motor with permanent magnets -- nothing else!)

    the manufactures always inflate these numbers, hopefully not as much as vacuum cleaners manage to get away with
    I have a shop vac that's rated "6.5 peak HP." Plugs straight into a regular 15A outlet. Pretty sure they failed math.



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    Default Re: Spindle power on the 440; what RPM is best?

    [QUOTE=jwatte;2264874
    I have a shop vac that's rated "6.5 peak HP." Plugs straight into a regular 15A outlet. Pretty sure they failed math.[/QUOTE]

    But got top marks in marketing.



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    Default Re: Spindle power on the 440; what RPM is best?

    Vacuum cleaner power ratings suffer from inflated power ratings very much like many audio amp power ratings. The Peak Power rating being quoted for a vacuum cleaner is the momentary peak power drawn from the outlet. With a nominal 15A 120VAC outlet (electrically rated at 12A continuous current) in the US, the vacuum cleaner motor can momentarily draw well over 15 Amps for a *very* short period of time during startup without tripping the breaker. The actual power *output* of the motor is actually very close to zero, being exactly zero before the motor shaft actually starts rotating. As far a cleaning power goes such peak HP ratings are worthless since the vacuum cleaner is operating no where even remotely near its maximum output power. Disregard such peak power ratings altogether as worthless marketing hype.



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Spindle power on the 440; what RPM is best?

Spindle power on the 440; what RPM is best?