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  1. #1
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    Upgrade driver or stepper?

    Hello, I have an sx2 I CNC'd years ago. Mistakes were made! Currently XY move OKish and Z is fine for 2.5 and some 3D milling in wood and aluminum. Most cutting is around 5 IPM rapids max out at about 15-20 IPM. I can cut steel if I take it really slow. Unfortunately I need to drill with it and it just can't drill at any reasonable IPM in aluminum. I'm at less than 0.5 IPM and even then it will miss steps.

    X and Y are on the original ACME .05 inch per turn lead screws and 1:1 pulleys. They are OK. a bit slow but usable. Z is on a ball screw. .5 inch per turn and a 2:1 pulley system I copied from Hossmachine. I should have taken his advice on steppers to use!

    At the time I used Kelling KL4030 drivers, 40V 3A max, with a 36V psu. I also used KL23H276-30-8B. The motor was a bad choice on my part. Nema 23.

    Best performance I can get with the combo is Bipolar Series at 2.1A and 8.8mH Supposedly 282 OZ-IN. I started with them Bipolar Parallel, 4.2A 2.2mH but driven at 3A because that's what the driver could do. That setup could not hardly move the table with out missing steps!

    I don't wan't to spend a bunch of money on this thing. It does OKish but I plan on changing machines at some point when budget allows. However I can justify a few bucks to get drilling to work. Maybe just upgrade the Z stepper. I can get Kellling 270OZ-IN steppers 2.8A and 3.6 mH that fit the controller better or I can get a bigger controller and give my current Z stepper 4.2A at the lower 2.2 mH configuration. These options are about the same price assuming I stay at 36V.

    I have also seen where people use lower holding torque steppers and get better performance because the inductance is so much lower. They claim the higher holding torque motors perform worse!

  2. #2
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    Re: Upgrade driver or stepper?

    Hi TJ - Why not change the Z pulley ratio to give you more force? Will be slower but at least you will be able to drill? try turning the pulleys around for a 1:2? or change the screw pitch 1/2" is a bit steep... Peter

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    Re: Upgrade driver or stepper?

    Quote Originally Posted by peteeng View Post
    Hi TJ - Why not change the Z pulley ratio to give you more force? Will be slower but at least you will be able to drill? try turning the pulleys around for a 1:2? or change the screw pitch 1/2" is a bit steep... Peter
    Can't really change much on the Z drive. The ball screw nut is directly threaded into the large pulley. That rides on a big bearing. The screw itself does not turn and is attached directly to the head stock. Sorry I don't have a pic handy and Hoss's site is long gone.

    I'm pretty sure my problem is a stepper and drive mismatch as I have seen similar conversions work much better. I am also hesitant to add more gear reduction to the system as I think that will push it further into the poor RPM range of the combination.

    I'm still looking over old threads on the math involved. I remember having a face palm moment the last time I tackled this problem. I learned enough to get it working good enough. Now I need to find that post again!

  4. #4
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    Re: Upgrade driver or stepper?

    Hi,
    if you want more speed and more 'authority' use a much higher voltage driver, 80V is the current norm among the 'big boys drivers',
    and use an 80V supply, don't mess around.

    Craig

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    Re: Upgrade driver or stepper?

    Quote Originally Posted by joeavaerage View Post
    Hi,
    if you want more speed and more 'authority' use a much higher voltage driver, 80V is the current norm among the 'big boys drivers',
    and use an 80V supply, don't mess around.

    Craig
    I take it that's a vote for keep the steppers and upgrade the control. More current and more voltage.

  6. #6
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    Re: Upgrade driver or stepper?

    Hi,
    yes a little more current so that you can exploit the current rating of your servos would be nice.

    Voltage on the other hand does not improve the torque but rather delays the onset of torque reduction with speed.
    All steppers lose torque the faster they go. Inductance is a good indicator how bad that torque reduction will be.

    For example a 1mH 23 size stepper with a 72V driver may retain 45-50% of its holding torque at 1000rpm.
    A near identical 2mH 23 size stepper might have 15-20% of its holding torque at 1000rpm.
    An 8mH stepper probably won't even get to 1000 rpm, and could have as little as 5% of its holding torque at only 500rpm.

    Low inductance steppers are highly regarded for CNC purposes but they tend to be high current units with only moderate torque
    specifications, and so only ever bought by people who know what they want. Most manufacturers make high torque units but with
    commensurately high inductance and sell to people who don't understand inductance. Don't be duped......you want the lowest inductance steppers
    you can get.

    Your existing 2.2mH (when wired in parallel) are probably OK. With a good high voltage driver and preferably a high current
    linear supply, you'll get the gest out of the steppers you've got.

    Craig

  7. #7
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    Re: Upgrade driver or stepper?

