Leonard I’ve got a background [degree] in Materials Science, which I hardly ever used in my professional life in computing and IT, from which I have now retired after 42 years. I consider myself an engineer first, programming is just something I do.. The machines I make are handled by people who often have little mechanical sympathy so sometimes they broke things - but three years of making these machines means I have tuned out all the problem areas where ham fisted operation broke parts. Below is what works for us... 3d prints are a bit like wood, strong in compression, not so good at resisting bending unless sized to suit the load, a 7mm thick walled cylinder with four printed walls inside and out is remarkably strong; Chamfer every edge, it looks nicer and helps to remove any corner ringing..; Chamfer every hole, for the same reason and it speeds up finishing,; Use cylinders, not cubes, they are stronger in bending, just like drinking straws / bamboo canes Lattice structures print faster than cubes if you need height and reach with minimal weight and good stiffness..; The strength of 3d prints are directional, so design your parts so when you print them the layers are in the direction of greatest load - which of course you can choose when slicing - have a look at CNC Kitchen and Makers Muse on You Tube; Fabricate big parts from smaller ones fastened together - if you want to take them apart use flanges so that you can put self tapping screws through the mating faces, if permanent is ok, make the parts to slide together - I always deduct a print allowance from the diameter of pegs which fit into properly sized holes - I use 0.3mm for my print allowance - then use a solvent adhesive to weld the parts together; Depending upon where you are in the world, you should be able to get Plastruct solvent weld - in the UK I use EMA Model Supplies Plastic Weld which works on PLA and PETG. Its way better than superglue as it melts the faces which then dry as a solid one piece; Avoid sharp corners - use a fillet - round or straight diagonal between faces; If you need low friction bearing surfaces, there are low friction 3d printing materials available - or I use flanged bearings pressed into my components - or put thrust bearings in where appropriate; I did design a large diameter thrust bearing using 6mm stainless balls, complete with cage and a means of removing backlash but never used it in a production machine - the central backlash adjust was a 110mm ISO screw thread with a 2mm pitch, low friction materials work better in sliding surface contact at the rotation speeds involved - you can reduce friction considerably by polishing to 2000 grit and using a PTFE spray on the contact surfaces; To get extra stiffness or bending resistance embed holes in parts through which you can pass a screw / bolt and put a nyloc nut and a load spreading washer underneath the nut face - this helps to overcome the directional weakness along the layers by putting everything into compression - same principle as pre-stressed concrete; Getting strength in 3D parts - You can fix a lot of issues at the slicing stage - increasing the number of walls is better [and faster] than increasing infill density, for example If you need to stiffen up a cylinder, hide another smaller cylinder inside using difference, or create a lattice structure which puts more walls than infill into your component - this is the absolute joy of OpenSCAD IMHO - reusable elements, rescaled without having to consider anchor lines or mess about with edges, I’ve tried FreeCAD, hated it, gave up. Use gyroid infill - I use Cura for slicing and you’ll hardly ever need more than 15%; Thicker layers are stronger than thin ones, use multiples of the minimum step 0.04 mm - this is a general rule which I avoid because I need parts that look good, so 0.2 mm is just about all I use now - I take the print time hit in preference for appearance; If you want to use a captive bolt, make a socket for the bolt head and some of the shaft then print a sleeve which holds the bolt head, over the shaft then solvent weld the sleeve and bolt into the socket, I use M5 hex headed bolts with printed hexagonal sockets and either a regular full nut or nyloc according to the application - the embedded bolt is stronger than a 3d printed thread which has a nice stress raiser all the way around it. if it looks right it generally is.. good luck,