.25 in screw hole doesn't fit .25 in screw

[janikows]

The screw is .25 inch and wont thread into a .25 inch threaded hole in fusion. I attached the example fusion file.

is this screw a different thread designation I tried printing 20UNC and 24UNC with the same results. My hole seems to small for he screw and .3125 inch is slightly to big the bolt wiggles a bit but threads fine.

What am I doing wrong here?

[johnswetz1982]

Measuring the crest would not be as helpful as what the pitch diameter would tell. Do you even know how many threads per in the actual screw is? 

Is is supposed to be a threaded hole or a clearance hole? Also even though i have no experience with 3D printing I know the resolution and the setup could lead to some inaccuracies with the actual print, I think.

[janikows]

I just ran to hardware store and the screw is a 1/4 inch diameter and 20 thread. So I think I'm creating the thread incorrectly in fusion or there's some scaling bug that's getting inserted.

[seth.madore · Accepted solution]

What is your printer?

I've got a Prusa FFF and I find that in most materials, the accuracy is not good enough to just accept bolts. If you need the threads to go together, you're likely going to have to modify the thread in the design space. This can be done with "offset face" and nudge the root out a smidge. 

That said, be aware that the shear strength on 3D printed threads isn't going to be that impressive. I'd suggest printing a female hex shape into the backside of it and using a steel nut instead, provided your design allows for that...

[jari.peltola]

I use printed threads every now and then. With my Prusa MK3S the threads come out pretty well, but need still to be cleaned with a tap.

In this example the thread is M5. The yellow locking nut has also printed thread.

But yes, if I need more strength then I usually use the "embedded steel nut" technique that Seth mentioned.

Br. JariP