Slaving axis (rotating nut, 2 steppers for Y-axis)

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08 May 2017 08:44 #92807 by denhen89
Hello guys,
soon i want to start to build an large cnc router (+2500mm x 1350mm, my first router) and want to rotate the ball nut instead of the screw. Before i start to build the router and order parts, i would like to know if its very difficult to setup 2 steppers for one axis, for someone without experience with LinuxCNC and generally cnc routers. Of course i have read through couple of threads where the same was asked, but for me it sounds very complicated. Maybe thats because i have never used LinuxCNC and all the special words are unknown for me.

Also i would like to know how many home switches i need. Do i need only 3 (one per axis)? Do i also need limit switches or could i use soft-limits ?

*Maybe it would be a lot easier to use only one, but big stepper (12Nm) for both 2510 ball screws (connected with an belt), but i think the travel speed would be too slow for that large router, or what do you think ?

I have already installed linuxcnc just to test the computer and the jitter test result was quiete good (jitter 6000-8000), so i am ready to go with LinuxCNC. As board+card i want to use MESA 5i25 + 7i76, even its very expensive (345€), but for me it sounds that the MESA hardware is very good and i am not sure if there is an cheap, but good alternative.

I dont want to make this thread endless long, and i am sorry for opening a thread just to ask if its difficult to setup slave axis, but i hope you understand that i dont want to start build/order parts just to later find out that i do not manage to setup it correctly.

Thanks in advance!

BR,
Denis

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08 May 2017 10:25 #92810 by BigJohnT
The Master branch (pre 2.8) is the best to use with a dual motor gantry. It can home the gantry to individual home switches and square the gantry.
linuxcnc.org/docs/devel/html/config/ini-....html#_home_sequence


JT
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08 May 2017 20:05 #92872 by denhen89
Sorry for the late reply, but i was thinking the whole day what i should do. Using one big motor to run both ball screws or rotary ball screw. I would like to have an dynamic and precise router, and not an extremly slow one, so the only option is to use rotary ball nut.
The only disadvantage i see when using one motor per ball screw, is that i fear to not get it done properly.

Maybe i need some motivation :D

@BigJohnT: Thank you very much! I read a bit about that, and i would like to try to install the Master branch pre 2.8 version, but the computer where the latest official linuxcnc version is installed has no VGA inside. For the Jitter tests i used the NVIDIA GTX 770 from my computer. I am not sure if it makes sense to install it until i have no parts (motors, drivers, supply, etc...), but i would like to know if i get it done..

I read all the time about HAL, .ini file, config, synaptic, etc. and that gives me the feeling that it will be very difficult or it will take very long until i get the setup done properly, so even its difficult to answer and for sure its an stupid questions, i would like to ask if the process is doable for someone without any experience with CNC.

I am quite good with PCs and never really had any problems which i could not solve by my self, and usually its sounds always more difficult as it is, so maybe its same in this case and i just need to do it, but the parts i order are depending on which system i use (1 motor for 2 screws or 2 motors for rotating the nuts). Just as example, for the Rotating nut system i would use 2505 (instead of 2510 with one motor for both screws) hiwin ball screws which are cheaper then 2510 and it should be fast enough with 4Nm steppers per nut..

Maybe someone can give me some good feeling haha :D

Thanks in advance.

BR,
Denis

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08 May 2017 20:25 - 08 May 2017 20:28 #92875 by Todd Zuercher
Just install Linuxcnc, and play with some of the included simulation configurations for a while and try to familiarize your with it. Get used to using Linux. Read the manuals. Once you build a little confidence, try building your own simulation config. then once you feel you are ready, try hooking it up to some real hardware. Most of the time the best way to learn how to do something is to just jump in and do it. (You'll never learn to swim just soaking your toe.)
Last edit: 08 May 2017 20:28 by Todd Zuercher.

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09 May 2017 10:31 #92904 by denhen89
Thanks Todd!

I didnt know that its possible to make simulations on linuxCNC. I will soon buy an VGA and start to play around with linuxcnc.

Just one more question: Would you go for rating nut (2 motors) or one motor rotating both screws (belt)?

Thanks in advance!

