Mechanical Rigidity

Mechanical Rigidity

This page goes into the basics, with illustrations, on how to ensure that a design is mechanically rigid, and also what to look for if buying an existing 3D printer. The key elements are:

• Rigidity must be achieved in all six degrees of freedom: Rotation ("twisting" or "screwing") about each of X, Y and Z, and Movement in X, Y and Z (shearing or "parallelogramming").
• The simplest rigid open structure is a triangle.
• Solid materials (plates, bars, extrusions, rods etc.) have rigidity that is proportional to their thickness (or length). Rods (or bars) in particular have lateral flex that is proportional to the square of their length.
• A "lever" effect on the way that two frame parts are connected together is critical to take into account. The further the distance the more critical the material strength of the parts (and their method of connection) becomes.

Frames

A quick guide to analysing an existing frame design is:

• If it is a cube design, is there support in all six faces of some kind? Either plates (polycarbonate, acrylic, plywood or hardboard at least 2.5mm thick) filling each face, diagonal struts that go fully to corners creating complete triangles, or even suitably strong (i.e. with no flex) very high-tension wires (again creating complete triangles to all vertices)
• If it is a Mendel style design, these are not rigid at all in their base, and rely completely on being on a flat surface, with gravity assisting to keep them down. This tends in practice to be ok, but it is not the only issue to watch out for.
• If it is an "open design" without triangles (or plates), is the frame of sufficient thickness for the size (3030 or preferably 4040 for a 200x200 printer, to 8020 extrusion for a 300x300 or greater) and are the frame struts sufficiently strongly connected together?
• 3D-printed plastic, if used at corners as the sole method to join frame parts together, should be definitively considered a "red flag" that warrants full investigation and a thorough analysis.
• If you are concerned at all about rigidity affecting build quality, avoid Kossel (delta printers) entirely.

For further illustration, examples have been split into their own sub-page, with a quick summary on each:

• Mechanical_Rigidity/Mendel - inspiration for the 3D printing movement but deeply flawed.
• Mechanical_Rigidity/Mendel90 - significant improvement, still not perfect but in practice perfectly fine.
• Mechanical_Rigidity/MendelFlex - extremely well-engineered variant of the Mendel: a different approach from the Mendel90
• Mechanical_Rigidity/Cube_Example - good demonstration of the flaws and the complexity inherent in a cube design.
• Mechanical_Rigidity/Ultimaker_2 - the "Gold Standard" for any cube design.
• Mechanical_Rigidity/Original_Kossel - low cost but deeply flawed: no rigidity whatsoever.
• Mechanical_Rigidity/Fisher - how a Delta 3D printer should be done.
• Mechanical_Rigidity/Folgertech_Kossel_Bracing - good effort but a Delta's struts are too long for bracing to be effective
• Mechanical_Rigidity/Voxel_Ox_8020 - how to "beef up" a frame to good effect.

Printbeds

A quick guide to printbeds:

• Mendel style printbeds are fine: just watch out for the plate under the printbed being made of sufficiently rigid material (see printbed section for details)
• Cantilevered printbeds should have linear rails or V-rollers, and if rods are in use they should be at least 10mm preferably 12mm. Most cantilevered designs are severely problematic, and they all rely on the mechanical properties (amount of bend) of the materials used.
• With dual z-screws (centrally-cantilevered printbed), look for four linear bearings/blocks (two per rod/rail) or twin V-rollers per rail with separation of at least 75mm vertical separation between centres, to ensure that the bed cannot wobble about (rotate). Ensure that there is a rigid cross-bar (plate or other assembly) to which all four bearings / blocks / V-rollers are mounted.
• Kossel (delta) printer beds are fixed (and so are fine): it's the print-head rods that require micro-millimetre accuracy and are a huge headache to calibrate.
• The best (vertically-moving) printbed arrangement is by far and above triple (or greater) lead screws and dual (or greater) rails/rods (even if the rods - if rods are used - are only 8mm and only have one bearing/block/roller per rod/rail).

Examples are illustrated here, with general relevant guidelines below:

• Mechanical_Rigidity/Triple_Lead_Dual_Support Triple lead screws, dual rails: excellent rigidity without requiring high-strength materials (does not rely on materials not bending under load). Most professional designs will use quadruple leads and quadruple rails.
• Mechanical_Rigidity/Boxframe_Cantilever_Printbed Innovative use of laser-cut panels. good example.
• Mechanical_Rigidity/Fusebox_classic_flawed_cantilever_printbed Fusebox: contains absolutely every single one of the design flaws which are important to avoid. Very useful example of what not to do.

Mendel style printbeds

Mendel style printbeds have one significant advantage: they are flat, only move in one direction, and need only three bearings and two rods (see photos above of Mendel90). One thing to watch out for however, particularly on cheap-cost China clones, is the use of an inadequate thickness metal plate to which the bearings (and the Y-belt) are attached. For a 200x200mm Printbed, anything less than a 3mm aluminium plate, 6mm acrylic or 4mm dibond is going to be completely inadequately stiff, resulting in flexing of the plate (to which the printbed is attached), thus in turn adversely affecting build quality.

Print Bed Mounting: 3 or 4 points

Although it is considered unnecessary to have 4 mount points for bed levelling, one of the problems with printbeds is that sometimes they will be warped. Depending on the degree of warping, 4-point levelling allows the warping to be corrected, whereas if a triple-mounted printbed is warped it will need to be repaired or entirely replaced. Sourcing a triple-mounted printbed plate that is sufficiently thick and properly machined (and adequately packaged when shipping) is therefore critical.