Mill's with VFD's, torque problem?
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    Default Mill's with VFD's, torque problem?

    I see some of the newer Bridgeport copies like the Sharp LMV can come with a VFD drive that goes up to 4500RPM. Have any of you used them ? I can only assume that it still has the Low and High range gear? Are there any torque issues when running at low RPM or is the motor big enough to still get some work done? would it quickly run out of torque if you put a 1.5" face mill and try to cut some stainless with it ? I also see that these mills with the VFD use 2 extra little fans in front of the motor to keep it cooler. Does the motor have its own separate enclosed fan on top that runs all the time(like it should?) or is there only these 2 little fans? looks like an after thought to me and like they could possibly get hit with a wrench when changing holders. Any thoughts on that?

    I wonder if its worth the extra money or if its better to stick to the Variable pulley drive.

    Thanks

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    Also, anyone know who makes this Sharp mill? I can only assume its made by First Machine in Taiwan? Hard to tell all those mills apart as there's a few manufacturers and they all look relatively the same, like the ARGO mills... I'm used to the First mills but never used the Sharp before.

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    I would get the belt drive unit. Then install your own vector drive later. This would also mean you could run either single or 3 phase input. There are a number of manufacturers that size their diodes and capacitors for single phase on the smaller drives. Maybe up to 5 HP.

    Any motor that is run at a speed substantially below base speed needs forced air cooling because there is not much self generated air flow.

    With a belt or gear drive you get approximately constant HP independent of spindle speed. With any electrical motor speed control and no variable mechanical gearing it is more like constant torque.

    .

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    There is no "torque problem" with a VFD properly matched to the motor it drives. The motor develops its full rated torque at full load amps regardless of RPM (within reason). The problem is when physics challenged operators attempt to run large cutters at low RPM by turing down the VFD instead of using the mechanical speed reduction.

    An electric motor is a constant torque device. Torque is twisting effort regardless of RPM. Motor HP is a function of RPM at full load Amps. If Torque is constant HP and RPM will be proportional. Halve the RPM and it develops half the HP. At 1/4 the RPM = 1/4 the HP etc. If the mechanical speed is set at 1100 RPM and you wish to run a 3" shell mill at 80 RPM by turning down the VFD the spindle will almost certainly stall even on a light cut. Your 2 HP spindle motor is developing about 0.14 HP - roughly the power of a battery drill - when turned down to 130 motor RPM. Naturally it will stall. The motor isn't "losing torque" it's operated under unrealistic expectations.

    Here's the general rule. For light cuts use the VFD's speed controls freely with the mechancial speed set at what's convenient. For heavier cuts you may have to use the mechanical speed setting for best effect. For capacity cuts you will have to run the VFD at or near 60 HZ output and adjust the mechanical speed to suit the desired cutter RPM.

    A motor run at near idle won't need a fan regardless of RPM. A machine tool does not operate under Hollywood physics; its motor will not explode into flames the instant its run below a certain speed. It may need supplemental fan cooling in proportion to load Vs RPM as it affects heat gain in the motor stator. If you wish to run a motor for a couple of minutes at full load at say 10 hz the motor will warm up but a few minutes of idle time and a share of the accumulated heat will dissipate into the ambient air. If the motor is to be run at full load for an hour at 10 HZ it will certainly need cooling air, especially in a warm environment.

    My favorite solution for the supplemental cooling problem for VFD controlled motors is to install a 115 volt, motor-sized biscuit fan on the motor's fan shroud and control it from a 60C* snap action thermostatic switch bonded to the stator iron. That said I have no suplementary cooling one any of my VFD controlled motors (I have 7) and have never even had an overheating problem.

    There are limitations for nearly everything; even magic boxes of electronics.

    My reccommendation is to buy the machine with the VFD and resolve to understand the limiting factors of VFD's and motors (few) and the advantages (many). Once you do, you will not have a single "loss of torque" problem for the rest of your life.

    * the switch rating will have to be adjusted to suit the motor's insulation class.

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    Forrest is spot on, of course. Good write up. I just went through this with a vector drive (fancy word for VFD) on a 450HP AC motor for a rediculously expensive twin screw extruder.

    The bottom line is to avoid using the combination below 60% the designed frequency of the motor and you will be fine. Current draw and the heat and resistance it brings are the primary concerns. If the motor is wound for 60Hz, do not try to use it for power under less than around 38-40Hz. You can run it up to 120Hz (overspeed) if your bearings are up to the task.

