Where can I learn a sequence of steps for sketching?

michael_mcewen

After hand drafting for 20 or so years, I'm learning Onshape.  This is my first CAD program.  I've been putting in hours of study, listening to tutorials, following and making test sketches, with a fair bit of success.

But as soon as I try to make my own sketches for my work (lighting design), I am quickly confounded by my lack of understanding about constraints, and the requirement of CAD to fully define all elements of a sketch.  Again, I have read all the tutorials, and I do have a basic understanding of the purposes of these elements.

What I have come to realize is that with hand drafting, the sequence of steps in making a sketch can be arbitrary, as once committed to paper, the relationships are clear, if dimensioned properly.  There's no history of steps, because it doesn't matter.  So, even though my approach to drafting may be unconventional (self-taught), my drawings don't care!

But with CAD, I realize that I'm missing a fundamental understanding of how to approach making a sketch.  I've come to realize that consideration of the point of origin, and the step by step of constraining of elements in order to fully define a sketch, is essential.

I continue to make sketches in Onshape as I always have approached sketching, and I end up with a sea of blue lines and arcs, with absolutely no idea of how to get to black, fully defined elements.  I've tried tugging on lines to see what comes loose, and then, frankly, thrashing about FOR HOURS in search of a fix.  

Here's an example: 

https://cad.onshape.com/documents/4a78b4d43bba9895005d0217/w/5d35f7e08d1b0971e81a8eee/e/f614241248b14b441e30bbe0

Can anyone point me to a basic course where I can begin from the bottom up to build a foundation of understanding of the basic elements and a rational sequence of steps in building a sketch in CAD?  I need to acquire a strategy of approach.

Thank you,

Michael

brucebartlett

Have you got a photo of what you want to model. By looking at your sketch I think you should maybe break it down into simple parts but hard to tell without the end goal in mind.

Design intent also comes into play as you structure your constraints. 

michael_mcewen

Hi Bruce,

Well the part I drew is a piece I will have waterjet cut in 1/8" copper. I haven't extruded it in my sketch yet, because it's woefully under defined. But in terms of my dimensions and how the sketch looks, it's perfect!  But not according to the program. 

The image below is a similar fixture with a similar part built into the assembly. The piece I drew needs to be one part. 

Thanks,

Michael

viru

michael_mcewen, Your sketch is under defined because you haven't given constraint relationship between sketch entity. Kindly refer below linked document and video to learn some basics things about Onshape.
https://cad.onshape.com/help/
https://cad.onshape.com/help/#sketching_basics_video.htm?TocPath=Onshape%20Videos|_____3
https://cad.onshape.com/help/#dimensions_constraints_video.htm?TocPath=Onshape%20Videos|_____4
I hope above information will be helpful to you.

brucebartlett

Fixed it up a bit for you. Not sure it's completely right but may help.

https://cad.onshape.com/documents/57049980e4b0491f10f2a277/w/21ddf79ea3cf2e06120ddcdd/e/7f0bb04feb510031b98f7841

I deleted some of the lines out, then mirrored in the sketch from side to side so now you will have symmetric constraints. You were also missing some constraints on to the origin which I add to turn everything black. 

When analysing dragging the blues to understand the constraints then ctrl z to remove the drag is handy, you can also tick the show constraints in the sketch dialogue . Practice on something simpler to start may help get your head around constraints, key word being practice. 

brucebartlett

Hi Michael,

I could not help myself so I just updated the sketch in my copy of your B Ring Parts, see my link above. I thought it would be good for you know how to add simple calculations in your sketches, see attached image for the segment angle calc.

I also moved the hole angle dimension now 3.5deg, from the centre line 49.5 to the outside of the segment, I felt this gave more control if in the future you want to change the number of light fittings. This is what I meant by design intent, however it's no mine so could be barking up the wrong tree on this one. 

I also added a variable for for thickness (#thickness) that I used when extruding the copper parts.  You could also use variables to help drive your sketch, if this is a product that can be customised to a set of sizes or number of feature (i.e amount of lights, overall diameter, height, etc) maybe worth looking at. 



