Epoxy granite in metal frame.


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    Default Epoxy granite in metal frame.

    Hi guys
    I am in the process of designing a frame for cnc gantry machine. I have this thought and I would like to ask if it is gonna work as I am thinking or if there is something I got wrong.
    My thought is to weld square metal tubes to build a frame for the table. On the top surface of the sides of those square tubes 2 flat steel plates will be welded too and those 2 surfaces will be where the linear guide rails will be bolted. Then cast epoxy granite inside of this frame. After the casting I think that the frame will be solid with no possible flex at all. Then I will get the surfaces of the steel plates for the linear guides ground flat and weld feet on the bottom side of the frame with the proper reinforcements. And this will be the frame for the gantry.

    Does it sound like a solid plan?
    Will the epoxy granite casting inside the frame make the frame solid so that after grinding the plates flat I can be sure they will stay flat and get no bend at all later?
    I attach a photo with a draft design of what I think.

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    Default Re: Epoxy granite in metal frame.

    Anyone any thoughts??



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    Default Re: Epoxy granite in metal frame.

    I don't know anything, but I've had the same thought. The added weight has to be a plus, and it should certainly damp any vibrations. But I wonder if you wouldn't still need to stress relieve the welded assembly first to ensure stability?



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    Default Re: Epoxy granite in metal frame.

    Well I was hoping that epoxy granite that would be cast into the frame would not allow the frame to move even if it has stress. Also I am thinking of welding the frame and let it sit for a while, like a month or two and then proceed with the casting, but there are welds that need to be done after the cast.



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    Default Re: Epoxy granite in metal frame.

    Quote Originally Posted by jimskeet View Post
    Hi guys
    I am in the process of designing a frame for cnc gantry machine. I have this thought and I would like to ask if it is gonna work as I am thinking or if there is something I got wrong.
    My thought is to weld square metal tubes to build a frame for the table. On the top surface of the sides of those square tubes 2 flat steel plates will be welded too and those 2 surfaces will be where the linear guide rails will be bolted. Then cast epoxy granite inside of this frame. After the casting I think that the frame will be solid with no possible flex at all. Then I will get the surfaces of the steel plates for the linear guides ground flat and weld feet on the bottom side of the frame with the proper reinforcements. And this will be the frame for the gantry.

    Does it sound like a solid plan?
    Will the epoxy granite casting inside the frame make the frame solid so that after grinding the plates flat I can be sure they will stay flat and get no bend at all later?
    I attach a photo with a draft design of what I think.
    I think you're going about this a little backwards and probably making more work for yourself without appreciable benefit.

    Your square tube will not be flat, especially after welding. Your flat plates will not be flat once welded to your non-flat tubes. You will need to grind the mounting surfaces flat once the welding is complete and any stress relieved. That will be necessary for any welded steel frame.

    It's not clear how you mean to fill the base - each tube individually or by encasing the whole assembly. The first method will be a real pain because you'll have to seal one end of each tube and somehow compact the granite down the entire length. You'll also need to wait a day between the tubes oriented X vs those Y. Although you will have increased the mass, and thus energy absorption, it won't be optimal.
    The second method needs you to build a mold to contain the entire assembly. I'm not sure what role the tube really plays then - you could have just embedded the mounting plates in a casting and saved yourself a bunch of welding hassle.

    There is one place where I could see this type of construction being useful - using the second method to embed a frame within concrete. It's a cheap and easy improvement to the bare frame, but you need the steel's properties because concrete by itself won't cut it.

    With epoxied granite done right, that steel is just getting in the way and hence just another opportunity to screw things up.

    A waxed mdf mold on a flat surface can hold pretty good tolerances on the finished part. Your mounting surfaces shouldn't need much scraping or grinding to make flat and co-planar.



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    Default Re: Epoxy granite in metal frame.

    Well I was leaning towards the first method. First weld the frame. But make square holes on the sides of the long tubes at the point the short tubes will be welded so the whole internal area communicates. Then weld the top steel plates. Ofc this will bend from the stress of the weld. Let it sit for a month or too and then fill in all the frame with epoxy granite. After this have the top of the steel plates ground flat. And then weld the lower perpendicular tubes. On those 3 I will bolt a 40mm aluminum plate as my table. And then Weld the feed and their reinforcements. (I will probably fill the rest of the tubes with epoxy granite too.
    The question is will the epoxy granite that will be inside the initial frame resist the stress of the welding that will be done after the casting???



