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Thread: What did I buy? Old machine, new tech, take a look.

  1. #21
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    Default Re: What did I buy? Old machine, new tech, take a look.

    Quote Originally Posted by Nicedesigns View Post
    Interesting, thank you! I asked the seller of the low profile assembly for more info on the Kirk lead screw.

    The first listing I posted is for a much cheaper chinese assembly. It says it uses a ball screw. Are there any other obviously disadvantages or concerns about that assembly?
    The person managing that ebay seller profile is not helpful.

    I called Avid CNC more more info about their assembly (it's missing from the product page). It's seems pretty awesome, it's got dual 15mm linear rails with two self greasing bearing blocks per rail. A 10mm pitch ball screw. A dust cover. And it's designed to handle their 8.7 hp spindle. What do you guys think about it?



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    Default Re: What did I buy? Old machine, new tech, take a look.

    with this actuator the servo is only a 100 watts which is not very big, with the 2:1 ratio the torque is better on the drive screw but half's the rpm of the servo motor which i think was rated at 3000 rpm so you end up with a working speed of 1,500 rpm with a 10 mm lead screw which would probably still give you decent rapids and feed rates on a z axis. the big concern would be does the small servo has enough power to lift the head? torque speed curve charts are helpful for all this. so you would need very specific data from the servo manufacturer to make a determination for power requirements.

    taking a wild A$$ guess i would tend to think it would be underpowered. the screw pitch has a lot to do with it too so you need to factor that into your decision making process as well.

    i can only offer basic considerations to take since i'm not an expert by any means.

    one other thing that comes to mind is with a timing belt you introduce backlash from the timing belt into the system. so you end up with more backlash as a result. timing belts probably have .002 to .003 of backlash. this is not a huge amount but in a cnc application it's enough to add problems if you want things to be accurate.

    Last edited by machinedude; 10-31-2020 at 12:51 AM. Reason: more information


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    Default Re: What did I buy? Old machine, new tech, take a look.

    There are lots of ball screws on the market. Some can legitimately be called "precision" ball screws. Others can't. Ball screws are made for use as skylight openers, etc.; they work fine and stress their motors less than other types of screw, but can't be counted on to produce a certain position at a certain degree of rotation. This latest linked ebay candidate seems closer than the previous ones. but that presupposes that the seller's assertions are true.That's possible, but not certain..

    Andrew Werby
    https://computersculpture.com/


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    Default Re: What did I buy? Old machine, new tech, take a look.

    Quote Originally Posted by machinedude View Post
    with this actuator the servo is only a 100 watts which is not very big, with the 2:1 ratio the torque is better on the drive screw but half's the rpm of the servo motor which i think was rated at 3000 rpm so you end up with a working speed of 1,500 rpm with a 10 mm lead screw which would probably still give you decent rapids and feed rates on a z axis. the big concern would be does the small servo has enough power to lift the head? torque speed curve charts are helpful for all this. so you would need very specific data from the servo manufacturer to make a determination for power requirements.

    taking a wild A$$ guess i would tend to think it would be underpowered. the screw pitch has a lot to do with it too so you need to factor that into your decision making process as well.

    i can only offer basic considerations to take since i'm not an expert by any means.

    one other thing that comes to mind is with a timing belt you introduce backlash from the timing belt into the system. so you end up with more backlash as a result. timing belts probably have .002 to .003 of backlash. this is not a huge amount but in a cnc application it's enough to add problems if you want things to be accurate.
    Backlash on Z is sometimes less of an issue as the weight of the assembly will always pull one way.

    My big Chinese CNC has I think a large +.010" backlash on the Z but it never really an issue except maybe when setting up my tool heights I want to make sure not to back up a few thousandths before entering values, as those would include the backlash and make the whole value off.

    Weight of the Z assembly (ie light) versus any friction or static friction would be the issue that would allow backlash to show up in more traditional ways.

    Sent from my motorola one using Tapatalk



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What did I buy? Old machine, new tech, take a look.

What did I buy? Old machine, new tech, take a look.

What did I buy? Old machine, new tech, take a look.