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IndustryArena Forum > MetalWorking Machines > Uncategorised MetalWorking Machines > Fixed gantry or column for mill rigidity?
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  1. #21
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    Re: Fixed gantry or column for mill rigidity?

    Quote Originally Posted by JRoque View Post
    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 ;-)
    For a table that width size you will have to have a bridge mill design.....a single column will not support the over hang for the Z axis to reach over the full width of the table you plan have but a double column or bridge will enable it to do so......but you could make the Y axis (under the bridge) the short size and the X axis (across the bridge) the longer size as the bridge can cover the long size easier as it's more supported.

    Column mills normally have a long narrow table because of that reason.

    BTW, the length of the mill base between the columns (Y axis) is usually twice the width of the table (X axis) to enable the full length of the table Y axis to move under the bridge. and to prevent too much overhang by the spindle (Z axis) from the face of the bridge

    Bridge mills also have less Z axis travel due to the height restrictions of the columns which can be massive to make them less liable to back and forth movement due to cutter torque forces.

    Attached is a pic of my favorite bridge or gantry type mill design.
    Ian.

  2. #22
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    Jun 2004
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    487

    Re: Fixed gantry or column for mill rigidity?

    Hi all,

    Thanks for the clarification on the external controllers. If those don't interpret G-code directly, they probably nearly do. It'd be great to understand the Mach (for example) protocol. If Mach4 no longer generates the timing pulses, it would have to tell the external controller how many steps and in what direction it needs to move, and at what speed each movement is made. Plus it probably sends other things like curve radii. Anyway, I'm mostly speculating here; need to read more on this.

    I'm still trying to decide on the machine design. One day fixed bridge wins, others vertical column wins. Right now, I'm going through what/ifs with vertical columns. It's hard to figure out how all would go together without having critical things like the spindle and its motor. The spindle cartridge (pictured previously) arrives in the next day or two. That will give me a chance to take measurements, figure out how I'm going to drive it, and build the support box/head for it. Of course, I can't finalize that until I have a spindle motor....

    I have been obsessing the past few days over the spindle motor. What I want and what I can get at a reasonable cost don't seem to intersect. I want a servo motor that can do about 8,000 RPM at the top. There are a million different high speed, VFD driven spindles everywhere. But I want to rigid tap, thread mill, maybe even automatic tool changing one day. Sure, there are tricks to get an induction machine to do these things but a servo has all that it needs natively.

    What I can get that meet my requirements is a 3.7kW servo package, like this:



    The darn thing is way too large for the small machine I'm planning. It's also 100lbs (46kg)! I can get a 2.2kW that's slightly lighter but it costs the same to ship and only about $80 cheaper than the 3.7kW. The Chinese are so imprecise with their quotes and leave so much details out. I wish they posted the complete specs of whatever they're selling, post the manual online so you can read it *before* buying and have a good explanation why a smaller servo package costs the same to ship as a larger one.

    Back to the Alibabas and their express train of confusion.

    Regards,
    JR

  3. #23
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    Re: Fixed gantry or column for mill rigidity?

    Hi JR - There are no "curves" in G code only point to point commands. They maybe very close together so the path looks like a curve. The g code generator has settings which define how curves are broken down into small straight lines. The motion controller looks ahead and keeps the tool moving according to the controller settings of tolerance, speed and accelerations. You'll figure it eventually. Peter

  4. #24
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    Re: Fixed gantry or column for mill rigidity?

    A question or two......do you really want to build one for the experience or for economy.......and do you want to use it for production for saleable items or mainly hobby use?

    You can go to great lengths for the ultimate and then find that you could buy the same machine on the open market for less and get it tomorrow, so the burning question is how much do you want to spend?
    Ian.

  5. #25
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    Jun 2004
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    487

    Re: Fixed gantry or column for mill rigidity?

    Hi all,

    Peter, maybe I've been confused all of this time but, something like this: https://www.haascnc.com/service/code...value=G02.html. Now, that the external controller can't interpret that is certainly a possibility. I do understand that in the end, it's all small lines. In fact, from where I stand, there are no circles in the universe, only short segments of straight lines.

