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Thread: Large Format 5-Axis Gantry Router

  1. #81
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    Default Re: Large Format 5-Axis Gantry Router

    Hallo Jono

    A few thoughts for you to consider.

    Just too look at it from a slightly different viewpoint.Whats sufficient precision for what you want to do?How little precision is enough?The figures you quote(1-1+1/2 thou in) are IMO flat enough for the bearing to tolerate.Provided you meet the requirements for precision over a total length,which is the most difficult,in measuring-for a start.Also its the "frequency" of the variation,you want a nice slow variation so lots of contact with your rail(spot/sq area).You can look up standard mills precision,its maximum probably 1/1000in per 12in..You have probably observed like me that many are bolting there rails straight to RHS,with no apparent ill effect.This suggests the lower limit for precision without shortening bearing life is substantially higher than we may think.


    The limiting factor in how unflat a surface you can use a bearing on is bearing life which is determined by the maximum individual ball loading .If exceed failure occurs through the classic bond breaking,metal flaking equals pitting mechanism.They do quote a figure in hiwin catalog from memory.Calculating is possible for experts?It will be affected by rail bowing(shifting load to fewer balls),preload(consuming some of load carrying capacity-so thats a point against using preloaded rails),acceleration deceleration rates(weight transfer)working conditions(vibration,chatter),load.Whats the worst case scenario?

    You need also to consider whether in fact the "static"precision achieved is not swamped under loaded "dynamic" conditions by buckling in the rail support or structure.You could test by creating a jig and measure deflections under realistic loads using DTI.The "buckling" stiffness really determines how worthwhile a higher precision/stiffer(wider,preloaded) rail is.

    Because of the substantial load carried by the rails this does provide some of the benefits of "preloading"to the x and y axis(but not the z)Especially on the longest axis.
    Medium preload=1/3maximumload,newtons/10=kg). Would this be enough?Preload is applied to all bearings all the time of course,whereas gravity doesnt achieve this.


    Larger rails may be stiff enough to pull the mounting to there flatness rather than the desired reverse,and more difficult to pull straight.They will not be true on arrival.


    Have you considered how temperature variation in work area will affect stability of dimensions?



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    Default Re: Large Format 5-Axis Gantry Router

    Quote Originally Posted by shedbob View Post
    The "buckling" stiffness really determines how worthwhile a higher precision/stiffer(wider,preloaded) rail is.

    Because of the substantial load carried by the rails this does provide some of the benefits of "preloading"to the x and y axis(but not the z)Especially on the longest axis.
    Medium preload=1/3maximumload,newtons/10=kg). Would this be enough?Preload is applied to all bearings all the time of course,whereas gravity doesnt achieve this.
    Good points, also worth considering whether heavy preloaded carriages can operate at sustained high speed used on some routers without overheating either themselves or the motors driving them. I assume that heavy preloads shorten service life, as they are working all the time.



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    Default Re: Large Format 5-Axis Gantry Router

    Hi John05

    Some thinking on your counterweight system to balance the Z axis design weight of 120 kg?

    The very basic approach would be to add a counterweight system right at the Z axis. This would work however it does add a fair amount of mass to all the axis motions that would have to move it around. Alternatively you can build an air or air over oil balancing system' that will work at the expense of complexity and a fair amount of weight.

    Below is a quick CAD concept lash up of your design that uses (4) Counterweights shown in red suspended within your support frame columns. These counterweights are connected to the z axis via cables to transfer the forces. (a 5mm cable would be overkill the thinner the better for less vibration).

    14 grooved pulleys are used easy to manufacture they should have ball bearings for low friction. They don't need shafts, Just ideally an eccentric boss to mount them to the frame.

    I used 4 counterweights balance the forces in both the X and Y direction. The Z axis balances itself.

    Using this system the 120kg mass increase remains, and that will affect the dynamic performance. Also there will be a small amount of friction. with ball bearings that should be minimal. However the steady state Loads on the drives are mostly balanced out.

    Once set up the maintenance will be minimal. and Gravity does not go out of adjustment.

    Regards
    John

    Large Format 5-Axis Gantry Router-gantry-z-counterbalance-jpg



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    Default Re: Large Format 5-Axis Gantry Router

    Should there be a pulley where the cables join the Z azis ?



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    Default Re: Large Format 5-Axis Gantry Router

    @JohnMcNamara

    That's interesting (as shown in the cad layout). Simple, and old school, reliable, good engineering - I like it. It doesn't add much mass to the moving/accelerating parts (gantry, Z-axis), just the pulleys/cable mass. But, the inertia/friction of accelerating X & Y against the relatively stationary cables/weights would become more of a factor with faster machine travels.

    Very interesting, but I will be using an air ram counterbalance system, which will add the mass of the ram to the Y-carriage, but will otherwise not influence rapid movement.

    And my beams are already full of epoxy granite.

    [note 1: ram = cylinder, hydraulic or pneumatic, ........]



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    Default Re: Large Format 5-Axis Gantry Router

    Hi Jon

    As understand it you have used composites to reduce the mass of of the moving components of your design. While a composite beam will be lighter it will have more deflection than than a hollow steel section of the same outer dimensions Its the outer dimensions that affect deflection the most.

    The weight of the gantry carriage and Z axis will be a lot more than 120 kg combined. probably over 200kg ?

    This carriage has to travel along a maybe 4.5 meter beam (to allow a 4 meter travel)

    When the carriage/z axis assembly is positioned in the center of the gantry beam have you calculated the deflection? You may find that to in order to obtain the performance numbers you are applying to the selection of the linear rails the beam will need to be surprisingly deep.

    A while back I looked at the same problem. It was not easy to solve. It was a measurement beam for an industrial process.

    Yeah I like old school sometimes, but not always.
    Here is a link to a little project I am working on
    https://www.model-engineer.co.uk/for....asp?th=139042
    I popped it in here because it also deals with mounting rails into composites and hand lapping as discussed in this Forum thread.
    Please restrict comments on it to parts that affect this Large gantry project I as others, are watching this large gantry build with interest!

    Regards
    John



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    Default Re: Large Format 5-Axis Gantry Router

    Hi Zorbit

    I drew a tube within tube Z axis The cables would simply attach to the moving member. Only the pulleys at the top are required.

    Many commercial machines use tube within tube however it does not matter, The same system would work with open rails on the outside or any other arrangement. Tube within tube is probably more expensive to manufacture.

    Regards
    John



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