Prints are smaller than the design by about 1mm

[swindelljd]

I have a Rostock Max v2 and I've tried to print a case for my raspberry pi and have found that even though the design in Tinkercad says the part is 61.30mm wide the actual print comes out to be only about 60mm wide. This results in the case interior being too small to fit the pi into and therefore has to be junked.

In another case we printed 5 boxes from the same stl file only varying the infill design and found that the one with 100% infill and the one with 0% infill were about 1mm smaller length and width from the other 3 boxes.

A third situation I tried printing a case to hold 4xAA batteries. Again the interior was about 0.5mm too small to fit the batteries.

What can account for these variances and what is the best way to adjust for them, especially in situations where the interior needs to be a minimum size for an object to fit.
Could it be :
* the material - PLA or
* the print temp - 200F or
* the print speed - 60mm/s for edges or
* the print software - MatterControl

Or is just within the margin of error for this printer?

thanks,
John

[Captain Starfish]

Look up the thermal coefficient of expansion for the plastic you're printing. You will probably a large part of th
e undersizedness of square parts comes down to shrinkage. You may find it's laying the plastic down at exactly the right size but the part shrinks as it cools.

Arcs and circles are a different matter and are well documented throughout the forum.

If you work out what that percentage is, you then need to allow for it on critical tolerance dimensions in your model by oversizing things appropriately. Or just scale the whole model by 61.3/60.0 before printing it.

[Polygonhell]

That error seems large for that size of part, shrinkage should be somewhat less than 1% even for ABS.
You should print a few test pieces and measure inside, outside sizes as well as the height of the parts.
The only things that really impact size are steps per mm and arm length and you shouldn't touch either unless you have after market arms. Over extruding can also affect the overall size, but usually only by a few tenths of a mm.

[jckrieger]

I've found that slic3r and matterslice both seem to have issues printing a cylinder with a 10 mm OD and 8 mm ID. Slic3r prints both dimensions 1 mm too small and matterslice uses 2 perimeters and makes the OD too large and the ID too small. Cura seems to print this part correctly. All other objects print within .1 to .01 mm of the intended size (such as 20 mm calibration cubes).