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Thread: Acceleration

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  1. #1
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    Acceleration

    I am a bit hesitant to ask, but I need to know. What is the acceleration rate for a current model PCNC 770? I see the feed rate, but not the acceleration. For some types of work acceleration makes a bigger difference than feed. I'm sure its out there somewhere in huge block letters with a flashing arrow pointing at it, but I didn't see it.
    Bob La Londe
    http://www.YumaBassMan.com

  2. #2
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    Re: Acceleration

    Quote Originally Posted by Bob La Londe View Post
    I am a bit hesitant to ask, but I need to know. What is the acceleration rate for a current model PCNC 770? I see the feed rate, but not the acceleration. For some types of work acceleration makes a bigger difference than feed. I'm sure its out there somewhere in huge block letters with a flashing arrow pointing at it, but I didn't see it.
    PathPilot accelerates the spindle speed during the rapid to the work, it appears to be up to speed before safe height is reached, so its up to speed before entering the work.
    mike sr

  3. #3
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    Re: Acceleration

    I wasn't asking anything about the spindle. The machine should be configured by default to accelerate the spindle to full speed before beginning cuts. For example in Mach 3 I have my heavy machine (3600 rpm) machine set to dwell for 4 seconds because that's what I programmed in the VFD. Even with a huge braking resistor that's the fastest I can stop the spindle consistently without faulting the VFD. On my high speed spindle machine (24K) I have it set to 11 seconds to dwell before moving on because that was the quickest I could set it to go to full speed or go from full speed to dead stop without faulting when I programmed the VFD.

    However, that is not what I was asking. I was asking about acceleration of the axis as (for those of us who built and retrofit machines getting started anyway) that's the important number. Its typically rated in units per second per second. My light weight machine accelerates at 25 i/s/s, and my heavy weight accelerates at 20 i/s/s but I am hoping newer heavier servo drivers that can provide higher current will allow it to accelerate faster.

    i/s/s or iss is the number of inches per second faster it is traveling after one second than it was travel at the beginning of the second. A 25iss machine can be traveling at 25 IPM after 1 second. 50 IPM after 2 seconds. 100 IPM after 4 seconds. If your Path Pilot machine has a semi real time display of actual IPM you will see that on complex 3D moves in ES mode or CV mode with a small rounding factor it might never actually reach the programmed feed rate. Also, during very short rapids it may never actually reach the rapid feed rate. Mach 3 displays this and I often see tool paths that calculate out at 95-100 ipm on never reaching more than 30-35 IPM on complex 3D tool paths.

    During a machine operation the spindle only has to come up to speed once, but the the axis may need to come up to speed thousands or even tens of thousands of times.
    Bob La Londe
    http://www.YumaBassMan.com

  4. #4
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    Re: Acceleration

    Bob,

    Just look at the XML file. There is a utility on the ArtSoft website that will display it in a more-or-less intelligible format. It *might* also be possible to simply open the Tormach XML file in a non-Tormach version of Mach3, then look in the Motor Tuning dialog.

    Regards,
    Ray L.

  5. #5
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    Re: Acceleration

    That is a good idea, but I do not see it available for download on a quick look around.
    Bob La Londe
    http://www.YumaBassMan.com

  6. #6
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    Re: Acceleration

    This post shows the ini - don't know if it changes from machine to machine but it is set to 15in/s^2

    http://www.cnczone.com/forums/tormac...ot-simple.html

  7. #7
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    Re: Acceleration

    I guess I had my head in the wrong place when I read the post Bob, sorry about that.
    mike sr

  8. #8
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    Re: Acceleration

    Quote Originally Posted by popspipes View Post
    I guess I had my head in the wrong place when I read the post Bob, sorry about that.
    No worries. I started to give a rude sarcastic reply, and then realized you just misunderstood the question. I figured you knew what I meant if you thought about it, but also figured it wouldn't hurt to explain it if somebody else didn't get it.
    Bob La Londe
    http://www.YumaBassMan.com

  9. #9
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    Re: Acceleration

    Quote Originally Posted by samco View Post
    This post shows the ini - don't know if it changes from machine to machine but it is set to 15in/s^2

    http://www.cnczone.com/forums/tormac...ot-simple.html
    Thanks. I read part of that thread earlier, but I didn't see acceleration numbers. I appreciate the extra feedback. I think I am going to have to ask Tormach directly. 15iss would be a deal breaker for me.
    Bob La Londe
    http://www.YumaBassMan.com

  10. #10
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    Re: Acceleration

    Bob

    I think you're explanation of acceleration has a few flaws. Maybe not intentionally so but your figures are off by a large factor. Maybe you were meaning to continue with the iss abbreviation when you moved to ipm?

