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IndustryArena Forum > Mechanical Engineering > T-Slot CNC building > Overwhelmed and want to build a 4x8
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  1. #1
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    Overwhelmed and want to build a 4x8

    First time posting so my apologies, this will be a newbie builder thing:drowning: :drowning:. After looking through the site for months, joining almost two years ago, I decided, I really need to ask for help.
    Attached is what i want to build (style wise) These are examples from xzero and just a basis to show you what I have in mind. I have tried to get a parts list from multiple parties with no luck. A BOM would allow me to get the parts over time to complete my build.

    So many questions, but where do i even start? so lets start here:

    I want to build a machine that is 4'w x 8'L with a t-slot table top. Haven't decided on the dual ball screws for the X axis or the single in the middle. I do like the support you get by running duals on the outside allowing for table support.

    I already know I will be running Nema34 stepper motors. I am assuming how i mount them will depend on the build?

    Ball screws and linear rails: I want to use 2525 and 20mm at x and y. But how do you order these to fit your machine? after other decisions are made, does it come down to overall length? Note: i am fortunate enough i live less than a mile from THK linear. I will probably order these items through them, couldn't ask for more local support than that and no shipping cost lol.

    Gantry: I will revisit this subject later, many questions just in this category. I know i will be running a spindle motor and not a router for my Z axis


    So where to start? I can sit here and read answers all day, my issue is, I dont know what questions to ask first, sometimes thats the hardest part with trying to learn something. I can run the machines, building them is a whole new ball game for me, so i decided to step back and take a breath. If you have a link on here that answers these questions, by all means put it here, i will gladly read it and go from there.

  2. #2
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    Re: Overwhelmed and want to build a 4x8

    and literally after posting this, "similar threads" shows some of what i am looking for lol. Well, thats a start.

  3. #3
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    Re: Overwhelmed and want to build a 4x8

    Usually people describe the long axis as the X axis.If yours is meant to have the long axis as the Y axis,one central ballscrew will almost certainly lead to the gantry trying to twist.

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    Re: Overwhelmed and want to build a 4x8

    Quote Originally Posted by routalot View Post
    Usually people describe the long axis as the X axis.If yours is meant to have the long axis as the Y axis,one central ballscrew will almost certainly lead to the gantry trying to twist.
    Got it. Any other helpful insight?

    Sent from my SM-G950U using Tapatalk

  5. #5
    Community Moderator ger21's Avatar
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    Re: Overwhelmed and want to build a 4x8

    Ball screws and linear rails: I want to use 2525 and 20mm at x and y. But how do you order these to fit your machine?
    Typically, a catalog from the manufacturer will have dimensioned drawings and specs of the end machining. This should give you the info you need for the overall length. You're going to need to have a final design with all the components in place, so you know the actual length you need. Most people do a complete CAD model before they start ordering parts and building.
    For an 8 ft axis, you'll need screws close to 9ft long. THK screws will be very expensive, probably well over $1000 each, possibly much more.
    Most people building 4x8 machines use the rack and pinion drives from Avid CNC.
    Gerry

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  6. #6
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    Re: Overwhelmed and want to build a 4x8

    Quote Originally Posted by ger21 View Post
    Typically, a catalog from the manufacturer will have dimensioned drawings and specs of the end machining. This should give you the info you need for the overall length. You're going to need to have a final design with all the components in place, so you know the actual length you need. Most people do a complete CAD model before they start ordering parts and building.
    For an 8 ft axis, you'll need screws close to 9ft long. THK screws will be very expensive, probably well over $1000 each, possibly much more.
    Most people building 4x8 machines use the rack and pinion drives from Avid CNC.
    I knew THK would be on the high end so another supplier may be in order. You mentioned the end machining. and i know im putting the cart before the horse here, Im guessing this will be based on whatever mount i need to use based on design of machine? these are the things that in the long run, hang me up, because i have absolutely no idea what i need to have in place to order something like this lol.

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    Re: Overwhelmed and want to build a 4x8

    https://www.vxb.com/4X8-Feet-CNC-Rou...SABEgL-gfD_BwE

    Just as an example, if i could figure out the extrusion specs i would need, i could do a first build with something like this. But again, cart before the horse in my head.

    Rover cnc offers some things pre built, like the gantry kit, z axis (ball screw included) but it is still up to me to do the base if buy those parts. which i am more than ok with

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    Re: Overwhelmed and want to build a 4x8

    Quote Originally Posted by CroweCues View Post
    https://www.vxb.com/4X8-Feet-CNC-Rou...SABEgL-gfD_BwE

    Just as an example, if i could figure out the extrusion specs i would need, i could do a first build with something like this. But again, cart before the horse in my head.

