Diameter for hole

[curious aardvark]

then READ THE FORUM - ALL YOUR QUESTIONS HAVE ALREADY BEEN ANSWERED MANY MANY MANY TIMES :-)

[LambdaFF]

My fellow "engineers" here seem to have different ideas of what "exactly" and "perfect" really mean. The various classifications of fit are something that machinists obsess about, as I tried to indicate with the link I posted. Doubtless with some trial and error one can come up with a hole that will adequately contain a metal rod for most purposes (with maybe a little sandpaper and glue to help). But producing a hole that is really a specified press or sliding fit to a cylinder is more complicated than printing, or even drilling will accomplish. Drilled holes tend to be slightly triangular; they aren't truly round or dimensionally correct. The common practice in machining when accurate holes are called for is to drill undersized, and ream to the correct size.

While this is indeed how I was taught to make stuff, I think that for the application range in consideration here, their solution is "good enough". But yeah, once you start talking fatigue cycles, high stress and so on... you need to plan the fit accordingly, that means tight tolerances, that means reaming.

[curious aardvark]

no it doesnt.
It just means making it the right size in the first place :-)

I can get down to repeatable tolearances of several hundredths of a millimetre. Now for most things you're going to make on a desktop 3d printer - that's more than sufficient.

For anything else - well you most likely wouldn't use plastic in the first place.

[awerby]

Several hundredths of a millimeter is the difference between a tight fit and a sliding fit. But maybe that doesn't matter to the person who wants to do this.

[LambdaFF]

For anything else - well you most likely wouldn't use plastic in the first place.

You're right about that.

I guess it's a proffessional bias. But in my line of work, an assembly that's not in spec can fail by fatigue overload in a matter of hours.