Hello,What is the reverse comand for Chamfer (working with 3D solids)?

There isn't any.  Use Chamfer with a "0" setting instead.

I  want to share this info about chamfer  with you.  If you have two solids  say a  circular  shaft  attached to a  circular plate  and if you  try to chamfer the  joint between the two solids , to make it look like a weld the chamfer will narrow  the  joint. However if you Union the two soilds  and then chamfer, you will get a a nice  weld like  chamfer at the joint.