I may have under sized my gantry beam. - Page 2


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Thread: I may have under sized my gantry beam.

  1. #13
    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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    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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    Community Moderator ger21's Avatar
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    Default Re: I may have under sized my gantry beam.

    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?
    That's something only you can answer. Different people have different expectations.

    The bigger issue is that it's going to twist when that same 30lb load is pushing on it from the end of the Z axis.
    And the sag or twist isn't the only issue. The fact that it can move, means it can vibrate as well, causing poor quality cuts.

    Gerry

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    (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.

    Right.
    That was static weight too. Add the weight o the Z axis as well. Then when you add in the driving forces of the weight of the whole gantry, it uses that weight against itself when it changes direction abruptly. Even though it is relatively light, the span makes it susceptible to many different types of error at the tool. I would venture to say the tool itself would not be rigid enough to get any kind of tool life from carbide tooling in aluminum. Anything less than rigid is going to quickly be fatal for carbide.

    Lee


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

    Quote Originally Posted by LeeWay View Post
    Right.
    That was static weight too. Add the weight o the Z axis as well. Then when you add in the driving forces of the weight of the whole gantry, it uses that weight against itself when it changes direction abruptly. Even though it is relatively light, the span makes it susceptible to many different types of error at the tool. I would venture to say the tool itself would not be rigid enough to get any kind of tool life from carbide tooling in aluminum. Anything less than rigid is going to quickly be fatal for carbide.
    OK so how do we define rigid?



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

    I think it may be easier to describe what is not rigid.
    And again it is about what the machine is designed to do and it's size as well as speed and accuracy while doing that.
    On the gantry in question, it might be able to machine aluminum up next to one of the gantry supports rather well if the spindle is strong, fast and we have very low backlash and run out.
    If cutting full size material, you will not see the same cut quality across the entire span. Cut quality will diminish toward the center and get better when approaching the opposite support.

    Lee


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I may have under sized my gantry beam.

I may have under sized my gantry beam.