HI all. I have just finished building my reprap mendel and my first prints have been very successful most of which have been basic cubes and circles but the quality has been very good. However today when I designed a cover for the Melzi circuit board and printed it I found that it was 3-4mm bigger on all axis. I checked my drawings a like a million times now and all my measurements are spot on. I thought that it might be an issue when converting the file to STL but when I opened it in netfabb all the dimensions were correct again spot on. Has anyone come across this issue before and is it easily solved?
Please bare in mind that I am completely new at this and if I have left out any relevant details apologises in advance.

How accurately is the printer calibrated. If the printer is not set up for the exact correct number of steps/millimeter in all 3 dimensions, you will see errors.

Also consider the software you sliced it with. Some slicers are better than others with respect to accuracy of the G code.

Finally, at least for the Z axis, the printer moves in discrete steps. Basically your final height will be a multiple of your layer height; chances are that is not going to line up correctly with your CAD height.

Given that your error is 3 or 4 millimeters, I would suspect the printer calibration. The inset issue and the Z height issue will introduce errors that are fractions of a millimeter.

I completely agree with the first reply, it sounds like a calibration issue. Since it was consistently off on all 3 axes, I would double check your scale setting in your slice software.

Keep in mind if it is a step/mm issue, it would be much more apparent on larger prints so that may be why your first few prints seemed good.

If you have access to a dial indicator, I would strongly recommend using it to properly calibrate all 3 axes, although I don't see any other way of accurately doing so.

Cheers for the help guys I believe that has sorted it. I was loading the file into slic3r by opening slic3r on its own and not through pronterface therefore the default printer settings where not being loaded .