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  1. #1
    As per the good advice from a thread I posted a few days ago I've been digging around in the build logs for good ideas for my CNC build. I've set myself a pretty tight schedule for a first build I think as I would like to have the mechanical parts of the build done by the end of the year. I reckon with some late nights and early starts I stand a reasonable chance though.

    I initially considered going for an MDF CNC machine (because it was cheap) but some excellent advice saw that idea consigned to the bin in no time flat. If I'm going to spend an age building a machine I might as well build it properly! I then considered a wooden sub-frame with the CNC being aluminium extrusion but that just seemed like a poor compromise so here we are with a frame designed in steel box section.

    The X-axis rails and the bed are 80x40x4mm the legs are 50x50x4mm. The bracing around the bottom is 20x20x2mm. Overall, the X dimension is 1500mm the Y is 980mm (900mm leg to leg) and it stand 800mm tall. I believe this should give me a maximum possible cutting area of about 1400x800x200 which is a little more than I was initially aiming for.

    Click image for larger version. 

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    The frame will, initially at least, be bolted together possibly with epoxy on the X-axis rails (inspiration from here: My First Router, Built in Steel - CNCzone.com-The Largest Machinist Community on the net!). I would rather it was welded but I need to be able to dismantle the machine for moving in the future and I'm pretty sure I couldn't get it down the stairs in one piece .

    The current plan is to put the rails on the top of the X-Axis rail with the screw on the outside (there will be a screw on each side). To keep the length down I'm considering mounting the steppers on the bottom rail and using timings belts but I can't help feeling that's just complicating matters.

    Looking at the design again I think I'll stick at least one vertical support from the bottom to top X-axis rail. Do you think this design needs any other bracing? My gut is telling me it could do with some diagonal bracing but I want to keep the weight down if I can so where would be best to put it? The big question I have though is regarding the straightness of steel box section. I've read quite a few posts that say it comes with a bit of a wobble on it, presumably I need to mount the rails onto something that is fairly straight and flat? How do I go about flattening out at least one side of the top X-axis rail?

    Any comments about the frame or any of my other rambling ideas gratefully received , cheers.

  2. #2
    i can't wait to see what you come up with to get 800mm cutting area from a 980mm Y axis... i may just be copying your design if all works out well..

    i have 654mm Y rails providing a cutting area of 381mm, so if you get your design up here and everyone says it's good i'll be doing whatever i can to reduce the size of my Y :D

    good luck with the build i look forward to see the outcome

  3. #3
    Looks familiar.....

  4. #4
    Now you mention it I remember seeing your build over the summer when I started seriously considering building a CNC. I liked the design so it probably stuck in my mind. Hope you don't mind a little creative borrowing :-).

    Out of interest what size box section are you using under your X-axis rails? I'm guessing it must be 50x50 as I've just noticed that the rails over hang on the design I posted above. Did you find that the box section was straight enough for the rails without any flattening?

  5. #5
    Having realized that SBR20 rails are 45mm wide I scrapped the 80x40x4 box section in favour of 50x50x4. I've plugged the figures into the gantry stiffness calculator and if I understand it correctly I should be getting a maximum deflection of about 50um with a 30kg gantry (I assumed the maximum span was 750mm, it's actually more like 665mm) - I've not studied engineering though so I might be reading the spreadsheet completely incorrectly :-). Should I add another support to reduce the span further?

    After seeing jon's design I've reduced the number of bed supports and thinned them down a bit, they are now 50x25x3mm. I needed to reduce the weight a little as this is going to be installed on an upstairs floor which already has a fair bit of weight on it. My calculations indicate the frame weighs about 85kg so far but that doesn't include the multitude of fixings and brackets I'm going to need.

    To keep down the mess I've added 2mm aluminium sheet on the sides and ends. I plan on making the ends hinged so that I can work on longer pieces if needed. The bed is sacrificial a sheet of ply or mdf.

    The length of the X-axis has been reduced slightly to 1480mm with the rails being 1450mm. This is so that I can hopefully get all for long rails out of a single 6000mm piece of box section and save a little money. The reduced cutting length on X doesn't worry me as I was only aiming for 1220mm.

    I'm pretty happy with this design as I think it's something I can achieve. Any comments are, as always, very welcome.


    Click image for larger version. 

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    P.S. Thanks to jcb121 for the sketchup models.

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  7. #6
    Quote Originally Posted by Wobblycogs View Post
    Now you mention it I remember seeing your build over the summer when I started seriously considering building a CNC. I liked the design so it probably stuck in my mind. Hope you don't mind a little creative borrowing :-).
    Of course not, its in the public domain now. Just thought id seen it before when I saw it. lol.


    Quote Originally Posted by Wobblycogs View Post
    Out of interest what size box section are you using under your X-axis rails? I'm guessing it must be 50x50 as I've just noticed that the rails over hang on the design I posted above. Did you find that the box section was straight enough for the rails without any flattening?
    60mm x 60mm x 4mm some welded some bolted as I have the same getting it upstairs issues as yourself. I didn't really have any issues at all regarding straightness. I just made a load of jigs for welding.

  8. #7
    Don't know where you will be buying your steel from but the standard length is 7.5 metres so might be best to work your figures on that rather than 6 mtrs. Think Jonathan will tell you that some diagonal bracing is needed but I don't have the experience to tell you what & where would be best.

  9. #8
    That's good to know. I've been working from the sizes I found at metals4u which seem to do 6m lengths max. I'd like to try and source the metal locally so hopefully I'll find a friendly place that will cut it down for me too. My comment about trying to get all four out of a single length is a bit moot now anyway as I realized that I wouldn't be able to hit my 1220mm cutting target on X, I've just finished stretching the frame up to 1800mm and adding an extra support column. It really brings home how important it is to design before building.

  10. #9
    Have a read of this from someone else's build, gives a bit of info about buying steel & should give you a better idea what you should be paying. Company you mentioned are probably ok if you want just a small length but are expensive if your buying full lengths, the 6 mtr limit will probably have something to do with maximum length they can get shipped. Most steel stockholders will only sell in full lengths but should be able to cut for you.

    http://www.mycncuk.com/forums/router...machine-4.html

  11. #10
    Quote Originally Posted by Wobblycogs View Post
    That's good to know. I've been working from the sizes I found at metals4u which seem to do 6m lengths max. I'd like to try and source the metal locally so hopefully I'll find a friendly place that will cut it down for me too. My comment about trying to get all four out of a single length is a bit moot now anyway as I realized that I wouldn't be able to hit my 1220mm cutting target on X, I've just finished stretching the frame up to 1800mm and adding an extra support column. It really brings home how important it is to design before building.
    If you struggle let me know. I'm only up the road from you in Hereford, the company that I work for builds grease, oil and water lubrication and cooling systems so we have a workshop that has a nice bandsaw for example! Let me know the lengths you need (and size) and I'll get a cost and will happily cut it up into the lengths you want?

    My baseframe was cut (and welded) at work.

    Neil...

    Build log...here

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