1. #1
    Hello everybody,
    first off I would like to say thanks to the dwellers of this fine forum for the amount of information Ivíe learnt during the countless hours of reading about and going through various build logs and threads!


    SHORT VERSION:
    I tend to write a lot, so feel free to skip the long preview and just take a look at my questions below, bottom line is I need a machine that will be able to cut wood and plastic at reasonable speed, have the option to cut aluminium (even at slower speeds as this will not be the main job) and have the option to modify later on if needed.



    LONG VERSION:
    I run a small product design/collaborative design space (kinda like an invitation only makerspace) and am now at the stage where I could really use a CNC.


    Iím hoping to build a semi-modular machine that will allow me to swap out parts and change the stuff it can do according to the type of job/material I want to work with:


    Mainly it should cut up wood, from ply to harder stuff, it should do this well. I will also need to work with plastics and foams and I might want to extend the Z later to allow for larger blocks or add a 4th axis at some point.
    It would be great to work with aluminium as well (until I have the chance to buy and convert a dedicated vertical mill).
    I would like to be able to work with full sheet material so Iím thinking of a 1300x1500x150 format (total size not workable area)


    Iím guessing it will work only a couple of hours a week.


    due to the modular nature of the build I think alu-profiles are probably the least worst material for the bed/gantry as this will allow me to bolt on and swap out parts as projects change.


    so hereís the part list Iíve got so far:


    PART LIST:
    80x120 profile for the Y axis
    80x40 profiles for the cross sections
    80x160 for the X gantry beam
    300mm Z axis with 150mm travel
    20mm alu machined parts to bolt everything together and for the Z axis
    HIWIN 20mm linear guides all over with wide carriages
    Belt driven 1610 ball screws with FK/FF type supports
    HTD/GT3 15mm wide belts / pulleys + idlers
    Nema 23 420 oz motors (have these from another project) coupled with maybe a Gecko 540 for simplicity?
    2.2Kw water cooled Chinese spindle with a hitachi or similar VFD


    For now the machine bed will sit (bolted to) a heavy 3000X1500 table (steel frame and quartz surface that i already use as one of the working tables in the studio. does the table have to be perfectly levelled out if the machine itself is squares?


    QUESTIONS:

    • single large profile for the X gantry beam like it is now, or rather break it up into an L shape as i see is popular?
    • ivíe seen some threads in which the Z axis was designed in a way that the bearing carriages are stationary and the rails move with the spindle - could anyone point clear this up or point me to the right thread?
    • are the alu profiles rigid enough for my expectations of the machine (i think maybe machining aluminium would be pushing it)?
    • bolting linear guides directly to the aluminium profiles - is this possible at all? It seems Iíve seen some have been successful at doing so and others have ended up placing ground bar stock in between the profiles and the guides. I could theoretically do the epoxy self levelling method on one face of the profiles, it would leave it modular yet levelled. Iím just not sure it theres enough material for threading later on as i will not be able to use the T-nuts anymore..?
    • can I swap out the machined 20mm aluminium parts (such as Z assembly, gantry Ďrisersí and motor brackets for welded 6mm steel?
    • 1610 ballscrew or 2010 for the Y axis?
    • filling the gantry profile main cavities with epoxy to add rigidity/dampening material, silly and messy or good idea and messy?
    • which axis should be planned with wide and which with long bearing blocks?
    • to keep things simple I was hoping to use the ready made ballscrew end supports, with the design I have now it seems the FK/FF type would work best, but I have also seen that there are the BK/BK type with mounting holes parallel to the ballscrew - so similar to FK/FF, is there any difference between the options?



    Iíve got a thread going on at cnczone, and have used some of the info there to iterate the design - I would love to get some feedback from you guys as well regarding the design/expectations/selected parts before I begin my actual build(i'm in no rush to order parts).

    Click image for larger version. 

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  2. #2
    Hi,
    Welcome! This will be one quite expensive baby.

    Answers:

    - the machine should be done so that will do from the beginning whatever will be meant to do. because "later" very soon becomes "never" to lack of time.

    -edited: ball screw size 2010 if i was doing that

    -70vdc drives for the motors, AM822 and similar digital drives

    -steel is 3 times heavier and stronger than aluminum, so design accordingly. Weight is no problem for Z as far as the gantry stays below 70-80kg, 90 at the most, where you will need servos to move that mass reasonably fast
    -normally Z axis travel is better achieved when rails move with plate and blocks are fixed. in that case the rails reinforce the moving plate also
    if you read my first build from signature, at the end somewhere you will see a correctly made strong Z axis from steel plate and dimensiones. The type i advise, with the moving rails

    -if you go with aluminum profile i believe L shape is popular because it offers better stiffness in all directions, plus some other minor benefits you will discover when designing how to mount the pieces

    -design the end supports of ball screws so that they have play in all directions and are easy to adjust. bk supports are better for me, as they will not squeeze between them the ball screw

    -cheapest and best brand for me VFD in EU is Toshiba, ebay Italy, inverters and motors. Chinese ones work also good.

