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  1. #97
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    Default Re: Need help - Fixed gantry design

    Quote Originally Posted by RCaffin View Post
    All solid - back of envelope suggests about 1 tonne.

    Cheers
    Roger
    2x2x0.25" is 5lbs/ft. 2x3 is 7, 2x4 is 9, 2x6 is 12. If the base is made up of 4-2x4 24" pieces, 72 lbs. 1 1/2 x 4 solid is 20lbs/ft, 160lbs.



  2. #98
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    Default Re: Need help - Fixed gantry design

    Yeah, hollow tubing is a LOT lighter, isn't it?
    Is there much difference in rigidity between solid bar and 1/4" tube? Some, but not a lot. The hollow tube will however ring a lot more I think.

    Cheers
    Roger



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    Default Re: Need help - Fixed gantry design

    Well it all depends on the work envelope to get the frame size.......I think a 6040 size mightn't come to that figure...........but it would be quite easy to build and the weight, once on a base, would not move the Earth beneath your feet.

    A Bridgeport weighs about a ton last time I moved one.........I doubt a 6040 to the design mentioned would come to that figure......and as it would be a moving table type, the frame would be the easiest part to build.

    This is just a concept idea, but when you consider the amount of work....and alignment factors...... that a composite in aluminium or steel takes, it is an interesting concept......the design is so simple, without being a compromise, and it could be up and running in a month......why spend 6 months on something only good enough to cut wood.

    I think this could be construed as getting out of the pail and thinking on the backs of Giants.......LOL.

    BTW, a lick of paint and you'd never know the makeup of the build.

    Just as a matter of interest, the concept of a solid build occurred when I was stacking some off cut pieces of 90 X 45 timber from a bathroom revival campaign........seeing the sections as they laid across one another generated the idea.
    Ian.



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    Default Re: Need help - Fixed gantry design

    What I have not seen yet mentioned is how you align one of these builds.

    Figure on spending quite a bit of money on a few precision instruments for this - and I do NOT mean a couple of $1 eBay string levels!
    A lot of eBay devices have a carpenter's resolution (or worse). A precision device can give you 0.0005 "/ft. It's the only way to get rid of twist in the guide-rails.

    Cheers
    Roger

    Last edited by RCaffin; 11-29-2017 at 01:37 AM.


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    Default Re: Need help - Fixed gantry design

    You laugh at string levels, but watch what the guy in the video below does to level a welding table.



    If all the pads (upright and rail) are at the same level, it seems to me it simplifies the machining. For good enough, you would work off a flat surface with machinist level to grind your way to perfection. Blanchard grinder ("Our largest 100 HP 60 inch chuck Blanchard grinding machine will cut the stock quickly and it allows us to grind parts up to 72” diagonally.") or even surface grinding ("We offer tight-tolerance surface grinders with sizes up to 32″ x 96″ and 16″ x 168″) as options at large outfits. They can also do stress relief if you think it needs it and/or don't want to setup a bake oven.

    But, yeah. This is one area that I haven't seen discussed a lot other than people using large mills where they work to complete the flatwork. I have a crude surface grinder fixture idea bouncing around in my head.



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    Default Re: Need help - Fixed gantry design

    Quote Originally Posted by RCaffin View Post
    Yeah, hollow tubing is a LOT lighter, isn't it?
    Is there much difference in rigidity between solid bar and 1/4" tube? Some, but not a lot. The hollow tube will however ring a lot more I think.

    Cheers
    Roger
    Right, it's actually pretty interesting.

    2x2x24" solid, 500lbs applied force
    Deflection: 0.0576000"

    2x2x24" .24 wall
    Deflection: 0.0842606"

    2.25x2.25x24" .24 wall
    Deflection: 0.0567138"

    3x3x24 .125 wall
    Deflection: 0.0387087"

    Increase the 2x2 1/4" wall tube by 1/4" and you have less defection than the solid.
    By 1" and you can halve the wall thickness and still be less than the solid.

    Of course, that's not the only characteristic we'd be chasing, but interesting none the less.



