I doubt that your simulation program understands the G51.1 code and as a result it won't be able to display what the machine is doing.It doesn't necessarily mean that your use of the code is wrong.If you have the code correct for one half of the object and you insert the G51.1 correctly and cancel the command correctly which probably requires a G50.1-as I expect your example does-it should produce viable code.I haven't used a mirror command in years,but used to when computers were a lot more basic and it was simpler to generate code for one half of a pair of objects and then mirror the program for the other.I did have a look at Machinery's handbook to see what the recommendation was but there was nothing specific and not all controls are the same.