    I use a 1HP BP with a VFD daily. I use the belt to get close and fine tune if necessary with the VFD.

    Scott

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    I guess my 1.5 face mill may not be the best scenario, as in reality I'd rather use a somewhat smaller carbide tool and run it fast where the machine has more power. Drilling would likely be a better scenario, Drilling stainless with HSS drills over 1" means using pretty slow RPM's so this may be where stalling may be more of a problem if trying to put much feed on it. Says it goes from 60 to 4500rpm, so theres gotta be some low range gearing. Unfortunately they don't say what the ratio it is so that I can know what rpm the motor would really be running at. I'll need to figure that out.

    My cyclematic lathe works on a VFD and the motor is made with a good fan on the end that has its own separate motor and as soon as you turn on the main switch it turns on and blows a lot of air on the motor at all times so its not dependent of motor RPM at all for cooling. So I haven't been too worried when I have to run the lathe at low rpm's.

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    "Drilling stainless with HSS drills over 1"..."

    IMHO you are asking to break the back-gear.

    I know you are limited in shop space, access, etc, but I'd try to find an old camelback drill press to punch out those big holes.

    A boring head can of course open them up but that's a long process if there's a lot of metal to be removed.

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    Quote Originally Posted by SND View Post
    I guess my 1.5 face mill may not be the best scenario, as in reality I'd rather use a somewhat smaller carbide tool and run it fast where the machine has more power. Drilling would likely be a better scenario, Drilling stainless with HSS drills over 1" means using pretty slow RPM's so this may be where stalling may be more of a problem if trying to put much feed on it. Says it goes from 60 to 4500rpm, so theres gotta be some low range gearing. Unfortunately they don't say what the ratio it is so that I can know what rpm the motor would really be running at. I'll need to figure that out.

    My cyclematic lathe works on a VFD and the motor is made with a good fan on the end that has its own separate motor and as soon as you turn on the main switch it turns on and blows a lot of air on the motor at all times so its not dependent of motor RPM at all for cooling. So I haven't been too worried when I have to run the lathe at low rpm's.

    As far as the RPMs go, why dont you get an inexpensive surface mount tach. I like this one from SenDec. Its real compact mounts easily.
    http://www.northerntool.com/webapp/w...2901_200322901

    And with drilling the big holes, why dont just drill small and step up the hole size. I never drill such big holes in one shot.

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    Holes do get stepped up, but still slow rpm happens at some point. Never had problems doing it with Variable speed mills so I was just wondering if VFD powered ones still gave the same type of performance. If there was anything gained, or lost. Doesn't seem to matter much so I guess so I'll figure it the last part when I see the final price for each of them. I do like that vfd's are so quiet.

    thanks

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    SND:

    All the comments so far tell you that your performance at low RPM will not be as good with a VFD as compared with a mechanical speed reducer. This is assuming both motors have the same HP rating at base speed (1800 RPM or whatever).

    Forest gave you examples.

    Here is mine said differently. If the motor base speed is 1800 RPM and you reduce this to 450 RPM with the variable speed belt drive, then you have 4 times the torque as comparable to using the VFD drive with direct mechanical drive. Basic mechanics.

    .

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    Belt varidrives like in the BP wear out over time and become real noisy the VFD's work great they do have back gears so you can drill big holes there are alot less parts to fail and they leave a better finish because of less vibration we are trying to sell the last of our varidrives and we will never buy another because the vfd is the way to go.

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    Arrow This is how I see it.

    I'd rather have a mill with a good selection of mechanical speeds. I have no idea what the gear ratio is on the back gear for sure, but I think it's probably 3:1 or so. It should be OK, but you're better off with a mill that has a step pulley arrangement allowing you to use the VFD to fine tune your speed. The vector drives can actually push the motor beyond what it's normal torque would be on sine wave AC (utility power) by adjusting the pulse width. I dunno if the VFDs built into the machines can do this, but the vector type VFDs can. Even with the motor pushed to 150% of it's rated torque, anything below about 300 rpm will have LESS torque than a machine with multiple speeds and a VFD, or one with mechanical variable speed. If you really need low RPM torque, you'll want more than two mechanical speeds. You can cheat when the machine is loaded lightly and adjust the VFD to whatever speed you want, but when it really needs torque, it's best to have the mechanical speed as close as you can get it, and fine tune with the VFD. Drilling 1" holes in stainless might be a little much for the electronic variable speed machine if that was pushing it with mechanical variable speed.

