GD&T Flatness Symbol

[@maryamgoli-x3w]

Thanks a lot sir. your video training is very helpful.

[@printlife9016]

Hi, what's the difference between flatness and surface roughness? if I use surface roughness Ra 1.6 for a feature surface, does it also mean flatness is controlled by surface roughness?

[@saurabhroy7100]

But why do I need flatness? I will control it by parallelism...it will be more logical right..why pay extra money to control smoothness

[@myigetit]

Great question. Flatness lets you control the "smoothness" without it being in relation to anything. Depending on your part and needs it could cost more to inspect for parallelism because you then have two surfaces you need to compare, and smooth or flat surface could fail because it was parallel. Often time you can use flatness to refine a surface that is also controlled for parallelism. But there are certainly parts where just controlling parallelism makes the most sense.