McNaughton coring question

[Dan Crafton]

I have been using the McNaughton system for several years and am able to core most woods with few to no catches. However, cherry continues to give me trouble. It’s always the same place in the blank, about half way in. This morning, after not being able to get beyond the halfway point, I hollowed out the blank with a gouge down to the point where the catches were occurring. I noticed the one of the two cutting edges of the coring blade is parallel to the wood grain at this point. I am using the large #3 blade with a spear point. The height of the blade is above center throughout the cut - 1/4 to 1/8 inch. Any thoughts on how fix this? Perhaps regrind the tip to less of a point or even a chisel like the old style blades?

 

[Richard Coers]

It's that transition from mixed end and long grain, to just long grain is where it gets tough. Bringing the cutter out more has helped me by clearing chips better.

 

[robo hippy]

I do have a video up on You Tube about coring with the McNaughton. So does Dale Bonertz, hope I spelled that correctly.... As I say in the video, I prefer a squared across cutter. The idea with the spear point came from Mike Mahoney, who also has a video out. The idea behind it is to 'fish tail' as you go deeper. This opens up the kerf. I prefer to take a straight cut in. Like Mike, we both use the medium curved blade for 90% of our coring. The catches, as far as I am concerned, come from being below center, or at least that makes them more prone to big dig ins. There is a lot of flex designed into the system. I keep mine at least 1/4 inch higher than the center line. If you have ever tried to remove the stub/tenon left when a bowl breaks out, you can see how much the system flexes when it is under load, and this is also why I keep it higher than the center point. With scrapers, if you are below center on the inside of a bowl, the catch gets a lot deeper because it can't fall out of the cut like it would if you are above center. Things that can add to the downward pressure are shavings inside the kerf, and the blades not being a perfect arc of a circle which causes the blade to drift to the outside of the cut and then kind of bind in the cut. You have seen the burn/burnished marks on the inside of your bowls I am sure. The tips tend to go straight rather than follow that perfect curve. In my experiments with different shaped tips, the actual shape of the tip didn't seem to make any difference in the coring path/kerf. Bevel to the inside, bevel to the outside, bevel square. I did have a lot of nasty catches before I figured this out. They are less frequent now days....

robo hippy

 

[Dan Crafton]

Richard and Robo, thanks for the input, Robo, I have seen yours and Mike’s videos and found them both very helpful. Once the catches start at the mid way point, I find it difficult to get any further. It only happens with cherry, green wood from different trees. In those cores where I do get beyond the mid point, the core comes out without a problem. I will pay more attention to making sure the blade is not rubbing on the side of the kerf and that the chips are cleared and see what happens.

 

[dbonertz]

Richard and Robo covered it pretty well. I'll add two things. First when you set the tool at center height or slightly above (I set it about 1/4") you should do so when the tool is fully extended from the turret. The tool runs down hill a bit and combined with the downward pressure while coring can drop you below center fairly easy. To test this set the tool on center when extended from the turret an inch or two. Now fully extend it and back the turret out so you can take the measurement again. Most likely it will now be below center. Make sure you hole the handle up like you are coring. As mentioned chip removal is very important especially at the location you're describing. I learned a trick through trial and error. At about the location you are describing while coring spray wd40 into the kerf (you can do this at about the 12:00 to 2:00 location in the kerf - does not need to be directly on the tool). This helps slick the kerf walls and cool the tool, both of those things in turn help with chip ejection. No the wd40 does not penetrate the wood and ruin it, you are not applying enough for that to happen. Even if it did penetrate the wood it wouldn't go very deep and you are twice turning anyway. Good luck.

 

[robo hippy]

I need to make forms, probably metal, and take a torch to my blades so the arcs are perfect. This would prevent the binding issues that happen with the McNaughton. Retipping my old blades with tantung has also helped. I tried a couple of different lubricants on my blades and they didn't really do enough to make it worth the effort, WD 40, walnut oil, Glide Coat, but not Ken Rizza's Slick Stick. I would still get burn marks on the outside of the core, and generally not on the core, and some times that blade was very hot when pulling it out. It has seemed that opening up the kerf would cause the shavings to plug up the cut more than if you had a single kerf. No easy solution here. As I like to say, every one who has tried to use the McNaughton swears at it. Every one who knows how to use it swears by it.

robo hippy

 

[Dan Crafton]

Thanks for your input. When I first got the McNaughton system, I made a spacer so the turret would always be at the correct height for my lathe, as you said, 1/4 inch above center fully extended. Over time I found that I needed to raise the the of the turret to compensate for a slight downward bend in the blade. It’s fairly easy to bend the blade left or right, but much harder to bend it back up. As for lubricantion, I use wax on the blade. Helps it slide through the pins on the turret and the kerf. Since this is the only blade I have trouble with, and it only happens with cherry, I am thinking the blade has a bend that is causing the catches. Before I replace the blade, I am thinking of grind off the point so that the cutting edge has three sides. it will be a little harder to sharpen, but will have less cutting edge in any one direction to catch.

 

[dbonertz]

Dan,
Something goofy going on if that is the only problem blade especially with Cherry. According to the Janka table cherry has the same hardness as maple and maple is great to core. I don't have cherry where I live so I don't know about the other properties of it. I spray into the kerf because it slicks the side walls and really gets the chips flying out again when you hit that fibre transition point. I hope you can get that frustration cured.