I got to watch Mike Mahoney at the last Oregon Woodturning Symposium here and he made an interesting comment. He said that he did over 90% of his coring with the medium curve blade. Me too, and we had not chatted about that before. I do grind my points off square rather than the 'spear' point, which was a Mike improvement, especially since the first sets were dog eared off to one side rather than centered on the blade. When Mike cores, he 'fishtails' which means he wiggles it a bit side to side as he cores. This opens up the kerf a bit. The problem I had with that was that I seemed to get a lot more clogging up in the kerf when I did that compared to when I just went straight in.
For setting the height of the coring blades, they tell you to use the longest blade you have and adjust till you are at center height. I actually have mine set 1/4 to 1/2 inch above that. The reason is that there is a lot of flex designed into the system. If you have ever tried to remove the tenon stub in the bottom of a cored bowl, first thing that happens is that the blade drops as much as half an inch. I got so I could 'feel' when the blade was below center, and this was a constant problem, so I made a collar to go around the tool rest to keep it at constant height and that height is above center to compensate for the flex. If you start getting catches near the bottom of your core, this is one likely problem.
If you are going through the bottom of a bowl, the main issue is that you went parallel to the side of the bowl rather than going in a bit. I lay the blade up against the outside of the bowl. If the arc of the bowl and the arc of the blade are pretty much the same, and if you try to match the curve of the bowl wall, you will put notches in the jaws of your chuck. I have a couple that have bite marks.... I saw one video, I think the turner's name was Brown, and he put pencil marks on the outside of the bowl to mark the desired depth he wanted for each of his cores. That seemed to be a good idea to me.
I do use lubricant on the blades some times. Mostly the Slick Stick from Woodturners Wonders. Great stuff, and it does wonders on the sole of your hand planes. I have heard of all sorts of different things being used, including Pam the vegetable spray. Some woods just have more sticky stuff in them than others, and that is when I use lubricant.
The biggest problem with the blades is that they do not follow a perfect arc of the curve. My guess is that they are profiled, as in rounding over the edges top and bottom first, then they are bent. In researching getting 'perfect' circle arcs, they will bend a much bigger length, like 4 to 5 inches, and then snip off the extra. Kel insists that they are 'spot on' but you don't have to look very closely to see that they are not, and the last inch or 2 goes pretty much straight. If you can't see it, you can lay it on a plastic circle template and it is obvious. I can't think of any way to get the perfect curve in my blades other than heating it with a torch and bending it a bit more. Not sure what that would do to the laminated cutting material on the tip. I was able to take one old blade and 'armstrong' it in a vide to get a little more bend, and I bent it a bit more than needed, so it actually tracked a bit to the inside. The blades as they come ALWAYS drift to the outside of the kerf. I have burned my fingers a number of times by handling the blade as I withdraw it, and more times than I can count, I get that burned edge on the bowl blank from heat/friction. I have thought about making my own blades and making sure they are perfect arcs of the curve. Just haven't gotten 'a round 2 it' yet. I did retip a couple of blades with some tantung, and it works well.
For sharpening them, a coarse, like 220 or more diamond card. They are scrapers, and they work better with a burr, on most woods anyway.
robo hippy