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    Default Re: Epoxy granite in metal frame.

    No. And if it gets over 250°F, the epoxy will break down.

    Gerry

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    Default Re: Epoxy granite in metal frame.

    It seems like a waste of expensive materials. Epoxy-granite is great for making monolithic castings, but if you're just trying to add weight to your hollow steel frame, you can use regular sand, packed in tight. The heat of welding won't bother it either.

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    Default Re: Epoxy granite in metal frame.

    Quote Originally Posted by ger21 View Post
    No. And if it gets over 250°F, the epoxy will break down.
    Why would it get to 250 degrees? Water boils at 212 degrees F. It would have to go in an oven to hit that temp.

    The inside of a parked car on a sunny day can reach over 160 degrees though and most room temp cure epoxies start to soften at 120 degrees F.

    My preferred room temp epoxy is Adtech 820 because it remains stable up to 180 degrees without post curing in an oven. I don't know of any other room temp epoxies that have temp resistance that high.

    For temperatures greater than 180 degrees F, you need to either post cure or cure the resin in a curing oven. There are heat cure epoxies that remain stable up to 500 degrees (although 350-400 degrees is more common for oven cure resins).

    Note that "starts to soften" does not mean it immediately loses stiffness. 120 degrees is just where the process starts. The temp usually needs to be a fair bit higher before you can visibly bend cured epoxy.

    The question on whether epoxy granite will add strength to steel tubes is a different one. In my experience (in other areas), filling a steel, aluminum or carbon fiber tube with anything rigid will add some stiffness. Even filling it with a 16lb urethane foam would make it harder to bend or twist.

    I think the more important question is if it's worth it. It might sound like there is nothing to lose from filling the frame with EG but.... after going through the process of partially filling my gantry with EG, I can tell you that it costs a fortune, takes foreever and is generally a horrible job.

    There is a piece on CNC cookbook about filling milling columns and other castings with epoxy granite but my understanding is that the aim was to improve vibration damping. Just look up the force required to bend 1/4" wall steel tubes or I beams. It's a lot..... Between steel and epoxy granite, steel is the stronger material. Epoxy granite is used for it's superior properties (in other areas) for precision machines.



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    Default Re: Epoxy granite in metal frame.

    Quote Originally Posted by Goemon View Post
    Why would it get to 250 degrees?
    Because he talked about welding after adding the epoxy granite.

    Gerry

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    Default Re: Epoxy granite in metal frame.

    Quote Originally Posted by awerby View Post
    It seems like a waste of expensive materials. Epoxy-granite is great for making monolithic castings, but if you're just trying to add weight to your hollow steel frame, you can use regular sand, packed in tight. The heat of welding won't bother it either.
    Somewhere on YouTube is a series where someone built a gantry router doing exactly that with sand, asked it appeared to have worked very well. Sand is a cost-effective improvement to a hollow frame. Just make sure it's totally dry!

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    Default Re: Epoxy granite in metal frame.

    Thanks for the info guys. My initial intention for the use of E/G was to hold the frame still and prevent possible movement over time due to stress. Also the added weight. But as it seems my thoughts were wrong. So for the added weight a good alternatives is to tightly fill the frame with sand.



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    Default Re: Epoxy granite in metal frame.

    Quote Originally Posted by genixia View Post
    Somewhere on YouTube is a series where someone built a gantry router doing exactly that with sand, asked it appeared to have worked very well. Sand is a cost-effective improvement to a hollow frame. Just make sure it's totally dry!

    Sent from my SAMSUNG-SM-G900A using Tapatalk
    I found this video. I'm not sure if he ever made a build log of it. It doesn't appear he made any other videos of the router.





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    Default Re: Epoxy granite in metal frame.

    Thanks for the video. I have enough time to think this through so any idea is welcome.



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    Default Re: Epoxy granite in metal frame.

    No, that wasn't it, although it shows a nice technique for vibrating the sand down. I remembered who it was. "This Old Tony". He's got some great videos out there, his style is very informative. He's not afraid to share his thoughts, concerns or mistakes, and the editing is very well done too leaving a well-paced video with a nice touch of humor. I don't know if he's a member here. Sand fill is in part 2.







