Setting helical head knives.

[Glen Butler]

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.

[glenn bradley]

I'm confused a bit. When I hear helical, I think carbide inserts (although there are HSS ones for some reason?). The screws seat and center mine so that an "alternate" position related to the screw center is not possible. The diagram shows what, to me, feels wrong unless the backs of the inserts are "keyed" but this does not sound like the case. Wrong inserts maybe? I would contact Jet and I would not go to grinding on things until I spoke to them. Please keep us posted.

[Glen Butler]

It is a helical head with carbide inserts. As you know for each insert there is a flat spot with a threaded hole and a back stop for the insert. On the jet head there is a tiny shelf that is preventing the insert from lining up properly. Simply tightening them down using the countersink is not always a guarantee that they are all set within a thou of each other. Especially when you have a little jag at the back corner that catches the insert when it is seating. There is a back stop opposite the cutting side. I make sure they are butted into that stop which is the also in line with the countersink of the screw. Powermatic head on the jointer butts in just fine. But on the jet planers' head there is that little shelf and the inserts will sit on that shelf and have a tiny void under them which I think could crack the inserts when torqued down. If I have the inserts sitting completely on the bottom and butted into the shelf, then they are not sitting completely against the "back stop" (as in the case of the powermatic machine) and the countersink of the screw does not line up as well. Do you see my conundrum now? That little shelf -- perhaps a machining problem, unless someone else out there can tell me if they have one -- is getting in the way.

[Glen Butler]

Hmm, well contacting jet customer service was quick and painless. They said the shelf is normal and they should sit on top of the shelf. 50 inch pounds isn't enough to break the inserts when tightened. I wonder why then the powermatic does not have the little shelf?

[Chip Lindley]

Only reasoning I can see for the *shelves* may be to insure better chance of seating the inserts if gunk or slight debris is present during factory assembly. One less manufacturing step in cleaning the seats!

Doubtful that the slight void allows torquing down of the insert in a more positive manner, as in the manner of cupped lock washers. Carbide is too brittle to have any spring to it.

[Glen Butler]

Only reasoning I can see for the *shelves* may be to insure better chance of seating the inserts if gunk or slight debris is present during factory assembly. One less manufacturing step in cleaning the seats! This was my thinking also.

Doubtful that the slight void allows torquing down of the insert in a more positive manner, as in the manner of cupped lock washers. Carbide is too brittle to have any spring to it. This was exaclty my fear, the void under the insert would cause it to crack when torqued down.

Well see how it goes tomorrow when I put everything back together.