Struggling to understand dimensioning

[abennxxx]

My wingtip fuel tank model is now producing a print exactly as I want it, but I'm conscious that I've achieved it by cheating!  Following advice from CADwhisperer I managed to achieve black lines and a red lock on several of my sketches, but I can't figure out the others:

One example is the Light recess sketch, which is green and doesn't have a red lock.  It's a 48 x 18 slot positioned 26mm from the origin in the X direction, and centered on the X axis.  What else do I need to do to make it black and give it a red lock?

Another example is the Wingtip sketch, which is green and has no dimensions on it, but does have a red lock.  Is green acceptable, or how do I make it dimensioned and black?

Then there's Sketch 28 which appears to be a copy of the Wingtip sketch;  I'm not sure how that got there, and I don't know whether Sketch 28 or the Wingtip sketch is dictating the geometry of the body as a whole.

[TheCADWhisperer]

@abennxxx 

It isn't a matter of just throwing on dimensions - you should use dimensions that make logical sense, dimensions you can actually measure if asked to reproduce the geometry exactly.

How do you measure 159.6392414584?

[TheCADWhisperer]

@abennxxx 

Observe Extrude2...

[TheCADWhisperer]

@abennxxx 

When geometry doesn't show as fully defined - click and drag and observe.

This is the single most powerful technique in all of parametric sketching.

I observe that there are no Coincident Constraints between the arcs and the Construction Line.

When properly constrained to the arcs - the 48 dimension is redundant.

[TheCADWhisperer]

Warning: M2.5 holes are very difficult to tap without breaking taps.  And don't specify bottom tap. Leave some clearance at the bottom of the hole.

[TheCADWhisperer]

@abennxxx 

Location of this circle is not specified?

[TheCADWhisperer]

@abennxxx 

Move is almost always the wrong move (as used by beginners).

Use Dimensions instead to locate your geometry.

[TheCADWhisperer · Accepted solution]

@abennxxx 

Working too hard - get lazy.

Duplicated hole rather than simply edit the original? Why create extra work?

Editing geometry is what parametric modeling is all about. That is one of the reasons we go to the effort of fully defining the geometry. So that it is east to edit with robust, entirely predictable results.

You are making progress but come back to this in 6 months and you will say to yourself, "Whatever in the world was I thinking. Man, I was doing wayyyy too much work".

[TheCADWhisperer]

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@abennxxx wrote:

One example is the Light recess sketch, which is green and doesn't have a red lock.  

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The green geometry is because you applied a Fix Constraint.

I almost never ever use a Fix Constraint (observers will note that I just suggested to someone else that they DO use a Fix Constraint in a particular instance).

If I remove your Fix Constraints I observe that the sketch is missing Tangent Constraints between the arcs and the lines.

The Slot sketch command would have taken care of all of this for you (get lazy, let the software do some of the work for you).

When you get proficient - most geometry constraints are applied for you automatically by Fusion, you merely need to add logical dimensions.

[abennxxx]

Thank you for going into so much detail.  I appreciate it.  I'm happy to simplify some of my dimensions to, say, 2 decimal places, but with freehand sketches why does it really matter if the dimension can't be measured?

[abennxxx]

Yes, I positioned that and the other magnet circle by eye against the background grid.  I couldn't (still can't!) figure out what points to reference it to.  I think I then also Moved it after printing a test piece to compare with the item I'm copying.

[abennxxx]

I thought I had edited the original, but obviously not.  That was possibly when I was struggling to view Sketches, and thought I'd lost it so made a new one.

[abennxxx]

Originally it was drawn as a Slot, and I thought it still is.  Perhaps another example of me 'losing' a sketch and then re-doing it!  But again I can't figure out what to reference it to to fix it in the body -- it, and other sketches, seem to be just hanging in space.  Shouldn't I be giving them x, y and z coordinates from the origin?

[TheCADWhisperer]

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@abennxxx wrote:...but with freehand sketches why does it really matter if the dimension can't be measured?

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@abennxxx wrote:

My wingtip fuel tank model is now producing a print exactly as I want it,

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Exactly huh?

Helps avoid unintended errors when you specify exactly what your Design Intent is.  Might be only a matter of discipline, but I have seen too many instances of real-world consequences.

If I get a chance - I will demonstrate how this could have been modeled with 2 sketches. (Not that trying to reduce the number of sketches is a good idea - you generally want more simple sketches rather than two complex sketches. Just to demonstrate.)

[TheCADWhisperer]

@abennxxx 

Don't you want to Shell the thickness to reduce print material, or is your slicer software automatically doing a sparse fill?

[abennxxx]

I'm doing a single pass perimeter and a 2% gyroid fill, using LW-PLA.

This afternoon I've started a small multi-sketch to try and improve my understanding of how things work.  I'll play around with it for a while, trying other Fusion functions.  Long way to go!