Do extrusions (non cut side) need to be polished?

[SamP]

I was reading through the Zenith construction standards and there was a mention of the need to remove all cutting marks as they may be stress risers. That makes sense to me. I wouldn't consider extrusion surfaces to be smooth. They often have lines that show the direction of extrusion. Do these surfaces need to be polished or brushed as well? Is that common practice?

 

[TFF]

No. Extrusions strength is including its mill surface. Assuming you have good material. Third or fourth hand stuff that’s been treated like angle at a farm, not so much. It’s the builder breaking the surface making the part that becomes the issue. Kind of like surgery. Are you trying to cut in with a Buck knife or a scalpel? It’s your nicks and scratches that cause problems.

 

[Victor Bravo]

Also... the aircraft industry definition of "polish" is different than the common idea of something being mirror-reflective. In aircraft structure, when you are instructed to "polish out all nicks and scratches", they mean smooth enough that you can't catch a fingernail on it, or feel an edge with the tip of your finger. They do not mean to make it shine.

 

[SamP]

Thanks very much. It makes sense to me. At the microscopic level, where all the fun really happen, the lines on the extrusion would be much smoother than where a cut was made.