Dimensions needed

[Anonymous]

I just created a new sketch, and I see the message displayed in the lower right corner "18 dimensions needed".  I keep staring at the drawing trying to figure out what is unspecified by either explicit dimension or constraint, and don't see anything.

Of course, the first thing I did was search the online docs for "dimensions needed" and NOTHING useful came up; in fact, instead of giving me all the entries that had both "dimension" and "needed", it gave me a ton of irrelevant entries that only matched "dimension".  If there is a message that appears on the screen, there MUST be a corresponding piece of documentation that is found when that message is seen.

I would have expected that if these dimensions are needed, it is because the program found unconstrained distances.  It knows precisely where these are.  I would have expected that if I clicked on "Dimensions needed", it would highlight in some way the places it was unhappy about.  Why won't it tell me where the problems are?  I don't see the problems.  What's the big secret?  Not documented, non-disclosing error messages should never, ever exist in a product (as someone who wrote large systems for fifty years, I guaranteed that every error message was unique, if only by displaying a numeric code with a boilerplate message.  Without this, field support and bug reporting could never have been possible.  Since I made sure every message was documented, there was never a question as to the cause and recovery for each message.  I know that it takes effort, but that effort paid off bigtime with both customers and vendor support sites.)

[Anonymous]

Your horizontal/vertical construction lines are floating (length & position required for each)

BW

[CCarreiras]

Hi!

It's due the construction lines.

check the video and see how you can see what's missing and how to repair.

[SBix26]

First: please always say what version of Inventor you're using.  I discovered that the file you posted is Inventor 2014 format, but it would be much easier for anyone considering offering help to know right away what version they should work with.

As @Anonymous pointed out, the unconstrained parts are the endpoints of your construction lines (mathematically speaking, they are actually line segments).  Inventor 2014 is just waiting for you to ask with the Show All Degrees of Freedom button as illustrated below:

Sam B

Inventor Professional 2016 Update 2
Vault Basic 2016
Windows 7 Enterprise 64-bit, SP1

[TheCADWhisperer]

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@Anonymous wrote:

....  What's the big secret?  

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Logic.

[TheCADWhisperer]

I recommend to beginners that at the very second something unexpected occurs that they STOP and investigate.  Find the logic behind the unexpected behavior and it will no longer be unexpected.

Logical Steps

1. Create one line, rectangle, or circle and dimension it immediately.  Fully constrained? Logic: Yes - continue. No - stop, and figure out why.

2. Create next line, rectangle, circle or arc and dimension it immediately. Fully constrained? Logic: Yes - continue. No - stop, and figure out why.

3. See #2....

Once the basic logic of sketching is understood, multiple entities can be created and then tidied up.

Kind of like writing code, or any proceedure that involves logic.

Included in the logic of design should be the manufacturing/inspection technique.

The Inventor Auto-dimension tool could be use to fully  constrain a sketch - but the design intent of the manufacturing/inspection process will be undefined.

See attached file.

[Anonymous]

Ah, I guess I missed reading the piece of documentation in the manual that explained this. You know, the piece that is found when I search for "Missing dimensions"

I was not aware that a construction line needed a length. I saw them as abstract line segments which could be constrained to geometries and which in turn could serve as geometric objects that I could use as constraints.
joe

[Anonymous]

In logic, there are a set of axioms, and from these axioms one derives a set of theorems and lemmas. I agree that your answer is correct; what is missing is a well-defined set of axioms. Instead, I am forced to infer the axioms, and therefore I lack critical facts necessary for correct inference. I treated construction lines as abstract line segments not requiring a length if their position were constrained. But then, I should have been able to learn this when I looked up the error in the documentation.
joe

[Anonymous]

Thank you! That was very informative, and I learned something new. Too bad the creators of the documentation failed to include such a useful example in the documentation. I didn't find anything this useful when I searched for "Missing dimensions ". No, wait...I didn't find ANYTHING that was remotely useful when I searched for "missing dimensions".
joe

[TheCADWhisperer]

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@Anonymous wrote:
.... what is missing is a well-defined set of axioms. Instead, I am forced to infer the axioms.....
joe

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noun

1.a self-evident truth that requires no proof. 

2.a universally accepted principle or rule.     y=mx+b  a line has a beginning point and an end point

[jtylerbc]

You seem to be correct that it isn't explicitly mentioned in the help files.  Have you looked at the tutorials on the "Get Started" tab?  This concept is taught very early in most Inventor training courses, and I expect it is somewhere in the sketching-related tutorials as well.  I am a little surprised that it isn't mentioned (and maybe it is buried in there somewhere), but maybe the writers of the help files overlooked it because it is typically covered in training.

One thing that is commonly confusing to new users is that the name of the field is somewhat misleading.  It does not literally mean "number of dimensions."  It actually represents the number of unconstrained degrees of freedom.  Constraining this geometry (and reducing the "dimensions needed" count to zero) typically requires a mixture of actual dimensions and geometric constraints.  For most of your construction lines, simply constraining the endpoints Coincident to the lines of the large rectangle would fully constrain them without adding any actual Dimensions (as Carlos did in the Screencast he posted).

Forgetting about constraining construction lines is an extremely common new-user mistake.  Also, I would point out:

• Two of your construction lines are coincident with an origin plane.  If you had Projected the plane (or axis) instead of drawing a construction line, it would be behaving exactly as you said you were expecting construction lines to behave.
• Some of your construction lines are unnecessary, as you could delete them and snap the dimension to a tangent point of the arc or circle instead.
• Other construction lines are unecessary because they could be replaced by a horizontal constraint between points (such as the center points of circles).

Hope this helps.