How can I overwrite class variables

Hi,

How can I overwrite a class variable?  What I have here is a class
called Machine which calculates how many times a rectngle fits in a 60 x
120 rectangular shape and its working, in the code below it could fit 36
times, the problems is that I'm going to have differnet methods which
are supose to overwrite @@kerf and make the calculation based on these
new values but I do not know how to overwrite the class variable @@kerf.

How can I overwrite variable @@kerf so that when I call method "laser
(machine1.laser) the result would be 6 instead of 36 and when the method
"rurret" (machine1.turret) is called the result would be 12 instead of
36?

------------------------------------
class Machine
 @@kerf = 0
  def initialize(prt_w, prt_l)
    part_width = prt_w + @@kerf
    part_length = prt_l + @@kerf
    sheet_width = 60
    sheet_length = 120
    parts_y = sheet_width / part_width
    parts_x = sheet_length / part_length
    @total = parts_y * parts_x
  end

  def laser
    @@kerf = 20
    puts "#{@total.floor}"
  end

  def turret
     @@kerf = 10
     puts "#{@total.floor}"
  end

end

machine1 = Machine.new(10 , 20)
machine1.laser
machine1.turret
-----------------------------------------

Sorry if my code doesnt make any sense but I'm practicing.

Thanks a lot for your help!

-- 
Posted via http://www.ruby-forum.com/.