Fixed gantry or column for mill rigidity?


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Thread: Fixed gantry or column for mill rigidity?

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    Default Fixed gantry or column for mill rigidity?

    Hello all,

    I'm looking to build a new CNC/manual mill. I have a ~1,000 pound (450 kg) table that has been ground flat, with a surface area of 42" x 36" x 1" thick (106 x 91 x 2.5cm).

    The build criteria is simple: be as rigid as possible, within limits.

    To that end, I'm leaning towards a fixed bridge design using heavy wall square tubing, 35 - 45mm guide rails, and as big a diameter ballscrew as I can practically fit under the axes.

    Here's a scratch design I've been noodling with while contemplating ideas. This is not a final design and I'm not really looking for feedback on it, just to serve as a conversation starter:



    Soooo... if you were building a mill from scratch that can hog through steel and not break a sweat (or end mills), what would you design? Is a fixed gantry a better design (within its compromises) than column? The commercial machines I've seen on YouTube easily cutting steel have all been column types... hmmm.

    Thanks!
    JR

    PS: good to be back. Looks like I've been bitten by the CNC bug again ;-)

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    Default Re: Fixed gantry or column for mill rigidity?

    A fixed gantry (or "bridge") design like you've drawn is more rigid than a moving gantry, and it can be made very massive, since it doesn't have to move. The "column" type mills are typically small machines; larger ones are usually massive castings in a "C" configuration. But mills like the one you've drawn are becoming more common in industry. They are easier to build if you don't have an iron foundry, and work well if attention is given to rigidity in all parts and assemblies.

    You say this is a "CNC/manual" mill; I'd suggest you choose one or the other. Ball screws are used for CNC, they're not used for manual mills because they tend to back-drive - you'll need a way to lock down any axes you aren't using for a particular move. Is this machine supposed to have dual spindles and another axis parallel to Z? If so, you'll need a way to drive the two separately, this gets complicated. Also, you'll need longer rails for X, and a footprint about twice as long as you've indicated if the tool is supposed to reach the whole table area.
    .

    Andrew Werby
    https://computersculpture.com/


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    Default Re: Fixed gantry or column for mill rigidity?

    Much appreciate the feedback, Andrew.

    I set out to build a moving gantry many moons ago to cut PCBs. It ended up being 52" x 60" (132x152cm) and mostly routed wood and aluminum parts for itself. I have cut a number of mild steel projects on it but need to go very slowly with that.

    Good to know about fixed bridge gantries. Looking at the different types, they seem to be the most rigid. Compared to a column, it's also likely easier to build in a garage.

    The sketch above is a moment in brainstorming time. Nothing's set, other than perhaps the table/base dimensions. The columns are 6" (152mm) wide, 1/2" (12mm) wall. The rails and trucks are 45mm, "winged" or wide type. Roughly, X and Y have 20" (508mm) travel. Z will have to use a smaller rail guide.

    I imported a drawing of my home-built ER25 spindle and it looks like a toy next to the 45mm trucks. Have a laugh:



    For reference, that spindle is 2.75" (70mm) diameter. I was so proud of my spindle...ha! Most of my current tooling is R8 since I run a Bridgeport, but it looks I'll eventually have to buy or make something like a BT30 spindle for this new machine.

    Yes, sorry, when I say "manual" I mean manually controlled motors. I have a DIY pendant I use in the wood router that let's me operate it manually, somewhat. That machine runs steppers and an old laptop that can't really keep up with faster manual inputs. I already have the motors for the new one: NEMA-52, 15NM, 2.3kW AC servos - though I think Z will use something smaller.

    Thanks,
    JR



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    Default Re: Fixed gantry or column for mill rigidity?

    I have a ~1,000 pound (450 kg) table that has been ground flat, with a surface area of 42" x 36" x 1" thick (106 x 91 x 2.5cm)
    Well that's a handy thing to have on hand !
    You're probably already aware of the WADE'O router but here's a link anyway:
    https://www.wadeodesign.com/design-details.html
    It's not designed to be a mill but maybe there's some useful info there.

    Bridge mills are great but I'm not sure that it's an easier DIY project than a C frame mill. WADE'O made it look easy but he cheated by starting off with all the good stuff ready made

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


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    Default Re: Fixed gantry or column for mill rigidity?

    Hi, and thanks for the post and link. I had watched his video on YouTube while searching for fixed gantries. Very nice build! It was going great until he said he built it "to cut aluminum" Hopefully he joins this conversation, if he's still around. Maybe I'm just dreaming that I'll be able to cut steel, but I'll try.

    Yes, mounting the square tubing does worry me. I'm also sort of cheating by starting with a Blanchard ground table that will be my datum reference for the rest. Grinding the square tubing flat will be a challenge, however.


    One thing I'd like perspective on is how I mounted the rail blocks upside-down:



    I'm not sure why more machines aren't built this way; I must be missing something. My rationale is that the Z and X axes will *always* be exerting pressure along the X plane range. It really doesn't matter where Y moves to, Z is always on the same X plane. If that is so, why bolt the supporting trucks to the moving table and having them be away from where Z and X are exerting pressure?

    In other words, if the trucks are mounted to the table and you're cutting at the edge of your Y travel, the trucks will not be under Z. But if the trucks are fixed to the base and along the X vector, there will always be maximum support from the rails and trucks regardless of where Y moves to. Sure, heavy loads from parts on one edge of the table might tilt it slightly (like Bridgeports do) but that's a lesser compromise compared to not having support under Z, IMHO. There's also the added advantage of not having the rails exposed to chips as they're always tucked under the table. I'm missing something, aren't I?

    Regards,
    JR



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    Default Re: Fixed gantry or column for mill rigidity?

    Seeing as no one else has replied I'll toss in 2 cents.
    The blocks you're looking at have huge capacity. The question may not be will they handle the load (?) but rather what is the lifespan of the components used in this orientation (?)
    It's not hard to imagine where the stress points will be on those blocks when the table is at the extremes of travel. These manufacturers have engineers to answer technical questions.
    I don't how responsive they are to questions from DIY builders but it's worth a shot ?

    I built a router using this concept but with linear bearings that are a joke. Deep groove bearings were added to the outside corners of the moving table (not shown in picture) to prevent the tilt.
    The original plan was to have the screw move with the table in rotating nut style but I may never get around to that modification. The router is lag bolted to a hefty torsion box.
    Fixed gantry or column for mill rigidity?-router_base_resized-jpg

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


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Fixed gantry or column for mill rigidity?

Fixed gantry or column for mill rigidity?

Fixed gantry or column for mill rigidity?