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    Turns out the low gear ratio is supposed to be 8:1. Which is pretty decent and would still keep the motor going fast enough for most of what I'd really ever need to do with it. High gear is probably 1:1 or very close to that.

    I also see that I had misused the word "torque" when meaning low power at low speed... point I was just trying to get at really was, does it have enough balls to still get most jobs done or not? and if anyone who has a vfd mill ever thought " should have bought vari speed instead ". Kpotter answered that quite well and so if possible and within budget I'll get a mill with the VFD.

    thanks

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    Here's a bit of my experience with a VFD on a lathe, FWIW...

    The machine is a Hardinge chucker converted to CNC with an Accuslide assembly for the X-Z axes, Fagor control. Originally I installed an 1800 rpm 3 hp motor belted 1:1 to the spindle. These machines only had 3,000rpm from the factory so overspeeding the motor to 100hz wasn't an issue. I don't know why Hardinge limited speed to 3K rpm, bearings(?), but I decided to stay with that max.

    Everything was cool with this setup. I ran for a few years, general turning, bar feeding with tapping. No problems tapping with spindle reversal down around 500 rpms. No extra cooling fans on the motor.

    Then I ran into a bunch of work with larger diameter aluminum, 6" diameter 7075. There was substantial amount of material to be removed and I was limited to around .075" depth of cut or less at the largest diameter on these parts. Greater depth of cut and the spindle bogged down something awful. This was for the birds......

    I went to my favorite surplus electric motor guys and found a rebuilt 1200rpm, 3hp motor, cast frame, older heavy motor. $180 out the door. Installed this one, no longer any problems hogging off the 6" diameter 7075. This setup did require overspeeding the motor to 150hz, no problems in eight years of at least 4 hours a day running.

    The only motor issue I've had at all is when bar feeding. I drop the rpm to around 150 when I open the collet to feed stock. If for whatever reason I pause the program with the low rpm on the spindle and get distracted leaving the spindle idling at this low speed for a few minutes the VFD faults out with an over current error. A reset of the VFD clears the fault and we're off and running again. The motor doesn't ever feel anything but slightly warm to the touch. The braking resistor does warm up a bit after a few hours of bar feeding with constant spindle reversal.

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    Expanding a bit on Doug's take. Gears and belts are great until you have to shift them reliably in machines whose up-time is at a premium. CNC machine tool manufacturers solved this problem with a single speed drive consisting of a huge motor running in a closed loop. What ever speed was needed was acheived by digital control of the motor drive. Since the motor was so monsterous and made for application requireing higher than usual torque there was no difficulty in running the 5000 RPM spindle at 500 RPM for example to punch in with a 3" inserted tooth drill. I seen 'em in action and they really buckshot the enclosure with flying chips.

    The economics work this way: A honkin' big motor and drive is cheaper in first cost and far more reliable over the life of the machine than a belted transmission or a multi-clutch constant mesh geared transmission. If there is a small loss of bottom end grunt it's acceptable given that bottom end grunt is seldom needed. An extra cut or two accomplished the same task at little cost.

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    Doug, I ran into the same problem on the same machine a few years back.

    My solution was to turn a smaller pulley for the motor, and used a slightly shorter belt, then changed the max rpm in the parameters. Lowered the max rpm to about 600 or so. I could change over from Hi to Low in just a few minutes.

    We also had the stalling problem on low rpms.

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    Thumbs up With 8:1 YOU'RE GOOD TO GO!

    That's a HELLUVA low gear, allthough that probably has an oddball motor on it if you think it's 1:1 with a 4500 RMP max speed. Anyways, with the low gear being THAT LOW, you're NEVER going to have torque problems. That should also give you 600 RPM or so in low gear if your drive ratio of 1:1 is correct. I couldn't find any data on the low speed, but you probably have .75 HP on the spindle @ 100 RPM.

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    Also just ran into the Hardinge Chucker problem with an older Omniturn retrofit. Original 2 speed .5 & 1.5 HP spdl motor. .050 DOC/ .012-IPR in Brass is becoming an issue at 3000 RPM. Wimpy 2HP inverter has been replaced with a 5HP, and I have a 5HP motor going into the machine. Hope this helps! I think it will.....

    dk


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