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Thread: Help with ballpark figures to use for sizing aluminum extrusion for DIY build

  1. #21
    Member peteeng's Avatar
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    Default Re: Help with ballpark figures to use for sizing aluminum extrusion for DIY build

    Hi P2N - A solution for your issue of connecting things square is to use the brackets that companies like 80/20 supply. They are cheap and square. If you use these you can get square stiff joints in plywood or sheet metal or plate machines. Use the ones with webs they are much stiffer then the simple L type ones. Time to see a sketch of your machine? Peter





  2. #22
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    Default Re: Help with ballpark figures to use for sizing aluminum extrusion for DIY build

    Peteng is right.
    Forget "strength".

    Strength has nothing to do with it.
    Rigidity is everything.

    My 1600 mm bridge mill is a dual-column machine.
    This is 16x stiffer than a typical C frame.

    And I used 20 mm thick steel sections, 200 mm tall, on the bridge, with 35 mm hiwin linear bearings.
    The bridge is 500 kg+ in mass.
    Each column could support 50 metric tons of weight, perhaps 100 tons, and the linear bearings on z are rated for over 50 metric tons, 50.000 kg, push force.
    After 5 iterations and over 10.000 work hours, this is what I ended up with, based on experience.

    My rigidity is somewhere around 40 N/um, pretty good.
    Using 32/5 mm screws.

    Forget the alu and use cast iron flats bolted together, and upsize your materials section area 20x+.

    If you stand on the alu structural members and they bend more than 0.05 mm, it will not work at all well as a mill.



  3. #23
    Member pipe2null's Avatar
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    Default Re: Help with ballpark figures to use for sizing aluminum extrusion for DIY build

    Thanks! I'm slowly working my way through the pile of info. Please excuse the crudeness of these incomplete sketches. I'm having laptop problems and have been going back and forth with Lenovo trying to get it fixed. Plus, just got FreeCAD installed and working on tutorials on how to do FEA.

    At this point I'm trying to figure out what a good first attempt at BASIC frame shape should be that I can use while I learn FEA software and whatnot. The need for a beefy moving platform and attempting to keep the top surface flat/open differentiates this build from the general build designs that I have seen. Basically, I'm starting from scratch with no known reference designs to start from.

    Much of this may prove infeasible, but too early to tell. What makes this frame different (and more PITA to do):
    - Spindle plus stationary CO2 laser tube
    - Typical spindle Z axis plus Z work platform for laser
    - Dual gantries: slow (spindle) and fast (laser head)
    - Attempting to keep top machine surface flat and open like a typical table/bench
    - Rigidity: Designing frame as-if milling is a target, but probably will not meet the bar. Won't know until I progress enough to start costing.
    - Achieving the above criteria enables just about any shop tool to be temporarily/permanently mounted on the common frame and/or work platform. Example: temporarily bolt an inverted radial saw to work platform to use as table saw, adjust platform Z for saw cut depth. And if you're a fan of kitchen sinks, why not add a 3DP extruder to the faster laser head gantry carriage? HAH! Well, the thought crossed my mind, but that is not an official target for this build.
    - Spindle gantry travel is extended a little beyond the front of the primary work envelop so additional tools can be mounted to the front of the frame. Example: drill press plate or auxiliary DIY rotary axis.

    X Axi:
    - Laser head carriage, belt driven
    - Spindle carriage, probably ballscrew

    Y Axi:
    - Laser gantry movement, belt driven
    - Spindle gantry movement, dual ballscrew

    Z Axi:
    - Work platform vertical movement, ACME/threaded rod/equivalent. NOTE: Current thought is angular contact bearings on the bottom and leave top of screw unsupported other than threaded nut attached to platform to maintain alignment.
    - Spindle vertical movement, probably ballscrew

    Colors:
    Black/Grey: Frame
    Green: Upper/slow/spindle and lower/fast/laser gantries
    Blue: Moving work platform with complete Z axis
    Red: Linear guide aka square rails
    Yellow: Threaded rod

    Current Z Work Platform thoughts:
    My current possibly completely wrong thought is that the linear rail and the screws are somewhat independent of each other and can be placed wherever they can optimally deal with their respective loads: linear rail for directional X and Y plus XY moment loads, threaded rod for directional Z plus XZ and YZ moment loads. For threaded rod, perhaps moving them inward in the X direction to reduce the span of the platform. For linear rail, one tentative thought is to place them on opposing sides of the same frame member to make it easy to preload them with a horizontal bolt. More preload more stiffness of the joint, but just a thought. Putting the rail at the corners doesn't make sense to me since most of the moment load with a Z component should be dealt with by the threaded rod... But I could easily be wrong about any of this.


    Any thoughts/advice on platform Z rail and screw placement is appreciated. Note: I'm trying to keep the back part of the frame as open as I can since that's where laser tube, fast gantry with related bits and clearances, and exhaust vents/ducting need to go. Same reason as I'm considering leaving the screw tops unsupported to avoid a horizontal frame member in that crowded zone.

