What tolerance should I allow for 3D printing?


How accurate are 3D printing machines in general?

I have yet to source my 3D printer, but I’m building a what is in effect a ‘piston’ (i.e. this consists of a hollow cylinder with another cylinder going up and down inside it.) And I need to know roughly what tolerance to allow. i.e. I need to know how much smaller outside diameter of the inner (i.e. ‘male’) cylinder should be compared to the inside diameter of the outer (i.e. ‘female’) cylinder. The inner diameter will be about 5mm.

I guess I am expecting to need to do a little light sanding to finish off 3D printouts… but hopefully not too much!

Printing will be done in standard plastic (e.g. ABS or Nylon or PLA etc)

Any advice?

Well I have used both ABS and PLA the fitting together is very good if parts are made on same 3D printer . . Made a bottom part of a 2 inch jar by someone here from ABS and the top from PLA they fit like they are supposed to ( On same 3D printer . . ) Had to run the setup for each one different as temps are higher for the ABS both bed and extruder temps
And the higher you have set infill the better the outside finish is . .
This is PLA at 90 % fill in just before I had a nozzle jam and the Extruder’s holders broke . . Got metal one now . .

To be honest, in order to get a snug fit, I always print at 100%, then sand to make it fit. Since the plastic warps and shrinks, it seems like I am going to end up sanding, anyway, so I model with full contact between pieces, then is some elbow grease and 150 grit to make everything fit.

I get tiny spider web thick ahumm things here and there where it left a very very small filament going from here to there when I use less then 90 % . . So 100% should really make it tight in fitting and hold a good shape . . They have clean up tools on Amazon to remove the tiny bits here and there, and a smoothing coating to make it really smooth I have been thinking of getting . . My sign to be when I get richer and can afford more colours of filaments ! just laid it out on the work bench to see what it looked like

I have another, related problem. I will need to 3D print an object that looks a bit like a small gun/pistol. (It isn’t a gun but that gives you the right idea!) The overall shape is a solid curved ‘handle’, boxy area, and then a tubular barrel. Something rather like this:

I imagine that the best way to print a tube is on its end. But the problem is that my shape will not balance very well on its end! On the other hand 3D printing of a circular section on it’s side will involve unsupported near horizontal geometry.

At this point I don’t know what type of 3D printers I will have access to. With the geometry above will I be force to use a ‘powder bed’ 3D printer rather than an ‘extrusion’ type?

I guess, if I am printing using a soft easily glued plastic like ABS, then I guess I could print the barrel on its end separately and then glue it with a solvent type cement (e.g. EMA Plastic Weld, Tamiya Extra Thin etc), however if I want a stronger plastic like Nylon or HDPE that will be much harder (impossible?) to glue.

To give an idea of size/scale, the radius of curves will be c. 5mm.

There are so many possible answers to your question. The type of printer, the material, the printer software etc all make for different solutions.
For example with my simple small printer I would position that sort of diagonal. So it’s sitting on the bend with the barrel at roughly 45deg.
My software would put in support structures to keep it upright and nothing would really be unsupported. But in the workshop with one of the other printers it would be different, and if I wanted it printed in metal then I would go and talk to my other guys …
It all depends on what you want as a final product. There is no one answer.

OK I get it - it’s tricky to answer. I hadn’t thought of support structures that could just be broken off after printing - yes that would be fine. I am expecting to do a bit of finishing work (presumably with sandpaper and a sharp blade).