I may have under sized my gantry beam.


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Thread: I may have under sized my gantry beam.

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    Default I may have under sized my gantry beam.

    I am building a 4x8 CNC router. My gantry beam is a 65 inch length of 3x4x.125 steel tube. I am using the CNCrouterparts linear rail so I have bolted and epoxied a length of 4x.250 CRS plate to the 3 inch wide side which is the vertical axis. I am beginning to think that this tube, even with the .250 CRS plate glued to one side may not resist twisting as well as I had hoped.
    I am hoping to put a 2.2kw spindle on it.
    So I have two questions.
    Is this beam strong enough?
    If not, would welding a length of 2.5x2.5x.375 steel angle across the rear vertical surface fix the issue?
    See attachment.
    ThanksI may have under sized my gantry beam.-gantry-jpg

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    Attached Thumbnails Attached Thumbnails I may have under sized my gantry beam.-gantry-jpg  
    Last edited by tkms002; 12-14-2018 at 02:19 PM. Reason: new drawing


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    Default Re: I may have under sized my gantry beam.

    It is likely marginal but a lot depends on additional information. One piece is what do you expect to machine The second is how high are you in Z. Perhaps most important is what are your expectations out of this machine.

    As for the angle iron if you do have a problem I wouldn’t try to correct it this way. Most likely I would just buy another beam of larger cross section. In any event I wouldn’t stress over it until you have considered how the machine will be used. I can see a machine with such a gantry beam as being usable in a number of circumstances. However if your goal is to push the machine hard in difficult materials and at the same time expect high quality you likely will not be happy.



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    Default Re: I may have under sized my gantry beam.

    OK I have about 6 inches travel in my z axis. I want to route aluminum but I wouldn't mind building a raised table for this to reduce the z twisting radius.
    If that is no good, My beam needs to be 3 inches in the vertical axis but could be larger than 4 inches in the horizontal. How thick of material do
    I need to make this work?
    Thanks



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    Community Moderator Jim Dawson's Avatar
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    Default Re: I may have under sized my gantry beam.

    As I recall a MultiCam 5x10 router has a 6x10x0.25 wall gantry beam. My 4x8 router had a 10x10x0.375 wall gantry beam. Both have about 6 inch Z

    Not sure why you are limited to 3 inch in the vertical, you could always space the rail off of the beam a bit for clearance if needed.

    Jim Dawson
    Sandy, Oregon, USA


  5. #5

    Default Re: I may have under sized my gantry beam.

    I probably would have gone with a thicker material, but why not try it as-is? You might be ok with it.

    If it does let you down, then you’ll be able to interpret better what is wrong and then apply a better, more directed fix.




    Sent from my iPhone using Tapatalk



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    Default Re: I may have under sized my gantry beam.

    You could possibly reinforce the beam with a X pattern of material down the inside of the existing beam.



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    Gold Member LeeWay's Avatar
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    Default Re: I may have under sized my gantry beam.

    It will flex and flex alot. Especially in the middle. It isn't really worth the effort to try and beef it up. It would be fine for a plasma cutter, but not for a machine with cutting forces. I don't think twist is the only concern. Up and down flexing would be just as bad.
    I would honestly start over.
    I have about a 5' wide gantry on my plasma cutter. It also uses a die grinder for engraving. I have used it to machine polycarbonate before as well. While I did get usable results, they were not perfect. The depth was a little hard to control and required a finish pass. I was using straight flute bits too. If I were to use a cutter with a helix, it would actually push or pull the tool deeper than wanted.
    My gantry beam is 3" x 3" aluminum extrusion with the 1/4" x 4" flat steel bar bolted to it. It works great at plasma cutting and engraving. Lots of easy speed.
    Not optimal as a router.

    I don't think I would rely on adhesive only for a structural joint either. I would back it up with at least a few mechanical fasteners. Especially given that this beam will be flexing.

    Lee


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    Default

    Well I have to admit I never thought that .25x4 plate would flex in the verticle. CNCRouterparts makes their 4x8 on a 3x3 chunk of 8020. Maybe that is a lot stronger. I looked at speedy metals for a 5x5x.25 tube and the price was over $300. I have to find a cheaper way. Thanks for all your input.



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    Default Re: I may have under sized my gantry beam.

    CNCRouterParts uses a 160mmx80mm about 6inch x 3inch aluminum extrusion.



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    Gold Member LeeWay's Avatar
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    Default Re: I may have under sized my gantry beam.

    Cost can certainly be an issue. You might want to proceed with the build and upgrade later.
    You can setup an easy test to see what you have. Support the beam on both ends.
    Put a dial indicator in the middle. Then add a little weight. I think you will be surprised by how much movement you see.

    Lee


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    Default Re: I may have under sized my gantry beam.

