and the whipstock the use of angles gt 5 u00b0 also result in a gradual erosion or wearing down of the lug l as the mill 12 advances dashed line 44 in fig5 indicates how much of the lug l is worn away during normal operation it is desirable to configure the lug 40 so that the transverse dimension from the initial point of contact shown schematically as 46 with the lug l to the outer periphery of the guide 40 should exceed the thickness of the casing to be milled stated differently dimension represented between the arrows 48 should exceed the thickness of the casing to be milled the dimension 48 represents the amount of expected horizontal movement of the guide 40 as the wear pattern illustrated by dashed line 44 is accomplished during the milling operation it should be noted that the window milling operation using the lug l as above described allows for successful lateral deflection of the guide 40 away from the whipstock 20 and toward the casing 18 omitted from fig5 for clarity the operator at the surface curtails the milling operation with the mill 12 has advanced a predetermined amount that amount is a distance generally about 3 feet which is sufficiently smaller than the initial gap between the cutters 42 and the top end 50 of the whipstock.