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  1. #21
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    Re: 150x100cm Steel frame router build

    Your design is much better than flat plates
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)

  2. #22
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    Re: 150x100cm Steel frame router build

    Hi Nico - Thin square tubes are very stiff globally but not locally. You are better off having a very thick angle in this position then a thin tube. This area is shear dominant and you need thickness to stop this area deflecting. If you do want to use those tubes then they need capping so they do not lozenge... but I'd go a thick angle, thickest I could get.

    Similiar theme - the pads that the rails are on need to go out to the sides. The little bit in between the radius and the pad deflects quite a bit relatively when you analyse it. Now is your chance to stiffen it.... Cheers Peter

  3. #23

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    Re: 150x100cm Steel frame router build

    I haven't really thought about local vs global rigidity. Thanks for that idea.

    I did some quick simulation in F360 to see what will happen if I used angle iron vs tubing. The results confirm what you said. I didn't cap the ends to exaggerate the deformation. I think I'll go with 10mm thick angle iron with capped ends (10mm is enough for tapping holes in the ends for m5-m6 screws so I don't have to weld the caps). And a big bonus is that angle iron is a lot easier to work with in this situation.

    For simulation I used 2000N of side force (along the X axis). 2000N in Y direction didn't show any noticeable deflection in the side risers, it was mostly in the gantry tube.

    I'm getting a bit worried about the ever increasing weight. As it stands now, the gantry without the Z axis weighs 70kg/150lbs. I thought about using dual Nema34 9Nm motors in a 2:1 belt reduction setup to have at least 10 meters/m and without going over 500-600rpm. But whatever... I'll design for stiffness and worry about the drive part later.
    Attached Thumbnails Attached Thumbnails 2000N .jpg   Side load.jpg   Side load close up.jpg   Angle iron.jpg  


  4. #24
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    Re: 150x100cm Steel frame router build

    Morning Nico - 2000N is 200kgf. This is an unlikely tool load. I use 1000N as a design load a nice round number. To help optimise your design using F360 you need to pick a target machine static stiffness. VMC's run at 150N/um small routers are <10N/um. Some small hobby mills are ~2N/um. If you aim at 25N/um then you will probably in reality achieve >10N/um which is quite stiff and will cut aluminium. So taking your current numbers 2000N in "push" direction deflects 158um. So the static stiffness is 2000/158= 12.7N/um which is quite stiff for a router but your model does not include bolts, bearings etc which are compliant. So you may get 6N/um in reality. So the point is pick a machine stiffness target and this will allow you to decide where you need to gain stiffness.

    The next thing to understand is structural loops. The usual loops on this sort of machine are the Z axis, the saddle, the gantry, the base. Each bit or loop will contribute to the global stiffness. The Z axis usually contributes most as its a cantilever. I hold the Z axis bearings and load that to calculate the loop deflection, then hold the saddle bearings (release the z axis restraint) do same, then the gantry bearings do same. In this way you can figure the proportional stiffness of each loop. Then the loop with the most deflection is the area you need to put work into. For instance my current design the Z axis deflects 30% the gantry is 30% and the base is 30%. Initially the Z was 50% of total so had to work on that. Keep making Peter

    Re: weight - you do have the tools to reduce weight, define what you want to cut then you will be able to define a design machine stiffness then you have a target to chase to reduce weight yet achieve stiffness. You did pick steel which is the "heaviest" material!!

    Its been very difficult getting info on modern machine stiffnesses especially routers. The image attached is from a very old research doc and modern machines are much stiffer then his chart suggests. I have sent inquiries to various toolmakers but they don't answer the question. They specify everything else it seems odd that they would not specify machine stiffness when its such an important thing. One large machine maker actually replied that they do not know!!

    Looking back at your #1 post you want to cut hardwood and Al. Al is the killer so you will need a machine of at least 5N/um and I would aim at a machine design stiffness of 15N/um maybe 25N/um up front and see how you go. You will not achieve commercial mill stiffness values.

  5. #25
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    Re: 150x100cm Steel frame router build

    Hi Niko - 10m/min is easy to achieve even with small motors. Its the acceleration to get there that requires the torque. Can't do the motor sizing until you know the structure masses and what accel you want/need so keep at it Peter

  6. #26

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    Re: 150x100cm Steel frame router build

    An interesting gantry design I've found online.

    I haven't seen anyone use triangular gantry even though a triange is a strong shape that resists change in it's shape more than the rectangular/square beam.

