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IndustryArena Forum > MetalWorking Machines > CNC "do-it-yourself" > How to build a UHPC 4 Ft x 7 Ft CNC Router?
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  1. #1

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    How to build a UHPC 4 Ft x 7 Ft CNC Router?

    Hello,

    I would like to build a CNC router with a dimension of 4 foot (x-axis) x 7 foot (y-axis). For the Z-axis I'm assuming maybe 1 foot of travel would be fine.

    It needs to be a precision based CNC router, with tolerances of accuracy and repeatability of at least 50-100 Microns range (0.002" to 0.004").
    Care about accuracy more than repeatability.

    From my research it seems ballscrews is out from the question because the Y-axis ball screws are no good at that length. Rack and pinion would make more sense. In this time era rack and pinion are very accurate and can hold tolerances I need. Not sure where to buy them from.

    Would like to build the CNC bed in a budget friendly manner, so planning to go with UHPC but not sure if it would hold tolerances in the long run as compared to epoxy granite.

    So question is, UHPC is a good idea for CNC router machine bed I have in plan?
    What kind of linear rails would be suffice and how I would go about in calibrating it of making it straight and parallel?

    Will use the CNC router to do fine art details of 3D carvings/engravings on wood to make chairs, tables, doors and etc.

    Any advice would be appreciated, Thanks.

  2. #2
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    Re: How to build a UHPC 4 Ft x 7 Ft CNC Router?

    You can do a fixed-fixed ballscrew with a rotating nut, the ballscrew doesn't rotate. As long as you choose a proper thickness for your length, there should be no issues with how long it is.(https://www.hiwin.de/en/Products/Bal...nit-AME/c/4522).

  3. #3

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    Re: How to build a UHPC 4 Ft x 7 Ft CNC Router?

    Quote Originally Posted by ardenum View Post
    You can do a fixed-fixed ballscrew with a rotating nut, the ballscrew doesn't rotate. As long as you choose a proper thickness for your length, there should be no issues with how long it is.(https://www.hiwin.de/en/Products/Bal...nit-AME/c/4522).
    Thanks for the reply. I do not think HIWIN has ballscrews at 7 feet (2133.6 mm) length.

  4. #4
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    Re: How to build a UHPC 4 Ft x 7 Ft CNC Router?

    Hi,
    the problem is less ballscrews than it is rigidity. If your machine frame flexes, and it will under cutting loads, then it will easily flex 50um-100um and no matter
    how accurate your ballscrews/rack/pinion is your accuracy is screwed.

    A 4' x 7' would have to weigh many hundreds of kilograms, maybe as much as a tonne to avoid flexure....and that's way more expensive than any ballscrew!!

    Craig

  5. #5
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    Re: How to build a UHPC 4 Ft x 7 Ft CNC Router?

    Hi Raj - ballscrews are still good at 7' just they will be extremely expensive as they will be big and you will need precision grades to achieve the accuracy you want. There are many companies making precision machines from UHPC and similar materials. As Joe says your machine rigidity will dominate your quest. Then all of your other parts need to be made to >0.005mm to achieve your 0.050mm accuracy. You need to use an engineering grout that is zero shrink. Class C I think its called. Chase the Grail... Peter

    I cover a lot of stuff about UHPC in the Milli composite mill kit thread look it up.

  6. #6
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    Re: How to build a UHPC 4 Ft x 7 Ft CNC Router?

    Quote Originally Posted by rajhlinux View Post
    Thanks for the reply. I do not think HIWIN has ballscrews at 7 feet (2133.6 mm) length.
    Hiwins ballscrews size 25 and up come in 2,5m and 4,5m so no problem on that front. Though for this length I think 32 is a minimum, 40/50 probably what you should get. Still it all depends on what and how fast you will be cutting. Also pay attention to the pitch, the ballscrew nut can only rotate so fast, so you will be limited by it, how fast the gantry can move.

