We investigate atomic force microscope nanolithography of single and bilayer
graphene. In situ tip current measurements show that cutting of graphene is not
current driven. Using a combination of transport measurements and scanning
electron microscopy we show that, while indentations accompanied by tip current
appear in the graphene lattice for a range of tip voltages, real cuts are
characterized by a strong reduction of the tip current above a threshold
voltage. The reliability and flexibility of the technique is demonstrated by
the fabrication, measurement, modification and re-measurement of graphene
nanodevices with resolution down to 15 nm