I have a 20" jet planer with a helical head. The machining is not done as well as the machining on my powermatic jointer. On the jointer there are no problems getting the inserts to seat where they should, but on the jet there is a extra tiny shelf at the bottom back corner where the knife sits. Maybe .0030 x axis and .003 on y axis. I don't know whether to butt the inserts to the back and slide them down so they are sitting on top of the shelf or slide them back so they are butting into the shelf. Sliding them down to sit on the shelf helps the screw countersink line up better, but would leave a gap under the insert and I fear cracking them when they are torqued down. Sliding them back is just as iffy because the screw heads do not always seat properly so can just as easily break out the back portions of the insert and they do not all seat the same. It is more likely that I will get ridging if sliding them back. I am about ready to whip out the dremel and grind off the shelf so they can all butt to the back of the head properly. Is this a machining error? It's no wonder I get less ridging from my jointers' head. It's cause they all seat properly.