Plywood frame? 300 x 200 bed?

Been planning on building a 90 for a while now, and the failure of a plastic clip on my current Prusa which requires re-building the whole corner and re-calibrating has made me decide it is time.

I am not a fan of MDF but I do rather like plywood and happen to have some very nice offcuts of 15mm Birch ply that I was planning to use. My feeling is that these will be just as dimensionally stable as MDF but more rigid, any reasons I shouldn't? I seem to remember that the compiler will allow you to adjust material thickness?

Also trying to decide whether to try a 200 x 300 print bed. I would probably put the 300 on the Y axis (so as to keep the weight off the lifted axis) and use 10mm instead of 8mm rod. I might also increase the build height slightly. Again any advice before I dive in is much appreciated.

George

My experience of plywood is that it doesn't come perfectly flat like MDF does. If you can find some that is then it should work. You would define a new sheet in the scad/vitamins/sheets.scad file and select it in the config.

If you increase the Y axis by 100mm it makes the machine 200mm deeper but if you increase the X axis by 100mm it only makes the machine 100mm longer. The weight of the lifted axis would only increase by the weight of 200mm of rod and I don't think that will be an issue with threaded rod drive. The weight of the Y carriage and bed increases 50% so it is better to have shorter Y bars than ones 200mm longer.

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[www.hydraraptor.blogspot.com]

Thanks for that. I will try to check the flatness of my sheets of ply.

Good point on the X vs Y axis. You are assuming that I would need to widen the spacing of the Y axis bearings as much as the increase in the bed size, whereas I had assumed that I would leave it as it is. I could also put a support in the middle of the long bar and space out the linear bearings even more so they effectively all have their own rail at the shortest possible length (300mm + bearing length). I will have to think about this.

Do you think 8mm would be OK for a X axis at 300mm rather than 200? If I size up to 10mm then the weight of the bars more than doubles, but as this axis doesn't need to move often or fast I think it would be fine.

Here's a heatbed:
[reprap.me]
and glass:
[reprap.me]
I think 8mm rod would sag with the longer length.

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gsport
Good point on the X vs Y axis. You are assuming that I would need to widen the spacing of the Y axis bearings as much as the increase in the bed size, whereas I had assumed that I would leave it as it is.

Yes if you keep the spacing the same it would only get 100mm deeper but the carriage would then move outside of the base. You would need to hack the code to do that.

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gsport
I could also put a support in the middle of the long bar and space out the linear bearings even more so they effectively all have their own rail at the shortest possible length (300mm + bearing length). I will have to think about this.

I think the bearings will always need to move past the centre unless you space them further apart than the travel, I.e. make the carriage longer than the bed.

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gsport
Do you think 8mm would be OK for a X axis at 300mm rather than 200? If I size up to 10mm then the weight of the bars more than doubles, but as this axis doesn't need to move often or fast I think it would be fine.

I also think 10mm rods would be a good idea but I haven't done the calculations as to how much they sag.

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[www.hydraraptor.blogspot.com]

Thanks.

I think the key consideration is going to be the weight of the X carriage vs the weight of the bed. I think the carriage is likely to end up considerably lighter so it makes sense for that to be the part moving furthest/most.

And thanks Ralph, that was the one I was looking at.

George

Intuition suggests that the weight of the rod would need to be increased in proportion to the length so that a 50% increase in length requires a roughly 25% increase in rod width. But intuition is probably inferior to the calculations of an engineer.

Bending moment will go up 50% roughly so ideally second moment of area needs to go up by a similar amount. Going from 8 being adequate on a 200mm run 9 would be fine for 300. But 9 is not a standard size so 10 should be a significant step up in stiffness even with the longer run.