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    Default Ground-up Mini Mill Question

    Hopefully this is the appropriate forum for this question.
    I would like to build a high (as practically possible) accuracy/precision/resolution mini mill (6"x 3"x 4" XYZ) to machine metals. I plan to base the design on the more conventional "L" shape like a Haas VMC with high quality ball screws, linear slides and semi-closed loop steppers on a frame machined from class 40 grey cast iron bar stock. Some early calculations and testing with a single axis test fixture look good so I intend to move forward with the design work. One aspect I really haven’t been able to find much info on is the X/Y arrangement. Lower end VMC's have the x axis linear slide blocks attached to an adapter that mounts them to the Y axis blocks. The X axis rails are inverted and mount to the bottom of the bed. The ball nut is stationary (also mounted to the adapter and stationary relative to X axis movement). The ball screw and motor travel with the table.
    On upper end VMC's there is a "saddle" instead of a simple adapter that mounts the slides, ball screw and motor driving the table that rides atop the Y axis slide blocks.
    What is the advantage of this arrangement? I'm thinking maybe some added vibration damping and rigidity but that's just a guess. Any feedback would be appreciated to help determine if it is worth incorporating the latter configuration in to my build. Thanks

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    Is it going to be similar in size to a sieg x2 mini mill?

    Will you use dovetaled slides? Have you considered buying a chinese cast iron cross slide for xy wich you could adapt.

    I have been thinking if it would be easier to get rigidity and precision in a mini mill, configuring it horizontal, instead of verticaly, perhaps not.

    Have you seen this:
    https://s.click.aliexpress.com/e/bUsfSI4p



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    Default Re: Ground-up Mini Mill Question

    Quote Originally Posted by Mackan View Post
    Is it going to be similar in size to a sieg x2 mini mill?

    Will you use dovetaled slides? Have you considered buying a chinese cast iron cross slide for xy wich you could adapt.

    I have been thinking if it would be easier to get rigidity and precision in a mini mill, configuring it horizontal, instead of verticaly, perhaps not.

    Have you seen this:
    https://s.click.aliexpress.com/e/bUsfSI4p
    I have the resources to build from a clean sheet of paper so buying a frame doesn't interest me. I have seen the frame you linked from Aliexexpress. That's a lot bigger than what I'm thinking but looks like a viable option for someone. Mine will probably be about half the size of an X2 also. Initial CAD work looks like it will be around 12-14" deep and 16" tall.
    I would like to do something that looks like a scaled down version of a Hurco pictured below which indecently shows the Saddle configuration I asked about in my original post.

    Attached Thumbnails Attached Thumbnails Ground-up Mini Mill Question-hurco-pic-cnc-cookbook-png  


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    Default Re: Ground-up Mini Mill Question

    The largest bearing spacing gives you the stiffest config. Then the support structures need to be as stiff as possible to achieve your accuracy requirements. What accuracy and precision are you aiming at? For such a small machine a moving table is the best design with fixed gantry. Plus use recirculating ball drive screws. May as well make it twice the size will cost the same and still fit on your breakfast table... You do mean 6 inches x 3 inches by 4 inches? Peter

    Last edited by peteeng; 03-10-2019 at 05:06 AM.


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    What about weight/stability, if you build that small/lightwight it will essentially be a model of a metalworking machine, you will not be able to actually machine metal.



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    Default Re: Ground-up Mini Mill Question

    Quote Originally Posted by busted bit View Post
    On upper end VMC's there is a "saddle" instead of a simple adapter that mounts the slides, ball screw and motor driving the table that rides atop the Y axis slide blocks.
    What is the advantage of this arrangement? I'm thinking maybe some added vibration damping and rigidity but that's just a guess.
    A long heavy table applies a lot of leverage at maximum extents of travel if the bearings are fixed to a central position on the mill. Add in a heavy fixture or so = more leverage. That's my guess for moving bearing/fixed rail designs on heavy machines, it's done to avoid 'overhang' effect.

    Anyone who says "It only goes together one way" has no imagination.


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    Default Re: Ground-up Mini Mill Question

    Quote Originally Posted by peteeng View Post
    The largest bearing spacing gives you the stiffest config. Then the support structures need to be as stiff as possible to achieve your accuracy requirements. What accuracy and precision are you aiming at? For such a small machine a moving table is the best design with fixed gantry. Plus use recirculating ball drive screws. May as well make it twice the size will cost the same and still fit on your breakfast table... You do mean 6 inches x 3 inches by 4 inches? Peter
    Yes. 6x3x4 inches. I'm trying to understand how robust to make the support structures and how wide to position the linear rails but unfortunately I'm not an Engineer. I'm a machinist so there may be some "looks about right" in the equation. On the test fixture (below) i saw about .0002" resolution and a following error of .0008" with 10lbs of load.That seems resonable for the parts I want to make.

    Quote Originally Posted by Mackan View Post
    What about weight/stability, if you build that small/lightwight it will essentially be a model of a metalworking machine, you will not be able to actually machine metal.
    Agreed. The actual frame will certainly be more robust looking than the Hurco pictured below rather than an exact scaled down replica.

    Quote Originally Posted by cyclestart View Post
    A long heavy table applies a lot of leverage at maximum extents of travel if the bearings are fixed to a central position on the mill. Add in a heavy fixture or so = more leverage. That's my guess for moving bearing/fixed rail designs on heavy machines, it's done to avoid 'overhang' effect.
    That seems like a logical reason

    I have a Haas VF2 but that is at the shop. I want to build something small to make small parts with at home but I do have a confession to make... the size of the machine is somewhat based on components I have on hand which are 8mm diameter ball screws and 7mm wide rails. The blocks for these rails have a dynamic load capacity of 190 lbs. The largest end mill I plan to use is a 3/16" but mostly 1/8". Using this Kennametal calculator https://www.kennametal.com/en/resour...and-power.html a 3/16" endmill slotting @ 3/16" axial DOC produces 10 lbs of cutting force in CP2 Titanium so I am considering that as the highest load the machine will see.The stepper/ball screw combination on the test fixture produces a maximum of 50 lbs of force.

    .

    Attached Thumbnails Attached Thumbnails Ground-up Mini Mill Question-20190310_064737-jpg  


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