Why am I not getting the expected surface area of a part? Constraints issue?

jay_sommers

I'm learning CAD and am currently working my way through the "Introduction to Part Studios" module in Onshape's "Learning Pathways." My goal is to really learn CAD, not just understand the instructions. In the exercise for Basic Features where you extrude a control arm sketch you create, I'm getting close to the surface area in the answer in the quiz, but not precisely: 50195.18 mm² and 50179.71mm², respectively. I'm investigating what things could influence the overall size when the dimensions I was given match up, the nuances of which seem vital for when you're making a part that you need in exactly the right size and profile.

One thing I identified is that where the perimeter lines of the sketch interface with the center circle appear to be located differently than in the sketch provided in the module. I made the line and then applied the "Tangent" constraint to the large outer center circle and the smaller outer lateral circle. I "Mirrored" that line to create all four lines between the circles. I get an error when trying to apply the "Coincident" constraint for the line and circle being different shapes, but it looks like there is a "Coincident" constraint in these spots on the module sketch.

Could this cause the difference in surface area I'm seeing? How do I get my lines to meet with the outer circles at the correct points? Is there anything else that seems off?

Onshape Extrusion Module_Surface area question_1.png

Onshape Extrusion Module_Surface area question_2.png

Onshape Extrusion Module_Surface area question_3.png

Onshape Extrusion Module_Surface area question_4.png

eric_pesty

The end of your lines are coincident with each other and/or the right plane so you can't also make them coincident with the circle.

Bet option is probably to use the trim tool, if you zoom in, you'll be able to see the part of the lines that go from the point where they are tangent to the circle to the midplane, get rid of that bit

eric_pesty

jay_sommers

@eric_pesty it looks like you just circled the areas I identified in my post, where I got an error trying to apply the coincident constraint. Would you take a look at the information I gave and my questions, and then respond in words if you have ideas on how to address my questions?

eric_pesty

The end of your lines are coincident with each other and/or the right plane so you can't also make them coincident with the circle.

Bet option is probably to use the trim tool, if you zoom in, you'll be able to see the part of the lines that go from the point where they are tangent to the circle to the midplane, get rid of that bit

jay_sommers

That did it! Thank you @eric_pesty .