I have one of these planers and it has been pretty good over the last 20+ years that I've had it. But I reckon it's about time I sharpened the blades in it. The bolts holding the blades on are not being very friendly to me and I think I will end up destroying them getting them off. the owners manual says the bolts are M6x17. What does that mean? 17mm long? What does M6 mean?