    Thanks Craig. If the rule of thumb from Gecko 32 * SQRT (L) = VMAX holds true that puts me at a 48V psu. 80V sounds high to me on such a motor. I don't want over the top speed at the expense of excessive heating. My Z is going to be pretty low RPM anyway, 100 RPM would get me 25ipm. It's a very small machine!

    A KL-5042 digital is $29. That would handle the 48V and the 4.2 A of my existing motors. That looks like my minimum cost upgrade.

    Now to look at what minimum plus a couple of bucks gets me.

  8. #8
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    Re: Upgrade driver or stepper?

    Hi,

    If the rule of thumb from Gecko 32 * SQRT (L) = VMAX holds true that puts me at a 48V psu. 80V sounds high to me on such a motor. I don't want over the top speed at the expense of excessive heating.
    Rubbish. Firstly that's Geckos Rule of Thumb....and I'm not quite sure that thumbs have much say in Physics. The higher the voltage the faster you go....that's Physics.
    Current is what causes heating, not voltage.

    A stepper has low resistance windings typically 1 Ohm or so. Thus for rated current of 3A you need only apply 3V. The high voltage is the 'Force' the current through the
    winding in the shortest possible time and once the rated current is flowing the drive uses pulse width modulation to reduce from 80 (or 48 or 36 etc) down to 3V necessary
    to maintain rated current only. Despite the high voltage power supply the stepper on average sees only 3V. The heating is 3V times 3A =9 Watts.

    I have Vexta 5 phase steppers on my mini-mill and they run through 10:1 low lash (<2 arc min) gearboxes using Vexta drivers. The drives are direct off-line 230VAC, and
    that gets rectified and buck regulated to 150VDC and that DC voltage is applied to the steppers via a PWM circuit. They whizz around at 2400 rpm and haven't missed a step in six years,
    except if I do something stupid. Voltage doesn't hurt steppers....excess current does.

    Quite frankly paying $29 to get another lousy 12V.......save your money.

    If speed is not your issue then get higher torque steppers with as low inductance as you can afford.

    Craig

  9. #9
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    Re: Upgrade driver or stepper?

    Hi TJ - agree with Craig - the typical dielectric strength voltage of a stepper is 500VAC so you can go much higher then the gecko eqn. Its current that heats it and voltage that drives it... but it only sees a very low average.. cheers Peter

  10. #10
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    Re: Upgrade driver or stepper?

    Quote Originally Posted by joeavaerage View Post
    Quite frankly paying $29 to get another lousy 12V.......save your money.

    If speed is not your issue then get higher torque steppers with as low inductance as you can afford.

    Craig
    Thanks for the extra info. What I was thinking is the $29 get's me is going from 8.8mH and 2.1A to 2.2mH and 4.2A. If I'm understanding you right that 8.8mH with the low voltage is killing my speed and torque. I'm also seeing that going from a 50V controller to 80V controller doubles the cost.

    I have been looking for stronger steppers with lower inductance. Most I see are 3mH or higher and would still need a new controller. I did find a 570oz-in with 2.5mH but it needs 5A $40 plus $35 for a 50V5.6A controller. That's tempting for my Z. I think I also made a mistake when I bought the ball screw. I did not realize it was .5 inches per turn. I may need to look at that setup as well. It was working ok till I needed more torque for drilling ops.

    I also have seen several places that claim higher voltage just gets me more speed for rapids. Actual cutting speeds in metal are not high enough to get the benefit of high voltage. Since this is a minimill I don't have the rigidity and horsepower to cut fast or deep. Feed and speed calculations put me under 10ipm for most of my cuts unless I'm doing wood or plastic.

    I do appreciate your time. I don't like to quibble over such small cost items but like many, I have been out of work for most of the last year. I'm just trying to get my hobby to put some stuff up on etsy type sites to earn some extra $$.

  11. #11
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    Re: Upgrade driver or stepper?

    Hi TJ - Lets look at your current system. You say you are drilling at a feed of 1/2" per min? and you have a 2:1 pulley to a 1/2" pitch screw. So the motor is doing 2rev/min? This is very slow, so what is your micro steps setting? Re power source is it adjustable? some are +/-10% so you can increase the 36V a bit. I think you can improve your system with the microsteps setting and also check that it is mechanically OK no jamming or sticking please... Peter

    Looking at your 283oz.in motor since its going so slow its near full torque.

    283ozin = 2Nm

    Force = (2x2PIx.9)/(9.81x0.012x2) = 48kgf. ( This neglects friction in the systems )You need much more then 48kgf to drill Al & steel.... get your bathroom scales under the z axis and measure its force. Then do same with a drill press and see the difference. Then you need to size your Z to the drill press force...

  12. #12
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    Re: Upgrade driver or stepper?

    I'd say that all three screws should be reconsidered.
    the Z axis at .5" pitch is more suited to a high speed wood or plastic router.
    The Acme screws on X and Y are much less energy efficient than ball screws and will wear out faster because CNC has a lot more move than manual milling.

