Mechanical Rigidity/Original Kossel

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Kossel Delta Printers

The Kossel is an innovative design that is unfortunately completely lacking in rigidity:

Kossel design: completely lacking in rigidity

Essentially its frame is a toblerone, however as can be seen each rectangular face has absolutely no bracing whatsoever. Twisting is therefore extremely easy to do about the Z-axis, and there will also be significant shearing in both X and Y. An original Kossel Delta printer should, for this reason alone, be avoided.

Properly fixing a Kossel's toblerone frame, by adding diagonal bracing from corner-to-corner across all three of the rectangular faces, would unfortunately impinge significantly onto the build area. However adding picture-frame-style panels with a border of at least 40mm and a material thickness of at least 6mm in acrylic (similar to the Ultimaker-2 front panel) would do the trick (which has actually been implemented, in the form of the Mechanical_Rigidity/Fisher. Due to the length of the uprights, the number of attachment points would however need to be increased.

The typical use of plastic parts for the corners in Kossel designs is also a major factor in the lack of rigidity. However efforts to replace these with metal, or to brace the twin base with external panels, really does not help because the uprights are so long in a Delta printer that the flexibility of the uprights themselves becomes significant.

Add to that the fact that Delta printers require extreme micro-millimetre-accurate lengths of rods and require extremely complex calibration and they're just not worth the hassle.

Proposed (Hexagon) Kossel Delta Frame

Aside from the addition of open-holed panels to a standard Kossel, a much better design would be to start from a hexagonal base (and top), with at least three consecutive sides completely filled in with panels, from top to bottom, with smaller panels on the remaining sides of height at least 50mm on the top and bottom hexagons, in order to ensure that the hexagons themselves are rigidly supported. The three full panels, by being at an angle of 60 degrees to each other, would provide self-supporting rigidity of the four uprights that they were attached to, but would transfer any "twisting" to an unsupported base (and top) hexagon. The smaller panels top and bottom would prevent such twisting effects.

The advantage of a hexagonal base would be rigidity without significantly reducing the build area. Panels with cutouts (similar to the Ultimaker 2 front) could also be considered, although the long height of a Delta design could result in twisting of any open (holed) panel that would need to be supported on the outside (with a 300 degree brace) so as not to interfere with the operation of the printer.