# Using global variables and equations to define sketch dimensions

Question asked by Dylan Studden on Aug 30, 2017
Latest reply on Aug 30, 2017 by Glenn Schroeder

I am trying to parametrically model the geometry of a volute for the turbine housing for a turbocharger on SW 2016, and basically when I am done want to be able to input a value for the turbine housing A/R and have the solid rebuild to that aspect ratio of the housing.

When I go to change the input value in my equations table and rebuild the document, nothing happens. The dimensions of the sketches don't change until I go into the sketch and double click on the value until the 'Modify' window appears, then the dimension changes to the value in the table. I have to go through each sketch that is dependent on this input dimension in order for the model to be correct. I can't figure out what i am doing wrong.

I probably did a terrible job of explaining this so if I need to clarify anything please let me know. I would appreciate any help.

Clarification:

So basically the volute is just a circular tube that is bent into a spiral. The A/R is the ratio of the cross sectional area of the tube inlet and the distance between the center axis of the spiral and the center of the tube inlet cross section.

Basically what I have done is made the changing diameter of the tube through the spiral dependent on the inlet diameter, by defining the dimensions as global variables, and then in my equations table (that you get when you go Tools > Equations) defining the relationship between them, i. e. "diameter_1" = 2in, "diameter_2" = "diameter_1" * 0.5.

I can go in the table and change the value for diameter_1, but after rebuilding (cntl + B/Q, or checking auto rebuild) nothing on my model changes until I go to edit the sketch which contains the dimension for diameter_1, and I'll also have to go into the other sketch which contains the dimension for diameter_2, and double click on that dimension for it to show the change.

Attached File Disclaimer:

The model is really messy and has a lot of suppressed unneeded features. My apologies.