    Replacing the screws with decent .2: ball screws would be a good idea in my opinion.

    if you want to rreduce " Mission Creep" and if your first and main problem is drilling in Z, just replace the Z screw without any pulley ratio reduction.

  13. #13
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    Re: Upgrade driver or stepper?

    Get a 570oz motor, and a drive like this.
    https://www.omc-stepperonline.com/di...per-motor.html
    Run it at 60V if possible, and you'll get a LOT more speed, for not much money.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

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    Re: Upgrade driver or stepper?

    Thank you all.

    Peter. I am using 4 micro steps. My understanding is less micro steps reduces torque loss to micro stepping. Friction in Z is higher than I'd like. I use good way oil and that helps. Gibs are as tight as I can have them and still have it move OK. Any looser and slop causes more problems than reducing friction helps. I have a spring counterbalance at about 50 lb (22kg) and that is an almost perfect match. I can take off the lead screw and move it up or down and it will stay. Takes about the same force in each direction, no binding I am aware of. It's been together for about 6 or 7 years. I plan on stripping down the machine for a good cleaning and tune up.

    Looking at the math on my Z axis I think I messed up when selecting components or what I got is not what I ordered. Took too long to assemble to realize that. I'm going to look for a better ball screw. I'm also going to take a good hard look at changing the pulleys and/or motor/drive combo.

    The Z axis is anemic but not totally useless. I surprised myself the other day with it. I had to bore out 4 holes in some 5/16" (8mm) steel. 1" bored out to 1.3" (25mm to 33mm) a well ground tool and it did not take too long. .02"DOC, 400RPM, 1.2 IPM. Steel is not normal for me on the CNC. It is just not made for it. Drilling aluminum is becoming a real necessity.


    Off topic but related. Since drilling is the one thing that really does not work well I have thought about making a CNC XY positioning table for the drill press. I have two of the ball screws I used for my Z. I could upgrade the mill and use the ball screws and other take offs or spares on the drill press. I actually use my CNC as a manual mill with power feed and DRO fairly often. I use Linux CNC with an add on that generates bolt patters. Some box ways with a lock would be pretty easy to construct. If I do so I will start another thread for that.

  15. #15
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    Re: Upgrade driver or stepper?

    Hi tjbaudio,

    I am using 4 micro steps. My understanding is less micro steps reduces torque loss to micro stepping.
    That is somewhat of a misunderstanding.

    Let me explain by way of an example. Lets say we have a two phase stepper with a holding torque of 100oz.in
    Between two full step positions, 1.8 degrees apart, the motor will exhibit its full torque, 100oz.in, after all that's the definition of holding
    torque. Between two 1/2 step positions, that is 0.9 degrees apart the stepper will exhibit either full torque or depending on the driver 141% of full
    torque, that is 141oz.in

    This occurs because our mythical stepper has 1A in each winding and the magnetic force is the vector sum of the two forces operating at 90 degrees to each other.

    Torque= (1002 +1002)1/2
    =141 oz.in

    The important point is that a stepper exhibits at least full holding torque between adjacent steps, be they fullsteps or half steps.

    Lets now imaging that you employ 10 microsteps per fullstep. Imagine also that the rotor is in a fullstep position with 1A flowing in
    one winding and no current in the other. The drive receives one step pulse and is therefore comamnded to rotate 1/10 x 1.8 = 0.18 degrees.
    The drive accomplishes that by decreasing the current in one winding and increasing the current in the other.

    Approximately the current in the energized winding will reduce from 1A to 0.9A and the current will increase from 0A to 0.1A in the other winding.
    Note the drive will apportion the currents using a sine/cosine distribution, but for our example this approach is close enough.

    The torque to rotate the rotor is provided by the current in the new winding, that is only 0.1A or about 10% of a fullstep.
    Torque = 10% of fullstep torque
    =10% of 100oz.in
    =10 oz.in (approx)

    The important point here is that the torque between adjacent micro steps is much reduced by comparison to fullstep/halfstep torque.
    In fact the torque is that whimpy that the rotor may not shift due to the load on it. The drive may receive one, two or even three more step pulses
    for a current distribution of 60%, 40% so now the torque has built up to nearly that of a halfstep and the rotor will overcome the load and rotate
    four microsteps at one go. It was intended that the rotor would turn 0.18 degrees with each step commanded but in fact it missed the first few until it rotated
    four microsteps in a bunch.

    For this reason the maximum achievable resolution of a stepper is considered 1/2 step, or 400 steps/rev, even if you'd like top believe that 10 microsteps means a resolution of
    10 x 200 =2000 steps/rev.

    What has not changed is that the motor still exhibits full, and in the case of half steps even more than full, torque between full and half steps, it just between adjacent
    micosteps that the torque is reduced.

