How much distance should there be between two moving parts?

[KERMlT]

One thing i don't find in the "Design Guidelines For Makerbots" [1]
is:
How much distance/tolerance/clearance should there be between two
moving parts?

For example:
1. Between a cylinder and a pipe to slide the cylinder into the pipe
(hollow cylinder)
2. Between two parts of a puzzle. They should hold together and not
fall apart easily.
3. Between a U-shaped profile and a wheel/sphere to roll without too
much resistance.

The two moving parts will be printed separately and will be put
together afterwards.
If i measure objects I downloaded at Thingiverse, there is a distance
between 0.5 and 0.1mm.
Is there a rule of thumb of how much space there should be?

I don't have a makerbot (yet) and don't know anyone to ask except you
guys.
But what I have is several years of experience in CAD and modeling. I
hope to design some nice things in the future and share them on
Thingiverse.

Thanks for you help
-KERMlT


[1] http://wiki.makerbot.com/design-guidelines-for-makerbots

[Aaron Double]

I always found that .35mm space between parts worked well.

Aaron Double

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[Chris Chiang]

Tolerances are usually designed to give yourself room for error from
the ideal.

It depends on how well tuned your printer is.
I am able to get my parts down to .25 mm in tolerance in some cases.
However, it also varies on the skeinforge settings, such as shells,
and variations in the diameter of plastic...

However, in general I feel:
0.25mm : a bit close may or may not work well... But you can sometimes
clean up and adjust after with acetone and/or Xacto knife
0.40mm : minimal risk of not fitting, not too loose.
0.50mm : spacious
1.00mm : wobbly and loose.

Also, remember that ABS is flexible material, you can easy design thin
walls that have flex, thus adding friction to axles or allowing for
snap-together parts.

-Chris

[Whosawhatsis]

The most common types of error on an imperfectly calibrated machine
are the same ones that increase the space you need to leave for
tolerances. I had a hacked-up profile for a while that reduced this to
zero, but the hack was disabled when I started using Print-o-Matic,
and the mechanical calibration of my bot is not as good as it was at
that point either. I want to work out a way to translate this hack
into something that can be added to Print-o-Matic and similar tools,
or possibly even become a skeinforge plugin, but I've gotten caught up
in a lot of yak shaving to make sure that my results will be
repeatable, rather than relying on trial, error, and voodoo as my
original hack did.

So, to answer your question, there is no standard because there
currently is not a good way to calibrate that particular aspect of a
machine's prints, but the .25-.5mm figures mentioned above are
probably typical for machines considered to be well-calibrated.

[KERMlT]

Thank you all for your help and advise! Now I know how to start.
I will start drawing a few models and share it later for improvement.