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  1. #181
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    Default Re: Yet another epoxy granite mill

    Hi Thomas - what spec is your servo and ballscvrew pitch? I don't think its a movement issue but its more of an acceleration issue. But I can calc the forces for you....Peter



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    Default Re: Yet another epoxy granite mill

    Quote Originally Posted by peteeng View Post
    what spec is your servo and ballscvrew pitch? I don't think its a movement issue but its more of an acceleration issue. But I can calc the forces for you....
    Yeah, defently acceleration issue, speed isn't much of an issue when max travel is 700mm:-) But i'd like to get as much as the system can, hence the idea of using aluminium for the subtable...

    The ballscrew is a 32mm with 10mm pitch. The servo is 2,2kW with 7Nm continuously, but 300% overload possible for bursts(like acceleration), so 21Nm max(if we ignore the cutoff at high RPM)... I've actually struggled a lot to determin if the sizing was correct :-/ But i've made the design around the ballscrews so there is physical room to go up one framesize of servos in case my current one isn't enough, then the next limit will be the powerline to my house ;-)



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    Default Re: Yet another epoxy granite mill

    Morning Thomas - Since the table is sliding around and not moving vertically its "supporting" its table weight and the job weight but not moving it. This is like you pushing your small car in a flat carpark, your not pushing 1 tonne around until you try to stop it rolling down a hill. Say the job is 500kg and the table is 100kg.

    So the motor only has to move the frictional component of the weight not the dead weight (moving slowly) plus the seal drag. Ignore the seal drag for now. Hiwin publish the friction coefficient of their ball cars as 0.004 so this means the motor only has to supply 600x0.004=2.4kgf to move the mass slowly. 7Nm with a 10mm pitch screw can push 400kgf so all good there. To check possible accelerations I need the length of the screw and the rotational inertia of the motor. But I suspect you'll have heaps of torque and accel... Peter

    Attached Thumbnails Attached Thumbnails Yet another epoxy granite mill-thomas-jpg  


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    Default Re: Yet another epoxy granite mill

    Quote Originally Posted by peteeng View Post
    Morning Thomas - Since the table is sliding around and not moving vertically its "supporting" its table weight and the job weight but not moving it. This is like you pushing your small car in a flat carpark, your not pushing 1 tonne around until you try to stop it rolling down a hill. Say the job is 500kg and the table is 100kg.

    So the motor only has to move the frictional component of the weight not the dead weight (moving slowly) plus the seal drag. Ignore the seal drag for now. Hiwin publish the friction coefficient of their ball cars as 0.004 so this means the motor only has to supply 600x0.004=2.4kgf to move the mass slowly. 7Nm with a 10mm pitch screw can push 400kgf so all good there. To check possible accelerations I need the length of the screw and the rotational inertia of the motor. But I suspect you'll have heaps of torque and accel... Peter
    Interesting! I've tested the motors and they can EASY move anything i'd throw at them... as your calculations illustrate there is pleanty of headroom.... and my crane above my mill cannot take more than 300kg, so 100+300 is probably max(give or take a Kurt wise or two ;-) )

    The motor inertia is specified as 15.2 (*10-4kg.m2). The travel for Y is 400mm and X is 680mm.

    /Thomas



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    Default Re: Yet another epoxy granite mill

    2.2kw servos are very powerful for an axis motor.

    Look at commercial VMCs and what servos they have.



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    Default Re: Yet another epoxy granite mill

    Hi Thomas - You have a small beast in there. At 400kg "payload" the 7Nm will accelerate it at >0.3g which is quite fast. It will get to full speed in about 10mm (by calc, say 20mm in reality) so no concern if you go steel table. Steel is tougher then aluminium for a table if that's your choice. Someone maybe able to check my calcs or use a web calculator to confirm the numbers. This also does not consider the 21Nm overload condition. But to consider this we need to look at the thermal constants for the motor to check it will cope with the intended thermal cycle. Cheers Peter

    edit --- I meant to write a spreadsheet for this next time I did it so next time!! But in this case 65% of the torque is required to turn the system and 35% is used to push/pull the system. This is one reason to keep the screw size as small as possible to minimise rotational inertias. At some size of machine its worthwhile going fixed table so the mass becomes the machine only so drive systems can be optimised closely to this mass for high speed machines....

