# 3D Design / 3D Scanning / 3D Modeling > 3D Modeling, Design, Scanners >  Diameter for hole

## viktor

Hi

I try to build some toy in freecad

If does anyone have any experience

So the object has a cylindrical cavity in which after printing I will put a cast metallic cylinder. What the diameter of the hole I need for cylinder with 10 mm diameter to sit tightly in the socket but not to bursting material, in other words, perfect. 10mm or with minus?

Question 2

Not very expensive  printer and plastics good  for mechanic details printing ?

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## curious aardvark

start with 10.5 internal diameter. 
voids - I find - are usually slightly smaller than the cad specifies. 

So you should find that a 10.5 mm stated diametre will give a hole that's a good fit for a 10 mm object. Depending on your printer you might have to go to 11mm. 

Bear in mind that a 10.5mm hole actually only gives 0.25 mm leeway over an actual 10mm hole.

Attached a small cylinder with an internal diameter of 10.5. 
try that - always easier to test before you do a large print :-) 
10.5mm hole.stl

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## awerby

I'd say print it a little small and ream to size if you want a good fit. 

Andrew Werby
www.computersculpture.com

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## curious aardvark

that's just stupid, when you can just print it the right size to start with. 

But apparently he doesn'e have a printer - so it's kinda moot :-)

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## awerby

You may not be aware of it, but there are various degrees of fit between cylindrical parts and holes. They vary by extremely small increments. Here's a page that might clarify this for you: http://www.mitcalc.com/doc/tolerance...erancestxt.htm

By definition, there's no such thing as a "perfect" fit; there are always going to be deviations between the absolute perfection of a CAD model and the results achieved in physical reality. These include dimensional deviations, shape deviations, position deviations, and surface roughness deviations.
 I don't know of any 3D printer that can produce holes to extremely precise fit tolerances, not to mention the "not very expensive" one the OP is considering. 

Andrew Werby
www.computersculpture.com

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## curious aardvark

well once you've had your printer a while and learnt to use it - you should be able to get exactly the sizes you want, consistently. 
Yes even with a cheap printer. 

The printer should be consistent (mine certainly are) so as long as you know it'll print slightly larger or slightly smaller - there is absolutely no reason you can't produce parts the size you want them time after time after time. 
I can :-)
And so should you. 

Why print something you know is the wrong size and will require post processing - when with a little thought you can simply print it the size you actually need.

Now maybe my cheapo creator x, is more reliable than every single other printer on the market and that's why I can get extremely consistent results every time. 
But that's probably not the case - more likely my obsessive attention to detail ad the fact that if I have a piece of kit - I'll learn to use it properly :-)

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## Marm

If you have a printer, then printing off a small test block with a variety of sizes, say 5-6 holes from 9.75mm to 11mm and see if the bolt fits the way you want.   When I do this, I try to make the hole surrounded by an amount of material similar to how the final product will be.  That way the shrinkage of the material as it cools (I do seem to encounter that a bit with ABS) will be similar to the final piece.    I've done ultra minimal test prints, only to find the amount of shrinkage in the final varied from the test.     Another option is to print to a tight fit, then drill it out with a 10mm bit.  The machined bit will ensure a perfect size.

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## awerby

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.

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## viktor

thank you for all answers ! 

for the first need more practical   info about printer and plastics good  for mechanic details  and also  how about elastic rubber materials?

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## viktor

thanks for  comprehensive information

need more practical  info about printer and plastics good  for mechanic details  and also  how about elastic rubber materials?

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## curious aardvark

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

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## 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.

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## 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.

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## 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.

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## 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.

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