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    Default Re: Epoxy granite in metal frame.

    Quote Originally Posted by ger21 View Post
    Because he talked about welding after adding the epoxy granite.
    Ah... gotcha. I was thinking that it would be filled after any welding because the pic of the design seemed to show open ends for the steel tubes.

    Still, I was under the impression that, aside from the momentary high temp around the joint being welded, the rest of the steel tube doesn't get nearly as hot and that it can be handled very soon after the welding is finished. Is this correct? (I don't weld).

    Momentry heat, even if it's well past the glass transition temp, usually doesn't harm epoxy much. It usually takes some time before it softens. Even when it softens from the high temp, it usually becomes rigid again when it cools. With that said, there will be limits to that. A very high temp can scold the epoxy which is why they advise people to keep the blow torch moving when using one to remove air bubbles from bar top epoxy.

    There are other options if ultra high temp resistance is needed in a filling. For example, there is a vendor here in NY that sells ultra high temp resistant ceramic resins for use in a foundry environment. Some of them can withstand more than 3000 degrees F.



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    Default Re: Epoxy granite in metal frame.

    My initial thought was to weld the frame. Then cast the EG in the frame and then grind the top surfaces. Then weld feet on the frame. The whole reason I was thinking of casting EG in the frame was to make it rigid so that the top surfaces (that would be where the X linear guide rails would mount) stay perfectly flat after welding the feet.

    Since as ger21 said the EG won't guarantee that I will try to think of a different way to have to build the table. Maybe weld the frame. Then let it sit for a couple of months so it stress relieves and then have it ground and fill it with sand for added weight.



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    Default Re: Epoxy granite in metal frame.

    Quote Originally Posted by jimskeet View Post
    Thanks for the info guys. My initial intention for the use of E/G was to hold the frame still and prevent possible movement over time due to stress. Also the added weight. But as it seems my thoughts were wrong. So for the added weight a good alternatives is to tightly fill the frame with sand.
    Check out what this guy says on the topic. It might help:

    https://www.cnccookbook.com/epoxy-gr...-machine-fill/


    If you just want to make it heavier then filling it with anything heavy would work. The 50lb sand and granite aggregate mixes from Home Depot are very cheap. I think I paid around $5 per bag.

    I was toying with the crazy idea of filling steel or aluminum tubes with oil (for steel) or water (for aluminum) to increase weight. I then wanted to fit a valve to connect a vacuum pump so it could be emptied when it was being moved. I abandoned that idea for various reasons but the main one was that just being heavier was not necessarily helpful.

    After doing (far too much) research on this, it seems like there is some confusion on this topic. I.e. The desired end goal is maximum stiffness and strength with minimum vibration. Just being heavier doesn't necessarily help on it's own. Some people seem to assume that extra weight is the end goal. It just happens that thicker walls in iron castings increase weight but stiffness is the aim.

    Cast iron, the typical material of choice for mills, is not as strong as steel so thicker castings are necessary to achieve comparable strength which makes good cast iron machines heavier. I don't know how much benefit you get from increasing weight by adding a material that is not strong or stiff (like loose sand or a liquid). It's worth a little research though.

    I don't know nearly as much about CNC machines as some of the experts here but one thing I do know is that many of these questions (like what should I fill steel tubes with) become a lot easier to research when you are clear on what the end goal is.

    Based on what I have been told here and confirmed with my own research, I think you were on the right track when you were looking for ways to make your frame stiffer.



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    Default Re: Epoxy granite in metal frame.

    Quote Originally Posted by Goemon View Post
    Check out what this guy says on the topic. It might help:

    https://www.cnccookbook.com/epoxy-gr...-machine-fill/


    If you just want to make it heavier then filling it with anything heavy would work. The 50lb sand and granite aggregate mixes from Home Depot are very cheap. I think I paid around $5 per bag.

    I was toying with the crazy idea of filling steel or aluminum tubes with oil (for steel) or water (for aluminum) to increase weight. I then wanted to fit a valve to connect a vacuum pump so it could be emptied when it was being moved. I abandoned that idea for various reasons but the main one was that just being heavier was not necessarily helpful.

    After doing (far too much) research on this, it seems like there is some confusion on this topic. I.e. The desired end goal is maximum stiffness and strength with minimum vibration. Just being heavier doesn't necessarily help on it's own. Some people seem to assume that extra weight is the end goal. It just happens that thicker walls in iron castings increase weight but stiffness is the aim.

