Size calibration error - I'm baffled!

I’m not a newby, Been printing on my CR-10S for months now and I’ve produced many great items, both from web designs and from my own designs created in SolidWorks. Now here’s my problem:
I am trying to make a simple box 161mm on a side. Using SolidWorks I lay out the box (I’ve been using SW for years, have no problem there).
Then I slice with Cura 3.4.1 as I’ve been doing. The printed box comes out about 7mm too small. So I went back to basics and printed the 20mm calibration cube </> and it comes out exactly right. it seems the slicer and printer are OK. Next I laid out my own 20mm cube and it printed exactly right. So I try the box again, same problem, it is too small. So I went back to SW and using the same box I reduced it to a 161mm rod 3mm by 3mm. IT CAME OUT RIGHT, exactly 161mm.

So what is going on here? SW makes the right size test rod, made from the same box, but the box is undersize about 7mm.

Ideas are needed

If it is a simple rectangular box, Cura will tell you the exterior dimensions of that box in the lower right of the printer window, prior to printing the part. If that dimension isn’t what you’re expecting, the problem likely lies upstream with the Solidworks file or STL export.

Agreed. Cura says 161mm which is what I want. But it prints 7mm to small. Other things print at 100%, this one doesn’t. Tried repeatedly.

Still baffled!

Maybe try to rotate it 90’ and print again. On my Folger FT5, the steps per unit was off in the firmware, causing an object to be correct in one orientation (say the x), but off in another (y). Or maybe a belt/stepper is slipping on or missing steps on one axis.

Those are the reasons I went back and printed the calibration cube, I came out exactly 20mm x 20mm x 20mm. Same for the similar cube I laid out in SolidWorks. It was fine too so I don’t think there is any problem between the printer mechanism and the printer software and the slicer and SW. They all seem in sync until I print this particular Item.

Still baffled!

20mm cubes are small items, that usually only print in the centre of the bed, so try printing multiple cubes around the bed, to ensure that all areas are correct.

This will show if there are any issues with the printer (belt slippage etc.)