    Rover cnc offers some things pre built, like the gantry kit, z axis (ball screw included) but it is still up to me to do the base if buy those parts. which i am more than ok with
    You could do a lot worse than that kit,it includes a lot for the price.There is a "but" coming-those screws have a 5mm pitch and if you are looking for normal feed rates they will be spinning pretty fast.There are others here who know much more than I ever will about harmonics and the chances of whip and flex occurring,I would guess most likely on the long axis,and that is another positive for the use of rack and pinion drives.

  9. #9
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    Re: Overwhelmed and want to build a 4x8

    Hi,
    the previous post about 5mm pitch screws is worth thinking about.

    Certainly to cause an axis to travel fast with 5mm pitch screws will require they spin quickly. For instance if, and that's a BIG IF,
    you could spin them at 1000 rpm then the axis travel is 5m/min or 200ipm. If you used 20mm pitch screws you could obtain four
    times the speed at the same rpm, ie 20m/min. Having said that, the torque required of the steppers is four times higher also.
    I personally am of the opinion that the torque demanded by 20 or 25mm pitch screws will demand more than its reasonable to
    expect from a stepper.

    The 'mechanical advantage' of a 5mm pitch 20mm diameter screw is 12. A 20 mm pitch in the same 20mm diameter the advantage drops
    to only 3.

    The probability of the ballscrews 'whipping' at 1000 rpm is quite high, I suggest looking on the NSK website for design information
    relating to ballscrew whip. I will try to track down the .pdf that had that info, it had a graph of permissible speeds vs ballscrew diameter
    and 'free length'.

    Craig

  10. #10
    Community Moderator ger21's Avatar
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    Re: Overwhelmed and want to build a 4x8

    I personally am of the opinion that the torque demanded by 20 or 25mm pitch screws will demand more than its reasonable to
    expect from a stepper.
    Yes, that's why most use rack and pinion. The inertia on big, long screws requires powerful, high speed motors.
    Big industrial routers that use ballscrews, use AC servos, and large, high pitch ballscrews that spin slowly when the machine is moving fast
    Gerry

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    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  11. #11
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    Re: Overwhelmed and want to build a 4x8

    Quote Originally Posted by ger21 View Post
    Yes, that's why most use rack and pinion. The inertia on big, long screws requires powerful, high speed motors.
    Big industrial routers that use ballscrews, use AC servos, and large, high pitch ballscrews that spin slowly when the machine is moving fast
    Screws are usually 32mm or 40mm and DONT spin when they are 8ft long as well. George had some issues when he made them 8 feet. The 2525 screws couldn't spin very fast - 500rpm -700rpm maybe was safe - and needed a very large servo (100mm frame minimum) to handle the inertia.

    I can get you the end frame and upright parts for the "last gen" predator. That could jump start the build. It will be, 24" or 36" "cross" travel if i remember. I can build you a gantry plate and a new z (slightly expensive, due to being one off and the new z uses a THK BNK20 screw and proper NSK end bearings). There are no more custom extrusions (beams or t slot tables), and they aren't strong enough for that span if there were. I made a set of beams with solid 3.5" square bar stock, but, we can only make those up to about 39" (29" travel). I also made the lower cross beam solid, which helps dramatically in torsional strength when using a single ball screw (you have the option to use 2 screws, but that can be bothersome in other ways).

    I would go as short as you can get away with. If your longest cue is 7'2", go 7'2" feet. Every extra inch at that length makes the machine exponentially weaker. The table top is also critical. It will become a structural member due to the span. A 25mm mic 6 top that spans the whole machine will cost you $1500, but might be the best choice - it only needs to be drilled and "cleaned" up on the edges, which you could do with a belt sander if it came to it. Don't bother with t slots, just drill and tap any mounting holes where you need them. You can then drill any new holes that come up in the future directly on the machine. Stick a vac grid table on top if you want to do flat things.

    I don't have any bright ideas on the beams - just brute force. 3.5" x 5" bar, 8 feet long, might cost about $2500 to machine - if you had someone that could mill it. Same would go for steel tubing - someone still has to machine it.
    MOST machines with that much travel build the whole base into a big welded structure that sits on the floor and no cross member than has to move underneath. Because you only have about 3.5" of height for the beams, you are pretty limited. making the whole base from a slab of 6061 plate with integrated would be fun, but cost a fortune... maybe (I'm often surprised by these things).
    wotzBotz

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