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.