    Speedy metals is expensive.
    You can get a 3/16 4x6 for a lot less.
    https://www.onlinemetals.com/merchan...41&top_cat=849

    Regardless, you'll want to reinforce the beam, maybe like this one.
    https://www.cnczone.com/forums/cnc-w...ml#post1689518

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: I may have under sized my gantry beam.

    Quote Originally Posted by nlancaster View Post
    CNCRouterParts uses a 160mmx80mm about 6inch x 3inch aluminum extrusion.
    the new ones do, yes. But at the school I mentored a robotics club at we bought one several years ago (a 4848) and it has a 3x3 gantry beam.



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    Gold Member LeeWay's Avatar
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    Default Re: I may have under sized my gantry beam.

    I designed my plasma cutter the same way. It does use CRP trucks and drive components. The First design they had uses roller skate bearings on flat bar rather than V wheels and later profile linear rails and trucks.

    Lee


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    Default Re: I may have under sized my gantry beam.

    Quote Originally Posted by tkms002 View Post
    the new ones do, yes. But at the school I mentored a robotics club at we bought one several years ago (a 4848) and it has a 3x3 gantry beam.
    The 3x3 heavy extrusion is quite a bit stiffer than the 1/8 in wall tube by itself. It has the added benefit of all the inner webbing to help support it.

    Lee


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    Default Re: I may have under sized my gantry beam.

    I have never seen a CNCROuterparts that used less then 3x6inch beam for the 4ft axis, they had plans for a machine that was 24x36 with a 3x3 beam. And I have been reading about cncrouterparts since he started by just making and selling his skate bearing units.



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    Default Re: I may have under sized my gantry beam.

    Quote Originally Posted by tkms002 View Post
    Well I have to admit I never thought that .25x4 plate would flex in the verticle. CNCRouterparts makes their 4x8 on a 3x3 chunk of 8020. Maybe that is a lot stronger. I looked at speedy metals for a 5x5x.25 tube and the price was over $300. I have to find a cheaper way. Thanks for all your input.
    How much flex you get will be a direct function of how you use the machine. In this case it isn’t so much as flexure in one direction but rather the combination of flex in all directions and twist. If you are this far along you might as well finish the build. Since “machine aluminum” means different things to different people you can always try and see if it meets your expectations.

    As for extrusions do not assume that they are stronger or even a good solution. More so You can’t rely upon the opinion of others as we comeback to the question of what it means to machine aluminum and what your expectations are.

    As for steel beams don’t buy from online vendors known for jacking up the prices few to the extreme. Find a local steel supplier and get your metals at more reasonable prices. If you are in the Rochester NY area, Klein is moving their metal outlet store where they sell a lot of drops, due to the move stuff is going at a discount. In not sure what the discounted price will be but they expect to be gone by the end of the month and setup in a new facility. Of course there is the issue of luck and the right beam being there.

    For what you are talking about here, an ideal steel beam would be at least 8x8 inches and relatively thick. The thickness is more about screw holding and machining allowance than anything. In any case the beam itself needs bracing to prevent it from going parallelogram like under load and to help resist twisting.

    There is a sticky thread that goes deeply into gantry beam design that would box be well worth reading.



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    Default Re: I may have under sized my gantry beam.

    Quote Originally Posted by LeeWay View Post
    The 3x3 heavy extrusion is quite a bit stiffer than the 1/8 in wall tube by itself. It has the added benefit of all the inner webbing to help support it.
    This likely should be qualified a bit as there are huge differences in extrusions from one manufacture to another. Even what is considered to be a heavy extrusion varies a bit.



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    Gold Member LeeWay's Avatar
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    Default Re: I may have under sized my gantry beam.

    This was what I call the standard. 80/20, but I know there are many sources. But when discussing CRP machines, they use 80/20 exclusively as far as I know.
    80/20 has a light version extrusion and what I call heavy. My term. If one is light/lite, then a thicker extrusion in similar dimensions would be heavy.
    I haven't seen anything CRP related that uses a lite variety of extrusion.

    Lee


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    Default Re: I may have under sized my gantry beam.

    Quote Originally Posted by nlancaster View Post
    I have never seen a CNCROuterparts that used less then 3x6inch beam for the 4ft axis, they had plans for a machine that was 24x36 with a 3x3 beam. And I have been reading about cncrouterparts since he started by just making and selling his skate bearing units.
    Well, we have one at Arrowhead High School in Hartland Wi.



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    Default Re: I may have under sized my gantry beam.

    Hmm Thought I posted this already but I dont see my update. I put my beam on the concrete floor supported at both ends and put a dial indicator against the top horizontal surface.
    I put a 30lb dumbbell on it in the middle and got .005 inches of flex. I get about .001 inches when moving my z axis across it with no spindle on it so the question is what is too much flex and what should I shoot for?
    Thanks



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I may have under sized my gantry beam.

I may have under sized my gantry beam.