1. #1
    I am on my 7th design iteration.

    My question is regarding motors. Is there any reason not to just use a servo on one axis?

    My thinking is that if I go the two X axis motor route, I can probably buy a servo + drive for roughly the same money as 2xnema and 2x drivers.

    Upside of this being I could run a heavier gantry (120KG).

    I am thinking I'd see more single servos driving an X axis, and steppers the rest, if that was the case?

  2. #2
    Two motors are used to keep the gantry square, and make it more rigid, not to provide twice the power.
    If you drive a gantry with a single screw in the center, it likely will exhibit racking when cutting at the ends of the gantry
    Gerry
    ______________________________________________
    UCCNC 2017 Screenset

    Mach3 2010 Screenset

    JointCAM - CAM for Woodworking Joints

  3. #3
    Quote Originally Posted by Ger21 View Post
    Two motors are used to keep the gantry square, and make it more rigid, not to provide twice the power.
    If you drive a gantry with a single screw in the center, it likely will exhibit racking when cutting at the ends of the gantry
    Sorry, I should have been clearer. I was thinking of using the servo with two ball screws either side connected with a timing belt. The machine is too large for a single central ball screw.

  4. #4
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 5 Hours Ago Has been a member for 9-10 years. Has a total post count of 2,149. Received thanks 237 times, giving thanks to others 5 times.
    There is no reason why you can't mix and match steppers and servos on different axes.

    You still want to avoid any unnecessary weight though.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  5. #5
    Thanks m_c

    I'm finding it hard to keep the weight down with an epoxy granite filled steel gantry of a suitably rigid shape.

    I'm probably over compensating quite a bit. I'm going to try and bring the weight down.

    I'm also failing to understand how to calculate which motor I'll need for a given linear system.

    I'm basing the servo on Boyan's massive purple machine. Which I believe used a 400w servo for a 120kg gantry.

  6. #6
    Weight is not an issue, as long as you have the power to move it.
    We're looking at a new industrial router that weighs about 4-5 tons, and rapids at 96m/min.

    There's no reason to fill the gantry beam with epoxy granite imo. A steel beam with a 10-12mm wall thickness is plenty strong enough, if you add some internal bracing.
    Gerry
    ______________________________________________
    UCCNC 2017 Screenset

    Mach3 2010 Screenset

    JointCAM - CAM for Woodworking Joints

  7. #7
    That sounds like a mightily impressive machine Ger21.

    I was thinking of going the EG route for vibration damping and resonance reasons rather than strength. In my case, internal bracing of a steel shs would be beyond my limited tools, skillset and budget. What with it being prohibitively expensive/time consuming to have made.

    If I can't bring the total gantry weight down to the 75kg range. I'll have to deliver the power to drive it via a suitable servo.

    Thanks for the help guys.

    I know there's a nice stepper calculator excel sheet.

    Can anyone point me in the right direction to start to calculate for choosing the right servo?

  8. #8
    Mix n match isn't a problem but how about giving us a clue to the design and size etc. Also what are you intending to do with this machine.?

    Heavy isn't always better.! . . . Done correctly then Mass can help but it's expensive to do correctly because to take full advantage it doesn't just stop at motors. What's the point of having a gantry built like tank when to take full advantage you need 25HP spindle.?
    Any machine is only as good as the weakist link and I guarantee there will be more weak links in your design than the gantry, Also if too heavy or not done correctly then it soon can become the achilles heal of the machine.

    The motor Calc spreadsheet is useless in the real world of DIY CNC, far too many variables come into play which the spreadsheet doesn't account for. The person who designed the sheet had never even built a CNC machine so couldn't posssibly know all the things which come into play.

  9. #9
    Quote Originally Posted by JAZZCNC View Post
    Mix n match isn't a problem but how about giving us a clue to the design and size etc. Also what are you intending to do with this machine.?

    Heavy isn't always better.! . . . Done correctly then Mass can help but it's expensive to do correctly because to take full advantage it doesn't just stop at motors. What's the point of having a gantry built like tank when to take full advantage you need 25HP spindle.?
    Any machine is only as good as the weakist link and I guarantee there will be more weak links in your design than the gantry, Also if too heavy or not done correctly then it soon can become the achilles heal of the machine.

