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Thread: My carbon fiber CNC gantry

  1. #97
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    Default Re: My carbon fiber CNC gantry

    I saw a post in this thread that mentioned using glass fibre and a layer of carbon fibre. Consider this carefully, the carbon can end up taking all the load. If the carbon is not specced to take all the load it can fail before the glass takes any load, transferring all the load to the glass, which will also fail if not specced for the entire load. Best not to mix carbon and glass on one part, unless you are looking to add impact resistance to a safety part that is likely to get damaged.



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    Default Re: My carbon fiber CNC gantry

    Quote Originally Posted by Zorbit View Post
    I saw a post in this thread that mentioned using glass fibre and a layer of carbon fibre. Consider this carefully, the carbon can end up taking all the load. If the carbon is not specced to take all the load it can fail before the glass takes any load, transferring all the load to the glass, which will also fail if not specced for the entire load. Best not to mix carbon and glass on one part, unless you are looking to add impact resistance to a safety part that is likely to get damaged.

    I would not obsess about the strength of carbon fiber VS fiberglass. The forces from an average diy Gecko G540 set-up are not that great and both materials are regularly used for far more demanding applications involving forces from significantly larger motors.

    Carbon fiber and fiberglass have different but complimentary properties. I.e. Carbon fiber is stiffer, fiberglass has superior impact resistance. They are regularly combined in parts to get the benefits of both. A well made composite of the two is appropriate for a cnc build.

    As with any of the commonly used materials (for cnc machines), you can compensate for differences in strength or stiffness by adjusting part thickness. Steel may be stiffer than aluminum but only if the parts have the same wall thickness. A 1" thick aluminum bar is as stiff as a 1/2" steel one.



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    Default Re: My carbon fiber CNC gantry

    I'm not obsessing about it, and the strength is not really the issue. For example, consider the properties of S Glass vs Carbon. S glass is damned strong, better than Carbon sometimes, but laminating carbon and glass together can give results that you might not expect. The stress can build really quickly in the carbon, leaving the glass doing very little until the carbon fails.



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    Default Re: My carbon fiber CNC gantry

    Quote Originally Posted by Zorbit View Post
    I'm not obsessing about it, and the strength is not really the issue. For example, consider the properties of S Glass vs Carbon. S glass is damned strong, better than Carbon sometimes, but laminating carbon and glass together can give results that you might not expect. The stress can build really quickly in the carbon, leaving the glass doing very little until the carbon fails.

    That simply does not happen with properly made parts unless they are not designed with a wall thickness and shape that is appropriate for the load. Nobody should be trying to cut it close.

    Making cf parts that are strong enough not to break is really not that hard. People here are doing just fine with cnc machines made from far weaker materials like plywood...

    The sort of force required to break a cf tube made with 7 layers of fabric is immense. It is certainly more than the force required to break most of the end mills I use...



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    Default Re: My carbon fiber CNC gantry

    Quote Originally Posted by Goemon View Post
    That simply does not happen with properly made parts unless they are not designed with a wall thickness and shape that is appropriate for the load. Nobody should be trying to cut it close.

    Making cf parts that are strong enough not to break is really not that hard. People here are doing just fine with cnc machines made from far weaker materials like plywood...

    The sort of force required to break a cf tube made with 7 layers of fabric is immense. It is certainly more than the force required to break most of the end mills I use...
    The point I'm making is that laminating CF and Glass together has to be considered carefully - the CF can be taking 90% of the load with the glass just adding weight, because the glass stretches so much more it does virtually no work until the CF fails. Adding a small amount of CF to "add stiffness" can fail badly.



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    Default Re: My carbon fiber CNC gantry

    Quote Originally Posted by Zorbit View Post
    The point I'm making is that laminating CF and Glass together has to be considered carefully - the CF can be taking 90% of the load with the glass just adding weight, because the glass stretches so much more it does virtually no work until the CF fails. Adding a small amount of CF to "add stiffness" can fail badly.
    That is true. I've seen that where people try skinning a wooden object with one layer of cf to "make it stronger". That sort of thing shows a misunderstanding of how the material works.

    I have also seen plenty of examples where people misinterpret or misunderstand things like the Young's modulus when comparing two materials. They see a chart showing that one is higher or lower for strength or stiffness and use it to choose their material.

    It is important to understand that critical part design is far more complicated than any one piece of data. Strength and stiffness is affected by part shape, part design, wall thickness, the manufacturing process, choice of resin etc.... as well as choice of material.

    If you ask most people here, "what is stiffer, a steel or an aluminum cnc machine frame?", most would answer "steel". But, what if the steel frame had a 1/4" wall thickness and the aluminum had 1" walls? What if one used plates and the other square tubes?There are at least as many complexities with composites. With a well made composite, there is synergy between multiple materials. A poorly designed ones could easily see one material drag the other down.



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