G52 code explanation please

[cossiegaz]

I have this week started operating a 5 axis routing machine with a Fanuc 15-M series controller. The method of programming on this machine is one that i am not familiar with and am having trouble getting my head around it.
At the start of the programmes there is a G52 code and a line of X,Y,Z, values, i am aware that this is in someway altering how the machine reads its datum positions, its almost as if it is reversing the 'plus' and 'minus' directions of the axis, i say this because on a particular job we run a test cut and then take a measurement before altering an 'X' co-ordinate in the programme by the required amount to produce the finish cut in the correct place. What is confusing me is that if the finish cut needs to be produced 1mm further over than the test cut in the 'X' plus direction we actually subtract 1mm from the 'X' co-ordinate in the programme to achieve this.
Can somebody please explain what this code does and of what benefit it is, also is it in any way similar to a G10 code? (I am familiar with the G10 code)

[beege]

It's closer to a G92 code than a G10. G52 is used to describe a "local coordinate" or "child coordinate" system. It moves the working system by the XYZ you see in your program. It's canceled by a G52 X0Y0Z0. A G52 can be called with any of the G54-G59 fixture offsets active. I haven't had the opportunity to use it on a miller, but I have used it for back turning with a groove tool, with the Z changing by the width of the tool. HTH...

[cossiegaz]

Thanks for your reply, i see the benefit of using this code for what you have described with the groove tool, i have done that myself in the past on a cnc lathe only it wasnt a fanuc controller it was a seicos controller and if i remember rightly the code we used was a G151 or G152 so i didnt relate this to the G52 until you just described how you used it but now it makes a little more sense.
So if i understand correctly then it is simply just a datum shift code? I still dont see what benefit it has on the job we are using it on and i still dont understand why it appears to have switched the 'plus' and 'minus' directions of the axes around.
I do also vaguely remember seeing a G92 in the programme but didnt realise it was anything to do with what i wanted to know at the time, i have only been on the machine for a short time and the operator who is training me doesnt really seem to understand this G52 code, he just says that the G52 is what causes the axis directions to appear to be the wrong way in the plus and minus directions. I am sure it will become clearer to me after a while.