Bridgeport BOSS steppers with Gecko?


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Thread: Bridgeport BOSS steppers with Gecko?

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    Registered damae's Avatar
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    Default Bridgeport BOSS steppers with Gecko?

    I'm about to start my Series I Boss3 retrofit, and weighing my motor options. I have three matching 2HP AC servo motors and controllers. But I have other CNC projects on the books that could use those motors instead.

    So before I completely toss out the option of using the original steppers, what do you guys think? Obviously, they were orginally sized for that machine and they're already mounted, ready to go.

    Should I spend another $134 x 3 (about $400) for a set of G202's? I know a lot of you went this route and I'm curious to hear how it turned out, and what the pitfalls (if any) are.

    Aparently in full step mode, the BOSS3 machine I have is setup for .001" resolution at best. Since geckos are microstepping, what is the maximum (and realistic/practical) resolution I could expect?

    Thanks!

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    Community Moderator ger21's Avatar
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    Quote Originally Posted by damae
    Aparently in full step mode, the BOSS3 machine I have is setup for .001" resolution at best. Since geckos are microstepping, what is the maximum (and realistic/practical) resolution I could expect?
    Thanks!
    Read this about microstepping.

    http://www.machinedesign.com/ASP/str...tedArticle.asp

    A lot of people use Gecko's and original bridgeport motors just fine. A lot of people have a lot of problems with Gecko's. When you run Geckos at their Maximum limits, everything must be set up perfectly, especially proper cooling. I'f you're going to go the Gecko route, I'd call Mariss and find out exactly what you need to do to reliably run Geckos at their full rated current and voltage.

    Gerry

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    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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    Registered damae's Avatar
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    Default microstepping myths

    That was an excellent article. I was totally unaware that microstepping reduces torque so much!

    Depending on coditions of load and detent position, the motor may at times not move at all when commanded to move with a microstep! If the bearing friction and detent torque are against you, the motor won't move until enough microsteps acculmulate to generate enough torque!

    Unless somebody can tell me they've had success with a microstepping controller on their original boss stepper motors, I'm going to go the servo route. After all, I already have them.

    If I'm jumping to conclusions, stop me before I put the old motors on ebay, under the "antiques" category =)

    -D



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    Community Moderator ger21's Avatar
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    Most of the time, a microstepping drive will outperform a half or full step drive. Even though the torque is slightly lower, the motors will run much smoother and won't suffer from resonance problems. The end result is that it may seem like they have more torque.

    I've heard you can usually count on every half step with a ten microstep drive. But that depends on the motor and load. If you're full step is .001, keep in mind that each microstep from the Gecko is .0001. Think about this for a second. If the machine is in motion, how fast are those .0001 steps being sent? Most likely fast enough that you'd never notice if it takes 3-5 microsteps for motion to occur. When you're accelerating, steps are being sent faster and faster, so again, you'd never notice. Remember, you're not losing any steps, if it takes 4 microsteps before it moves, it will still move those 4 microsteps. The only time this would really be a problem is if you really needed to move .0001.

    The reason I posted that link was because you asked about an increase in resolution. I think the main focus of that article is that microstepping doesnt increase resolution. With full steps you get .001. With microstepping, you won't get .0001. You might get .0005, you might not. You will get smoother running motors, though, which is a good thing.

    I can't tell you whether servos would be a better choice, that's you're call.

    Gerry

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

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    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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    Quote Originally Posted by ger21
    ...With full steps you get .001. With microstepping, you won't get .0001. You might get .0005, you might not. You will get smoother running motors, though, which is a good thing.
    Thanks Gerry for the good information! Good points about the overall effect, that these delayed (not missed) microsteps will very likely still improve the effective resolution. I am hoping that the ground ballscrews on this machine are of good accuracy and that my efforts to get better than .001" resolution will have a real effect, regardless of which path I take -- steppers or servos.

    I started pulling the steppers off the machine and ended up disassembling the X and Y belt assemblies to learn how it all goes together. There's a mechanical odometer on those two axes that seems to measure total distance traveled on an axis. Taking it apart, I learned a lot, in particular, I learned that whatever motor I use for each axis, it's goint to get VERY dirty! Those old steppers were just soaked in oil, and when I removed the rear cover, it looks like none of it got in there. The white PCB had not a hint of dust on it!

    Can anyone here offer some experience with converting their BOSS machine to geckos? What kind of accuracy and repeatability are you getting? I think that so many people must have converted their BPs using geckos that someone can tell us what the measured accuracy and resolution are. If you measured the repeatablility of your machine with a dial indicator, that would be great info to share!

    As Gerry pointed out, the delayed steps may have an even less pronounced effect when the machine is in motion. I imagine that the inertia of the table, which is likely a few hundred pounds, will help smooth out the weaker microstepping steps. The way to measure this accuracy would be to measure real parts machined using a BOSS motor and Gecko drive.

    Can anyone who has retrofit one help out with some data?
    Thanks!



