How to get a desired thickness of elbow
Thickness of raw pipe is important if you want to ensure qualified elbow.
Thickness of  tolerance on elbow and pipe are the same,it is +/- 12.5% . Thickness reduced problem will not be happend normally,but due to mandrel and operating skill, some area in an elbow will has thickness reduced problem. To avoid the problem, to add more extra thickness when choosing raw pipe. For example : Elbow thickness is 8.18mm,you can use raw pipe of thickness 8.5mm.