with a production tube a second embodiment of the milling head is shown in fig6 this embodiment of the milling head 140 can be fitted with a check valve comprising a ball seat 142 a check ball 144 and a spring 146 it can be seen that as milling fluid passes downhole through the fluid path in the milling head 140 the check ball 144 can be lifted off its seat 142 against the bias of the spring 146 to allow flow out the lower end of the work string a kick or pressure excursion sometimes occurs in the formation fluids which could create an undesirable flow in the uphole direction through the work string to prevent this the spring 146 biases the check ball 144 toward engagement with its seat 142 as pressure below the milling head 140 increases above the drilling fluid pressure this causes the check ball 144 to seat more securely thereby preventing flow in the uphole direction a third embodiment of the apparatus 210 of the present invention is shown in fig7 and 8 in this embodiment the mill body 212 is secured to the milling head 214 by shear pins 216 in shear pin bores 224 and 226 in the mill body 212 and the milling head 214 respectively flow passages 228 are provided through the milling head 214 however in this embodiment the check valve comprises a swing check type valve with a check valve body 262 assembled in the milling head 214 and with a flapper valve 264 which is pivotably mounted to the check valve body 262 by a pivot pin 266 the check valve body 262 can be retained in the milling head 214 by one or more snap rings or pins as is known in the art.