What do you mean by sheet?
What i understand is you had drawn a sketch and then revolved it then applied thicken option to top face .
Is it right?
See my file i have done the same thing but im not getting that extra sheet.
Answered with a tutorial: https://grabcad.com/tutorials/does-this-look-normal-there-seems-to-be-an-extra-surface
Yes, the same thing happens to me in Solidworks 2014 sometimes. My extra surfaces appeared after using the "knit" surface command. I used "knit" 16 times in the last model, there are two of them that have left extra surfaces. Both times I knitted one additional surface to others that were already knitted together.
I haven't figured out the reason yet, but deleting the extra surface worked fine for me so far. I've checked multiple times, when reordering features or making structural changes to the model, but it never caused any other troubles. Also, the appearances of these extra surfaces are fairly consistent, once they appeared, they always will under the given circumstances, no matter what you change on the model otherwise.
You can use the "Delete face" command, or - if this extra surface is a surface body itself, like in your case - the "Delete Solid/Surface" feature.
hope this helps
thanx i will try it next time it shows its ugly "face"
i hope this will help...
Answered with a tutorial: https://grabcad.com/tutorials/does-this-look-normal-there-seems-to-be-an-extra-surface--1