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Thread: My carbon fiber CNC gantry

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    Default Re: My carbon fiber CNC gantry

    Quote Originally Posted by ack1 View Post
    This is really interesting. I have a Multicam router with 2x3 meter work area so the gantry is very long and flexes if i engrave too fast so ivve been thinking about how to stiffen it up.

    Will be following closely!
    What is the gantry made of and what are you cutting when it flexes?

    Those Multicam branded machines look well made in the pics. The gantry beams look thick, it's surprising that flex would be a noticeable issue.



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    Default Re: My carbon fiber CNC gantry

    Quote Originally Posted by Goemon View Post
    What is the gantry made of and what are you cutting when it flexes?

    Those Multicam branded machines look well made in the pics. The gantry beams look thick, it's surprising that flex would be a noticeable issue.


    This is my machine when i got it.


    Side view of the gantry with the cap off. IF i remember correcly steel was about 8mm thick.



    Sometimes i do whole alu sheets of engraved pieces and if I go too fast (gantry makes a sudden change from forward to reverse or vice versa, in the Y direction) the gantry flexes just by its own. Specially if im in the middle of the table.
    The gantry span is somewhere around 2.3meter so its not surprising.
    These numbers are very small but you can cleary see what i mean. Look at number 7.




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    Default Re: My carbon fiber CNC gantry

    This is an example of engraving. In the last few seconds i pan out and you see im only half way of a full sheet and it takes forever. And I cant go faster than this basically.




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    Default Re: My carbon fiber CNC gantry

    Quote Originally Posted by ger21 View Post
    My biggest issue with most CNC parts and kits for sale, is that the people selling them don't know what they are doing in most cases. Not only can you usually build the same thing for less money, in most cases, you can build something better for less money.
    That's so true.

    There is some Canadian company selling a "heavy duty gantry kit" on eBay right now for $4,000... you can see in the pic that it's just a couple of aluminum extrusions and two of the thinnest looking aluminum plate risers I have seen.

    Now... I'm no communist... a company has to make some profit but.... for that sort of cash I would expect some quality custom design with thick steel components and I certainly wouldn't want to see a list of things it didn't include....

    You can buy 1000mm aluminum extrusions for $10....



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    Default Re: My carbon fiber CNC gantry

    Quote Originally Posted by ack1 View Post
    I think there may be a number of reasons why this is happening. I'm skeptical that this all because of your gantry tube flexing. Interesting that they don't have any ribs in the gantry tube. Could this be caused by servo over and undershoot? The straight lines look almost like sine waves. My first guess would be servo over / undershoot, and the lines that look like sine waves is the closed loop control trying to bring things back in order each time it misses the mark.

    I've been reading some of the posts here, so here's my 2 cents.

    Lots of mention of strength. Strength doesn't matter when building a CNC machine. If you're designing for strength you've already failed because stiffness is the important factor. The modulus of elasticity and the geometry of the tube determine a tube's stiffness.

    The stiffness to weight ratio of carbon fiber is going to be superior to steel.

    The rotor inertia of the motor, any gear reducers, or ball screws will still be need to be accounted for, and these use a significant amount of torque to accelerate, so you can get better acceleration for a given motor with a lighter gantry but the improvement will be far less than you might at first assume. Also, there may be a need to consider over and undershoot of the system if the load to motor inertias have too much of a miss match.

    I can see carbon fiber gantries being beneficial in a couple instances.

    If you can make molds that are easy to mass produce parts that are nice and flat for a reasonable price. Shipping would be less expensive than a comparable steel part as well. Or for the DIY person, if you can make inexpensive molds that have good mounting surfaces out of wood for example, perhaps that could be easier than working with steel or aluminum.

    Also with a mold, you have more ability to change the geometry, for example, to mount your motor and run a ball screw in between the rails in such a way that you don't have a large offset from the gantry structure. Big commercial machines can do this with custom castings. Working with square steel tube sections, this is difficult to come up with good easy ways to do the same thing.

    Or for gantries that have a very long span. The deflection of a beam is determined by the length cubed, so the relationship is not linear. For a long gantry on a specialized machine, the superior stiffness to weight ratio of the carbon fiber could be a good design choice.

    I wish I had more experience working with carbon fiber, not for building a CNC machine, but for some other projects.

    Goeman,

    It's interesting what you've come up with. It looks like your gantry is more of a flat plate than a box so far. Are you going to add more pieces to it? It's definitely nice that you're pioneering this and sharing your results. I'm sure many people on this forum, including myself, are looking forward to seeing more posts from you.



