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  1. #1
    Hi,

    I have been lurking here for quite a while and learned a lot in the process, so thanks to everyone for that!

    I've spent a while designing my first ever CNC router based on what I have read here and elsewhere on the interwebs, albeit with absolutely zero experience in the CNC field, so would appreciate comments, criticisms and anything else you care to throw at it, bearing in mind that I have probably little if any idea of what you are talking about.

    This machine will be used for milling soft and hardwoods, plastics and probably some shallow aluminium engraving. The work are is 900mm x 600mm. Much of it is based around 120mm x 80mm t-slot profiles. I have not made any decisions regarding the electronics yet, since I want to get the design to a workable state first. All suggestions regarding the electronics are gratefully received nonetheless.

    The rails (20mm) for the gantry are located under the bed in order to save some width and reduce the amount of dust contact that will inevitably occur. No idea if this is a good idea, or a disaster in the making. Gantry walls are 20mm thick.

    All the motors will be drive belts, which will allow me to play around with pulley ratios. I have read a considerable amount regarding the belts-versus-direct-drive argument, but for my purposes I'm keen on using belts.

    Some piccies below, looking forward to comments, sagacious advice and no doubt some humorous comments with regards to the sheer noobiness of the design. Please be gentle :-).

    Click image for larger version. 

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  2. #2
    Tom J's Avatar
    Lives in Melksham, United Kingdom. Last Activity: 3 Weeks Ago Has been a member for 4-5 years. Has a total post count of 148. Received thanks 7 times, giving thanks to others 18 times.
    Hi
    In general this is easy to build design but I would change it slightly:

    1. Put rails on spindle plate and blocks on Z plate (this will give zero torque at top spindle position, your design create torque on spindle plate regardless of the spindle position)
    Click image for larger version. 

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    2. Y blocks I would install on top rather on the bottom (it easy to make guard if you want to protect blocks against dust) - too big arm from bottom blocks to top of the gantry - think about forces
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    3. Direct drive is transfer vibration to the motor better to use gear and belt.
    4. I would use one motor for Y drive driven by pulley
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    5. Gantry should be connected on the bottom - not open. This make it stiffer and side plate a bit thin
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  3. The Following User Says Thank You to Tom J For This Useful Post:


  4. #3
    Loads of luvverly advice, thanks!

    Quote Originally Posted by Tom J View Post
    Hi
    In general this is easy to build design but I would change it slightly:

    1. Put rails on spindle plate and blocks on Z plate (this will give zero torque at top spindle position, your design create torque on spindle plate regardless of the spindle position)
    Great idea. I don't remember seeing this anywhere, or probably I did, but simply didn't notice it!

    Quote Originally Posted by Tom J View Post
    2. Y blocks I would install on top rather on the bottom (it easy to make guard if you want to protect blocks against dust) - too big arm from bottom blocks to top of the gantry - think about forces
    I was wondering about the pros and cons, so that's very helpful. Will have to look around for ideas to protect the rails from dust, or at least mitigate the issue.

    Quote Originally Posted by Tom J View Post
    3. Direct drive is transfer vibration to the motor better to use gear and belt.
    Is the direct drive on the z axis such an issue? Should I make that belt drive as well?

    Quote Originally Posted by Tom J View Post
    4. I would use one motor for Y drive driven by pulley
    Is there a disadvantage of using two Y motors? Sure, they can get out of sync, but surely two homing switches/sensors should take care of that between jobs?

    Quote Originally Posted by Tom J View Post
    5. Gantry should be connected on the bottom - not open. This make it stiffer and side plate a bit thin
    I was not sure about that. My thinking was that by using two Y motors, I could save some weight, and the two motors and lead screws would compensate for the added stiffness the connecting plate would provide. Is this incorrect?

  5. #4
    Tom J's Avatar
    Lives in Melksham, United Kingdom. Last Activity: 3 Weeks Ago Has been a member for 4-5 years. Has a total post count of 148. Received thanks 7 times, giving thanks to others 18 times.
    Quote Originally Posted by danilius View Post
    Loads of luvverly advice, thanks!

    Is the direct drive on the z axis such an issue? Should I make that belt drive as well?

    Is there a disadvantage of using two Y motors? Sure, they can get out of sync, but surely two homing switches/sensors should take care of that between jobs?