    Are there any obvious flaws with this design that I'm overlooking? Or is the lack of this design being used just a result of the lack of the availability of this beam; or that you have to produce it yourself from plate.

    I don't have the time now to do any simulations, would a 200x200mm triangualr beam be stronger than 200x200mm square tubing?


  7. #27
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    Re: 150x100cm Steel frame router build

    Hi Niko - There are commercial machines that use triangles and it is a very good design. I've done a lot of analysis on gantries and if you can make the shape then its a good shape to use. Its not about strength its about stiffness. A Sq or rectanglar tube "lozenges" due to the shear loads. This means you have to use thick walls or thick corners to stop this., The tri design is locally stiffer in this case. Cheers Peter

  8. #28

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    Re: 150x100cm Steel frame router build

    I guess the easiest way is to laser cut all the parts and than weld them and heat threat it?

    Or maybe CNC bending a flat sheet while leaving the ends in such a way as to bolt them together...

    I now see why I haven't seen this design anywhere

    Sent from my SM-G970F using Tapatalk

  9. #29
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    Re: 150x100cm Steel frame router build

    Hi Niko - If you are using Fusion360 you can run a simulation to see what an optimum shape maybe. I haven't done it on a gantry but plan to. Peter

  10. #30

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    Re: 150x100cm Steel frame router build

    I think Peter said in another thread that it's better to spend 500 hours in CAD than to spend that time in the workshop and thousands of dollars to figure out that something is wrong with the design.

    Since my decision to mount the rails on the side I've investigated the accuracy of the machine that will make those sides parallel. It turns out the machine isn't that accurate; across the whole length they can't guarantee parallelism of more than +- .3mm. If my gantry was made from aluminium that could probably work, but with a rigid steel gantry I think that error would put a LOT of pressure on the bearings. I could shim the rails to make them parallel but what was the point of machining them?!

    So I've came up with another design. I'll still use their machine (since they'll do it for cheap for me) to make my construction flat for mounting linear rails. I think this is an interesting design. It prevents a big problem most routers have and that's sawdust and chips falling on the rails and ballscrews. Here they are completely covered.

    But I have a dilemma. I hate that I have this cantilevered portion of the table top. I intend to use 1" - 1,1/2" aluminium plate here (20-30mm). Is this going to make some problems that I'm not aware of? Only during drilling operation you have a straight down force on the table...

    I could raise the top an inch more and bolt some angle iron underside the table to give it some support against flex and vibration...

    What are your thoughts?
    Attached Thumbnails Attached Thumbnails Without top.jpg   Topless.jpg   CNC front close up.jpg   CNC front.jpg  

    Close up 2.jpg  

  11. #31
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    Re: 150x100cm Steel frame router build

    Hi Niko - Its a good design and used on commercial machines but the columns get longer and less stiff. Your Z axis will be quite wide and you won't be able to get over to that area to "push" down on. But it will make a good clamping area or rest space for parts. If its 1" plate it will be very stiff but check if the tool can actually get here. Here's a similar bed idea... Peter

    I have run some shape generations in Inventor for fun and if you start with a square solid it makes it hollow with thick corners. It keeps the ends on ie capped and it makes a rib behind the bearings from one side to the diagonal other. This can't be done in reality as the Z axis moves. I have not started with a triangular section yet to see what happens. I have Inventor which runs the same isogeometric analysis as F360 does. But why not go high rail? (like you had originally sort of medium rail) makes it stiffer and puts the rails and cars up out of the muck?? The sides contain dust and all round I think its a better config. Its remarkable the stiffness you gain for not much effort using high rails...Just means you have to load from the front... or rear

  12. #32
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    Re: 150x100cm Steel frame router build

    Chips and dust are not that big of a problem.

    Don't make a weak design just to try to avoid dust.

    Linear bearings have good seals. You can cover them with bellows.

    You should trap the dust at the source.
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)

  13. #33
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    Re: 150x100cm Steel frame router build

    Quote Originally Posted by pippin88 View Post
    Chips and dust are not that big of a problem.

    Don't make a weak design just to try to avoid dust.

    Linear bearings have good seals. You can cover them with bellows.

    You should trap the dust at the source.
    This is good advice,not just for the health of the bearings,but also for the good of the people around the machine.It isn't too hard to arrange extraction from a brush sealed zone around the spindle and there are all sorts of designs out there if you do a little searching.If you work with several different thicknesses of material,you will have to use slotted mounts and adjust to suit.

    DIY project: Some cool dust shoe designs
    https://www.instructables.com/id/CNC-router-dust-boot/

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