Why there is an error in my sketch?
Because the arrowed dimension is trying to move the sketch point to a position it can't go, If you delete that dimension & drag that point it will only move along the curve of the helmet so far before other relations stop it moving. If you set the dimension to 100 as I have below the sketch doesn't give any errors.
If you're following a tutorial & that dimension is supposed to be less, then you most probably have done something wrong earlier in the model.
This particular dimension is not shown in the video. Because Fully Define Sketch tool is used to fully define the sketch at the last stage. Also in the video I couldn't see any additional dimension after fully defined sketch.
Solidworks tutorial | sketch football Helmet in Solidworks (Advanced Surfacing) - YouTube
When you converted entities you didn't pick the very outside edge that would be on plane, you selected one further around the helmet on the edge of the rib. Also there should be a 40 dimension from circle center to vertex point that is missing. The final point is set to your preference & then fixed using Fully define sketch, I assume using relations only as no dimensions are added but when I tried it this didn't work. There is something strange about the sketch geometry that keeps causing it to over define when I try to do the final part.
EDIT: Just to let you know, you have missed out the fillet at 36:00 which should be between the mirror & sweep.
I couldn't understand this. I purposely deleted 40 mm to make things working.
Even though there is a problem with the sketch, I am able to revolve. Does sketch problem is a matter in this case?
When I opened your model & looked at the sketch & compared it with the video there is a difference. Look at the image below, you converted entities on the orange line & orange arrow, where according to the video it should of been the very outside edge, indicated by the red arrow. Now this doesn't really matter in this case but it could cause issues in some circumstances when trying to follow instructions & dimensions based off of the edge or resulting face/feature.
To answer your other question, if the part revolves I wouldn't worry too much about the sketch unless other features further on are based off of it. I would get rid of the errors & fully define it in my own way if following the video didn't seem to work out correctly.
I had confusion over selecting the edge. Therefore I decided to proceed without fillet but the result was same. You can see this in my attached file. Do lines created by fillet make any different?
Even without the fillet you didn't pick the outer most edge shown in the video (the fillet is not shown in my screenshot of previous post either). Effectively you picked the edge shown below with the orange arrow when it should of been the edge shown with the red arrow. Make sure you are viewing the model Normal to the front plane & zoomed in to pick the correct edge. The fillet doesn't make any difference as to whether the outer most edge (red arrow) is available for selection.
Before I did like that, but I repeat it again as you advised but the result is same with same error massage.
I found a method to escape from the problem, but I do not know whether it is correct or not, but it is working without any problem.
Dimensioning the sketch with relation after that deleting all the dimension only (& not relations) then using “Fully Define Sketch” tool to make the sketch a fully defined one.
Why is the "Fully Define Sketch" tool being used rather than learning to fully define sketches for yourself? As you probably know, this tool is NOT always good practice.
Usually I never use. But this particular model is giving me colossal problems, that is why I am trying many methods. I do not want to leave the model as well because it is challenging.
Not sure on error but I set the 99.67 dimension to driven and back to driving which make everything good.
I couldn't understand what you mean please explain.
I think you should delete that sketch and do it again
As it is challenging model. I am going to redo it again from scratch.
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