You'll also need the RMB on a face and export as DXF/DWG for your water jet cutter

michael_mcewen

Thanks Viru, I'll watch those tutorials.

michael_mcewen

Hi Bruce,

Thanks for the useful tips.  I do understand what you mean by design intent, in terms of setting up the initial sketch in such a way that I can make future mods without getting stuck or having to start from scratch, etc.  

I think because I'm new to CAD, and don't have any experience with the myriad ways one can paint themselves into a corner, I really have no strategies for how to initially approach a sketch, in terms of 1st step, proper sequence of actions, necessary constraints, and other relationships.  Again, in hand drafting, I never had any such concerns; these rules were not necessary; it simply didn't matter.

I'm still hoping someone can point me to some kind of comprehensive, step by step beginners guide to parametric modelling.  My preference would be to get completely solid with these rules, rather than stumble upon them as I draft, which has been quite frustrating.

Thanks again Bruce,

Michael

brucebartlett

Have you got a photo of what you want to model. By looking at your sketch I think you should maybe break it down into simple parts but hard to tell without the end goal in mind.

Design intent also comes into play as you structure your constraints. 

michael_mcewen

Hi Bruce,

Well the part I drew is a piece I will have waterjet cut in 1/8" copper. I haven't extruded it in my sketch yet, because it's woefully under defined. But in terms of my dimensions and how the sketch looks, it's perfect!  But not according to the program. 

The image below is a similar fixture with a similar part built into the assembly. The piece I drew needs to be one part. 

Thanks,

Michael

viru

michael_mcewen, Your sketch is under defined because you haven't given constraint relationship between sketch entity. Kindly refer below linked document and video to learn some basics things about Onshape.
https://cad.onshape.com/help/
https://cad.onshape.com/help/#sketching_basics_video.htm?TocPath=Onshape%20Videos|_____3
https://cad.onshape.com/help/#dimensions_constraints_video.htm?TocPath=Onshape%20Videos|_____4
I hope above information will be helpful to you.

brucebartlett

Fixed it up a bit for you. Not sure it's completely right but may help.

https://cad.onshape.com/documents/57049980e4b0491f10f2a277/w/21ddf79ea3cf2e06120ddcdd/e/7f0bb04feb510031b98f7841

I deleted some of the lines out, then mirrored in the sketch from side to side so now you will have symmetric constraints. You were also missing some constraints on to the origin which I add to turn everything black. 

When analysing dragging the blues to understand the constraints then ctrl z to remove the drag is handy, you can also tick the show constraints in the sketch dialogue . Practice on something simpler to start may help get your head around constraints, key word being practice. 

brucebartlett

Hi Michael,

I could not help myself so I just updated the sketch in my copy of your B Ring Parts, see my link above. I thought it would be good for you know how to add simple calculations in your sketches, see attached image for the segment angle calc.

I also moved the hole angle dimension now 3.5deg, from the centre line 49.5 to the outside of the segment, I felt this gave more control if in the future you want to change the number of light fittings. This is what I meant by design intent, however it's no mine so could be barking up the wrong tree on this one. 

I also added a variable for for thickness (#thickness) that I used when extruding the copper parts.  You could also use variables to help drive your sketch, if this is a product that can be customised to a set of sizes or number of feature (i.e amount of lights, overall diameter, height, etc) maybe worth looking at. 



You'll also need the RMB on a face and export as DXF/DWG for your water jet cutter

michael_mcewen

Thanks Viru, I'll watch those tutorials.

michael_mcewen

Hi Bruce,

Thanks for the useful tips.  I do understand what you mean by design intent, in terms of setting up the initial sketch in such a way that I can make future mods without getting stuck or having to start from scratch, etc.  

I think because I'm new to CAD, and don't have any experience with the myriad ways one can paint themselves into a corner, I really have no strategies for how to initially approach a sketch, in terms of 1st step, proper sequence of actions, necessary constraints, and other relationships.  Again, in hand drafting, I never had any such concerns; these rules were not necessary; it simply didn't matter.

I'm still hoping someone can point me to some kind of comprehensive, step by step beginners guide to parametric modelling.  My preference would be to get completely solid with these rules, rather than stumble upon them as I draft, which has been quite frustrating.

Thanks again Bruce,

Michael