Relief ground cylinders will last considerably longer than those spun ground. When you grind a relief on a unit you are removing metal from the back of the blade therefore the diameter of the cylinder is not effected. The cylinder diameter is only affected when you spin the cutting land onto it, as already established a unit which has been spun ground only, has to be ground at least 3 times more often, which equates to them wearing out 3 times more quickly.
Back lapping is one of those contentious areas where some people champion its merits and some bemoan. Like spin grinding it can be a quick, albeit temporary, fix! However one point that should be made is that if you do back lap as part of your cylinder maintenance programme then your units must have a relief angle. The relief angle is essential to force the cutting paste to the cutting edge; without it you will only succeed in sharpening the back of your cylinder blade, in other words its non-cutting edge.
Setting Up Your Units On the recommendation of some manufacturers, such as John Deere and Jacobson, units should be set up to have no contact between the cylinder and the bedknife. Other manufacturers, including Dennis, suggest that set up should, ideally, have very light contact between the cylinder and bedknife. Unfortunately, if you only spin grind, this very light contact is almost impossible to achieve! This is because the cutting land is the full width of the blade, unlike the small l and which is achieved when you relief grind. This has a very undesirable effect on the bedknife because contact generates heat which, in turn, produces even greater contact. In other words the bedknife loses its sharpness far quicker and, therefore, needs to be ground more often and replaced far more frequently.
Spin grinding is only a quick fix and like all quick fixes it has its limitations! Short term gains are nearly always long term pains. Don’t lose sight of a very important point here. You only need to regrind a unit when it’s no longer sharp, not because the grinding method you’ve chosen dictates it!