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I have a reprap printer with 0.3mm nozzle. It prints quite well, I am really surprised with quality of all the surfaces and the general precision of the parts printed. BUT I HAVE a problem: when making (for example) a 10mm x 10mm x 10mm cube with a 2.8mm diameter hole from top to bottom (to fit a screw) after I print it gets a size of 3mm diameter.

I know this is related to extrusion width but cant the slicer software (I am using s3d) know that it is using a specific extrusion width and compensate for that in order to get the diamter right?

OBS: this printer is supposed to get 0.05mm precision.

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    $\begingroup$ You may need to double check this, but last time I looked at getting a new printer the precision that is rated is in reference to the position precision of the nozzle, while the part precision will be substantially worse than that. TO get the precision of the final part I had to call the manufacturers directly and get them to fess up to what they were claiming $\endgroup$ – Diesel Apr 17 '17 at 20:34
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It's a generally accepted fact that FDM/FFF printers will have deviations when it comes to holes and perimeters. Typically, holes print smaller than designed and external surfaces end up larger than designed.

In your case, it seems to be the opposite: the hole is too big. It could be that you're just printing too big overall. You might want to make sure that your printer is printing the 10mm X/Y dimension in you example correctly. If it's too big, part of it may be just the typical oversized perimeter, but some of it might be due to incorrect firmware X/Y 'steps/mm' or extrusion like you mentioned.

See also: "Are you printing undersized holes?"

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This is a common problem with many different types of 3d printers. When ever you print something the physical object will never be the exact same size as it is in the computer. However, you can get it to be very close to what you need. By tweaking certain things you can get the sizes almost perfect. It could be something slight like your stepper motors are mis-calibrated even a little bit. The distance between the build plate and the extruder could be off causing your print sizes to vary. Also the temperature of your extruder and your build plate could be affecting your overall size after printing. How long your print takes to cool and what temperature it is at at when it prints can affect sizes of your print.
This link could be of some assistance:

https://ultimaker.com/en/community/11526-problem-with-correct-size
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    $\begingroup$ If you include a link in your answer, please summarize the linked content, so we know what to expect at the linked site and your answer doesn't depend on the external source (i.e. the key information is contained in your answer and your answer remains useful, even if the external site becomes unavailable). Also, you can use markdown to format your answer (and include a link like this). $\endgroup$ – Tom van der Zanden Apr 18 '17 at 14:50
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Simplify3D has a feature that makes holes a little bigger or smaller (for adjusting tolerances).

I don't remember what it's called, I think it's something-offset (if someone can help me with this in the comments I'll update the answer).

It's possible your silcer is set to enlarge the holes and the printer is just fine.

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For one thing, the entire part generally shrinks, as it's printed (at whatever precision of nozzle location) as molten plastic, and then cools and solidifies.

Given that you are getting a bigger hole in the finished part, I'd start by measuring the part's actual overall outside size, as printed (is that 10mm, or might it be 10.5 or 11mm?), and suspect some calibration tweaks are needed.

If the outside of the cube is actually 10mm, then you may simply need to find the right fudge-factor for hole sizing - if 2.8 is becoming 3, you might need 2.65 to get 2.8 as printed - or 2.63, or...

Alternatively, plan for drilling when a precision hole is needed, and print a deliberately undersized hole with a thick shell so it can be drilled out precisely.

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