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IndustryArena Forum > CNC Electronics > Servo Motors / Drives > Delta servo drives and servos.
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  1. #61
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    Re: Delta servo drives and servos.

    Quote Originally Posted by joeavaerage View Post
    Hi,
    It too have 5mm pitch ballscrews and I use electronic gearing to arrive at 5000 pulse/rev or 1um per step. The finish is perfect. Just because the servos
    have a 160,000 count per rev does not mean you have to use it, and even if you did then you would require a motion controller that could produce
    step/direction signals in the low mega-Hertz range, at huge cost......and to what advantage.......none.
    Electronic Gearing does not change the Encoder you still have 160,000 PPR at the motor your electronic Gearing of 5000 step / per is 32:1 Ratio


    Quote Originally Posted by joeavaerage View Post
    If you went and bought new high resolution servos you would see no, or virtually no difference to surface finish or smoothness. Later model
    servos may have better tuning aids and control modes that may help but the extra encoder resolution will make no difference. Craig
    That is total BS
    Mactec54

  2. #62
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    Re: Delta servo drives and servos.

    Thanks for the feedback guys albeit opposing, it adds some perspective, and this post in general has been really helpful.

    May I ask for another opinion which is "What Control System?", it's for a clone BT40 bed mill. I did another post here but would be interested to hear your views (opposing or not) on this. There's the mainline suppliers (too expensive??) Chinese GSK and others, maybe with little support, and Acu-ritie and Protract which are 3 axis only, for all the install work it would be preferable to have options for more axis IMHO. . Over Xmas I had some spare time and played with LinuxCNC, all very interesting and potentially powerful, and got an installation running on the computer which can be complied but it was a strain and time soaking, and not sure if the end result would be an easy to use fully sorted system. and that comment is as much about my knowledge as LinuxCNC. I still had to get a card (6 axis) and get it running which didn't have the FPGA setups done, and learn their INI file system and the setup for talking to the outside world etc and thought it would have been still very time consuming to get it up and running. I've also looked hard at Centroid Oak which seems pretty good and sorted system and is good for DIY folks although the 5th full axis is sort of blocked by US regs. Not dirt cheap with options.

    I'm aware of motion control cards but you have to get some CNC software to talk to them I guess, not sure how play and play the whole setup is but with limited tech skills I don't think I wanted to get into stuff is too complex. SO if you've got any comment on this or other control ideas let me know. Thanks

  3. #63
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    Re: Delta servo drives and servos.

    Hi,
    not so many years ago, say 10-15 years, the state of the art servos had 2500 line (10,000 count/rev) encoders.
    Mactec54 seems to suggest that a servo with an encoder of that description is incapable of smooth movement
    and lacks the resolution required for a good surface finish, and that as we know is BS.
    Its enirely probable tha your Panasonic servos have encoders or resolvers in that same category and they were
    certainly very capable of great surface finishes, Panasonic was and remains a significant brand in the servo market.

    I use Mach4Hobby ($200) and an Ethernet SmoothStepper ($180) as motion controller, I like it and reccomend it.
    Others like UCCNC ($60?) and a UCxxx ($150?) motion controller.
    Yet others use Centroid Acorn ($300).
    Yet others use LinuxCNC (free) plus extra parallel hardware like Mesa (approx $300).

    All have their strengths and weaknesses but all are pretty capable and all are within a hobbyists budget.

    Craig

  4. #64
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    Re: Delta servo drives and servos.

    Quote Originally Posted by joeavaerage View Post
    Hi,
    not so many years ago, say 10-15 years, the state of the art servos had 2500 line (10,000 count/rev) encoders.
    Mactec54 seems to suggest that a servo with an encoder of that description is incapable of smooth movement
    and lacks the resolution required for a good surface finish, and that as we know is BS.
    Its enirely probable tha your Panasonic servos have encoders or resolvers in that same category and they were
    certainly very capable of great surface finishes, Panasonic was and remains a significant brand in the servo market.
    Craig


    More of your uneducated BS (wedge)

    The Panasonic are a level up on your Delta so would help if you new what you where posting about

    20 years ago Yaskawa which was the most used servo system for machining centers where using 17Bit Encoders, and yes the sigma II system has been in use for that long

    2500 line incremental encoders for wood working would be ok, but everyone had to start somewhere, it sure was not smooth in anyway as you say

    Bridgeport which was one of the first CNC Machines ever built they used Stepper motors and used no Encoders anything is posable depending on what you want out of a machine as for smooth cutting no that was not posable but was a start of the CNC evolution

    To see how far out of touch you are Fanuc have been using 10million CPR for the last 10 years on there standard model machines and offer much higher here is some of there spec's
    Mactec54

  5. #65
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    Re: Delta servo drives and servos.

