Mechanical Rigidity/Triple Lead Dual Support
Triple lead screws, dual rails
This type of design is discussed on the reprap forum here: http://forums.reprap.org/read.php?397,726304,766930 and the design files may be obtained here: https://www.youmagine.com/designs/2040-laser-cut-core-xy-v0-8
Note that this particular design's frame suffers from lack of rigidity (no side support, insufficient upright bracing - see above) however the bed support is excellent. Here is a photo highlighting the use of dual rails:
A six degrees of freedom analysis of this design is as follows:
- Triple lead screws stop X and Y rotation of the printbed and control (allow) Z linear movement but do not prevent X or Y linear movement (sideways sliding) and do not prevent Z rotation of the printbed (not least, the Z-screws can bend).
- Dual rails will stop X and Y linear movement, will permit the required Z linear movement, will stop Z rotation, and will stop EITHER X OR Y rotation (but not both). Specifically: without the lead screws, one rail could go up and the other could go down, causing the bed to rotate about the axis that's perpendicular to the two rails.
The COMBINATION (and only the combination) of these two sets of constraints happens by a happy coincidence to provide the full set of required control over all six degrees of freedom.
Note that a single rail will not do the job, because the Z-screws can bend about the middle and the printbed could rotate around the centre of a single rail. Triple (or greater) rails would be redundant. Quadruple z-screws or greater would also be redundant.
For a moving bed like this, one or two lead screws are not sufficient: with only one or two lead screws the burden then falls onto the z-rails to prevent rotation in X Y and Z, and that places significant strain on the block.
One of the big, big advantages of the "triple lead, dual rails/rods" design is that there is no critical dependence on the mechanical rigidity (along the length) of the leads screws or rails or rods utilised. Cantilevered bed designs on the other hand tend to require 10mm or even 12mm rods in order to ensure that the weight of the bed (and object as it is printed) does not cause significant bending.