如何实现主窗体自动编号改变，子窗体编号从1开始递增

Private Sub 手臂_AfterUpdate()
Dim bh As Long
    Dim a, b As String
    b = Me.一级编号
    If b <> a Then
        bh = Val(Nz(Right(DMax("二级编号", "20", "一级编号 = " & Me.一级编号 & ""), 2))) + 1
        Me.二级编号 = Me.一级编号 & Format(bh, "00")
    Else
        Me.二级编号 = Me.一级编号 & "01"
    End If
End Sub