1 out of 10 parts way to high?!

Very wierd issue, maby some of you know what can cause this?

Im designing a hexacopter and ive printed some parts/prototypes the last few days. Sometimes I chance just a very little detail and print it again. Ive noticed that some parts are a lot bigger (higher) than ive designed. To be clear: its the exact same part, I just make a hole 3.5mm instead of 3.3mm, I make a wall 2.1mm instead of 2.5mm, etc etc.

Ive printed 22 versions right now and 19 of them are 14,98-15,08 mm high, 3 of them are 15,91-15,98 mm. Ive used two different slicer settings but dont know when i used which setting. The only difference between the two settings has to do with speed. For example: infill at 30 or 45.

Ive no clue?!

I have seen this issue before and I'm printing some Nylon bushings and confronting same issue. Now there is several things that can make for this differences:

1. Different layer height (but you didn't change it) 10mm at 0.1 and 0.2 will be 10mm but at 0.3 will be either 9.90 or 10.2 been 10.2 most standard.
2. Different Materials (My case now) my 24mm bushing need to be scaled down to 23.65mm to come exact at 24. (my printer is calibrated to the tee).
3. Changes from standard to metric values in the slicer or between design softwares (seen this before) If a design was made originally in standard measurements with high tolerances "0.0001" those decimals conversions may add to real dimension when converted to millimeters.
4. scaling errors in slicer (became aware of this little thing recently) when I made a part in Inventor and generate the STL file, when I transfer it to the slicer, some of the measurements aren't exact. Take a look at the attached pictures.

The best way to be sure that what you see is what you get is to always look at your G code, be sure that every value is correct.

I extracted this values out my G code:

layer 81, Z = 24.3483 layer 1, Z = 0.277778 layerHeight,0.3.

Total layers= 81
Layer Height= 0.3
total height of the part= 24.070522

Out of those 81 layers 2 are for a raft so 79 are actually the part 79x0.3 = 23.7 (look at the attached picture) my part is 23.65 so it will have an error of only 0.05 but in reality comes at 24mm.

Hope this help you to understand the beauty of what we do every day for fun........ Or business.

Edited 1 time(s). Last edit at 03/14/2015 06:29PM by ggherbaz.

Hey thanks for you reaction. Made me understand a lot.

Still, the problem remains and ive seen more extreme prints right now. A few days ago I printed a part of 7 cm high. It turned out to be 9cm!!!! You can clearly see the distortion, there is a hole on the side of the print, its shaped as this -> 0

Then I turned off the printer, sliced again, turned on and printed again. Size: 70,06mm

Maby stupid I sliced it again, now I still dont know if its the printer or slicer..

Did you compared both g codes? That could resolve the question of slicer problems.
Do you print from computer or from sd card?
What size threaded rod you use and how the bolt is mounted in the carriage?
When you set your printer originally and printed a calibration piece, what slicer did you used and did the part came right?
Did the printer suddently started doing this or happened after a modification?
If you print from SD card, how old is the card and have you format it recently?

If you printed the same part twice using the same file (same g code) and one comes 9cm and the other 7cm, the problem can only be in either the printer hardware or the SD card reading process, but most likely in the hardware.