In sixth volume, in last chapter it shows that
Doraemon goes to the future and didn't come back again. The chapter shows how Nobita trying his best to do anything without Doraemon, he got beaten with Giant and still refuse to call Doraemon. Doraemon find him still fighting Giant and help him to get home. It end with Nobita looking at his desk, where Doraemon come at the first time through his drawer. His drawer become a regular drawer without Doraemon's Time Machine.
This is the picture of that chapter
With that, it looks like Doraemon is supposed to end at volume 6. But we know that it didn't happen because it still continue and they didn't make an ending for Doraemon.
So, is Doraemon manga series are suppose to end at sixth volume or it just another story from Doraemon?