    Cast iron, the typical material of choice for mills, is not as strong as steel so thicker castings are necessary to achieve comparable strength which makes good cast iron machines heavier. I don't know how much benefit you get from increasing weight by adding a material that is not strong or stiff (like loose sand or a liquid). It's worth a little research though.

    I don't know nearly as much about CNC machines as some of the experts here but one thing I do know is that many of these questions (like what should I fill steel tubes with) become a lot easier to research when you are clear on what the end goal is.

    Based on what I have been told here and confirmed with my own research, I think you were on the right track when you were looking for ways to make your frame stiffer.
    True, the filling isn't going to add strength. It is going to damp vibrations leading to better rigidity. Resonant frequencies exist for every beam based upon its length and mass. If you are taking a cut where the cutter tooth hits the workpiece at a resonant frequency then that beam will start resonating. A small amount of energy at the resonant frequency can lead to a large deflection. Think of a child on a swing set - a small push each swing results in a large seeing movement. That large beam vibration will cause the finish to suffer, could cause the machine to walk across whatever surface it sits on, and in extreme cases with enough time, cause the beam or weld to work-harden and crack. The filler absorbs vibrational energy across a large frequency range and attenuates that resonance. Finishes are improved, and more extreme outcomes prevented.

    Even doing this or using EG frames won't entirely eliminate resonance in the machine. Linear rails have resonance, balllscrews have resonance, spindle shafts have resonance. Running into any of them can affect finish and shorten the lifespan of that component. Spindle resonance is especially tough on expensive spindle bearings, and cutting tool life.
    At the high end of CNC machining it is becoming increasingly common to map the machine's resonant frequencies and to tweak speeds and feeds to avoid them. In a production environment where a machine is making one part for years, tool life and spindle rebuilds go straight to the bottom line.

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    Default Re: Epoxy granite in metal frame.

    Quote Originally Posted by genixia View Post
    True, the filling isn't going to add strength. It is going to damp vibrations leading to better rigidity. Resonant frequencies exist for every beam based upon its length and mass. If you are taking a cut where the cutter tooth hits the workpiece at a resonant frequency then that beam will start resonating. A small amount of energy at the resonant frequency can lead to a large deflection. Think of a child on a swing set - a small push each swing results in a large seeing movement. That large beam vibration will cause the finish to suffer, could cause the machine to walk across whatever surface it sits on, and in extreme cases with enough time, cause the beam or weld to work-harden and crack. The filler absorbs vibrational energy across a large frequency range and attenuates that resonance. Finishes are improved, and more extreme outcomes prevented.

    Even doing this or using EG frames won't entirely eliminate resonance in the machine. Linear rails have resonance, balllscrews have resonance, spindle shafts have resonance. Running into any of them can affect finish and shorten the lifespan of that component. Spindle resonance is especially tough on expensive spindle bearings, and cutting tool life.
    At the high end of CNC machining it is becoming increasingly common to map the machine's resonant frequencies and to tweak speeds and feeds to avoid them. In a production environment where a machine is making one part for years, tool life and spindle rebuilds go straight to the bottom line.

    Sent from my SAMSUNG-SM-G900A using Tapatalk
    Maybe but I am not convinced that loose sand is the right filling to damp vibrations in a steel tube. It's not that I have specific info that states that sand is a poor damping material. It's just that none of the commercial vibration damping products I have seen have properties that are similar to sand. Plus, if it was that simple, commercial machine builders would use sand filled steel instead of cast iron as it's stiffer but they don't. They choose cast iron even though it isn't as strong or stiff as steel because of it's superior vibration damping properties.

    Where steel is used in precision machines, they reduce vibration (rather than damp it) by using more steel. I.e. Steel doesn't vibrate much when you use 10 tons of it.

    When I was researching this for my build, most of what I read indicated that, superior vibration damping is best achieved by choosing materials that vibrate less - like granite, epoxy granite, cast iron and carbon fiber - i.e. Not steel or aluminum where precision is required.

    I couldn't find any viable method of testing vibration damping at home though. It would be interesting (ok not exactly interesting...) for someone with the means to do proper testing to provide the results from a steel tube with and without a sand filling vs epoxy granite and cast iron.



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