I'm not sure about the "stacking" stuff in the error since you're not stacking columns here, but I would expect an error from concatenating numeric values without converting them to character strings first. Wrap each numeric column in your formula in char().
dt_summary << New Column( "Category",
Character,
Formula(
:Name( "Frame Config" ) || "_" || char(:Name( "Bar Spacing (mm)" )) || "_" ||
char(:Name( "Clamping Spacing (mm)") )
)
);
-- Cameron Willden