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 ....

How to print a good 3D print out ...

Let me share with everyone how to get a good print out.
First since 3D printer is a precision equipment, if your components are not precise, you will never get a good printout for a reasonable time. eg, use of  threaded rod may not be ideal for a long time.


Let's assume, you have a structurally stable and good print head or extruder.

First in order to print well, look at the first layer. If it print well, you will more likely to get a good printout. If it does not, the structure will move and you will a pile of rubbish.. what do I mean by a good first layer. Assume that you had calibrated the flow rate and you do not get something like this....

1) Print bed slanted - You will see one side is crushed and the other side does not stick well.
Remedy : make the bed flat or it could be your print head  is slanted.

2) Print head too high - the print out just does not stick to surface .
Remedy : I prefer to adjust the Z offset in my slicer program. it is easier because your are talking about probably 0.1 to 0.4 mm. Adjust your stop switch will be tough.

3) print head too low - will cause excessive adhesion to surface. Filament will get compressed on the surface and if you see carefully, you will see that it is very flat and enjoy your hard time getting the pieces out of the heated bed.  I prefer to adjust this by the Z-offset also. It is easier.

This print out is good. A little more will crush and compressed the print out.


Next let's talk about the various type of surface to print on.
1) kapton tape:  My experience is, it is not very sticky and in the surface is too little , the printout will dropout. So I will definitely put a raft layer as a interface layer.  I use it on the Felix printer.

2) Uhu Glue on glass - The prusa I have is using UHu glue on glass. It sticks really well. even when the base is small. Usually I apply 1 time, I can use it for many print.

3) Blue painter tape: the adhesion is pretty strong also, but sometime, it just come out with the printout. So I find it quite a trouble to reapply the blue tape.

I will edit with more pictures when I gather them ...