    Hi,
    well there you have it, according to Mactec54 your Panasonic's are incapable of smooth running.
    Makes you wonder how it is that Panasonic became one of the top tier manufacturuers?

    Craig

  6. #66
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    Re: Delta servo drives and servos.

    Quote Originally Posted by joeavaerage View Post
    Hi,
    well there you have it, according to Mactec54 your Panasonic's are incapable of smooth running.
    Makes you wonder how it is that Panasonic became one of the top tier manufacturuers?

    Craig
    How did you figure that out by yourself, as I said nothing about what a Panasonic servo system can do or can't do (nuts)

    Who said it was a top tier manufacture !! are you sure you can read what is posted, you seem to be good at making stuff up
    Mactec54

  7. #67
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    Re: Delta servo drives and servos.

    Quote Originally Posted by joeavaerage View Post
    Hi,
    not so many years ago, say 10-15 years, the state of the art servos had 2500 line (10,000 count/rev) encoders.
    Mactec54 seems to suggest that a servo with an encoder of that description is incapable of smooth movement
    and lacks the resolution required for a good surface finish, and that as we know is BS.
    Its enirely probable tha your Panasonic servos have encoders or resolvers in that same category and they were
    certainly very capable of great surface finishes, Panasonic was and remains a significant brand in the servo market.

    I use Mach4Hobby ($200) and an Ethernet SmoothStepper ($180) as motion controller, I like it and reccomend it.
    Others like UCCNC ($60?) and a UCxxx ($150?) motion controller.
    Yet others use Centroid Acorn ($300).
    Yet others use LinuxCNC (free) plus extra parallel hardware like Mesa (approx $300).

    All have their strengths and weaknesses but all are pretty capable and all are within a hobbyists budget.

    Craig
    My older brother tapping center (1992?) has 1024 line encoders. 5 pitch screws, 700ipm, 400w.
    My newer brother tapping centre has 2500 line encoders, 10 pitch screws, 1400ipm, 720w.

    Doesn't take long to realise they are "the same".

    The encoder resolution does help with high dynamics, but, your machines (sorry), don't qualify in that realm

    Things to know where though, before this devolves into... well, I cant read them, I've had mactec on ignore for the last 5 years.

    1: Just like with a stepper, you want your servo to have the resolution that you need in practice. If you want 0.001" precision , then you want an encoder resolution of 0.0001" or better. The end resolution entirely depends on your machine. My brother tc225 has a 5 pitch screw, so that means I would "like" 2048 encoder counts. I have 1024, which is 0.00019". "close enough" for a machine that was meant to do straight line tapping. if you have a rack and pinion, you'll want a much higher resolution encoder, or a gear reduction - although at the same time, that type of machine isn't aiming for 0.001" precision usually.

    2: Do NOT use a second linear encoder on a machine that has high backlash and linear guides. you will get "jitter" as the machine tries to find its place. So, if you have rack and pinion, no scales. There are exceptions of course.

    3: In addition to being a complete twat who thinks the hobby market is beneath him, the north American Yaskawa rep will NOT support drives not bought from them. If you mention you got them from china (the drive and motor models are tagged with the country btw) they wont even reply. So, be prepared to have no support if you go for the cheap kits for fast2buy

    4: fast2buy are great, don't be afraid to buy from them

    5: Delta A series drives, are "more or less" the same as the equivalent Yaskawa drive feature wise. the A2 is similar to the sigma 5, and the a3 is almost identical to the sigma 7. The Delta is just 2/3 the cost.

    6: Yaskawa own both Omron and Siemens automation.

    7: When choosing a servo size, inertia will likely be your main concern, not power. if you have rack and pinion, you may need much larger servos than you think, even though you don't actually need the power. This is one area where the Yaskawa sigma 7 does shine (and delta A3) because it can dynamically tune larger inertia rations. Drives like the delta B2 can only handle small ratios. So you might need for example a 750w delta B2, where a 200w Yaskawa or delta a3 will work.

