It helps to make a flat end of corrugated stainless steel pipe and make a good sealing surface with washerwe supply DN16 , DN18 . A special tool is required to flatten the corrugations on the end of the tubing. There are a few different types of tools available. There is an economical tool which can be used for flattening the corrugations by striking it with a hammer. There is also more expensive tools or toolboxes which are capable of performing the flattening by hand and they can also do multiple sizes of piping installs. We recommend the economical tool for anyone installing fittings to a single size of tubing who will not be doing many fitting installs, and the professional kit for multiple tube sizings or when many fitting installations are to be performed.