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    Can't help at the moment with a boss conversion but I am running a Beaver mill which is similar.
    The Beaver is on the same Superior drives as the Boss but drives 1:1 onto 0.200" ball screws.

    I'm also in the process of doing a Bridgy MDI but this was on servo's with two bad motors.
    This is going back onto steppers as i have all the parts nessesary.

    To go back to the Beaver, This is on Gecko 210's as it was all I could get here in the UK quickly. 201's would have done as well but they were not available.
    If I did one now, with Gecko's, I would use the 202's as they do have some protection.

    Drives are set to x10 microstepping and are wired in parallel, no current resistor fitted and running at the full 7amps.
    Power supply is a dual wound torriod at 47 volts AC which when smoothed and rectified gives 59v DC.
    Each side has a 22,000 mfd cap rated at 100v working.
    One side powers X and Z, the other Y and A.
    The idea behind this is that you never normally use X and Z or Y and A together giving each axis access to full power.
    Each side has a 5v fuse fitted to the Gecko input. Once setup and the axis tuned I was getting far faster rapids and far, far smoother that the old bi-level drive fitted to this machine.
    This machine is normally in use every week, I wo't say every day as I'm a jobbing shop and it is type of work dependant.

    The MDI is going onto steppers as I have these, going back to servo's is a no go as they were special on this and I would need two new motors, three drivers and three encoders as the old encoders are analogue.
    Although I have spare type 42 steppers from scrapped macines I am fitting three new type 34 motors to this.
    The reason is that modern magnets have improved motors since these old 42's were made. The 34's are more powerful, accelerate faster and have less detent torque than the 42's.
    The drives on this will be Chinese drives rated at 7.8 amps and 80 volts. The reason to use these is they are easily available here and cheaper than importing Gecko's.
    I have run one on the Beaver to test it and it runs fine, no worse, no better than the 210.
    If I do blow a drive I can collect and fit a new drive in under two hours.
    HTH.

    John S.



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    John S,
    the drives you are talking about, are they the ones from Arc Euro Trade?
    I am myself just about to embark on a Series 1 retro but already have my manual series 1 converted so it will just be a case of swapping over all the hardware but I am interested in the drives you mention for any future projects I may have ;-)

    Regards
    Hood



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    Hood, Yes these are the same ones.
    They are only about 3/4 of an hour away from me so they are very handy.
    John P also has some of these but I don't think he's used any in anger yet.

    John S.



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    Great, thanks. Only thing I thought bad was that they can't cut the current unless on full steps, do the motors not heat up?
    Just ordered a small stepper from them so hopefully they can deliver pretty fast to Scotland as the shaft on my quill stepper snapped (flaw in shaft I think) and although I will purchase a larger stepper for the Boss quill this is the easiest and cheapest option to get me going again.

    Hood



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    Thanks John,

    I'll be using X,Y,Z simultaneously, so I'll have to size the power supply accordingly. But that really is a pretty innovative way to save ont the power supply if you're not doing 3d contouring. Did you recycle any of the transformers from the original control? I'm peering into the power cabinet on my macine and wondering what I can salvage, considering I want to go to single phase power.

    How is your accuracy and repeatability with that setup? Do your finished parts measure within .001" of the design or better? I guess that's what I'm really trying to get at. =)

    Thanks!
    -D



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    Quote from article:

    Few if any stepper motors have a pure sinusoidal torque vs. shaft position curve and all have higher-order harmonics that distort the curve and affect accuracy. And even though microstepping drives have come a long way, they still only approximate a true sine wave. Significant too is that any load torque will result in a magnetic backlash, displacing the rotor from the intended position until sufficient torque is generated.

    End of quote.

    This is a kind of unfortunate article. The author is only partially correct on some points, grossly out of date on some others. To wit:

    1) "Few if any stepper motors have a pure sinusoidal torque vs. shaft position curve." This is flat-out wrong with the new, square motors. They are nearly perfectly (<1%) sinusoidal. This means torque varies less than 1% between the full-step locations.

    2)"And even though microstepping drives have come a long way, they still only approximate a true sine wave." Wrong again. Our drives quantify the sine-cosine currents to within 1% of ideal. The author shouldn't extrapolate; this may be a problem with his company's drives, not other's.

    3) "Significant too is that any load torque will result in a magnetic backlash, displacing the rotor from the intended position until sufficient torque is generated." True but uninformed when it comes to actual applications.

    If you are hogging-out a part, you may be 1.8 degrees out of position (0.001" on a 5-TPI leadscrew). If the motor has 1,000 in-oz of torque, that would be the same as applying 32 lbs of force on a 4" diamater handwheel on a manual machine. That is a fist-full of force.

    When you do a finish cut, it would take 156 in-oz of torque to be off-position by 0.0001" (1/10th of a full step). That would translate into 5 lbs of handwheel force on a manual machine.

    I'm not a machinist but most people I talk to say a finish cut is at less than 1-2 lbs.

    Mariss



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Bridgeport BOSS steppers with Gecko?

Bridgeport BOSS steppers with Gecko?