First ambitious cnc build


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  1. #1
    MikeTerasT's Avatar
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    Default First ambitious cnc build

    Hello people!!
    Nice to be on here! First post.

    We are building a quite ambitious cnc for wood,soft metals and sheet metal.
    We are at the part where we should start ordering things. But to be sure that we dont have any...mishaps. we have some questions. I will start off with a few simple ones.

    We are using nema34 1600oz motors. RP drive. Table bed is 2.1*3meters Planned on using shielded wire and stranded. But what awg should it be. Motors are 7.8amps and awg 22 is rated for 7 amps. But there are quite alot of variabales to consider so what do you recommend? I think awg 16-18?

    Same goes with the spindle wire. We have a 4500w vfd air cooled spindle.

    An actual example of the wire would be ofcourse perfect but any advice is greatly apprecciated

    Cheers!



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    Default Re: First ambitious cnc build

    There's not that much current going through the wires, so it's not the AWG rating that's most crucial, but the flexing that they have to withstand. Make sure you get finely stranded wire that can take a lot of it, and use cable carriers to limit the radius it has to bend to. The spindle should have a braided shield over the whole bundle, that's grounded only at the VFD end, to avoid creating a ground loop. Make sure the insulation on your wires can take a bit of heat as well; steppers often will run rather hot.

    Check the inductance on those motors. Often steppers with a lot of holding torque like those also have a lot of inductance, and require a lot of voltage to overcome it and get up to top speed - which you'll need for cutting wood effectively. Drivers that can tolerate that much voltage tend to be more expensive than ones that can't. The formula is: Square root of the inductance (in mH) times 32 equals optimum voltage to run motor at top speed.

    Andrew Werby
    https://computersculpture.com/


  3. #3
    MikeTerasT's Avatar
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    Hey!
    Thankyou very much for such a detailed answer.
    We were põanning on going with awg16-18 does that sound okay?

    And with the spindle wire. My idea was to use one cable with 4 wires tohether with shielding. But from your respone I understand that you recommend using separate wires and to shield the whole bunch?



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    Default Re: First ambitious cnc build

    I would look into 'continuous flex' cable. It's a bit pricey, but should last a lot longer. Normal flexible cable isn't meant to be flexed thousands or millions of times like will happen on a CNC machine. Something like this: https://www.igus.com/product/1084 You could use 18AWG without much trouble I wouldn't think.
    If you can use much larger bend radii on your e-chains you can probably get away with standard cable.
    Similar cable for the spindle, is it a 220V or 380V spindle? Probably 12Awg if 220V, 14AWG if 380.



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    MikeTerasT's Avatar
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    Hello again!

    So, the machine is running!! Still needs fine adjustment, but we are getting there! It has been an amazing journey!

    We have some problems that have come up.
    Mainly Z axis, which uses 1605 ballscrew direct drive with NEMA34 motor.

    We cannot get a feedrate higher then 1170mm/min. That is not during running a job, it is just jogging with the keyboard. Even if we put just 1175mm/min for travel speed. It gets stuck. It starts moving but then suddenly slams stuck with no warning, then you let go of the jog, and press again, and it starts moving again. It might slam 2-3 times between the full length which is 300mm. It sounds like it is almost like hitting something, and of course it looses steps at that point. We have not runned any long jobs, we do not know if it happens with lower speeds over the long run.

    What could it be and what could solve this?
    Counterbalance springs?
    Lack of grease?
    1610 screw?
    Settings on the driver?

    Would appreciate it much if someone could point us to the right direction.

    The second question for what we are waiting for the response of the manufacturer, is what could be the max running temp for our spindle? https://www.shop.cncdrive.com/index.php?productID=910

    And the Nema34 max running temp should be around 80celsius??

    I am currently away from the machine for a few weeks and when I get back I would like to have a full set of fresh ideas where to start from.

    All the best guys!
    MIKE



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    Member peteeng's Avatar
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    Default Re: First ambitious cnc build

    Hi Mike - Sounds like your acceleration is set too high. Its ramping up and then it can't accelerate anymore (as it can't overcome the inertia at that acceleration) so it stalls. So dial the Z acceleration way down and bring it up slowly. Peter



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    Thanks alot for the idea! I will try it!
    Any other ideas?

    What about max temp for spindle and nema 34?



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    Default Re: First ambitious cnc build

    The manufacturer of the spindle and the motor will spec their max service temps. In general speak, if its too hot to touch it's too hot. Peter



  9. #9
    MikeTerasT's Avatar
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    Spindle manufacturer does not specify and does not answer emails.
    On the Nema specs it is specified only that stocking temp is +70. Which means that until 70 for sure everything is okay. But for max short term I am interested.



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