    When the Z is considered we have to include the mass being lifted so this calc would have to be adjusted if done for the Z...Your Y screw is nearly half the inertia of the X so it will perform better as well...

    Attached Files Attached Files
    Last edited by peteeng; 12-06-2021 at 06:17 PM.


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    Default Re: Yet another epoxy granite mill

    Quote Originally Posted by peteeng View Post
    Hi Thomas - You have a small beast in there. At 400kg "payload" the 7Nm will accelerate it at >0.3g which is quite fast. It will get to full speed in about 10mm (by calc, say 20mm in reality) so no concern if you go steel table. Steel is tougher then aluminium for a table if that's your choice. Someone maybe able to check my calcs or use a web calculator to confirm the numbers. This also does not consider the 21Nm overload condition. But to consider this we need to look at the thermal constants for the motor to check it will cope with the intended thermal cycle. Cheers Peter

    edit --- I meant to write a spreadsheet for this next time I did it so next time!! But in this case 65% of the torque is required to turn the system and 35% is used to push/pull the system. This is one reason to keep the screw size as small as possible to minimise rotational inertias. At some size of machine its worthwhile going fixed table so the mass becomes the machine only so drive systems can be optimised closely to this mass for high speed machines....

    When the Z is considered we have to include the mass being lifted so this calc would have to be adjusted if done for the Z...Your Y screw is nearly half the inertia of the X so it will perform better as well...
    Sorry for not getting back to you! I appreciate all the calculations, but i'm finding it a bit hard to believe that my servos will be able to accelerate 400kg to full tilt in 1-2cm... i haven't gotten to do the math myself yet, but i have a feeling that something is a bit off... i'll get back to you on this :-)

    ...and by the way, the project ain't dead, here is a sneak peek from this week:
    Yet another epoxy granite mill-image-ios-42-jpg
    ....bigger update will follow...hopefully soon :-)



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    Default Re: Yet another epoxy granite mill

    You are progressing good, nice build.
    About servo power you really don’t need to worry you have plenty of power to move 500kg of mass.

    I would say a little oversized but as always, it’s never too much of power

    You can make simple calculation using this oriental motors sizing tool – very useful.
    https://www.orientalmotor.com/motor-...ml#QuickReport

    I have made quick calculation and for 400kg load on 3210 ballscrew, with moving speed of 500mm/s and acceleration 0.2s you need 4.5Nm.
    You will find out that we all pretty much oversize our motors way too much



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    Default Re: Yet another epoxy granite mill

    hey badhabit

    how much did the servo drives cost per piece? Do yours have vector control as an option?



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    Default Re: Yet another epoxy granite mill

    Quote Originally Posted by ardenum View Post
    how much did the servo drives cost per piece? Do yours have vector control as an option?
    I paid 650usd per servo plus shipping. I'm a bit unsure of the vectorcontrol-option you mentioned? Isn't this only for asynchronous motors with VFD's?



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    Default Re: Yet another epoxy granite mill

    Quote Originally Posted by badhabit View Post
    I paid 650usd per servo plus shipping. I'm a bit unsure of the vectorcontrol-option you mentioned? Isn't this only for asynchronous motors with VFD's?
    650 just for the drive? or together with the servomotor?

    I'm a bit confused about CTB's drives, they seem to have only 2 versions available, one regular and one multi axis but then I have their manuals where the same drive is used for both regular axis servos and also for their direct drive motors. However in one manual there's vector control specified and in the other there isn't for the same drive number. Confusing.