    8: External braking resistors are only really needed when you have very high acceleration and inertia. My brother TC229 has 500w resistors per motor, and 1.0G acceleration. It also has a 1kw resistor on the spindle, because, reasons . If tour machine is not particularly heavy, and not accelerating very fast, you wont need a resistor other than the built in one.

    Ok, you can now go back to your regularly scheduled bickering over nonsense.

  8. #68
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    Re: Delta servo drives and servos.

    Quote Originally Posted by ihavenofish View Post
    6: Yaskawa own both Omron and Siemens automation.
    Fact check Yaskawa have never owned Omron they had a partnership a few years ago that Omron did sell rebadged Yaskawa servo products, but no longer do

    Siemens they do own 100%
    Mactec54

  9. #69
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    Re: Delta servo drives and servos.

    "....Finally during the fall of August 2004, Yaskawa brought the last branding partner, Omron IDM, into the fold as well. Yaskawa did so by purchasing the rights to Omron IDM Controls’ HVAC and Industrial channels"

    The relationships get really screwy (they do with most big Japanese companies), because Omron safety is a different company. Doesn't really matter, other than knowing they are very closely related and the products are "the same". (there were rebranded siemens Yaskawa drives for a while too, though they weren't machine tool class)


    Back to delta... a few more good points:
    - You can buy them at digikey with next day shipping. So if one blows out, and your in the middle of a paying job, you can get replacements easy.
    - The "B3" is out now. 24 bit encoder, much shorter motor, 350% overload, 6000rpm, smaller drives. Price seems to be slightly more than the B2, but not as much as the A series.

    fast2buy has some in stock - I'm gonna try a 400w model for my spindle, should be entertaining

  10. #70
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    Re: Delta servo drives and servos.

    Quote Originally Posted by ihavenofish View Post
    "....Finally during the fall of August 2004, Yaskawa brought the last branding partner, Omron IDM, into the fold as well. Yaskawa did so by purchasing the rights to Omron IDM Controls’ HVAC and Industrial channels"

    The relationships get really screwy (they do with most big Japanese companies), because Omron safety is a different company. Doesn't really matter, other than knowing they are very closely related and the products are "the same". (there were rebranded siemens Yaskawa drives for a while too, though they weren't machine tool class)


    Back to delta... a few more good points:
    - You can buy them at digikey with next day shipping. So if one blows out, and your in the middle of a paying job, you can get replacements easy.
    - The "B3" is out now. 24 bit encoder, much shorter motor, 350% overload, 6000rpm, smaller drives. Price seems to be slightly more than the B2, but not as much as the A series.

    fast2buy has some in stock - I'm gonna try a 400w model for my spindle, should be entertaining



    Yaskawa ended the Partnership a little over 10 years ago, and it was only a Partnership, Omron now sell Rebadged Panasonic servo drive and Hitachi VFD Drives, they don't sell any Yaskawa drives Etc of any kind , here is a
    Snip from 2009

    After months of speculation and rumour, Omron and Yaskawa Electric have announced that they are ending their European drives joint venture, Omron Yaskawa Motion Control, which they established in 2003. In a terse statement issued earlier this month

    In future, Omron will sell badged inverters made by Hitachi and servo drives from Panasonic Full details are expected to be released in November at the SPS/IPC/Drives show in Germany. 2007

    Yaskawa have only had ownership of Siemens since 2018 have not seen any siemens rebranded drive not in the USA anyway may be something in Europe I can't see them changing any of the Siemens CNC Control system

    I like the Delta Servos drives also, the knockoff's from China though is a concern, you can even get Yaskawa and most other brands of servo from China also but not all are knockoff's
    Mactec54

  11. #71
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    Re: Delta servo drives and servos.

    A few years ago I delved into ballscrew math on the forums. Been away from the forums for a while. Worked alot of overtime during the past two years and now I find myself, like many others, with time on my hands to work on my projects and look for a new job. So in the process of making my own thread about a problem I am now having, I came across this thread (please feel free to help me with my problems in my thread if you know anything about AC servo drives ).

    Did any of the interested parties in this thread find any of my previous posts where I went through the equations I used, to accommodate for inertia, including sample calculations and graphs, in your searching of the forums? They are the same equations that are available in all of the major ballscrew manufacturers literature, although some use different units.

    I was hoping that someone else would jump in on the forums and validate my spreadsheets with their own spreadsheets, to compare results, but no one did, at that time.