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    Default Re: My carbon fiber CNC gantry

    Quote Originally Posted by NIC 77 View Post
    I think there may be a number of reasons why this is happening. I'm skeptical that this all because of your gantry tube flexing. Interesting that they don't have any ribs in the gantry tube. Could this be caused by servo over and undershoot? The straight lines look almost like sine waves. My first guess would be servo over / undershoot, and the lines that look like sine waves is the closed loop control trying to bring things back in order each time it misses the mark.

    I've been reading some of the posts here, so here's my 2 cents.

    Lots of mention of strength. Strength doesn't matter when building a CNC machine. If you're designing for strength you've already failed because stiffness is the important factor. The modulus of elasticity and the geometry of the tube determine a tube's stiffness.

    The stiffness to weight ratio of carbon fiber is going to be superior to steel.

    The rotor inertia of the motor, any gear reducers, or ball screws will still be need to be accounted for, and these use a significant amount of torque to accelerate, so you can get better acceleration for a given motor with a lighter gantry but the improvement will be far less than you might at first assume. Also, there may be a need to consider over and undershoot of the system if the load to motor inertias have too much of a miss match.

    I can see carbon fiber gantries being beneficial in a couple instances.

    If you can make molds that are easy to mass produce parts that are nice and flat for a reasonable price. Shipping would be less expensive than a comparable steel part as well. Or for the DIY person, if you can make inexpensive molds that have good mounting surfaces out of wood for example, perhaps that could be easier than working with steel or aluminum.

    Also with a mold, you have more ability to change the geometry, for example, to mount your motor and run a ball screw in between the rails in such a way that you don't have a large offset from the gantry structure. Big commercial machines can do this with custom castings. Working with square steel tube sections, this is difficult to come up with good easy ways to do the same thing.

    Or for gantries that have a very long span. The deflection of a beam is determined by the length cubed, so the relationship is not linear. For a long gantry on a specialized machine, the superior stiffness to weight ratio of the carbon fiber could be a good design choice.

    I wish I had more experience working with carbon fiber, not for building a CNC machine, but for some other projects.

    Goeman,

    It's interesting what you've come up with. It looks like your gantry is more of a flat plate than a box so far. Are you going to add more pieces to it? It's definitely nice that you're pioneering this and sharing your results. I'm sure many people on this forum, including myself, are looking forward to seeing more posts from you.

    My gantry beam is being made in two parts. I haven't finished the rear half yet so all you can see in the pic so far is the face plate which will hold the linear rails.

    The face plate itself is not a flat structure. If you look at it from the rear, it has small crenellations which are almost like a couple of steps that follow the contours of the two steel plates (which are sandwiched between the layers of cf). There is around .75" of solid compressed carbon fiber as it stands today so that plus the choice (and orientation) of the materials makes the face place extremely rigid on it's own.

    Even though the face plate is likely more than stiff enough for the purpose on it's own, the bulk of the stiffness will come from the rear half and the design of the filler that connects them.

    Specifically, there is a layer of 1" long x .75" diameter cf tubes with .25" wall thickness made from 4x4 twill. 120 of these little tubes will be mounted at 90 degrees to the rear of the face plate and embedded / encapsulated in a mix of cf strands wetted out with epoxy (to fill the gaps between tubes). Behind the 1" tubes will be 6 additional tubes mounted parallel to the face plate that run across the whole length of the gantry beam. Aside from their length, the long tube specs are the same as the short tubes. This tube filler will be sandwiched in between another face plate of similar specs to the front plate.

    I haven't decided yet but I am considering filling the inside of the tubes with urethane rubber to help dampen vibrations. I make the tubes using a roll wrapping process (which is very easy).

    Obviously I will need to test my design but I am fairly confident that it will have enough stiffness and strength. My thinking is based on my general experience that rounded edges and cylindrical shapes produce maximum stiffness in cf parts. Plus, I can orient the weave in the tubes in a way that delivers maximum stiffness in the directions of the forces the gantry beam will be subjected to.

    The way I see it, being able to create custom designs to put the strength where you need it most, is the key advantage to carbon fiber or fiberglass parts, so I'd be foolish not to make the most of it. Plus... I think a little extra creativity in the design is needed for long flat parts like a gantry beam. Long flat surface aren't naturally optimal for stiffness.

    For anyone interested in designing their own cf gantry, those Compotech guys have a section on their website which describes their approach to engineering cf gantry beams and other cf machine parts for max stiffness. While I don't think that their cf gantry beam designs are the best they could possibly be, they have some interesting ideas that are worth borrowing, copying and steeling (like incorporating tubes into the inside walls of their beams).

    As a general point, the only way to get experience with carbon fiber is to have a go at making something. It's a lot of fun and not overly difficult if the pattern is chosen wisely. By "wisely" I mean avoiding anything with small details, deep crevices and tight sharp corners until you have more experience. You would need to have a general interest though. The investment required to get started is not huge but you wouldn't want to buy all the resins, mold making materials, mold release products and polishes etc without a good reason.