    With your machines 25iss setting it would not take 4 seconds to reach 100 ipm. In 4 seconds if you're machine were capable it would be traveling at 6000 ipm. I don't know you're rapid speed so I will assume 300 ipm for a example. You would then have 25x60=1500ipm after 1 second. Then 1500/300=5 to give you full acceleration in .2 seconds or 1/5th of a second.

    You are correct that acceleration is important but you also have to take into account deceleration as well which would at least double your accel time to move from start to stop in a move. Not a big deal when moving in larger increments but taxing when moving .1inch or .001inch per move and made worse if a full reverse is needed.

    What are you using for cad and cam? This could be a much better investment to speed your process then a faster machine. A tool changer could be as effective of a investment as well. Normally I wouldn't make these suggestions but since I have followed your progress for many years I know you have quite a few machines that are pretty capable. If a faster machine is needed then with what you already have I would think it's time to move to a decent used vmc at the very least.

    Hope I'm not coming off as a jerk. That's not my intention at all.

    Ben

  11. #11
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    Re: Acceleration

    If doing 3D profiling, 15 in/sec/sec is painfully slow, and will dramatically increase program run times. For 2D work, I normally run 25-30 in/sec/sec to reduce the amount of machine shaking, due to the acceleration and deceleration. But if I were going to do 3D work, I'd definitely need to crank that up quite a bit, ideally to something more in the 75-100 in/sec/sec range.

    Regards,
    Ray L.

  12. #12
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    Re: Acceleration

    Ben, we are good. I'll need to read up on acceleration more. I thought I understood it. Obviously I don't.

    Units
    Acceleration has the dimensions of velocity (L/T) divided by time, i.e., L/T2. The SI unit of acceleration is the metre per second squared (m/s2); this can be called more meaningfully "metre per second per second", as the velocity in metres per second changes by the acceleration value, every second.
    SCz 75-100 iss is screaming fast. What machines do that? That's a huge G load.

    Even with my mills the difference between 20 and 25iss is huge in regards to material removal on 3D work.
    Bob La Londe
    http://www.YumaBassMan.com

  13. #13
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    Re: Acceleration

    Quote Originally Posted by Bob La Londe View Post
    Ben, we are good. I'll need to read up on acceleration more. I thought I understood it. Obviously I don't.



    SCz 75-100 iss is screaming fast. What machines do that? That's a huge G load.

    Even with my mills the difference between 20 and 25iss is huge in regards to material removal on 3D work.
    Bob,

    I was able to run at least 50 on my knee mill with DC servos. I expect I could do the same on my Novakons, with AC servos. How fast you NEED to accelerate is a function of how fast you're cutting (and, at some point, how will your machine is anchored to the floor). Commercial VMCs can run at those acceleration, because they can be cutting at 300-500 IPM. Not likely you'll get near that high with steppers, but acceleration will be what ultimately limits your 3D cutting performance, so you want to get it as high as the machine will tolerate reliably without losing position. Given the 110IPM max velocity of the Tormach, something in the 25-30 range may well be at the point of diminishing returns. The only way to know for sure is to profile some 3D toolpaths at different accelerations.

    Regards,
    Ray L.

  14. #14
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    Re: Acceleration

    Quote Originally Posted by SCzEngrgGroup View Post
    Bob,

    I was able to run at least 50 on my knee mill with DC servos. I expect I could do the same on my Novakons, with AC servos. How fast you NEED to accelerate is a function of how fast you're cutting (and, at some point, how will your machine is anchored to the floor). Commercial VMCs can run at those acceleration, because they can be cutting at 300-500 IPM. Not likely you'll get near that high with steppers, but acceleration will be what ultimately limits your 3D cutting performance, so you want to get it as high as the machine will tolerate reliably without losing position. Given the 110IPM max velocity of the Tormach, something in the 25-30 range may well be at the point of diminishing returns. The only way to know for sure is to profile some 3D toolpaths at different accelerations.