  7. #7
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    Re: How to build a UHPC 4 Ft x 7 Ft CNC Router?

    Hi,
    if the ballnut is rotating and not the ballscrew why go to 32mm?

    My new build mill has 32mm diameter 5mm pitch C5 double nut ballscrews and the screw is direct coupled to 750W servos. The servos are 3000rpm rated and
    5000rpm max, and I have and do run them at 5000rpm for g0's. So the ballscrew rotates at 5000 rpm while the nut does not, which is equivalent to the nut rotating
    at 5000rpm and the ballscrew not rotating....is it not?

    The second thing about 32mm ballscrews is the large rotational inertia. While OP may intend on having the ballnut rotate rather than the ballscrew on the two long axes,
    he surely will not go to that trouble for the shorter Z axis, thus the rotational inertia of the Z axis screw cannot be ignored.

    My ballscrews are 700mm long and have a first moment of inertia of about 4.7 x 10-4 kg.m-2.
    The armature of the servo has a first moment of 1.13 x 10-4 kg.m-2.
    The cast iron axis beds weigh 115kg and its first moment (linear to rotational conversion) is 0.55 x 10-4 kg.m-2.

    So the total inertia of the setup is dominated by the ballscrew, being 80%, whereas the armature is 15% and the entire 115kg axis bed is only 5%!!!!

    The takeaway of this is that large diammeter screws have HUGE rotational inertia and therefore your servo/stepper must likwsie have HUGE torque to accelerate
    said screw.

    Note the the first moment of inertia of a ballscrew is proportional to the fourth power of diammeter. Thus my 32mm screw (4.7 x 10-4 kg.m-2)
    would reduce to 37% or 1.75 x 10-4 kg.m-2 for a 25mm diameter screw of the same length, and down to 0.7 x 10-4 kg.m-2
    for a 20mm diameter screw of the same length..

    So be careful about selecting large diameter screws for stiffness and prevention of whip WITHOUT considering the rotational inertia. The fourth power relationship
    catches up mighty quick!

    Craig

  8. #8
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    Re: How to build a UHPC 4 Ft x 7 Ft CNC Router?

    Quote Originally Posted by joeavaerage View Post
    Hi,

    So be careful about selecting large diameter screws for stiffness and prevention of whip WITHOUT considering the rotational inertia. The fourth power relationship
    catches up mighty quick!

    Craig
    I would not rotate a ballscrew that is over 2m exactly because of that, if its that long might as well rotate the nut and go thicker on the ballscrew so it doesn't flex at high speeds, see example :

    https://www.youtube.com/watch?v=HF-OtLCGphg&

  9. #9
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    Re: How to build a UHPC 4 Ft x 7 Ft CNC Router?

    Hi All - Screw whip or whirl is caused by the ballscrew sagging in the middle creating a banana shape that is then allowed to fling around at the critical speed. There are several whip preventers in the forum. These are plastic rests that prevent the screw from sagging but allow the nut to pass it by. Don't place one in the middle but place it offset from middle so it is not at a node or two maybe better. I have not looked at the figures but this would seem to be a good solution for long screws.... Peter

  10. #10
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    Re: How to build a UHPC 4 Ft x 7 Ft CNC Router?

    Hi,

    I would not rotate a ballscrew that is over 2m exactly because of that
    I did acknowledge that OP was not intending to spin the 2m long screws, but how about the Z axis screw?. At 32mm diameter you'd be unwise to overlook its angular inertia.

    Craig

  11. #11
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    Re: How to build a UHPC 4 Ft x 7 Ft CNC Router?

    Hi All - quick calcs using the Hiwin formulas show a Dia25mm screw won't whirl and it won't buckle if used under 1000rpm and use a motor of 3Nm. If the screw is 10mm pitch this gives 10m/min which is not slow. Its doubtful if you use a stepper you will get to 1000rpm anyway, so whirl is out. Buckling has a big reserve so I'd be happy to use a std ballscrew on this machine. Depends on the operators requirement for rapids... plus the operators requirement for acceleration. Spinning these screws fast will take up most of the motors torque Peter

    as usual someone check the math....