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09 May 2017 13:11 #92921 by Todd Zuercher
If you want to go fast on a long screw, it is usually better to rotate the nut rather than the screw. (The whole problem of critical speeds for a ball screw.)
You may still find that the low pitch of a 5mm screw is going to limit your speed too much even with a rotating nut. (Don't forget step motors are not particularly fast as well.)
What are the max velocities you are hoping to achieve?

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09 May 2017 14:27 #92927 by denhen89
Good, so i think i will use the rotating nut system and i will take the 2510.
The reason why i wanted to use the 2505 ball screws is, because i can only buy 2500mm long or 3000mm long ball screws, so my plan was to buy 2505 3000mm long and shorten both screws to about 2600-2700mm, so i could use one of the cutted pieces for the Z-axis (want to have about 20mm travel)
The travel only need to be about 2550mm x 1300mm, because the sheet metal plates and wood plates are mostly in 2500x1250mm. When i will buy the 2510, then i am not sure if i should use an 2510 ball screw (cutted piece from 3000mm long screw) for the z-axis, because i want direct driver the screw.
The other piece i could someday use for an other machine, thats why i also wanted to use the 2505 instead of 2510, but maybe i will just buy the 2510 and keep the total lenght and buy an 1605 ball screw for the z-axis.
Hope you understand me, and maybe you have some good solution for that.

To your question about the velocity: I think 10m/min would be fine, but currently i dont know hot to make the calculations when using ratios (not sure which ratio i should use for the rotating nut). The gantry ball screw i would like to direct drive, but i want to be honest, i have really no glue if that will work well, i mean rotating nut on long axis and direct drive for gantry. Also i dont know if 4 Nm - 4,5 Nm steppers are enough for all screws.

I think it would be a lot easier to use one motor for both screws, but i fear it will be too slow.

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09 May 2017 14:40 #92928 by tommylight
Just as a comparison
I have plenty of 2505, had one 2.4meters long mounted on a machine with direct drive from the nema 34 motor it could do 2m/minute before starting to vibrate, 3m/m was unusable. So i cobled together a rotating nut assemby with a nema 23 motor, it was wobly but moved so i pushed it to 16.8m/m. Could not push it further as one of the axial bearings was not seated properly so it was causing that woble. Still abot 8 times faster.
One detail i found in that test was: axial bearings are realy sensitive to the amount of preload, litle preload and it has to much baclash, more preload and it barely moves.
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09 May 2017 15:02 #92929 by denhen89
Thank you very much for the comparison. So, it sounds like rotating nut is the only way to go for that lenght. 16,8m/min is really fast, but i think 10-12 is totally fine for me. As long as i do not do it commercial its fast enough, but if i would do it commercialy, then i would for sure use servos. For now, its not necessary. What i am worrying about is the lubrication of the nut/screw.

My cousins husband works daily on manual milling and lathe machines with his father on their homeshop (gear production), and today (in about an hour) i want to show him some pictures and ask what he needs to make 2 pieces for me. I am just printing some pictures of Jonathans rotating ball nut (from mycncuk forum) and hope it will be not an problem to do that.

This is the rotating nut system from Jonathan (Post #74) : www.mycncuk.com/threads/3340-Rotating-Ba...t-design-ideas/page8
I want to build the exact same one.

One question i have:
You are saying that the axial bearing was not seated properly. Could you tell me how much bigger the hole has to be where the bearing will be seated in, so that it will fit properly ? The housing will be from aluminum and i dont know if the bearing needs to be "cold" pressed inside the housing or to first heat up the housing and then push it inside.


Thanks in advance.

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09 May 2017 15:06 #92932 by Todd Zuercher
Here is a good ball screw selection reference.
www.nookindustries.com/LinearLibraryItem/Crit_Speed_w_EZM_
(10m/min is outside of the acceptable range of even a 2510 screw at 2500mm length)
But if you rotate the nut instead both the 2505 and 2510 would be fine. (2505 up to 14m/min 2510 up to 28m/min)

To move at 10m/min with the 2505 screw you will have to rotate the nut at 2000RPM. A NEMA 23 step motor might be able to do that, but there won't be a lot of extra torque left. With the right drivers and power supply it should work though.
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