    Quote Originally Posted by joeavaerage View Post
    I've done the calclations for the acceleration potential of my machine including rotational inertia. I've had my calculation checked by peteeng, a professional mechanical engineer. Additionally I have Hiwins calculation formulas and can double check all the calculations and they all point to the same conclusion.
    Who is this peteeng? Where was he when I wanted to compare notes with someone? LOL. Hey, feel free to ask him to join in the conversation here.

    From your full post here's what I got:

    Motor rotational inertia = 1.13 x 10^-4 kg*m^2 = 0.000113 kg*m^2

    Ballscrew rotational inertia = 5.252 x 10^-4 kg*m^2 = 0.0005252 kg*m^2

    0.0005252/650 for a 650mm long ballscrew gives

    0.000000808 (kg*m^2)/mm

    From the THK rolled ballscrew moments of inertia pdf that I have, a 32mm ballscrew has....

    8.08 x 10^-3 (kg*cm^2) / mm = 0.00808 (kg*cm^2) / mm = 0.000000808 (kg*m^2) / mm

    And that is the exact value that you have posted. Did you use the same source? Ground ballscrews and other manufacturers will be slightly different, but pretty close.

    pitch = 5mm

    mass = 110kg = 242 lbs

    Quote Originally Posted by joeavaerage View Post
    dw/dt= 2.4 /7.07 .10-4
    =3395 rad/s2
    Ok, I see, you are using 2.4 N*m as the torque. So 2.4 N*m as the continuous and around 7 N*m as the peak? So hard to figure out what you have available for acceleration when you have peak and continuous but the amount of time spent in peak depends on the geometry of what you are cutting. I haven't figured out a simple answer for this yet. But in general, you will be able to do better than the continuous torque.

    2.7 / 9.8 = 0.275G

    With a ballscrew efficiency of 1, neglecting couplers, neglecting preload drag, neglecting linear bearing friction, etc, I get almost exactly that. Around zero (-0.34 lbs) available cutting force at 0.275G using 2.4 N*m as the available torque. It's almost as if he used the exact same equations as I did . That is assuming that the axis you are referring to is being run by a single motor. If this axis is being run by two motors (do you have a build log, I clicked on your profile for latest started threads but it said there were no results?), it doesn't change much. So yeah, 32mm ballscrews are a bad idea unless you really need them (which you don't...I can say this with confidence because your axis would in all probability weigh way more than 242 lbs if you did, or if that would make any difference to your design). A 32mm diameter, 5mm lead ballscrew is pretty horrible for most CNC applications.

    Of course, I'm just doing this as fast as I can, and I've had several beers. So I might need to double check all of this later. I could have made an error. Standard disclaimers apply.

    Keep in mind with these graphs, that they don't include many factors such as preload torque, bearing friction, coupler inertia, etc. Also, this math is just theoretical, and doesn't incorporate many many other factors, like machine stiffness, spindle performance, etc, that can make or break a machine.

    If you look at the last graph, you can see that a 25mm diameter ballscrew with 10mm lead would have been a far better choice IMO. I'm guessing (not really knowing anything about your build) that even a 20mm diameter ballscrew would have worked.

    The good news for you is that, IMO, from what I've seen, 0.25G (more considering peak power) should still give you OK results. Your ballscrew lengths are fairly short, and your servos are large, and that goes in your favor.

    Quote Originally Posted by NordicCnc View Post

    Anyway, I quickly made and excel spreadsheet for calculating horizontal axis maximum acceleration, based on user inputs (yellow cells). You can even spec 1:1 direct drive (with coupling OR belt drive) and any other gear ratio with belt drive. Just insert rotation moment inertia for the pulleys OR the coupling. The calculations will include these values automatically.
    I tried having a look at your excel spreadsheet to see what you have done but couldn't get it to open. No worries, I was just curious.

    To the OP; I really don't know too much about servo drives. I am actually looking for some help on that subject myself. One option to consider as an alternative would be the Clearpath SDSK Stepper servos. I'm guessing that if you are buying your servos new it would be around the same cost or less. Plus you can get them in Nema 34 size factor so you shouldn't have to change your mounts, and with the higher torques at low RPM's and the 20mm lead of your ballscrews, you might not need any gearing. 20m/minute rapids is (20000 / 25.4) IPM = 787.4 IPM or 20000 / 20 = 1000 RPM, you may be able to even buy some Nema 34's that have better characteristics at 1000 rpm than the ones you currently have. What control software are you using? Mach 3 only lets you use one value for acceleration, but there are others, I forget which ones, that allow you to use two, one for cutting and one for rapids, and that could work in your favor.