  12. #12
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    Re: How to build a UHPC 4 Ft x 7 Ft CNC Router?

    Hi,
    the mass of a 25mm steel screw of 2.2m length is:

    (0.025/2)2 x PI x 2.2 x 8000 (steel=8000kg/m3)
    =8.64kg

    The first moment of inertia of a long cylinder of mass M and radius R is:

    J= M . R2/2 (Physics, Alonso & Finn, Addison-Wesley Publishing 1975, pg 215)
    = 8.64 x (0.025/2)2/2
    =6.75 x10-4 kg.m2.

    The armature of my Delta 750W servo (2.4Nm rated) is 1.13 x10-4 kg.m2

    The ratio of the servo inertia to the ballscrew inertia is 5.97 And that does not include the inertia of the axis mass.
    Good servos will tolerate inertia ratios of 10:1 but 5:1 is better. Thus if you used gear or belt reduction of 3:1 then the
    inertia ratio is 2.....maybe 3 once you include the axis mass, which is highly doable.

    Lets say you went with 2.4Nm (rated) 750W servos, and reduced them by 3:1, then the torque is:
    T=2.4 x 3
    =7.2Nm
    So the angular acceleration is:
    dw/dt=T/J
    =7.2/6.75 x10-4
    =10666 rad/s2

    At 10mm pitch the linear acceleration is:
    dx2/dt2= 0.01 x 10666 /2.PI
    =16.97 m/s2 which is MEGA!!!!

    So using peteeng's calcs the whip and buckling are no trouble under 1000 rpm. With 750W servos reduced by 3:1 would result in 1000rpm
    of the ballscrew and with 10mm pitch that's 10m/min. Using no more than rated torque you could get easily 1g acceleration.....all very encouraging.

    Craig

  13. #13
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    Re: How to build a UHPC 4 Ft x 7 Ft CNC Router?

    Hi,
    just to add some authenticity to this calculation we must include the linear momentum of the axis. Lets equate to rotational kinetic energy and the linear
    kinetic energy:

    1/2 . Jlinear. w2=1/2 m .v2 where Jlinear is the effective first moment of inertia, m is the axis mass and v is the axis velocity.

    But the angular velocity, w, and the linear velocity, v, are related by the pitch of the ballscrew,p:

    v= (w/2.PI).p

    Substituting in the energy balance equation and solving for Jlinear:

    1/2. Jlinear.w2 =1/2.m.(p. w/2.PI)2
    Jlinear=m. p2/4.PI2

    If the pitch is 10mm or 0.01m, and assume the axis mass is 100kg:
    Jlinear=100 x 0.012/4.PI2
    =100 x 1 x10-4/39.478
    =2.53x 10-4 kg.m2

    Comparing that to the first moment of the ballscrew of 6.75 x 10-4 kg.m2 shows that the linear momentum of a 100kg axis is only
    37% of the inertia of the ballscrew alone.

    This is very much what I found with my new mill. My ballscrews are 5mm pitch which means that the axis mass plays an even smaller role in the momentum equation.
    This might be counterintuitive that the ballscrew should have double or triple the momentum of the axis....but that is exactly what the calculation tells us.

    Craig

  14. #14
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    Re: How to build a UHPC 4 Ft x 7 Ft CNC Router?

    Quote Originally Posted by joeavaerage View Post
    Hi,

    I did acknowledge that OP was not intending to spin the 2m long screws, but how about the Z axis screw?. At 32mm diameter you'd be unwise to overlook its angular inertia.

    Craig
    It's hard to tell how rajh will build his Z axis, but I'd go with lets say 2x20 on both sides instead of one 32, if it were to be massive, but knowing diy builders, the Z axis is usually the weak link so one 25 will probably work for him.

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