    Attached Thumbnails Attached Thumbnails Help with ballpark figures to use for sizing aluminum extrusion for DIY build-frameroughsketch-v1-jpg   Help with ballpark figures to use for sizing aluminum extrusion for DIY build-railconfig1-jpg   Help with ballpark figures to use for sizing aluminum extrusion for DIY build-railconfig2-jpg   Help with ballpark figures to use for sizing aluminum extrusion for DIY build-railconfig3-jpg  

    Help with ballpark figures to use for sizing aluminum extrusion for DIY build-screwconfig1-jpg   Help with ballpark figures to use for sizing aluminum extrusion for DIY build-screwconfig2-jpg   Help with ballpark figures to use for sizing aluminum extrusion for DIY build-screwconfig3-jpg  
    Last edited by pipe2null; 08-27-2020 at 08:26 PM.


  4. #24
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    Default Re: Help with ballpark figures to use for sizing aluminum extrusion for DIY build

    Hi P2N - You have chosen an elephant and a giraffe to chew at the same time. The closest real machine to this is a knee mill. But the knee gets locked when milling. It is only used to adjust the part height relative to the tool. Co-ordinating 4 corners of a lifting bed accurately will be quite a feat. I have been involved with 4 poster presses and they can present trouble even made to toolmaker micron standards.

    Lifting bed for a laser or 3D printer is fine as there are no contact forces. Cutting metal requires very high contact forces so its usual to move the least things possible. You have about 1 year of development to go, enjoy the journey. Peter You will build two std machines for much less $$$ then this complex multi beastie. You can have a laser stacked above the router for instance and both could co-exist in the same footprint, where your heading is the path and destination of pain. Peter

    Look up Mike F he made a router/mill that bolted to a wall. You could do similar and have laser at one end and router at other. Took him 3 years to sort, buckle in for the long ride.

    https://www.cnczone.com/forums/diy-c.../2849-cnc.html

    https://www.cnczone.com/forums/cnc-w...cad-forum.html

    Last edited by peteeng; 08-27-2020 at 09:12 PM.


  5. #25
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    Default Re: Help with ballpark figures to use for sizing aluminum extrusion for DIY build

    Elephant and giraffe? I'm curious how your analysis resulted in exactly those specific animals, hehe, but no arguments there. Attempting a Z platform like this for a mill makes about as much sense as building an elevator by standing in a stationary box and moving the building around you.

    Well, I need to learn how to do FEA and get used to the software tools anyway, so might as well use the crazy shop-in-a-box design concept to cut my teeth with. Worst case scenario any hope of cutting metal gets scrapped (pun intended), I can use any moving Z platform design work toward a wide format 3DP down the road if I ever venture that way, and I'll learn a lot while debunking my own project. Arguably, the skill of self-debunking is one of the most valuable.

    Peter, I'm curious general types of problems you dealt with when using the 4 poster presses? I'm at least a *tiny* bit familiar with basic difficulties of mechanical bed leveling in terms of 3DP and I've been thinking about the problems of keeping screws aligned or if it is even possible to use multiple steppers (what if only one motor skips a step?). Or use one independent motor per screw and deal with mechanical bed leveling in firmware, but in the end that is leveling and alignment under zero non-gravity load. Like you said, it's a whole lot different with contact force.

    Funny you should liken the design to animals... I've been seriously thinking of naming it "The Platypus" due to the bizarre mashup. Or more likely "The Dodo" since the project will likely go extinct the moment costing is done...



  6. #26
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    Default Re: Help with ballpark figures to use for sizing aluminum extrusion for DIY build

    Hi P2N - two phenomena in particular. backlash and stick slip. Backlash - Trying to get 4 screws to go one way then all agree, stop and go the other way is tricky in terms of displacement control and accuracy. You mention ACME thread which means you will be using a frictioned drive nut (or compensating nut) of some description. Friction can be a friend and an enemy. Here its the enemy because it will create backlash and hysteresis (static friction vs dynamic friction) . In one case of a 4 poster I was working with it would regularly freeze. We extended the bearings well past the 2:1 rule and varied the fits and in the end had to go to linear rails. This was because of stick slip in the bushings. Any bush, drive nut or way that is sliding vs rolling is a candidate for stick slip. This will give you chatter or lock-ups. If your unsure of cutting a bit of wood straight then getting 4 posts parallel and the motion systems parallel, is well outside of the capability statement. Then there is the not so trivial discussion on the machine tolerance you want to achieve. Building in CAD universe is perfect, building in the real world is imperfect. Peter

    https://www.linearmotiontips.com/faq...is-stick-slip/



  7. #27
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    Default Re: Help with ballpark figures to use for sizing aluminum extrusion for DIY build

    Hi - Excellent formply build. Uses aluminium brackets. I like formply, Don't have to paint it just seal the edges. In Melbourne there is a company that sells form ply with coloured coatings. But its too far for me to ship. Local suppliers just have the brown... Peter

    https://www.instructables.com/id/LOW...-500-CNC-MILL/



  8. #28
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    Default Re: Help with ballpark figures to use for sizing aluminum extrusion for DIY build

    Hi Peter. Interesting info on the 4 poster press, very helpful info.

