Reinforce base of Tenon where meets body
When I need to print a something too long for the bed, I often split into two pieces and join with a Mortise & Tenon joint, especially when the Tenon is oriented vertically.
However, I have found that the joint is brittle with the Tenon breaking off exactly at the flat shoulder where the Tenon meets the body. Actually it probably breaks a couple layers below where the infill starts.
I believe the problem is that while the Tenon and Mortise are each individually strong as tube-like structures, the shoulder where the transition occurs is only a few perimeters thick and the Tenon which is perpendicular to it can thus easily rip off, especially if the Tenon is oriented vertically (which it needs to be to print nicely without supports). Since the vertical layer adhesion is weak, the Tenon essentially rips away where it meets the infill.
I can manually compensate by adding a cyclindrical modifier with 100% infill to span the area where the Tenon joins the body, but it would be nice if the slicer could recognize such geometry and add extra fill and/or perimeters when you have a transition between a horizontal and vertical body.
Any suggestions or experience here?
RE: Reinforce base of Tenon where meets body
Based on your description, and my experience, I would have to suggest that you are not printing with the correct temperature settings for the filament you using.
Good Luck
Swiss_Cheese
The Filament Whisperer
RE: Reinforce base of Tenon where meets body
I have had the problem with both PLA and with ASA using the default Prusa temperature settings.
In a mortise & tenon joint when there is torque exerted perpendicular to the line of the joint, the force is maximal where the tenon joins the body (i.e. at the shoulder). Since the shoulder is only the thickness of the perimeter, this is the weak point.
Even if I am off by a couple of degrees either way, the weakness at the tenon shoulder is profound and easily explainable by the geometry when there is no solid fill joining the strength of the tenon (as a tubular structure) with the strength of the body (as a semi-tubular structure).