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    Default Re: My carbon fiber CNC gantry

    NIC77, its accually running steppers. And the way the phenomena is most prominently present at the middle of the gantry makes me somewhat certain its flexing back and forth after a hasty change of direction.

    EDIT: I feel we're a bit OT discussing my gantry by now so in closing i'd just like to add that if anyone has any tips on how to best stiffen this sucker up, shoot me a pm and i'd be most grateful.



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    Default Re: My carbon fiber CNC gantry

    If you mount a dial indicator under the middle of the gantry beam and put some weight on it, you can measure the deflection from the horizontal. That should tell you whether the irregularities in those letters are due to flex in the gantry or something else. If there's not enough deflection to account for your issues, then do the same thing with the spindle. To me, it looks more like something is flexing or oscillating side-to-side than up-and-down. It could be that something's come loose somewhere.

    Andrew Werby
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    Default Re: My carbon fiber CNC gantry

    Quote Originally Posted by Goemon View Post
    What is the gantry made of and what are you cutting when it flexes?

    Those Multicam branded machines look well made in the pics. The gantry beams look thick, it's surprising that flex would be a noticeable issue.
    From the pictures it looks like the Multicams are well made for the most part but those gantry beams just look spindly to me. For the most part gantry beams are the weak links in most router designs (this includes the beam supports). This is one reason why I'm following your thread closely to better understand how your beam design will work. Generally you would want a rather large cross sectional area, a big box beam if you will, to get the required stiffness. This is the same no matter what material you make the beam out of.

    When it comes to a laminated carbon fiber beam I simply don't have the experience to say how one with a thin cross section will work in real life. You really can't tolerate much twist or deflection in the beam else you compromise with respect to the amount of driving force you can apply. That is you end up having to lower feed rate or more likely cut depth to avoid beam deflection.

    In any event keep posting, I'm learning something new overtime I view this thread.



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    Default Re: My carbon fiber CNC gantry

    Awerby, it is flexing side to side. Well front to back if you are standing in front of the machine.



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    Default Re: My carbon fiber CNC gantry

    Glue some ribs inside the gantry tube. They can be as simple as plywood, with a good quality construction adhesive. Make sure they are a snug fit.
    3-5 ribs evenly spaced should make it quite a bit more rigid.

    Gerry

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    Default Re: My carbon fiber CNC gantry

    Quote Originally Posted by wizard View Post
    From the pictures it looks like the Multicams are well made for the most part but those gantry beams just look spindly to me. For the most part gantry beams are the weak links in most router designs (this includes the beam supports). This is one reason why I'm following your thread closely to better understand how your beam design will work. Generally you would want a rather large cross sectional area, a big box beam if you will, to get the required stiffness. This is the same no matter what material you make the beam out of.

    When it comes to a laminated carbon fiber beam I simply don't have the experience to say how one with a thin cross section will work in real life. You really can't tolerate much twist or deflection in the beam else you compromise with respect to the amount of driving force you can apply. That is you end up having to lower feed rate or more likely cut depth to avoid beam deflection.

    In any event keep posting, I'm learning something new overtime I view this thread.
    In some respects carbon fiber is like any other material in that a thicker part will be stiffer than the same part design with fewer layers but it differs greatly from metals and woods in how the part design, choice of weave and orientation of the lay-up effects strength and stiffness.

    Essentially, it is easily possible for a 2" thick cf part to be stiffer than a 3" thick part that uses the same volume of cf.

    My finished gantry beam will end up being around 3" or maybe 4" thick. It is being designed in a way that maximizes stiffness and strength in the direction of the forces it will be subjected to.

    I will find out for sure when I start testing but my assumption is that forces hitting the center of the gantry at 90 degrees would present the greatest stiffness challenge. I.e. You don't want it to bend in the middle. This is why I am using carbon fiber tubes to reinforce the beam at 0 and 90 degrees.

    The other thing that is key to my design is the choice of using random matrix fabric for at least half of the thickness. Woven fabrics provide superior tensile strength and directional stiffness but the random matrix fiber's provide multi-directional stiffness. The mix of both types of cf produces really strong and stiff parts. I'm a huge fan of this approach. It's really cost effective too as the random matrix fabric costs 4 times less than woven 2x2 twill.

    Another important area where cf differs from most metals is that it doesn't bend much before it breaks. With steel, there is a noticeable flex / bend as a warning before a part breaks. Cf parts just snap if the load exceeds their capability so it's important to build in more strength than is necessary so there is a good margin of safety. If there is any noticeable flex at all then the part is almost certainly too weak to be safe imo.



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