Back extension bench, or the Roman chair is one simple and yet effective exercise machine to develop and straighten your lower back, as well as abdomen, glutes, and hamstrings.
If you decide to make this Roman bench plan, be sure to read some of the instruction on the internet how to use this fitness equipment in order to avoid unnecessary injuries.  

NOTE: The measurements within this text and 2D documentation are given both in millimeters and inches (in brackets).

Use various tools and clamps in process of welding, so you could weld the items together with accuracy.