linear rails advice for a 200x200 design

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What do you mean by noughts and crosses?

i since drew it up:

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Is that like the ultimaker arrangement?

the ultimaker only has one 6mm rod per axis, i am looking at two to prevent the rotation that the gears and 500mm closed loop belts prevent in the ultimaker

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Linear guides are traditionally selected based on the forces the blocks can withstand. Even MGN7 can survive roughly 600lb of force so when designing a printer we select on other factors.

wow that's enormous. have to translate to kg to get a feel for that but it's still enormous.

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MGN7 is too narrow to easily mount in 2020 extrusion (plus standard nuts only go down to m3) so it typically gets avoided. MGN9-15 have similar prices so you are basically making a trade between weight and rigidity.

ok. MGN7 is out then. the screw depth is only 2mm on MGN7 and it's M2. i figured to try MGN9 because the blocks are less weight. i'm not so concerned about the rail weight.

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If the rail is fixed and you have space, there is no reason not to use the largest rail. If the rail is moving you probably want the smallest rail since rigidity will be provided by the extrusion it is attached to.

rails are fixed, but i want to keep the block weight down as i will be experimenting with speeds of 500mm/s.

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The decision becomes complicated if the rail is unsupported, since in that case you probably want to do the math or FEA to see how much bending to expect. Unsupported rail is not ideal design because the holes drilled into the rail dramatically reduce its strength.

that makes a lot of sense. i considered using unsupported rails instead of the twin 6mm rods per axis to support the central carriage but was not confident that there would be any weight saving (compared to 6mm rods plus LM6UU, or compared to square 8mm plus v-rollers) nor in my ability to assess the materials strength.

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In unsupported applications you are probably better off buying name brand linear shaft and bearings instead. Linear shaft is also available in hollow or ceramic coated aluminum variants when weight matters. Ebay junk on out of spec drill rod is not representative of the performance you can achieve with properly chosen components. The real thing is very smooth.

For example misumi part number PSPJ10-280 is $13/ea and is almost certainly lighter and more rigid than unsupported MGN9 rail.

niiiiice, wooooww, hollow precision-engineered rods, wowww :) i feel almost embarrassed to admit being fascinated and amazed that such parts are available. thank you, because i will definitely investigate those.
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