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    Default Re: Need help - Fixed gantry design

    Pretty sure it has to do with surface area.
    I had some 7/8" stainless hex bar that I was sure was fairly rigid at short lengths. I was amazed at just how much I could deflect that supported on the ends on a bench. 30" length. I won't say wet noodle, but it was not far off.
    It deflected more than 1" x 1/8" square 6061 aluminum tube the same length.

    Lee


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    Default Re: Need help - Fixed gantry design

    Hi.....the concept of solid steel bar for a build is mainly to offset the factors that go for an assembly of the various frame parts.

    It takes many hours of machine work (if you have one) to interface bolted items but with an all welded solid steel bar build it's just put together, align by whatever, and hit the welder.

    Very true there is a weight factor to consider, but given the choice of a relatively light construction
    ;cos the tubing wall cannot be bought any thicker and the fact that having a tubing wall will allow distortion from the rapid heat build up as opposed to a solid bar not moving ........I proposed the solid bar as an option to consider.

    It comes down to proportions for the actual frame size.

    I don't think I'd consider going to a bigger than a 6040 build without first estimating the overall weight of the frame.......but it would not approach a Bridgeport weight.....ever.

    In the aforementioned design I would go to two bars for the X axis cross beam.....this would be more in proportion than a single solid lump of ferrous.

    I doubt anyone would be brave enough to build in solid bar stock........but the concept of an immovable frame is one that is the holy grail for mill or router design and the fact that the machining input is of the minutest makes it a quick build by any standards.

    I would even go so far as to state that no machining for the linear rail seats on such a build is a possibility if you know your fitting by the judicious application of an angle grinder and a straight edge......BTDT.
    Ian.



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    Default Re: Need help - Fixed gantry design

    Quote Originally Posted by LeeWay View Post
    Pretty sure it has to do with surface area.
    I had some 7/8" stainless hex bar that I was sure was fairly rigid at short lengths. I was amazed at just how much I could deflect that supported on the ends on a bench. 30" length. I won't say wet noodle, but it was not far off.
    It deflected more than 1" x 1/8" square 6061 aluminum tube the same length.
    It has been a long time since physics, the biggest thing I remember was make a beam taller if you can.
    https://www.engineeringtoolbox.com/b...on-d_1312.html

    So it is based on surface area, but the height of the object plays a bigger role. For a rectangular solid, the Area Moment of Inertia Ix = b h^3 / 12 (where h is the height and b is the width in cross section). Height being cubed plays a big part next. When calculating the deflection, the beam length and load are divided {in part} by the moment of inertia thus a larger AMOI means a smaller deflection.

    You can also think about an iBeam. One side in tension, the other compression. As you approach the center, these forces drop to zero. Extra stuff between the flanges not helping soo much.

    As far as solids go a 6' 2x4 in 1/4" cost me ~$42 and 52lbs not the easiest thing to handle, but not too bad. I've never seen thick, solid bar stock in any useful lengths. I'm sure you'd have to purchase new and have cut to length as 6' would be 163lbs. If available as drops ($0.80 clean metal) that would cost $130.



  10. #106
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    Default Re: Need help - Fixed gantry design

    Hi, in the design we are looking at, there are no unsupported columns like you have on a mill which is all about a toffee apple on a stick syndrome..... the higher the top the more wobble you get etc.

    I proposed solid black hot rolled bar for a CNC router frame build........that puts it in a box like structure where all the frame components are supported by one another once welded together into a composite structure.

    It is quite easy to list and get a precut supply of solid bar from the supplier ready for the welder, and this makes it very transportable.

    The main advantage would be the miniscule amount of actual milling work you'd have to indulge in to produce the main X and Y axes for the linear rail seats........the frame requires practically no other milling to make it a frame.

    I'm thinking in terms of a moving table 3040 work envelope to mill ANY material.

    That gives you 300mm X 400mm and approx 200mm under the gantry crossbeam.

    Apart from the height, 300 X 400 full table coverage is quite a job size.......all column mills do not (cannot) cover their entire table length......a 400mm X axis coverage on a column mill is a very big table length, and this router proposal is designed to cut all metals not just wood and maybe aluminium.

    I think the design that has so far appeared is an ideal layout for assembly and forms a rigid base for the Y axis on top of the 2 bars that go between the column uprights........even if it was made in steel tube.
    Ian.



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Need help - Fixed gantry design
Need help - Fixed gantry design