linear rails advice for a 200x200 design

hi ok so i'm starting a new printer design and would like to use 4 rails mounted to the top of a 20x20 box, to support an experimental XY arrangement that involves a "Noughts and Crosses" style carriage: two pairs of runners in X and Y through an 80x80mm hotend carriage. so i will need 4 linear guides in a square with a total of 8 linear blocks, 2 per rail. travel speeds i would like to be sustaining around 350mm/s, and would like to experiment with around 500mm/s just to see what happens.

my question is: would MGN7C do the job (which requires M2 bolts to a depth of 2.5mm, mounting points are 12x13mm) or would it be better to go with MGN9C (M3 bolts to a depth of 3mm, 4 mounting points @ 15x10), or even larger? i particularly want to keep the weight down.

tia,
l.

i would go for something more like mgn12 or mgn15

Edited 1 time(s). Last edit at 05/30/2017 06:29PM by chris33.

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hiya chris, thanks for this - ahh in the intervening time (before you replied) i had already ordered MGN9C. i'll have to see how those get on and upgrade if they're inadequate. do you happen to know of a technical (engineering) reason why MGN12C would be better?

What do you mean by noughts and crosses? Is that like the ultimaker arrangement?

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.

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.

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.


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.

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.

Edited 1 time(s). Last edit at 06/07/2017 12:12PM by 691175002.

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691175002
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.

Just a standard size mgn12 i used of my mini kossel and on that there won't be as much load on the bearings as a gantry style printer so i would find out how much load your gantry will put on the bearings as mgn9 might be out of spec

also prob fitting to 20x20 extrusion might be a issure but i have not looked into the size of smaller rail then mgn12.

there is actually a mod on the ulitmaker to get rid of the closed loop belt

Edited 1 time(s). Last edit at 06/13/2017 06:19AM by chris33.

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Check my rubbish blog for my prusa i3

up and running
[3dimetech.blogspot.co.uk]