    I need a bit more info if you want a better answer about my opinion on direct drive coupling a 750 W Servo.

    2020 ballscrews

    What are the approximate lengths?

    The 750W servo that you are contemplating, what is the Motor rotational inertia?

    70KG X axis...OK.

    2.4NM and 7NM (continuous and peak torque, is that correct?)

    Off the top of my head, I am thinking that either option (400W, 3:1 Belt, or 750W direct couple should be fine) and that the steppers you currently have, the torque falls off very quickly with RPM....something that is common with many high torque steppers....actually many steppers with a lower holding torque will provide much better torque at increased RPM, which is where you need it to decelerate the machine if you want fast rapids and good acceleration.

  12. #72
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    Re: Delta servo drives and servos.

    Yaskawa has an app that you enter in all the data and it tells you what motor combinations work. You can then approximate for other brands that have similar specs. Bit easier and less prone to errors than doing all the math by hand.

  13. #73
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    Re: Delta servo drives and servos.

    Attachment 443630

    The picture file didn't attach for some reason in my last post. Here it is. You'll need to save it and zoom in on it to see any details.

  14. #74
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    Re: Delta servo drives and servos.

    I used the Yaskawa app to size motors for my router which is an ultra heavy version of the xzero predator. 2525 screws (though, not very long), and a 130kg "x" axis moving mass. I wound up with an 86mm sanyo 500w motor (I have sanyo stuff already cause of my brother machines). If I was buying new motors, they would be 750w 80mm delta b3's for that axis. the Y axis, with only about 50kg on it got a 400w 76mm motor. In this case, I would probably still go 750w on the new motors. The older sanyo motors are built for high overload dynamics, and have more low end torque than another motor of equal "watts". This are all direct drive, no belt reduction.

    Main difference between my machine and his is length, and therefore inertia in the ball screw.

  15. #75
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    Re: Delta servo drives and servos.

    Hi,
    I was unaware just how significant rotational inertia would be when I bought those ballscrews.

    I reasoned the a 115kg mass axis would dominate the acceleration equation....but it does not, in fact it represents only about 10% of the inertia.

    I really wanted C5 ballscrews and my budget did not allow for new ballscrews. Thus when I found three 32mm C5 double-nut ballscrews in near new order
    with thrust bearings all for $1000 including shipping I jumped. I would have preferred 25mm but 32mm was what was available.

    It was only later that I realised just how sensitive acceleration is to ballscrew diameter.

    J=m .r2/2

    m=mass in kg
    r= ballscrew radius in m

    But note the ballscrew mass is:
    m=PI. r2.l.(density of steel)
    l=length of ballscrew

    J= PI/2 .r4.l. (density of steel)

    So rotational inertia is proportional to the FOURTH power of radius.
    Whereas rotational inertia is proportional to the square of the ballscrew pitch.

    The upshot here is that ballscrew diameter (equivalently radius) is vastly more important than ballscrew pitch.

    For example, if my ballscrews were the same length but 25mm rather than 32mm:

    J= 5.252 x (25/32)4
    =1.956 x 10-4 kg.m2
    which would reduce the total overall rotational inertia by 46%.

    I have just got the cast iron axis beds from the foundry. I have supplied the drawings to the a company to rough machine them prior to
    anneal heat treatment, and then they will machine to final size.

    I am still working on the pattern for the frame...its looking to be about 300kg of SG cast iron, it won't be cheap, I'm guessing molding and
    pouring will be over $3000NZD.

    Craig

  16. #76
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    Re: Delta servo drives and servos.

    Hi,

    A 32mm diameter, 5mm lead ballscrew is pretty horrible for most CNC applications.
    I disagree. Did you not see the calculation, my new build mill will traverse from one limit to the other in 1.5 seconds.

    The sheer amount of mass accelerating at 1/4g will still require that I bolt the machine down to the floor. I'm not sure
    that even higher accelerations will give me any advantages. My existing mini-mill does a good job of following a toolpath
    at only 375mm/s2 and I believe my new build mill will do a fine job of toolpath following at 2500mm/s2,
    a casual 7 times what I have currently.

    Even with 5mm pitch I will still end up with full rated torque G0's of 15m/min and in field weakening mode 25m/min....is that not
    fast enough for a machine weighing around 600kg?.

