Use existing feature dimensions, like total extrusion distance, in new feature

[david.antliff]

Let's say I'm making a box, and I have the wall/floor thickness set as a user parameter - "thickness".  I want the internal depth to be, say, 20mm, but I have to take into account the floor thickness, so I extrude by "thickness + internal_depth". This is now the external depth of the box.

Later I want to extrude something else, but I want to extrude it by the result of this previous extrusion. Now, I can just extrude it by "thickness + internal_depth" but that seems like I'm repeating myself. If I were editing a sketch I could click on an existing dimension and use that in a new dimension calculation, but how do I do this with features like extrusion? As far as I can see, there's no "3D dimension" tool that creates a dimension I can click on when entering the extrusion amount.

Now I know this is a trivial example, and I could just create an "external_depth" parameter equal to "thickness + internal_depth" and use that in both places, but in reality I actually have a much more complicated extrusion sequence, which gives me a final extrusion that is not a simple expression. I'd quite like to just be able to click on the final depth of that extrusion sequence and use that value for a new extrusion - can this be done?

Another situation is where the extrusion has been done "by eye", perhaps as part of a more ad-hoc sculpting workflow, so the exact extrusion distance is arbitrary and not easy to calculate with parameters. I can use the measuring tool to get the numeric value, but that won't be linked to the actual extrusion, in case it's changed later. So a way to somehow link a 3D dimension to a feature specification would be very useful.

[etfrench]

User User Parameters.  If the available math functions aren't available in Fusion 360, then use Excel (or your spreadsheet of choice) to create the formulas.  Parameter IO add-in in the App Store allows you to import and export parameters.

[david.antliff]

@etfrenchyes, I am using User Parameters but I consider that a workaround because it does not apply in my second example, where an existing feature's extent is arbitrary, or the extent is a complex combination of many other features (imagine a cascade of extrusions).

I've never seen "3D dimensions" in Fusion 360 so I assume they don't exist, but I wanted to check. In my mind, it would be possible to add such a dimension and then refer to it when setting an extrusion depth. I guess this becomes a Feature Request at this point.

Another situation where User Parameters doesn't work so well is when working with imported models such as STL files, where the dimensions aren't already parameterised.

Using Excel to calculate parameters breaks the parametric link between values, unfortunately. At that point I might as well just type floating point numbers into the design.

[etfrench]

Using the Parameter IO add-in along with Excel does not break the parametric nature of the User Parameters.

Here's an example of the .csv file created by Parameter IO:

"aParam","mm","( bParam / aVal ) * 1 mm"," "
"bParam","mm","65 mm"," "
"aVal","mm","6 mm",""
"d3","mm","bParam"," "
"d4","mm","aParam"," "