    For the record, I'm not beating a dead horse here. Even if the milling material on top of a movable platform simply will not work, I need to learn the right way to design a moving Z platform for any laser or 3DP builds I do down the road. The original intention for current design was the spindle's Z axis would handle all Z movements while cutting material that is statically held on the not-actively-moving platform, but that idea neglected contact forces and that the platform needs to be just as rigid whether or not it is actively moving during cutting. For my Z platform, I had assumed that going with ACME for moving a load vertically was the right way to go due to backdrive plus not worrying about backlash since gravity acts as an automatic anti-backlash nut, but it appears I was wrong on both accounts. Using 4 screws seemed like a good idea for stability and distributing the load, but I didn't think about inconsistent thread pitch or the issues you mentioned regarding the 4 poster press. I had also assumed that using linear guide/square rails would be required, and perhaps I'm on the right track on that at least?

    The current Z platform "spitballing" configuration I'm scratching my head about is putting a linear guide/square rail at the platform corners (4 rails total) and 1 ballscrew (2 total) on the two opposite short sides aligned/centered with the long axis of the platform. But using exactly 2 balls screws might create an undesirable pivot axis down the middle of the platform. Which brings up the next area of "discovered-undiscovered" knowledge regarding linear rail bearing blocks/cars, AKA I have discovered that I do not know how to relate the specified load ratings of linear guide/square rail in terms of rigidity and displacement, especially when looking at moment load ratings. Using Hiwin HGH20CA for instance, it is rated at 27.1kN dynamic, 36.68kN static, 0.27kN-m radial moment, and 0.2kN-m for both remaining moment loads. Stiffness rating of the specific rail/bearing block combo based on preload makes sense to me ranging from 232N/um to 678N/um, but I do not know how to determine if this specific rail and bearing block combination is capable of countering contact forces with a Z moment component in order to keep the platform level with only 2 ballscrews for vertical support (thus creating the pivot axis for the spindle to seesaw the material/platform over). It feels weird to me to consider using exactly 3 ball screws to vertically support a long rectangular platform, but maybe that would be a better direction.

    Formply build you linked: Interesting. From my perspective, seeing linear guide mounted on T slot mounted on wood like in the formply build makes me think 2 sets of opportunity to get the rail alignment right... I like that. Not sure how much rigidity is gained from the configuration, but might be a good direction for me to go either way. Hmm. The formply build also reminds me of the al/mdf/al laminate with 2mm/16mm/2mm proportions you mentioned earlier in the thread. Is there a rule of thumb on the proportion of materials used in the sandwich, or is it more important to just use 2mm aluminum with some arbitrary thickness mdf in between?

    My laser beam combiners arrived today so I can start prototyping the optical side of things. They are an unusual, harder to find configuration (within budget) and I got a good deal on them, otherwise I would have waited until much later before ordering. Should be interesting, hopefully not too painful to get the mount design going. Even if my unrealistic many-in-one build doesn't work out, I'll end up using these combiners in any laser build I do, so the optics are more or less their own project.

    As always, thanks for the info and advice



  9. #29
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    Default Re: Help with ballpark figures to use for sizing aluminum extrusion for DIY build

    Hi P2N -
    1) Putting bearings on 4 corners creates an "indeterminate" geometry. This means the system is inherently unstable plus you can't hand calculate the forces on the bearings or the separate drives. I suggest you look at knee mills for the solution. This means you will have a sturdy column with two rails on it. The Z axis will cantilever out from the two vertical rails and have 4 cars on it like all platforms do. This can be controlled using one ballscrew. This is determinate and therefore stable and therefore easy to statically analyse on a napkin at your favorite café. What working X and Y where you aiming at? So its like a forklift truck...
    2) The dynamic ratings allow you to calculate the life of the bearing (fatigue). The static ratings tell you how close you are to damaging the bearing (load events). Hiwin publish guidelines on bearing design in their manuals. You will need heavy preload bearings to remove backlash and motion hysteresis in your mill side of things plus improve the bearing stiffness. The laser could use light preload. I always use heavy preload.
    3) If you look at the calculation of inertia I have done before this will tell you how to figure out equivalent rigidity of sandwiches. Form ply is great for a prototype but I'd go with the al/ply/al sandwich if your up for it. If you model the sandwich and "bond" the surfaces then fea should give you the correct deflections. I'm not sure how freecad FE handles connections or assemblies....Cheers Peter

    see the z axis of the big mill image but the Z will be your Z platform not your tool holder.

    combo mil is another way to do it. Gives you very high Z but std XY table.

    edit- ideally the bearings take all forces and the drive screw just moves the payload around. In the case of vertical bearings the drive screw resists vertical loads and the bearings take the eccentric loads.

    Attached Thumbnails Attached Thumbnails Help with ballpark figures to use for sizing aluminum extrusion for DIY build-big-mill-jpg   Help with ballpark figures to use for sizing aluminum extrusion for DIY build-combo-mill-jpg  
    Last edited by peteeng; 08-29-2020 at 10:36 PM.


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Help with ballpark figures to use for sizing aluminum extrusion for DIY build

Help with ballpark figures to use for sizing aluminum extrusion for DIY build

Help with ballpark figures to use for sizing aluminum extrusion for DIY build