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I built a 3D printer kit a while back but now I want a large printer to play with. I would like to build a 300x300x400 mm clone of a 3D printer. But I am not sure what size lead screws to buy. They come in 300 mm length but once you connect the coupler and the to the screw that takes 20 mm's that the axis can not travel too so isn't it more like 280 mm length? Or do most people round up to the next size?

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    $\begingroup$ If your printer is 200x200x180, how much you need to add to get 300x300x400? So that simple. $\endgroup$ Feb 8, 2018 at 7:03

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You'll need to do some calculation to figure out how long of a lead screw you need. The best solution would be to mock up the entire printer in CAD so you can visualize how everything fits together. Not only is the coupler going to take up some space, but the nut also takes up some space, and perhaps (due to design constraints) you won't be able to have the nut go up right against the coupler so you'll need some more space. Unfortunately, there isn't a general "just take the length of your Z-axis and add X millimeters"-type formula.

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  • $\begingroup$ Thanks for your anwser Tom life took over and forgot I had posted on stack exchange about this. When Life gets done with me I will give that a shot $\endgroup$ Apr 15, 2018 at 12:01
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CAD visualization like mentioned in this answer is the safest route. If you are not equiped with CAD skills you could do some manual calculations as well.

You already confirmed that you need 20 mm for the coupler, and you have to include space for the distance between the coupler and the trapezoid nut, this should be added to the travel length you want.

You can buy the screws in virtually any size you want (many far Eastern vendors, you can ask whatever length you want). You could even buy oversized and grind them to the final desired length.

I once bought oversized screws but was a little too lazy to grind them down, so they are still sticking out, no problem.

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