    Hey Ian, no production here. I'm looking to replace a fairly large CNC router I built many years ago and a worn out Bridgeport that's more of a drill than a mill these days. I don't want a 10,000 pounds (4500kg) VMC but neither an aluminum extrusion frame machine from eBay. I do want the most rigidity for the money I can get, hence this thread. Like most of us on the DIY CNC pill, I will probably end up making parts for the machine itself in an endless loop


    PS: between the time I started typing this post and when I actually submitted it, I bought a spindle motor! https://www.alibaba.com/product-deta...791875146.html It's a smaller 2.2kW instead of the 3.7kW I had in my shopping cart before. It's 30kg instead of the 46kg of the larger unit.

    I picked this package for several reasons. First, the factory representative, Donald, was absolutely fantastic to work with. Remember my previous post where I complained about typical Chinese not being precise or forthcoming? Not Donald Chen. First thing he did was send me to his factory website to download the manual and drive software: Spindle catalogs-Hangzhou Bergerda Automation Technology Co., Ltd.. Nice! Then he stayed online while I asked a million and a half questions about the unit. The listed motor is for 380V but they are going to wind me a special version for 220V. Ha! How awesome is that?

    The unit itself looks great, with stuff you would expect from a model servo drive, but not necessarily in a spindle. It has encoder output which I can feed back to the controller NC software to read precise spindle position and speed. Tapping is easy if you know where the tool is. ATC is also a breeze when you can command a position and lock the motor there. The drive also has step and direction, optically isolated inputs and a rigid tapping mode built in.

    Regards,
    JR

  6. #26
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    Re: Fixed gantry or column for mill rigidity?

    Hi JR - Even though that code is for a circle it gets decomposed into small lines according to how the controller is set up. The "arc" code is to make manual programming easier and more succinct. If you use a CAM package it won't generate G02 or G03 code. Attached is the code from the first couple of mm's from a 25mm dia circle created in uccnc for instance.

    The external device is not a machine controller, its a velocity planner. The machine controller or CAM package generates g code which is a string of co-ordinates with no velocity attached just a starting velocity. The planner looks ahead (say 200 lines) looking for roadblocks and tight corners and slows the machine down accordingly to the road rules (set velocities, tolerances and accelerations) then speeds it back up to the last posted speed. Cheers Peter
    Attached Thumbnails Attached Thumbnails circle.JPG  

  7. #27
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    Re: Fixed gantry or column for mill rigidity?

    That makes perfect sense, Peter, thank you. Not exactly "dumb" but not as smart as I thought they were.

    Update on the spindle motor (already): the gentleman from the Bergerda factory called wanting to know if I preferred the motor being optimized for vertical or horizontal mount. I really hope this turns out to be a great system because I'm very impressed with these guys so far.

    JR

  8. #28
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    May 2005
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    1662

    Re: Fixed gantry or column for mill rigidity?

    peteeng
    Using Fusion 360 I have posts containing many G2/G3 moves. Some tool paths like bearing pockets are almost entirely G2/G3 arcs. If you get no arcs programmed it may be something to do with your settings in the CAM software. I've never given much thought to how G-code interpreters handle arcs internally but the idea that it amounts to a bunch of small straight segments makes sense.

    My (very) limited understanding of the various motion control cards is that they allow commands to be cued up in a buffer. Linuxcnc does not work this way which is why I called the available hardware 'dumb'. I'm surprised no one has chewed on my ear for that
    A possibly helpful link ? : https://www.cnccookbook.com/motion-c...-grade-part-1/
    Anyone who says "It only goes together one way" has no imagination.

  9. #29
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    Re: Fixed gantry or column for mill rigidity?

    Hi Cyclestart - Yes but then these "arcs" are decomposed somewhere into point to points. The machine itself cannot move in arcs it can only move in discrete axis increments that summed look like an arc. The other part of this is that the motion is not exact. The controller has rules about tolerances, velocities and accelerations and creates a 'road". The road maybe narrow or wide. Just like you cut a corner in your car to smooth the ride (if your a considerate driver or a race car driver ) The controller uses the entire road width (tolerance) to create a smooth path. So the actual machine cut maybe quite different to the CAD geometry, especially doing fast roughing. But then roughing does not matter. Finishing cuts will be slower and more precise. So even though the Gcode says (XYZ) the motion controller may/will miss this by the tolerance to smooth the ride....Peter

  10. #30
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    Jun 2004
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    487

    Re: Fixed gantry or column for mill rigidity?

    Hi all,

    I'm still playing the CAD and mouse game with designs. I'm working through vertical at the moment. First, let me describe my newly acquired spindle by saying: it's insanely big!