    Of course the 5mm pitch also results in great thrust for cutting toolpaths.

    Craig

  17. #77
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    Re: Delta servo drives and servos.

    Quote Originally Posted by NIC 77 View Post
    A 32mm diameter, 5mm lead ballscrew is pretty horrible for most CNC applications.
    Quote Originally Posted by joeavaerage View Post
    I disagree.
    The key word there was most. I'm not trying to put you down, apologies if it came off that way, nothing personal . Most people who build machines on this forum use stepper motors and many make routers that have long axis. I've seen people with large diameter 5mm lead ballscrews and huge high mH steppers trying to drive the long axis of their large routers at reasonable speeds and being so disappointed.

    I don't think we disagree that if some other options had been available to you regarding lead and diameter, they could have been a better choice.

    But I get it, you bought some good quality stuff and are making it work. I do the same kinds of things. And at the end of the day, if you're happy, it's a win. With those 750W servos, and custom castings, etc, yeah, from what you've said about it, it sounds like a very nice build that should do what you want it to do.

    And hey, in your specific case, perhaps there even some advantages to having those large diameter ballscrews, like being less prone to damage during accidental crashes? I don't actually know, I'm certainly not an expert on every subject, and I'm definitely not always right.

    Quote Originally Posted by joeavaerage View Post
    Of course the 5mm pitch also results in great thrust for cutting toolpaths.
    Did you have a look at the graphs I made?

    If you look at the second graph from the top, 32mm ballscrew at 0.25G acceleration, and 2.4Nm continuous torque, the 5mm lead has close to zero available cutting force, while the 10mm lead has over 100lbs of available cutting force.

    If you look at the second graph from the bottom, 32mm ballscrew at 0.733G acceleration, and 7Nm peak torque, the 5mm lead has close to zero available cutting force, while the 10mm lead has over 300lbs of available cutting force.

    IMO, you accelerate while you are cutting, so the 5mm lead actually has the least available amount of cutting force.

    But yeah, you should still have a great machine. I just find it interesting to make comparisons.

    And now I kind of feel bad because I think we derailed this thread from the OP. I wonder if he'll come back or if he got what he needed?

  18. #78
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    Re: Delta servo drives and servos.

    Hi,

    If you look at the second graph from the top, 32mm ballscrew at 0.25G acceleration, and 2.4Nm continuous torque, the 5mm lead has close to zero available cutting force, while the 10mm lead has over 100lbs of available cutting force.

    If you look at the second graph from the bottom, 32mm ballscrew at 0.733G acceleration, and 7Nm peak torque, the 5mm lead has close to zero available cutting force, while the 10mm lead has over 300lbs of available cutting force.
    I had a look but they came out so small that I couldn't read them.

    I find it hard to credit that your calculation shows more thrust with 10mm pitch screws verses 5mm pitch. After all finer pitch screws result in greater mechanical advantage and therefore more thrust
    for a given amount of torque.

    Thrust = (torque / radius) x ( 2 x PI x radius / pitch)

    For 32mm with 5mm pitch and 1Nm (unit) torque:
    Thrust (per unit) =1/0.016 x (2 x PI x 0.016 /0.005)
    =1256.4N

    For a 32mm with a 10mm pitch and the same 1Nm torque:
    Thrust (per unit)=1/0.016 x (2 x PI x0.16 / 0.01)
    =628.2N

    Whatever torque is available for generation of thrust over and above that required for accelerating the axis generates thrust, and a 5mm screw generates twice
    the thrust than a 10mm screw.

    I have allowed rated torque available for acceleration and any extra required for cutting will be supplied by overload capacity. Note that if I demanded a G1 of 15m/min
    feed rate the axis will accelerate for 90 ms to reach that speed and thereafter the required torque to maintain that speed is just the friction and cutting forces,
    a much smaller proportion of the rated torque than the acceleration phase. Thus while I have nominated that the extra torque for cutting come from the overload capacity
    it will in actual fact only NEED be supplied by the overload capacity for maximum of 90ms bursts.

    This mill is designed for cutting metals and I find it hard to imagine that I could cut aluminum or steel at 15m/min, hogging steel at 1m/min seems more realistic.
    In that situation the peak torque demand periods are even shorter, something like 5ms, and thereafter nearly all the avilable torque is to counteract cutting and frictional forces alone.

    Craig

  19. #79
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    Re: Delta servo drives and servos.