    The motor Calc spreadsheet is useless in the real world of DIY CNC, far too many variables come into play which the spreadsheet doesn't account for. The person who designed the sheet had never even built a CNC machine so couldn't posssibly know all the things which come into play.
    I am very much here to defer to the hands on experience of guys like yourself Jazz. So, thanks for prodding me for more details.

    The overall size of the machine is currently 940x1200mm. I am hoping for a machining area of 900x600x200mm. I intend to mainly route Aluminium, Copper, Plastics, the occasional bit of wood and I'd like to be able to tickle steel enough to surface it. I am hoping for an accuracy of as much as I can get.

    The super wide Z axis is to incorporate a standard 2.2kw chinese spindle and one of the cheap BT30 low 3000-6000 RPM spindles and a servo. I'd kinda like this machine to be as versatile as possible and would want the low RPM for tapping and possible fly cutting. I'd prefer this over a larger machining area. I expect I can shrink the Z width further to increase machining area and reduce weight.

    Cheap BT30: https://www.aliexpress.com/item/BT30...ceBeautifyAB=0


    The pictures show my current progress. The frame is 80x80x5mm SHS. The Gantry is a single 120x120x5 section with the rail mounts welded on 60x10mm flat bar, and either milled or epoxy levelled. The side sections are 15mm steel plate that will be bolted on and either mill surfaced in situ for rail carriages or epoxied level and in the same plane. The side plates are bolted to make milling the rail supports possible without crashing the mill. I am intending to fill the 120mm SHS with epoxy granite. I am yet to design the router floor stand.

    Mass of the gantry+Z is currently 65KG without any spindles or EG. Spindles, steppers and the EG will easily add another 40KG. Hence the question about servo controlling the X axis.


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  10. #10
    Quote Originally Posted by Electroconducive View Post
    I am very much here to defer to the hands on experience of guys like yourself Jazz. So, thanks for prodding me for more details.

    The overall size of the machine is currently 940x1200mm. I am hoping for a machining area of 900x600x200mm. I intend to mainly route Aluminium, Copper, Plastics, the occasional bit of wood and I'd like to be able to tickle steel enough to surface it. I am hoping for an accuracy of as much as I can get.

    The super wide Z axis is to incorporate a standard 2.2kw chinese spindle and one of the cheap BT30 low 3000-6000 RPM spindles and a servo. I'd kinda like this machine to be as versatile as possible and would want the low RPM for tapping and possible fly cutting. I'd prefer this over a larger machining area. I expect I can shrink the Z width further to increase machining area and reduce weight.

    Cheap BT30: https://www.aliexpress.com/item/BT30...ceBeautifyAB=0


    The pictures show my current progress. The frame is 80x80x5mm SHS. The Gantry is a single 120x120x5 section with the rail mounts welded on 60x10mm flat bar, and either milled or epoxy levelled. The side sections are 15mm steel plate that will be bolted on and either mill surfaced in situ for rail carriages or epoxied level and in the same plane. The side plates are bolted to make milling the rail supports possible without crashing the mill. I am intending to fill the 120mm SHS with epoxy granite. I am yet to design the router floor stand.

    Mass of the gantry+Z is currently 65KG without any spindles or EG. Spindles, steppers and the EG will easily add another 40KG. Hence the question about servo controlling the X axis.


    Click image for larger version. 

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    Click image for larger version. 

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    I have ditched this gantry design. I think instead of trying to make a router do more than it really should, I'm just going to go with a box Z style set up with the standard 2.2kw spindle. I'll go with single point thread mills or tap by hand.

    I thought about building another gantry out of extrusion and with a hand router, to be able to machine the box Z. I am probably better just paying to get the box Z machined from the start.

    Click image for larger version. 

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    There's where I am with the design.

    I am kinda shamelessly ripping off ideas from RouterCNC and Jonathan.


    Again, I intend to fill the steel tube parts of the gantry with EG. I already ordered the epoxy and aggregate to mess around with. I am also entertaining the idea of casting a bed out of EG with embedded steel and replaceable strips. It would still work out cheaper than machining a massive aluminium slab, with the added benefit of maybe improving surface finish through vibration dampening.

    When I am done with the design and start welding the frame, I'll start a build log.

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