    Some key measurements on the spindle are: ~20" (~50cm) long x 8" (~20cm) wide. The upper shaft, where the drive pulley would go is 60mm (2.3") in diameter, and though I found a possible match, options are limited for pulleys with that large of a bore. That piece at the top that looks like the top bearing, it's not. That's fixed to the rotating shaft so some kind of bearing would have to be added if I want to support that end. It's not really necessary for the spindle itself but I'm not sure about leaving it unsupported way up there.

    Frankly, if it weren't for the drawbar and it's many Belleville washers, I'd machine that shaft down to mere mortal size. Thoughts of springing for a new Chinese BT40 spindle and air cylinder have crossed my mind, though shipping costs to US are ridiculous at the moment (~$550).



    Here's the vertical sketch I've been working through:



    I added a CAT40 holder, 1/2" (12mm) end mill and my Kurt vise to see how it would work in the real world. With Z raised all the way up, I'd have ~12" (~300mm) between the tip of the end mill and the top of the vise, and ~21" (~500mm) from the bottom of the spindle to the top of the table (no vise).

    Struggling to learn basic Fusion 360, I managed to run a couple of stress tests on the vertical columns. Putting about 500 pounds (226kg) of horizontal force at the top of the columns makes them deflect a little over 0.001" (0.02mm) which is not too bad. Bracing the columns to each other will help, and so will filling them with epoxy granite. If I can figure out how to add granite in F360, I'll give that simulation a go.

    Peter, how's your moving X/Z bridge design coming?

    Regards,
    JR

  11. #31
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    Re: Fixed gantry or column for mill rigidity?

    Hi JR - Its still in thought bubbles. I have investigated the controller side as I will need 5 axes and I'd like to do many of the parts in composite so a few variables at the moment. Currently I'm committed to building a new router called YaG to replace the router I just sold. I pick up the bench materials today. YaG will be able to cut aluminium so then I can make the Mill parts. I was close to finishing the Mill design (but its Z height got out of control see image attached) then decided to go the lifting gantry way so the Mill is stalled. Its a long haul designing a machine in CAD as you are finding. But once done its great the rest moves along nicely. Then there's life ( I'm building a big workshop so I can make bigger machines and my commercial work has increased due to Covid when I thought it would slow down) so need to sleep less I think. Peter

  12. #32
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    Re: Fixed gantry or column for mill rigidity?

    Hey JR _ Heres one I think I'll copy. Peter

    https://www.waldrichsiegen.com/news/...comes-to-life/

  13. #33
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    487

    Re: Fixed gantry or column for mill rigidity?

    Hey Peter,

    Holy macaroni... you can mill your house on that thing. And is that a mill and entertainment ride? Why else would they add a ride-in cab to X/Z??

    Joking aside, that is the design you had proposed earlier. The big advantage being that you can mount Z rails right on the vertical bridge columns which takes care of rigidity issues in Z. It does add the complexity of the extra motor and how to keep those aligned over the travel distance, however. Accurately homing/leveling both sides of Z becomes critically important at each start, but doable.

    Another advantage towards bridge designs is not having to build an overly complex top of "C" to hold the spindle and motor. As I'm finding out going through my vertical design, it can get messy trying to fit spindle and motor, reach out all the way to the middle of the machine and keep it rigid enough. Lots of bracing, heavy thickness material and associated weight that have to be carried by the Z motor and screw.

    On the flip side, what do you do with your belt-drive spindle motor in a bridge design? If you want to keep your spindle close to the bridge support (as it should be), the motor needs to be off-set to the side, throwing your balance off some.

    Regards,
    JR

  14. #34
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    Re: Fixed gantry or column for mill rigidity?

    Hi JR - Machining's got to be fun I suppose and now with physical distancing requirements the operator is well away from others , the view is probably good from up there to. Aligning Z axis no different to a dual Y axis. I have been trying to get rid of the saddle for some time and the lifting gantry does this. I wouldn't worry about balance by the time you look at tool loads the offset weight will be minimal. But since the Z doesn't go up and down can't the spindle motor be mounted above the gantry improving the balance? I've decided to go 100% composite on the lifting gantry design. It will be thick skinned infused fiberglass with a steel fibre core. I have also looked at steel grit shot which is much cheaper then steel fibre. The FG skin can be post machined (if needed) and drilled/threaded so I don't have to worry about inserts. The FG is same stiffness and damp as epoxy granite is. The moulds can be cut on YaG so I'm looking fwd to that design/build... I also have a lot of carbon fibre offcuts which will be good in stiffness critical areas....Peter

    attached is a test block of the steel fibre material.

