Concantenating values selected from comboboxes

Hi all,
Having a bit of a problem trying to get this to work: I have a userform with several comboboxes. When I select a value from combo_material a second combo box appears (combo_material_type). When I make a selection from this combobox I would like that value to be added (concatenated) to the value from combo_material. Here is a snippet of code:

Private Sub combo_material_click()
R1 = Sheets("Sheet2").Range("D65536").End(xlUp).Row
R2 = Sheets("Sheet2").Range("E65536").End(xlUp).Row
R3 = Sheets("Sheet2").Range("F65536").End(xlUp).Row
R4 = Sheets("Sheet2").Range("F65536").End(xlUp).Row
NR = Application.WorksheetFunction.Max(R1, R2, R3, R4) + 1
'BR = Application.WorksheetFunction.Max(R1, R2, R3, R4) + 1
'If NR = R1 Then NR = NR + 1
On Error Resume Next
   combo_thickness.Visible = True
   Select Case combo_material.Value
      Case "Angle"
         combo_material_width.RowSource = "sheet3!h1:h30"
         combo_thickness.RowSource = "sheet3!k1:k40"
         combo_material_type.Visible = True
         combo_material_type.RowSource = "sheet3!e1:e4"
  
         Case combo_material_type.Value
        
          Case Is = "CRS"
          combo_material.Value = combo_material.Value & "CRS"
         combo_material_type.Visible = False
    Exit Sub

When I run it, the second combo box appears, however when I select "CRS" it is not added to that of the value selected in combo_material

Thanks in advance!
Chris

Bump, maybe I mistitled this...............