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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    Default Re: Fixed gantry or column for mill rigidity?

    Hi JR - Its usual to mount the table rails on the bed and have the cars on the table. Probably easier to assemble. But your arrangement will work, this will keep the load in the cars "kern" ie centre. Seems you have oversized everything so it will be a right beastie. Bridge, gantry or double column mills are common in very large machines. There is a width at which the column cantilever becomes too wide for stability so then the "bridge configuration" takes over. But it scales up and down very well. To answer your original Q there is no doubt that a fixed bridge config is one of the stiffest configs. Its downside is the larger footprint required for the sliding table but all designs are a bunch of compromises that produces a "best" design. Peter

    The next discussion is if you are going to weld this structure together are you going to stress relieve it? This is associated with what accuracy are you aiming at for the machine?



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    Default Re: Fixed gantry or column for mill rigidity?

    Hi, and thanks both for your insightful replies.

    Cycle, the idea behind mounting the trucks fixed to the bed is so they are always under the cutting forces. With the trucks moving, there is a rail there under the spindle but the truck could be elsewhere, not supporting the work. As I see it, the compromise of possibly flexing when the table is at the end of travel is a lesser evil than cutting with little support underneath. At that point, the table will really flex.

    Peter, the trucks I'm looking to get are the "wide" variety that can be specified with over or under mounting. True, it's easier to mount the rails down and then slide the Y table over it.

    I have been reading a really insightful paper on the topic, the "Principles of Rapid Machine design" by Eberhard Bamberg, PhD. <spoiler alert> It essentially concludes a variation of a fixed bridge machine to be the most rigid from the group evaluated, which also included a column mill.

    While the analysis proposed requirements include a 5-axis design - a far more complicated design that skews the conclusions as it takes this into account - it opened my eyes to two important factors: the square/rectangular beam I had chosen is in itself a compromise (it twists vs a round tube), and the Z axis in a bridge design makes it much weaker than a column design. This is contradictory to the paper's conclusion; bridge is better.

    Here's a quick depiction of what I see as a weak point in bridge designs, with my apologies for the crude drawing:



    Note how the spindle is lowered. The closer it is to cutting, the weaker its support becomes, just when it needs it the most.

    But a column Z is fully supported throughout and, in fact, it is *best* supported when cutting the work:



    The column is a weaker support structure when looked side by side a bridge, but not so much when the machine is cutting the work, IMHO. Another issue with bridge design is that it puts the weak cross of Y and Z up on the bridge, the weakest point of the machine. Contrast this with a column where the cross axes are supported by the machine base.

    Peter is right, this is all a series on compromises vs function and requirements. I'll probably change my mind a few more times and battle my OCD before settling on a compromised pick.

    Regards,
    JR



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    Default Re: Fixed gantry or column for mill rigidity?

    Hi JR - I have analysed many gantry and machine configs and each has + and -'s... One thing that can be done is to not have the top rail on the gantry face but move it to the top and back of it. This achieves two things 1) It makes the force on the rails smaller as their distance is further apart so the local deflection is less 2) It makes the centre of action of the moment closer to the shear center of the beam which means it reduces the lozenging of the section. But this then means the saddle is L shaped but my modelling shows its a bit stiffer. This also gives the opportunity of putting the drive on top of the gantry vs on the front if this is of use.... so many combinations.

    https://en.wiktionary.org/wiki/shear_centre

    https://www.quora.com/What-is-Shear-center

    bambachs thesis is a must for any engineer in this field.

    To extend your logic on the column config a rising Z bridge solves some problems. Effectively its a double column machine. I intend to do this next machine...

    Cheers Peter

    Attached Thumbnails Attached Thumbnails Fixed gantry or column for mill rigidity?-rising-z-jpg   Fixed gantry or column for mill rigidity?-gantry-jpg  


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    Default Re: Fixed gantry or column for mill rigidity?

    JR, the problem with a C frame (what you are referring to as a column mill) is the lever length needed to get decent Y travel.

    If you don't need much Y travel and want lots of Z travel, a C frame can be a good choice.

    If you want lots of Y, a bridge (dual column) may be a better choice as the spindle is much closer in as the table can travel under the bridge.