  15. #35
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    487

    Re: Fixed gantry or column for mill rigidity?

    Hey Peter,

    Very interesting indeed as I'm going to need some sort of composite to fill in my columns, vertical or bridge.

    Returning to an earlier topic regarding the external motion and breakout boards, you mentioned you were looking at one that you preferred. Is the Ethernet SmoothStepper, plus a C11G expansion board a good option in your opinion? I'm not sure Ethernet is a necessity here since bandwidth requirements are relatively low (USB might be sufficient) but Ethernet does provide extra flexibility in terms of connectivity and location of the controlling PC.

    Regards,
    JR

  16. #36
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    Re: Fixed gantry or column for mill rigidity?

    Hi JR - Haven't used a smooth stepper so can't comment. Others in the electronics area will be able to help. I've used UC100's and this BOB.

    https://www.omc-stepperonline.com/cn...kit-st-v2.html Peter

    I've looked at the following options so far:
    UCCNC products https://cncdrive.com/products.html particularly the AXBB-E
    https://www.aliexpress.com/item/559920306.html

    using the 5 axis BOB and 5 drivers and UC100 or similiar

  17. #37
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    Re: Fixed gantry or column for mill rigidity?

    Quote Originally Posted by peteeng View Post
    Hi JR - Machining's got to be fun I suppose and now with physical distancing requirements the operator is well away from others , the view is probably good from up there to. Aligning Z axis no different to a dual Y axis. I have been trying to get rid of the saddle for some time and the lifting gantry does this. I wouldn't worry about balance by the time you look at tool loads the offset weight will be minimal. But since the Z doesn't go up and down can't the spindle motor be mounted above the gantry improving the balance? I've decided to go 100% composite on the lifting gantry design. It will be thick skinned infused fiberglass with a steel fibre core. I have also looked at steel grit shot which is much cheaper then steel fibre. The FG skin can be post machined (if needed) and drilled/threaded so I don't have to worry about inserts. The FG is same stiffness and damp as epoxy granite is. The moulds can be cut on YaG so I'm looking fwd to that design/build... I also have a lot of carbon fibre offcuts which will be good in stiffness critical areas....Peter

    attached is a test block of the steel fibre material.
    Is this a new block different from the first one, looks good I thought you used CF in the first one is this better than the CF or cheaper to make
    Mactec54

  18. #38
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    Re: Fixed gantry or column for mill rigidity?

    Hi Mactec - I've made two small blocks and a large block. The first blocks had steel and a nano fibre but the nano fibre has worked out extremely expensive. So the fallback is all steel (shot or fibre) or all aluminium powder. Steel is heavy (4500kg/k3) and stiff. Got to build my new router so I can make moulds for the next machine. I think I'll move to all composite machines in future. My workshop should be finished in 8 weeks and then I can do more stuff. Peter

  19. #39
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    Re: Fixed gantry or column for mill rigidity?

    Quote Originally Posted by peteeng View Post
    Hi Mactec - I've made two small blocks and a large block. The first blocks had steel and a nano fibre but the nano fibre has worked out extremely expensive. So the fallback is all steel (shot or fibre) or all aluminium powder. Steel is heavy (4500kg/k3) and stiff. Got to build my new router so I can make moulds for the next machine. I think I'll move to all composite machines in future. My workshop should be finished in 8 weeks and then I can do more stuff. Peter
    So that is the same steel fiber they put in concrete you used, what about CF mesh or light steel mesh that you could bend into shapes to suit your mold and stack it in layers
    Mactec54

  20. #40
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    Re: Fixed gantry or column for mill rigidity?

    Hi Mactec - The steel fibres used in concrete are too big. The fibres I use are used in brake pads. If I were to go CF I'd just make solid CF laminates (1500kg/m3 and E=60-90GPa). But these would be expensive and light. So I think the go is infused fibreglass laminates on outside (35-45GPa at 1800kg/m3) say 10-20mm thick (I have infused up to 75mm thick in the past) (easy to machine and drill and thread) then backfill with steel shot and infuse for weight and stiffness. E=50GPa or 60GPa (have to test that yet) and 4500kg/m3.

    The aim is something that can be stacked dry so small fibres or particles, prefer being able to be poured into mould dry. Then tamp then vac bag and infuse. Plus it needs to be able to be post machined easily. Making all those insert armatures etc is too labour intensive for production. Peter

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