Printing a tube 120 mm diameter - print is undersized  

Cube 20mm

If it helps, I have printed the measurement cube. x measures 20mm, y measures 20.1mm and z measures 20.2mm

You could slice your model, use file/save project as/ and upload that file here after zipping it

We could see if you did something to get unusual results if you uploaded your post-slice project from prusa slicer.  Dimensional accuracy is something that you may have to do some scaling if you are looking to get a different result.  Suggest you look at the dimensions in the slicer after slicing the model - it may be you didn't model what you think you modeled.  Hard to tell

My slicer model and actual measurements

dust_port_prusa_forums.3mf

Thanks Dan, see attached. Correction to initial post.

Bottom of model, diameter, as drawn, == 123.5mm

as measured == 122.8mm

-----

At the top of model, as drawn == 121.5mm

As measured == 120.7mm

Print outside first

I seem to remember that the print outside first is intended to help with dimensional accuracy.

Of course you could play with scale factors.

Print outside first

where can I select the print outside first option?

External perimeters first

You can enable 'external perimeters first'. You need to be in 'expert' mode and then you will find this towards the bottom of the print settings page. 

Remember you are printing hot but measuring cold.  There will be thermal contraction It varies with different filaments. Cylinders have all strain vectors pulling to the centre, there will be stress contraction.

The answer is to print a test part and calibrate.  FDM is very repeatable so you only need do so once per part/filament combination.

Cheerio,

Prototype

Thanks all, I have applied the advice from Diem, and decided to print a shorter / smaller prototype for sizing and adjusting from there. Although i haven't got the accuracy from the 3d CAD model to the print, I am reasonably happy with the initial prototype method!

RE:

It is possible that your expectations are too high for the tolerance of your first 3D printer. However, there are several settings that can affect the accuracy of your prints, such as the nozzle size, the layer height, and the print speed. Additionally, factors such as the type of filament and the quality of the build surface can also affect the accuracy of your prints. You can try adjusting these settings and experimenting with different materials to see if it improves the accuracy of your prints. Also, you can check the calibration, bed leveling and the extrusion rate of your printer. it will help to get accurate prints. 

Cheers,

RE: Printing a tube 120 mm diameter - print is undersized

If you're slicing using Cura, you can try the experimental mode called 'slicing tolerance', the slicer usually takes some liberty with the 3D model when slicing them into layers. Use the 'Inclusive' setting if your print is undersized. 

Here's a video that explains slicing tolerance, and why it's an issue sometimes