Njerwa ndi zida zomangira zodziwika bwino, ndipo zimagwiritsidwa ntchito kwambiri m'magawo ambiri. Monga imodzi mwa zigoba zomangira, kufunikira kwa njerwa kukukulirakulira pang'onopang'ono. Inde, njirayi ndi yosasiyanitsidwa ndi kugwiritsa ntchito makina opangira njerwa. Ndi ver
Ma midadada konkire ndi zida zomangira zofunika pantchito yomanga ndipo kupanga midadadayi kumafuna kugwiritsa ntchito makina apadera monga makina opangira simenti ndi makina osindikizira. Makinawa amagwira ntchito yofunika kwambiri