    I was not sure about that. My thinking was that by using two Y motors, I could save some weight, and the two motors and lead screws would compensate for the added stiffness the connecting plate would provide. Is this incorrect?
    belt transmission gives you extra flexibility, you can adjust gear ratio means=feed, easier do deal with resonance if any occurs, allows to mount motor in two position (up side down and shaft up).
    Z axis takes most vibration as is driving spindle up and down, so those shaky shaky gets to the motor and disturb in high speed. My first machine was driven by coupling - cheap Chinese, than with plastic insert, odiham finally end up with 1:1 pulley.

    Y drive, if your power supply can handle biggest nema 23 3Nm motor without extra voltage than one motor should be ok and will wait only few hundreds grams more than 1.9Nm. Advantage of having one motor and two ballscrew is lack of need for squaring the gantry (one motor less, 2 sensor less)
    Motor have to be powerful enough so they do not loos steps, if you loose steps you loose your job, material, time.

    Gantry - more braces make it stiffer - joining them under bed will increase the weight but benefit you gaining by stiffer, more accurate machine are far more important.
    I hope somebody else will give you some inside, I only build 3 machines and even on last one I would make something different, now I designed another one will post it soon.

    Good luck with the build

  6. #5
    Thanks for that. I'm a little concerned that a belt that long will introduce other issues, so will see what to do about that. Will attempt to alter the z axis for a belt drive. I don't suppose it should be too difficult.

    Quote Originally Posted by Tom J View Post
    belt transmission gives you extra flexibility, you can adjust gear ratio means=feed, easier do deal with resonance if any occurs, allows to mount motor in two position (up side down and shaft up).
    Z axis takes most vibration as is driving spindle up and down, so those shaky shaky gets to the motor and disturb in high speed. My first machine was driven by coupling - cheap Chinese, than with plastic insert, odiham finally end up with 1:1 pulley.

    Y drive, if your power supply can handle biggest nema 23 3Nm motor without extra voltage than one motor should be ok and will wait only few hundreds grams more than 1.9Nm. Advantage of having one motor and two ballscrew is lack of need for squaring the gantry (one motor less, 2 sensor less)
    Motor have to be powerful enough so they do not loos steps, if you loose steps you loose your job, material, time.

    Gantry - more braces make it stiffer - joining them under bed will increase the weight but benefit you gaining by stiffer, more accurate machine are far more important.
    I hope somebody else will give you some inside, I only build 3 machines and even on last one I would make something different, now I designed another one will post it soon.

    Good luck with the build

  7. #6
    Tom J's Avatar
    Lives in Melksham, United Kingdom. Last Activity: 3 Weeks Ago Has been a member for 4-5 years. Has a total post count of 148. Received thanks 7 times, giving thanks to others 18 times.
    Quote Originally Posted by danilius View Post
    Thanks for that. I'm a little concerned that a belt that long will introduce other issues, so will see what to do about that. Will attempt to alter the z axis for a belt drive. I don't suppose it should be too difficult.
    use good quality gates HDT 15mm braided one - there will be flex which shouldn't give you error bigger than 0.02mm (I referring here to my steel machine which was 115kg)

  8. #7
    Food for thought. Thanks! Time to get back to drawing :-)

    Quote Originally Posted by Tom J View Post
    use good quality gates HDT 15mm braided one - there will be flex which shouldn't give you error bigger than 0.02mm (I referring here to my steel machine which was 115kg)

  9. #8
    Hi,

    Connecting the gantry at the bottom will give little benefits but, by moving the legs far apart, the base will be a lot weaker.

    Why would you put the X rails "hanging"? Only for dust/swarf protection? The side plates extension for guide blocks will deflect a lot and put pressure on the rails/blocks trying to twist them. I don't think it's worth it.

    The gantry side plates are beefy but only when looking from one direction. If you cut out one third longitudinally and bolt it perpendicular you will have more rigidity in the Y direction without any loss or weight gain. And you can connect to your t-slot profile overhang.

  10. #9
    OK, I think I have understood all this correctly. So, I'll move the rails up top, and stiffen up the gantry walls.

  11. #10
    So, I have made some changes. I wasn't convinced that the extrusion on the gantry was going to be rigid, so that has been dropped in favour of some solid lumps of metal. It is also much shorter, and the rails have been moved up to the top.

    Click image for larger version. 

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    What I'm wondering is if there is an optimum distance between carriages for the gantry.

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    I dont have any worksop equipment beyond the basic tools, so don't want to drill into the extrusions or anything, but I'm getting a bit lathered about the hseer size of the machine, given that it's only going to have a work area of 600 x 900. Currently the overall dimensions are roughly 1400mm x 940mm. I can trim a small amount off the length, but is there any way of getting this more compact?

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