How to design a good 3D printable part.

Many people design 3D parts for art, beauty or decoration. I found that plastic injection molding do a better job. My primary 3D printing is for engineering purpose. So I am more concern about the dimension accuracy and strength of objects. I like to use PLA and ABS if I really need to.

Did you ever had the frustration where by you took something from thingiverse and JUST PRINT IT.
Oh my god, it print nicely but it does not fit.

           First, you need to know your printer print out accuracy. It applied to external dimension ( overall length, width and height)  and internal  like holes, slot, fillet etc.

              
If you design a hinge , you need to know how much offset  it will really print. Let take this as an example. If your thickness of the yellow colour hinge is wrong, you can never fit in. Lets say the space between 2 blue hinge is 4 mm. so you thickness cannot be 4mm. if you  know you print  out will be about 4.1mm, then you can safely set you width to be 3.7 to  3.5. Always reduce your thickness by about 0.2 mm or more.



Next the hole. You might ask HOW BIG SHOULD THE HOLE SIZE BE ?
This guidelines is exactly the same as normal drilling. If you screw is 3mm, you hole should be 3.5 mm. Unless you can get a very accurate printout, but remember, beside the hole size, your position will be offset. I do not like to do too much post procession like drilling to enlarge the hole, because you will cause the hole structure to weaken.  I find that some Thingiverse user draw hole which is EXACTLY the same size as screw....

Next adding fillet to the parts. what is a fillet. fillet has 2 used. one is for strength, one is for smoothness. 
to be continued ....