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    Default Is this a bad idea. Extended nut housing

    Hi,

    Just looking for a quick bit of advice. I'm currently in the process of designing a fixed gantry machine. After looking at a few more designs it seems quite common to conceal the screws and bearings round the back to avoid chips etc.

    I'm wanting to extend the nut housing on the X axis fabricating a custom one out of a piece of 1 3/4" square solid aluminium bar which is 140mm long. As seen in this video:



    Is this extension something to avoid or would it be okay?

    Any advice or comments will be gratefully received

    Similar Threads:


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    Community Moderator ger21's Avatar
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    Default Re: Is this a bad idea. Extended nut housing

    I don't think that's a good idea, but the entire Z axis is going to be less than ideal, with the bearings so high up, and the spindle hanging down so far.

    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: Is this a bad idea. Extended nut housing

    I see what you mean with the bearings. Is that why you sometimes see people swap the positions of the rails and bearings. Have the rails mounted to the spindle mount-plate. Otherwise you sacrifice a lot of Z travel...

    Any suggestions on what to do about the nut housing?



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    Default Re: Is this a bad idea. Extended nut housing

    Had a quick redesign. Would love to know if you think this is any better:

    Is this a bad idea. Extended nut housing-perspective1-jpgIs this a bad idea. Extended nut housing-perspective2-jpg

    Attached Thumbnails Attached Thumbnails Is this a bad idea. Extended nut housing-perspective1-jpg   Is this a bad idea. Extended nut housing-perspective2-jpg  


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    Default Re: Is this a bad idea. Extended nut housing

    I'd lift the top tube in the gantry, and put the screw between the two tubes. Maybe move the z motor behind the gantry so the Z axis doesn't need to be so tall?

    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: Is this a bad idea. Extended nut housing

    My perspective on the first issue is do not do it as suggested in your images and text. The thing is this, the extension arm becomes effectively a lever arm. The axis slamming back and forth would effectively try to bend the fastening method or even pull the screws out of the aluminum. Small cross section bar stock does not give you a lot of flexibility in screw placement nor size.

    If you really want to place the ball screw so far away from the Y axis saddle you need to consider a more robust solution. Generally though people tend to keed the screws as close to the saddle as possible.


    As to your other questions i see some of the design choices as preferences. For instance i like to encourage people to think in box sections but wrapping the Y saddle around the gantry beam bothers me a bit. Mainly for a couple of reasons, one is that you loose clearance under the gantry beam. The other is issues of accessibility for maintenance.

    The issues of maintenance are worthy of explanation. If you are machining dusty materials you will need to perform regular ball screw and linear bearing maintenance including cleaning and lubrication. Easy access means the work is far more likely to be done and done properly.

    Another issue with the wrap around design is your motor position which impacts cooling. If vertical height is not an issue id move the motor top side simply to promote cooling. Top side also makes maintenance a tiny bit easier.

    You are right to be concerned about dust! However moving the screw around doesn't do a lot for the dust that impacts screws the most. That is the dust that forms a cloud around your machine. It gets everywhere and highlights the need for dust extraction. The only other solution for dust is a bellows around the screw and they have their own issues. In other words screw position alone doesn't solve the dust issue.



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    Default Re: Is this a bad idea. Extended nut housing

    Hi wizard,

    Thank you for your detailed reply. It felt like a bad idea from the beginning

    Since posting this I have kept redrawing the design and have drawn a box section as you say for the saddle, I have wrapped it around the gantry though. Not sure how to achieve a box section without doing this. Any precedents you can show me?

    Here is a sample of my latest design:



    I'm reluctant to place the Z axis motor up top since I don't know where it will live and space will almost certainly be an issue. I've seen people heatsink their motors, is this a way of getting round the issue of over heating?



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    Default Re: Is this a bad idea. Extended nut housing

    Well on my opinion every belt is a backlash, you dont want that to happen when machining precision parts, I think it`s just better to direct drive everything and find a way to close all the ball screws for dust debris contamination, the most important thing on a CNC is the ball screws, steppers drives are sealed so there`s a less chance to get contaminated even if you soak it with oil or diesel and run all week it will be just fine but the ball screws....just a little metal chips being eaten on the bearings and your material is done (garbage)



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    Default Re: Is this a bad idea. Extended nut housing

    I am just curious... why do you have belt between steppers and screw? I mean, I would understand if you would have pulleys which are not 1:1, but in your model it looks like you have 1:1 pulleys, so what is the benefit compared to direct drive?

    I would also change the design so that the centre of the spindle ends up at the centre of the frame. That gives you the maximum table movement and the maximum work area. Another thing you should consider is to add some more support for the gantry beam, pointing to the front of the machine. You design looks like a moving gantry design, and one of the reasons to build a fixed gantry machine is to be able to build it as rigid as possible. The weight of the gantry is not an issue in a fixed gantry design, so there is no reason for not increasing the rigidity.



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    Default Re: Is this a bad idea. Extended nut housing

    THere's nothing wrong with using belts like that. I think that's a lot better.

    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: Is this a bad idea. Extended nut housing

    Quote Originally Posted by ger21 View Post
    THere's nothing wrong with using belts like that. I think that's a lot better.
    Yep I agree with you boss probably on wood, plastic and some other soft materials it`s OK, but for gun parts and other hard steel with pilot holes that had to be very precise (0.2mm offset is considered material reject) I dont think the belt will still cling on that accuracy when cutting / engraving hard metals.



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    Default Re: Is this a bad idea. Extended nut housing

    I disagree. If you choose the right belts, they'll work fine.

    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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Is this a bad idea. Extended nut housing

Is this a bad idea. Extended nut housing