    -HTD 5M 15mm belts is best. min pulley ~20mm, has enough tooth engaged for this job, no less than 18t. calculator Avoid idlers. keep it simple.every element introduces imprecision.

    -reinforcing the gantry profile with bolted steel plate at the back or across is better than filling with epoxy. could lead to need of servos though due to weight, so calculate well

    -shimming seems best to me in combination with aluminum profiles. everything is possible. you could even straighten perfectly a profile using plastic hammer, DTI and 2m straight edge, if you are good with hammer like me.

    -you will need that 2m straight edge, 50cm square, 30cm square, 10cm square. / read my second build log why/



    That should keep you busy for a while
    Last edited by Boyan Silyavski; 12-11-2016 at 02:00 PM.
    project 1 , 2, Dust Shoe ...

  3. #3
    Boyan I don't know why you always insist on rotating ball nuts for this size of machine my router is 1600 x 1000
    using 1610 on X and YClick image for larger version. 

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    I can easily get 10m/min with 2 x nema 23s the screws don't need to be as long as the table.
    Although I use two motors many people use one nema 34 with a belt connecting the screws.

    -you will need that 2m straight edge, 50cm square, 30cm square, 10cm square. / read my second build log why/
    Very few people have a 2 mtr straight edge and still get perfect good machines just have a look around the forum.

    • ivíe seen some threads in which the Z axis was designed in a way that the bearing carriages are stationary and the rails move with the spindle - could anyone point clear this up or point me to the right thread?
    One of the reasons for this, ie putting the rails on the spindle plate is that it stiffens the plate up Click image for larger version. 

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    ..Clive
    The more you know, The better you know, How little you know

  4. #4
    Quote Originally Posted by Clive S View Post
    Boyan I don't know why you always insist on rotating ball nuts for this size of machine my router is 1600 x 1000

    I can easily get 10m/min with 2 x nema 23s the screws don't need to be as long as the table.
    I agree with Clive on this point. Rotating nuts only start making sense if the axis travel you want is over about 1.6m and start looking like the best option at around 2m. Otherwise they're just unnecessary complexity and will result in worse performance due to the higher inertia of the assembly.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  5. The Following User Says Thank You to Jonathan For This Useful Post:


  6. #5
    I missed that cause he said " 3000X1500 table" so i missed that the router is quite smaller. Sorry for that. Then all else what i said stays same except ballscrew size

    I never have insisted on rotating nuts for a small table less than 1.8m???
    project 1 , 2, Dust Shoe ...

  7. #6
    Quote Originally Posted by jeronimo View Post
    [*]single large profile for the X gantry beam like it is now, or rather break it up into an L shape as i see is popular?
    Better stiffness in vertical and fore/aft directions, plus better torsional stiffness than if you just used one of the single profiles used for the | or the _ part of the L
    To get these benefits they need to be joined to each other along the edges where they meet.

    Quote Originally Posted by jeronimo View Post
    [*]ivíe seen some threads in which the Z axis was designed in a way that the bearing carriages are stationary and the rails move with the spindle - could anyone point clear this up or point me to the right thread?
    With the bearing carriages stationary on the Y axis and the rails on Z you always have a support nearest to the tool, with minimum offset and therefore it's stiffer. Consider 3 scenarios of cutting height:
    If you are cutting at full Z extension then both methods are the ~same (low stiffness).
    If you are cutting at mid extension then rails on Z is much stiffer - offset from bearing to tool is lower.
    If you are cutting with minimal extension then rails on Z is very much stiffer - offset is very small.

    Sometime rails on Y axis is called the 'tuning fork' arrangement. Draw it out and you will see why, and you will see the difference at the different cutting extensions.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  8. #7
    Quote Originally Posted by Boyan Silyavski View Post
    I missed that cause he said " 3000X1500 table" so i missed that the router is quite smaller. Sorry for that. Then all else what i said stays same except ballscrew size

    I never have insisted on rotating nuts for a small table less than 1.8m???
    While we don't agree on exactly whats needed for good machine I'm not so sure you need to appologise Boyan because of this statement.

    Quote Originally Posted by jeronimo View Post
    I would like to be able to work with full sheet material so Iím thinking of a 1300x1500x150 format (total size not workable area)
    Full sheet to me means 8 x 4(2440 x 1220) so think the OP has got his sizing mixed up.?

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