    Regards,
    Ray L.
    Ray

    Wow! What controllers were you running on your knee mill. I am running DC servos on my knee mill (Hurco KMB1) with G320X drivers and could never get over 20iss and a top rapid feed of 200 IPM reliably. I am changing out the Geckos on that mill by the way. (unable to maintain performance over time) It does have a pretty heavy table and saddle. The two are a significant part of the weight of the machine.

    An example of the type of cut I might do that needs higher IPM than I am getting. I routinely take cuts that chip thinning calculations say could be done at 85-95 ipm, but due to acceleration never reaches above 25-30 ipm along a 3D toolpath. I can increase that to 32-35 (depending on the machine) by running in CV mode, but if I set the rounding value small enough to still get reasonably accurate parts its not much faster than cutting in ES mode as you can see by my numbers. (I almost always run in CV mode.)
    Bob La Londe
    http://www.YumaBassMan.com

  15. #15
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    Re: Acceleration

    Quote Originally Posted by Bob La Londe View Post
    Wow! What controllers were you running on your knee mill. I am running DC servos on my knee mill (Hurco KMB1) with G320X drivers and could never get over 20iss and a top rapid feed of 200 IPM reliably. I am changing out the Geckos on that mill by the way. (unable to maintain performance over time) It does have a pretty heavy table and saddle. The two are a significant part of the weight of the machine.
    It's a Chi-wanese Bridgeport clone, using G320Xs and 850 oz-in DC servos, running at 72V, 4-pitch 1" Nook XPR screws. It runs 350IPM rapids with ease on X, Y and quill (the knee runs only 70 IPM, using the same motor and drive).

    Regards,
    Ray L.

  16. #16
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    Re: Acceleration

    I feel like we are comparing bass boats now. LOL. My Cougar FTD w/ a 250 ProXS runs mid 70s with a normal load, but there are guys who swear they are running the same config in the low to mid 80s. The only way I could run over 80 is to take everything out of the boat, remove the extra batteries, pull the trolling motor off the bow and only fill the left gas tank. (to balance the driver) LOL.

    Seriously. I am impressed. The thing is my numbers are similar to what other people have gotten out of KMB1 retrofits, so I figured that was the max a G320X could do. I am running 78VDC with the original Electrocraft motors. (direct 1:1 on 5TPI ballscrews) Don't know their torque rating, but they are pretty large.
    Bob La Londe
    http://www.YumaBassMan.com

  17. #17
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    Re: Acceleration

    Come on guys. If you're worried about acceleration, you have obviously bought the wrong machine.

    These are STEPPER motors, not servos.

    If acceleration is an issue, maybe you should have bought a Mori or a Mazak. Except those machines would have cost another 100 to 150K.

    My 4 year old PCNC 1100 has done an amazing job for me. I could not have asked for anything better and my PCNC1100 didn't cost $75,000.00.

  18. #18
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    Re: Acceleration

    I find it interesting that nobody has been able to provide an answer, and that even after I explained from first hand experience how as little as 5iss can make a huge difference that people don't understand that it can be important to know.

    FYI: My stepper mill has better acceleration than my servo mill.

    I did contact Tormach, but haven't heard back yet.
    Bob La Londe
    http://www.YumaBassMan.com

  19. #19
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    Re: Acceleration

    Quote Originally Posted by Bob La Londe View Post
    I find it interesting that nobody has been able to provide an answer, and that even after I explained from first hand experience how as little as 5iss can make a huge difference that people don't understand that it can be important to know.
    Sacco already gave you the answer (at least for the 1100). Maybe someone with the 770 can follow the steps that BAMCNC showed in his video to get to the PP ini file.

    Click image for larger version. 

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    Quote Originally Posted by samco View Post
    This post shows the ini - don't know if it changes from machine to machine but it is set to 15in/s^2

    http://www.cnczone.com/forums/tormac...ot-simple.html
    Tim
    Tormach 1100-3, Grizzly G0709 lathe, Clausing 8520 mill, SolidWorks, HSMWorks.

  20. #20
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    Re: Acceleration

    The acceleration values in PP for the 770 are the same as for the 1100-3. The velocity values are obviously slightly higher.
    Step

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