scaling issue, model always to small (3 replies)

Hello,
i have a problem with my prints. Overall, they look very good but the problem is, that they are a way to small.
If i print my parts in slic3r with a scaling of 102%, the dimensions fit very well, if not, everything is to small.
For example, the 20mm box is printed out at 19.60mm in x and y. I have now calibrated everything again and again, my prints are always to small. If go 100mm in printrun, it exactly goes 100mm in x and y, i think there is no problem with that... I have no more ideas to solve this, because i can't print mechanical parts because of non fitting,...

calibration:
I have set the steps for x and y to 80 for t5 pulleys and nema17 1.8° with 1/16 stepping. My esteps are calibrated, if i want to extrude 20mm of filament it meaures 20mm. i have make the perimeter calibration, so my thin wall object has 0.7mm and slicer says, in the gcode, 0.7mm for the perimeter. (0.5mm nozzle and a layer high of 0.3mm)

Can anyone give me some hints what to do to get the right scaling, exept to scale the model with a factor of 1.02?