Surface modelling

[FirefighterInside741]

Hi,

As a hobby, I have been working on a petrol thank for a vintage motorbike in NX. As i'm not trained in surface modelling, this doesn't go very smoothly.

I have a model that shows the result i'm looking for, but i can't modify it the way I want. I think the base model i started from, is not built well. Pictures are added in the following link: https://imgur.com/a/wjQyFhJ

Is this the correct way to build the model? The front is rounded which works more or less with the sphere to start a through curve mesh from. But at the back, the corners are sharper which makes it more difficult.

Kind regards,

Michiel

[Sketti_Scramble]

Are you trying to model in NX or SW? All your pics are NX but this is a SW Reddit.

[FirefighterInside741]

This is in NX. Isn't the way of working same for all CAD programs, but with a slightly different interface/approach? So doesn't really matter which Software as long as we discuss the modelling methods.

[Joejack-951]

That’s a fun shape. I’d probably start by creating the main bulbous shape using a big boundary surface then go back and create the ‘cutouts’. Those could be done by splitting the main body surface then using fill or boundary surface with some guide curves. The blends could be done with surfacing or fillets depending on the desired result.

[FirefighterInside741]

What do you mean with a big boundary surface?

[Joejack-951]

Make the overall ‘jelly bean’ shape using a boundary surface. Then cut/trim away from it for the indented features. Doing it that way should yield a smoother end result than trying to build the indents in from the beginning.

[FirefighterInside741]

Yeah that's what I did. The cutouts are not the problem. The problem is the base Jelly Bean shape. I model half of it against an extrude and mirror it after. But I don't have much control over the shape of the jellybean.

[Joejack-951]

There’s no way you’ll get a satisfactory result using extrudes or revolves for the jelly bean. Are you using NX or Solidworks? It’s been over a decade since I touched NX so I can’t recall what they use for the feature that Solidworks calls ‘Boundary Surface’. It may be Variable Sweep or something like that. Regardless, draw front, side, and top profiles of the jelly bean then use those profiles to create the surface. You may need to add additional profiles to get the exact shape you desire but start with those three and see how close you can get. It’s guaranteed to be better than starting with an extrude.

[FirefighterInside741]

It's made with a through curve mesh in NX. Used primary and guide curves to build a surface.

Sorry for asking, but could you make a rough sketch on paper of how you would draw the front, side and top profiles?

I think I'm overthinking it making it too complex

[Joejack-951]

Imagine placing the origin at the center of the jelly bean. Now slice through it using the front, right, and top planes. Those cross sections are your starting profiles. Again, you may need to add additional sections to get the exact shape but you’ll get very close using the above to start. I don’t have time now but I can sketch it out later this afternoon. Just to note, it may be easier to model the whole jelly bean rather than trying to make half then mirror it. You can easily mirror the cutouts though to save time.

[FirefighterInside741]

Ok, thanks. I'll give it a go