    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)


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    Default Re: Fixed gantry or column for mill rigidity?



    Here's a large scale mill with moving everything. The rising gantry is called axis W. Peter



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    Default Re: Fixed gantry or column for mill rigidity?

    Quote Originally Posted by peteeng View Post
    Hi JR - I have analysed many gantry and machine configs and each has + and -'s... One thing that can be done is to not have the top rail on the gantry face but move it to the top and back of it. This achieves two things 1) It makes the force on the rails smaller as their distance is further apart so the local deflection is less 2) It makes the centre of action of the moment closer to the shear center of the beam which means it reduces the lozenging of the section. But this then means the saddle is L shaped but my modelling shows its a bit stiffer. This also gives the opportunity of putting the drive on top of the gantry vs on the front if this is of use.... so many combinations.
    Indeed. I came to the same conclusion when I built my first CNC router back in 2004. The execution was entirely lame, as I had neither the tools or the knowledge, but something was telling me that second rail on top was better than coplanar to the first. Since this pic, I've moved the second rail further back and added a 1/2" (12.5mm) steel plate across Y:



    On that last design with the blue machine, I was loving the fact that the entire Y axis dropped down on Z. That does solve many of the issues with stiffness and makes for a nice hybrid of both types of machines. But of course, the double ballscrew and belt add their own issues.

    Regards,
    JR



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    Default Re: Fixed gantry or column for mill rigidity?

    Quote Originally Posted by pippin88 View Post
    JR, the problem with a C frame (what you are referring to as a column mill) is the lever length needed to get decent Y travel.

    If you don't need much Y travel and want lots of Z travel, a C frame can be a good choice.

    If you want lots of Y, a bridge (dual column) may be a better choice as the spindle is much closer in as the table can travel under the bridge.
    Yes, I agree. C-frames do limit Y compared to a bridge, and this was another factor leaning me towards a fixed bridge design. As for Z, I guess I would be looking at Z height rather than total axis travel. In the years I've been using my current CNC router, I've never needed much of Z travel. Of course, if I had it, I'm sure I'd be saying I couldn't live without it. I think if I account for a vise, maybe a future 4th axis, holders and tools, the actual travel of Z shouldn't be more than a few inches.

    Regards,
    JR



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    Default Re: Fixed gantry or column for mill rigidity?

    Quote Originally Posted by peteeng View Post

    Here's a large scale mill with moving everything. The rising gantry is called axis W. Peter
    That is just amazing. Super complex to DIY, however, and I suppose if you threw enough mass at a machine it could probably be made out of marshmallows

    Cheers,
    JR



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    Default Re: Fixed gantry or column for mill rigidity?

    The commercial machines I've seen on YouTube easily cutting steel have all been column types... hmmm.
    Back in another lifetime I ran a Mazak VQC 20/50B bridge type mill. Pound-for-Pound it was a rigid machine. Unfortunately I can't find photos of the bridge with the swarf protection removed.
    Cycle, the idea behind mounting the trucks fixed to the bed is so they are always under the cutting forces
    The same as the router drawing I posted earlier, the tool is either over one of the middle bearings or somewhere in between the pair of middle bearings. Why 3 bearings/axis ? I had 6 blocks and didn't know what to do with 2 spares, no other reason. Like the Mazak my router is decidedly rectangular.

    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 JR - I'd use independent motors vs the belt and pulleys. Motors are cheap. Then I'd use UCCNC and an AX-BB controller so you have 6 axis to play with. I've started drawing up the mill already...Peter



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    Default Re: Fixed gantry or column for mill rigidity?

    Hi JR - Its a bit tricky figuring out where square bearings should be. This is because they can support a moment. This means the reaction can be in any direction (its an indeterminate problem). If the rails are the round type then the reaction is easy to predict visually. The sq bearing closest to the load will be the dominant load bearer (ie you could remove the secondary bearing and it will still work, just not as good). The next one will be secondary. Need to do FEA to figure out exact reaction trajectories and how the load is shared then try to get these through the shear centre.



    DMS use a flat front face but semicircular rear shape for their gantries...Peter



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Fixed gantry or column for mill rigidity?

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Fixed gantry or column for mill rigidity?