Printing with tolerances can be tough. There are settings in the software as well as settings in your printers firmware that may control these tolerances. If you have a prototype that needs to fit with other parts, or you want to make something fits with a certain part, it can be frustrating when your part comes out skewed. There are two different sections in todays Tip, "Off by a large margin" and "Off by a small margin". If you're printing 20 mm calibration cubes and finding that your measurements are 20.5 mm or larger in the X-Y or Z direction, I would definitely recommend starting off in the "Off by a large margin" category first!
Off by a large margin
If your print is off by a large margin, in the X-Y or even Z- axis, it points to one thing. Steps-per-mm values. When the G-Code Simplify3D makes is sent to your printers firmware, your printer uses your Steps-per-mm values to decide how much to turn the stepper motor to move one mm. If the Steps-per-mm value is off, than you will get parts that are not to scale.
If you are using a RepRap style printer, you can open the Configuration.H file of the firmware in Arduino and find the steps-per-mm values there. For MakerBot/Sailfish style printers, you will need to use a custom GPX ini file in your Simplify3D folder and reference it using the post processing script option in Simplify3D which is a bit more invovled.
My recommendation for calibrating Steps per mm if necessary is to do the following:
1) Print a 20 mm cube
2) Measure the cube with calipers
http://www.amazon.com/Inch-Digital-Cali ... 848&sr=1-1
3) Use this formula for correction for the axis you'd like to calibrate. This formula is simply for a 20 mm cube, using cross multiplication to find the Corrected Steps per mm value
CorrectedStepsPerMMValue = (20 mm*CurrentStepsPerMMValue)/(MeasuredSide)
4) Use new StepsPerMM values and reprint the 20 mm cube to validate results!
Quick and Dirty Fix: If you don't want to change your printers firmware or use a custom gpx file, and you need correct tolerances NOW for a deadline. Just use the formula below and then double click your part and scale it accordingly in the X-Y and Z-axis. (Not recommended, but it would most likely be a good enough temporary solution)
Scaling= CorrectSideLength/PrintedSideLength
Off by a small margin
Small margins can be tough! Since any number of factors may be at play.
-If your extrusion multiplier is too high, your outer perimeters could be about too thick and lead to tolerancing issues due to that
-Warping due to too much heat could cause your outer perimeters to lose their shape
-Print speed may be too high
Ensuring your extrusion multiplier isn't too high and also remembering not to print at a speed higher than your printer can handle are great starting points for small margins.
Also, if shifting is causing your parts to be off:
-If your belts are too loose, the motor could be turning and not turning the belt with it
-If your belts are too tight, the motor could be turning and once again, not turning the belt with it
-The voltage to the stepper motors may be off
To Conclude
I think changing and using the formula for Steps-per-mm values and working with those is very important. For small margins, you're really just changing general printing settings, that would affect many aspects of your print, not just size. If your extrusion multiplier is off, you'll probably see it in more spots than just the tolerancing of your part. Really calibrating and setting your Steps-per-mm value absolutely right and as close to as possible as your calipers will let you can provide amazing results and really take your printing to the next level.
From a CAD/design stand-point, Make Magazine has a good article with a section designated for this exact topic which I've pasted below.
http://makezine.com/2013/12/11/top-ten- ... -printing/
FIT TOLERANCES FOR INTERLOCKING PARTS
For objects with multiple interlocking parts, design in your fit tolerance. Getting tolerances correct can be difficult. Kacie’s tips for creating correct tolerances: use a 0.2mm offset for tight fit (press fit parts, connecters) and use a 0.4mm offset for lose fit (hinges, box lids). You will have to test it yourself with your particular model to determine what is the right tolerance for the thing you are creating.