    You might reasonably ask 'why bother with microsteps at all if they don't really give you genuine resolution increases?' The answer is smoothness.
    A stepper full or half stepping exhibits jerky motion, whereas microstpping smooths that motion out.

    Astronomers have long used stepper motors to drive there telescope mounts and they first proposed and used micrstepping so that the jerky motion they
    had hitherto suffered was eliminated. They also discovered the 'reduced adjacent torque effect'.

    For CNC purposes microstepping reduces 'midband oscillation' which often plagues steppers, it does not really offer genuine repeatable
    resolution beyond halfstepping. Four, eight and ten microsteps are usually considered sufficient to get the most 'smoothing' effect without
    having to increase the step rate to impractical extents.

    Craig

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    Re: Upgrade driver or stepper?

    Hi TJ - With a bathroom scale measure the force that the drive can provide. I appreciate its balanced so the drive force should be easy to measure. Getting rid of the 2:1 step up owuld be good....

    Craig thanks for the explanation, is the conclusion to increase the stepping to 10? Regards to all Peter

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    Re: Upgrade driver or stepper?

    Hi,

    Craig thanks for the explanation, is the conclusion to increase the stepping to 10? Regards to all Peter
    No, not at all, I selected ten because it made the discussion of 'adjacent torque effect' as simple and as plain as I could make it.

    If you follow my argument you do not, in practical effect, get more resolution by increasing microstepping beyond half steps, but you do get an
    improvement in 'smoothness' of motion. Most CNCers have come to the conclusion that 4, 8 and 10 microsteps per fullstep gain the
    most smoothing effect without having to signal your drives at hundreds of kHz.

    If you wanted smoother motion again you could choose 64 microsteps per fullstep for instance. To have your stepper rotate at 600rpm say,
    or 10 rev/sec then the step rate is:
    step rate= 10 x (200 x 64)
    =128kHz

    If on the other hand you elect to use a somewhat less smooth 4 microsteps per fullstep:

    step rate= 10 x (200 x4)
    =8kHz

    And 8kHz step signals are easier to generate than 128kHz!

    Craig

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    Re: Upgrade driver or stepper?

    Quote Originally Posted by peteeng View Post
    Hi TJ - With a bathroom scale measure the force that the drive can provide. I appreciate its balanced so the drive force should be easy to measure. Getting rid of the 2:1 step up owuld be good....

    Craig thanks for the explanation, is the conclusion to increase the stepping to 10? Regards to all Peter
    I just tested with a scale and it skips steps at 110lb (50kg) moving 10ipm or .5 ipm. With the same scale I get about 3 times that on my drill press with out much effort. It also shows your calculated force and what I am getting are very close.

    The 2:1 belt is a step down, 1/2 speed and double the torque. It's the equivalent of a .25 inch per turn lead screw with a 1:1. If anything I would think it would help to get more stepdown.

    Moving to a 570 oz-in motor and appropriate driver would get me past 200lb of down force. That my solve my problem for about $90 plus shipping. I also see some hiwin 16mm ball screws on ebay that may work for $30. Seller says "about 5 turns per inch" I assume I can make the change in my Linux CNC config to adapt if they are metric. Mounting them would be easy enough.

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    Re: Upgrade driver or stepper?

    Hi TJ - I was thinking in diameters not turns. So its small to big. So should be somewhere around 96kgf so your losing some force somewhere... but you now have the drift so onward.. Peter

    Ballscrew Force =( torque x 2 x PI x efficency) / (9.81 x pitch) = kgf

    pitch in meters, torque Nm, efficiency of ballscrews is about 0.9, leadscrew efficiency can be quite bad depends on the pressure angle...

  20. #20
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    Re: Upgrade driver or stepper?

    Quote Originally Posted by peteeng View Post
    Hi TJ - I was thinking in diameters not turns. So its small to big. So should be somewhere around 96kgf so your losing some force somewhere... but you now have the drift so onward.. Peter

    Ballscrew Force =( torque x 2 x PI x efficency) / (9.81 x pitch) = kgf

    pitch in meters, torque Nm, efficiency of ballscrews is about 0.9, leadscrew efficiency can be quite bad depends on the pressure angle...
    It's even worse than that, its .5 lead, 2 start. so .25 pitch. https://www.applied.com/c-brands/c-n...ut/p/101640711

    So I should have just over 400lb (180kg) - friction at my disposal in Z. Given I measured about 1/4th of that I must have something really wrong! Time to strip it down and see where my fault is. I'm guessing mechanical. The counter weight spring and lead screw are on opposit sides of the ways. Maybe that's inducing a twist. I could also have some shmoo not visible from out side.

    Any way, thank you for putting me on the path to the real problem. How much force would you guys expect the z axis to take to move with relatively tight gib adjustment?

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