DIY CNC Rigidity/Flex


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    Default DIY CNC Rigidity/Flex

    I read every forum i could find on the required rigidity for cnc routers planning to mill aluminum but i couldn't find anything useful. I'm building a cnc router with a cutting area of 15"x15or20"x4" for use in mainly aluminum(and anything easier to machine). First question, does this seem like a good size? If it's mostly for aluminum I can't see myself making anything bigger than that(because of budget) since it's being made purely for fun and for my other hobbies. It's going to be constructed of all aluminum(except for the spoil board) so I can get the best possible rigidity. Second question, how much rigidity should I have? In other words how much allowed flex should it have? I'm designing the parts to have under .0003" of flex under a 10lbs cutting load(.25" endmill, 2 flute, .05 axial depth, .25" radial width, .003" chip load, 60ipm ,10,000 rpm spindle(if that sounds right, i can give a link to the calculator if you want)). So when the gantry is in the middle of the table it will sag no more than .0003" (probably not necessary since i'll likely get more flex from my linear rails (rollon compact rail)). when the the Z gantry is in the middle of its travel the force will flex the rails and supports no more than .0003(will likely be more like .0001-2) in the x direction. When the z travel is fully extended-assuming the gantry is unmovable-the z plate can bend no more than .0003(it'll be more like .0001). Similar values will be used for all other ways you can possibly imagine the machine can flex. If these numbers sound too big or too small please let me know. I'd like to have as much accuracy as possible and as little chatter. If it's overkill i won't adjust the system since the savings will be small (about $50 will way reduce rigidity) considering all the aluminum plus shipping will be around $250 but I would be glad to make it stronger if needed. I've put more thought into this machine than probably 95% of diyers since most of them say "this looks rigid enough" or "I hope it'll be good enough" where I gone through the trouble of calculating everything. At this point I think my accuracy is limited to my linear rails which were never actually designed for cnc but you can take a look(they're actually probably more rigid). Sorry for the long post but I like to give as much information as possible. Any help is greatly appreciated.

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    Clearly no one wants to read a question that long so here are the simplified questions.

    Question 1-would it be worth the money to increase the cutting size from 15x15 to 15x20 or is 15x15 good enough since i likely won't make aluminum parts bigger than that.(mostly opinion)

    Question 2- What's a good level of rigidity for a machine made to mill mainly aluminum with high accuracy and hopefully no chatter? What should be the maximum amount of flex at the cutting tip with about a 10 lbs cutting load?

    If you want all the details just read my first post. Thank You in advance.



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    CNC mill build thread: http://www.cnczone.com/forums/vertical_mill_lathe_project_log/110305-gantry_mill.html


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    Thanks for the response. If your data is right it seems like I have nothing to worry about. I had to read those several times to understand the units you were using. On your first link I think with your calculation of my machine you were off by a factor of 200 compared to my calculation and the one you had in your third link. If it is that stiff I have nothing to worry about because it is at least 3 times as rigid as it needs to be. Even if I allow a fair amount of slop in the router it is still very rigid. The linear rails I'm using are not riding on ball bearing but instead 5 large precision roller bearings which are made off center from the shaft so you turn it and it adjusts preload allowing zero slop. The actual flex in the bearings from the forces from even a heavy aluminum cutting load still will only introduce only about .0001" of flex assuming worst case scenarios. It's really hard to calculate the exact amount of flex but in the end it seems like my machine is plenty rigid enough. Even if I have too much flex i'll just reduce cutting depth since it will lower the flex in the system. That won't mean the rigidity is higher meaning less chatter but my machine is plenty rigid to avoid chatter. Once I'm done building it I'll probably to a lot of enhancements since it'll be easier if i have a cnc router. Currently I have to design the machine so almost all I need to do is drill and tap aluminum. I'm just rambling on now so I finish by saying thank you.



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    Quote Originally Posted by bassman2914 View Post
    Question 1-would it be worth the money to increase the cutting size from 15x15 to 15x20 or is 15x15 good enough since i likely won't make aluminum parts bigger than that.(mostly opinion)
    Once you have your machine built, you will undoubtedly find additional uses that will cause you to want a larger machine. Maybe you are constrained on the aluminum front, but what about cutting foam? Or making molds? Or wood? I suspect that very few people are really fully satisfied with their machines on one attribute or another. The question is time, budget and space available. You can probably go up to a slightly larger machine with little appreciable impact on price or build methods. A much larger machine will blow-up your budget in a hurry once you start having to worry about deflection over long members.

    The best way to work through this is to design your machine relatively fully at your smaller size, and then scale it up to the larger size. Pay attention to both the physics/mechanics of the design and the cost. Since your electronics will mostly be insensitive to the size of the machine in this size category, the total cost of the machine will not scale 1:1 with changes in cutting envelope.

    All that being said, if you want to go with ball screws, buy those off of eBay first, and then design your cutting area around your screws. Make sure that any ball screws you buy have both supports, as well. A $70 ball screw isn't a great deal if you have to buy $200 worth of support blocks at full retail. Also, don't bother with the Chinese stuff in this size range. There are usually a ton of ball screws on eBay that are <$100/each in this size range. If you want to go really high-end for a small machine, then you can also get linear rails/guides to make a truly precision milling machine.

    See the 5bears site (5 Bears - Home) for the art of the possible.



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    It all depends on the type of machine you want to build. My steel router cut almost any material with no problem. I don't see the stiffness of the machine as a problem, to me "twisting" of the portal is something that must be taken care of. Even though I have heavy duty linear rails the twist exist, okay I have to push pretty hard on the side of the portal to get a twist, but when plowing through hard materials it must be stiff to stay in tolerance.
    I have successfully solved it with the wire crossing support solution that some other guys at this forum implemented (from cabinet sliding door building), don't really know what to call it to find it in a search but I'm sure someone else knows what I mean and could make a fill-in.



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    Quote Originally Posted by svenakela View Post
    It all depends on the type of machine you want to build. My steel router cut almost any material with no problem. I don't see the stiffness of the machine as a problem, to me "twisting" of the portal is something that must be taken care of. Even though I have heavy duty linear rails the twist exist, okay I have to push pretty hard on the side of the portal to get a twist, but when plowing through hard materials it must be stiff to stay in tolerance.
    I have successfully solved it with the wire crossing support solution that some other guys at this forum implemented (from cabinet sliding door building), don't really know what to call it to find it in a search but I'm sure someone else knows what I mean and could make a fill-in.
    Moving knot



  8. #8

    Default Re: DIY CNC Rigidity/Flex

    This is an old thread, but I was also trying to find info on making an aluminum extrusion CNC machine more rigid. I've added hardened steel v-track and bearings to the Z-axis for a reasonable cost and I've stiffened the X-axis c-beam with stainless channel. You can see my progress here as I'm working on it:
    I'd love to hear from you if you've successfully made your aluminum extrusion CNC rigid enough to comfortably mill metals.
    Cheers,
    Tyler



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