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    Default Re: Yet another epoxy granite mill

    Quote Originally Posted by ardenum View Post
    650 just for the drive? or together with the servomotor?
    Ah! No, thats for motor + driver + cables :-)

    I think the multiaxis driver is just so save a bit on space and cost. Otherwise i guess they are identical. I talked to "Jeff Zhang" via Alibaba and WeChat and he was very helpfull with any questions. If there was something he couldn't answer i got to "talk" to the "tech guys" behind him directly on WeChat....albeit a bit challenging at times since some of them could not write in english and my mandarin is....ehm....a bit rusty? ;-) However, Jeff helped translate, so no worries....



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    Default Re: Yet another epoxy granite mill

    FINALLY! I got the castings back....

    A couple of pictures of it at its final destination:
    Yet another epoxy granite mill-image-ios-jpgYet another epoxy granite mill-image-ios-1-jpgYet another epoxy granite mill-image-ios-2-jpg
    ...i just got it off the truck 10minutes ago, so haven't even had the time to clean it up yet...

    Hopefully I'll be able to put a bit more action into this thread again :-)



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    Default Re: Yet another epoxy granite mill

    Quote Originally Posted by badhabit View Post
    FINALLY! I got the castings back....

    A couple of pictures of it at its final destination:
    Yet another epoxy granite mill-image-ios-jpgYet another epoxy granite mill-image-ios-1-jpgYet another epoxy granite mill-image-ios-2-jpg
    ...i just got it off the truck 10minutes ago, so haven't even had the time to clean it up yet...

    Hopefully I'll be able to put a bit more action into this thread again :-)
    looks majestic, was it coated with something, wax maybe? it looks shiny for a UHPC...



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    Default Re: Yet another epoxy granite mill

    Quote Originally Posted by ardenum View Post
    looks majestic, was it coated with something, wax maybe? it looks shiny for a UHPC...
    It is shiny because it is epoxy granite...not UHPC :-)



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    Default Re: Yet another epoxy granite mill

    Quote Originally Posted by badhabit View Post
    It is shiny because it is epoxy granite...not UHPC :-)
    It's that damn color, keeps confusing me...



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    Default Re: Yet another epoxy granite mill

    Looking great.

    Watching with interest



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    Default Re: Yet another epoxy granite mill

    I just picked up all the steel parts(cross table and the 2-part Z-sled) from heat-treating(stress relief) this week and had them sandblasted where i'm going to fill in the voids with epoxy granite next week during the easter holidays...if everything goes to plan anyways:-) Will shot a couple of pictures during this process aswell...

    So hopefully this project will pick up a bit more speed now...



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    Default Re: Yet another epoxy granite mill

    A little progress today! I got the crosstable, z-sled and spindlenose back from heat treating and today i filled them with epoxy granite:
    Yet another epoxy granite mill-image-ios-3-jpg
    ... i've been very concerned with the parts "ringing" A LOT when i hit it with something, it is after all just plain steel. BUT after i filled them today, the ringing is almost completely gone! I hope the result is as good when the EG is cured, but even if it is not quite as good, i bet it is still a huuuge improvement...

    I've used the opportunity to test if could degas the epoxy granite mix using vacuum... but no, i could not:-( I have a vacuum champer from my previous carbon-fiber endeavours and i tried pulling a vacuum on a small bucket of EG. The EG grew like 20-30%, so there is a lot of air inside, but the air wouldn't degas from the EG. It didn't buble as it do when i degas resin for carbonfiber infusion, it seems like the EG-mix isn't fluid enough to let the air out, so it's trapped in the mix, even at full vacuum.... So vacuum-degassing EG- is apparently a no-go... Just thought anyone passing by should know :-)

    /Thomas



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    Default Re: Yet another epoxy granite mill

    ...blasted and painted...off to machining!
    Yet another epoxy granite mill-img_1855-jpg



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