To be honest you do not need class variables for this and you should
not do the calculation in the initialize method as you do not know
what value to use for kerf. This is a better solution.

class Machine
  def initialize(part_width, part_length)
    @part_width = part_width
    @part_length = part_length
  end

  def laser
    puts calc(20)
  end

  def turret
    puts calc(10)
  end

  private

  def calc(kerf)
    sheet_width = 60
    sheet_length = 120

    parts_y = sheet_width / (@part_width + kerf)
    parts_x = sheet_length / (@part_length + kerf)

    return parts_y * parts_x
  end
end

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