Creating (and using) an involute coordinate system

I've defined an involute coordinate system using an interpolation function. For an unstructured grid of x and z coordinates, I have defined two new coordinates t_coord and h_coord which have vectors everywhere parallel to a circular involute and everywhere perpindicular to a circular involute, respectively.

I can't figure out to make COMSOL recognize them as a new reference frame however. For instance, in the general projection model coupling feature I would like to be able to use t_coord and h_coord as my directions of value storage and integration. Also, in output plots I would like to be able to enter them as expressions to use along my x and y plot axes.

I've tried making variables out of the interpolation functions (t_coord_var = t_coord(x,z) ...) but that still does not seem to work. I've also tried making a new coordinate system with the option under "definitions" using these new coordinates, but that doesn't seem to work either. It's also possible that one of these methods have worked but I am calling the resultant variables by the wrong thing when attempting to access them later. But, for instance, if the variable t_coord_var is valid then I would think I should be able to access it simply by typing "t_coord_var" into the coordinate expression field.

Any help is appreciated.

Thanks

Adam