    I think he's implying at the same speed, the 10mm has more available torque because it has less inertia - you would reduce the belt ratio by 1/2 on the 10mm. If you used the same reduction, you'd of course have much more force on the 5mm, and half the speed.

    Long axes are tricky with spinning ball screws, which is why most commercial machines avoid them. Most big machines with screws spin the nut instead. Both inertia and whip get solved there.

    https://www.yaskawa.com/products/mot...ls/sigmaselect

    A run through for 3205 screw, 650mm long, no belt reduction, 110kg load, 0.25G:

    - can be run by a 60mm 400w sigma 7 motors, except the inertia is 12:1 and the motor can deal with 10:1 so it may not tune well. You also aren't left with much for cutting force during acceleration moves. You will need a 360w resistor for braking from 6000rpm.
    - can be run comfortably by a 60mm 600w sigma 7 as its intertia handling is 20:1. you get considerable cutting forces available with this selection. You will need a 360w resistor for braking from 6000rpm.
    - can be run by a 80mm 750w sigma 7 well within the inertia ratio, and with ample spare torque - you barely breach continuous torque for acceleration moves. You will need a 420w resistor for braking from 6000rpm.

    The wise choice would be the 750w, but the other *could* work. Of course, if you do a belt reduction, everything changes. Likewise, you need to gauge your purpose. My MAHO has 3210 screw, and 2.5kw 180mm frame motor on the X and Y with a 1.6:1 reduction. It had to provide 10000N cutting force while slinging both the rotary table, and 600kg of work piece at 0.5G. The Z was 7.5kw. Most hobby class routers do not need more than 300-500N cutting force.

    For the original poster, who has a 2020 screw - 90kg moving mas, 0.25g acceleration, limiting speed of 800rpm, max travel of 1600mm (per NSK compact FA data):

    - direct drive, Yaskawa gives you anything down to a 150w sigma 7 motor for "power" needs, but you need the 600w or 800w models for inertia handling. Neither of those motors need an external resistor in this case.
    - 3:1 reduction, you can use the 400w within its inertia ratio. the 200w and 150w (40mm) are still outside their inertia ratios. No resistors are needed here.

    The Delta B3 models have MUCH higher inertia handling abilities, so you might be able to get away with a 200w version there, but of course you have less cutting forces available. 400w is probably the smart choice with this reduction. 750 (80mm) is likely overkill.

    It's probably worth noting from that - the OP likely cant get any extra speed from his machine because the screws can't be spun that fast without resonance (screw whip) issues. The screw is really too small for that machine (i assume it's 8 feet). The nsk compact fa catalogue is a decent guide as they tell you directly what length and what speeds are allows with what type of supports - a bit easier than the normal graphs. They top out at 25mm though, and don't offer then above 2000mm, which will spin at 700rpm or 1000rpm with fixed mounting at both ends.

  20. #80
    Join Date
    Nov 2013
    Posts
    4280

    Re: Delta servo drives and servos.

    Hi,
    when I was doing the rough sketch calculations for this build I decided to try to stick within the 'rated' torque envelope rather than rely on the overload capacity.

    As I posted earlier the 32mm ballscrews were really because they were available rather than a conscious choice. I did not at that time know about, or rather consider,
    rotational inertia. As it turns out 25mm would have been a better choice but having said that 32mm is not outrageous and I think I will get away with them.

    My existing mini-mill has 2005 ballscrews and I really REALLY like the mechanical advantage they give, I never consider my machine thrust limited....just rigidity limited.
    When these 3205's came up I did not for a moment consider that the 5mm pitch was too fine. At rated servo speed (3000rpm) G0's are 15m/min, which is plenty for me.
    It's true however that I did not appreciate how much torque would be absorbed in spinning 32mm screws to that speed.

    In some respects I lucked out with the choice of 34size 750W B2 servos. The inertia ratio is about 5:1 so its getting up there but well within it's capacity. I have
    made no allowance, yet, for braking resistors.

    I had already bought one 400W B2 servo and had been experimenting with it. To say that I was impressed is an understatement, they seem to have much more power than
    the specs suggest. I was of the opinion that they would be adequate for my new build mill as well. When it came time to order them however I found that I could have
    750W servos of the same series for scarce another $40 each....so what do you do? I got the 750W's.

    Given my more recent exploration of the physics of ballscrew driven axes I'm glad that iI did get 750W's.

    Craig

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