problem with mcnaughton height setting.

[Nathan Hawkes]

So, I'm having some issues......I've recently (today after coring several bowls in a row) become aware that when I'm using my McNaughton Center Saver, the height of the tip goes down as it travels in its arc towards the center, and ends up about 1/2" below where it starts. It seems to happen with at least two of the curved knives, and I'm relatively certain with the straight knife, although in honesty I didn't notice the problem until after using the straight knife, and didn't feel like putting it back on to check it. I had a couple serious catches that led me to check the height at the end of the cut, although I'd set it at the beginning. I had used the Mcnaughton on my old lathe a lot before getting my 3520B, but didn't ever notice the problem before now. I've cored probably 200 bowls with it, and love it, and really don't feel like buying a new one.I really doubt that I've somehow bent all the tangs on the knives without realizing it along the way. The odds of that are impossible. Something else is going on all the sudden.

I don't have a machinist's square to check it, but I'm thinking that the top of the banjo might have been milled at a little bit of a slope, which would explain the descending tip height. I came up with the hypothesis while sitting here on the computer, so honestly I haven't even looked at the banjo top to see if it looks out of square. Each time I used the McNaughton, the banjo was roughly perpendicular to the lathe bed, thus leading me to my suspicion. Something is wrong here. Has anyone else had problems that sound anything like this?




Side note, when is Craft Supplies going to get the jumbo curved knives in???!!! Theyve been backordered for months. I've been wanting a couple to complete my set for a while now (assuming I figure out what's up with the thing quickly)

[Dale Miner]

I usually set the heigth of the tip with the knife fully extended, by lining it up with the tailstock center point. My knives do seem to run downhill as they extend. Cutting on center is more important as the diameter of the core gets smaller. Definately do not want to be below center as the knive gets fully extended.

A way to check the banjo for squareness:

Set the banjo in the center of the bed lengthwise, with out anything in the lathe chuck.

Put the Mcnaughton toolrest in the banjo and tighten the banjo tool rest clamp screw. At this point heigth is not important.

Using the longest knife, insert it into the Mcnaughton toolrest, and extend it out as far as it will go. While lifting up on the back end of the knife and maintaining full extension, swing the tip of the knife to point towards the headstock, and measure the distance from the cutting tip of the knife to the lathe bed. Swing the knive aroung to a right angle with the lathe bed, and measure the knife tip to bed distance again. Swing the knife aroung to point towards the tailstock, and again measure the knife tip to bed heigth. During this, all the revolving of the knife is done with the swiveling motion of the Mcnaughton toolrest. The toolrest post and banjo position should not be disturbed.

If the banjo is at fault, the heigth of the knife will change as the knife is swept around. If the knife or the Mcnaughton toolrest is at fault, the heigth will remain unchanged when the knife is swept around.


Another quick and dirty, but perhaps not as accurate way to check the banjo:

Position the banjo in the middle of the bed lenghtwise with the banjo at a few degrees off perpendicular to the bed. Drop the longest toolrest you have into the banjo and let it bottom out. Using the same end of the toolrest for all measurement, swing the toolrest around and measure as in the procedure above. Using the same end of the toolrest is important, as it removes the possibility for a measurement discrepancy from an off square toolrest. If the heigth changes, the banjo is at fault.

Later,
Dale M

[Peter Lamb]

excellant suggestion
Peter L

[Jeff Nicol]

Nathan, I found out right away that if I start a bit above center it will cut better and not catch. Also I notice that the top of the McNaughton rest that pivots, tips a little bit on mine. I am not sure if this is normal or not, but if it tips 1/32" at the handle by the time it gets to the cutter it may be 3/8" that could be the problem. I think Reed Gray a.k.a. "Robo HIppie" says he locks his in place, I will have to watch the video again and see what he meant. Also a spacer cut the right length to put on the tool post that puts it at the correct hieght each time will help too.

Good luck,

Jeff

[Nathan Hawkes]

Thanks guys. Dale, you hit the nail on the head for checking the problem. I posted this on another forum, and the suggestion was almost the same. Engineering minds think alike. Glad to hear not only mine slope downhill. I may actually have bent them over time. They've been used a lot. In regards to setting the post height, I always use a spacer made from 1" sch. 40 PVC pipe that keeps the tip just above center. Jeff, I too have a little play in the tool rest top that pivots. This could be the culprit, but I'm leaning towards the knives being bent, unfortunately. I'll report back later today.

Thanks again folks.

[Reed Gray]

I answered this on another forum, and should answer here as well.

There is some play/flexing in the tool rest. This is by design. If it was totally rigid, there would be more catches and chattering transfered to the handle and user. So, if you start at center height, by the time you get to the center, you will be below. The blade also flexes in use. If you want to see this in action, try to remove the stub left when the bowl breaks out/off with the coring blade. In retrospect, I probably should have shown this in the DVD.

Another thing is that the blades will bend over time, and the bend is a twisting bend, not a straight down bend. Of course, a nasty catch or three will do the same thing. This is also by design, that it will bend but not break. Lay your blade on your table saw, or any other flat surface, and if it rocks, it is bent. This is easy to fix. Put the tang in a vice, and using a big monkey wrench or big adjustable wrench, twist it back into shape. I do show this in my DVD.

Another cause can be that with the vibrations in the cutting, the tool rest can lower in the banjo. I set the center height by extending the blade to its full extent, then lifting up on the handle. I make an end grain turned tube to go around the tool rest post to make sure it stays at proper height. The height is the same for all blades as near as I can tell. Another benefit is you don't have to adjust every time you core a bowl.

For locking the tool rest in place, I prefer the fingers to be locked. I just didn't like them pivoting while I am coring. It made it easier for me to keep the blade on track. The over the blade bar has to pivot as you core because the handle swings through an arc.

robo hippy

[Nathan Hawkes]

Well, the answer was different than I suspected. The banjo is just fine, milled dead perpendicular to the lathe bed, and the top perfectly flat. All but one cutter is flat, and the other is only very slightly bent. I can't believe it isn't more bent, to be honest. The last couple catches were pretty violent. When I measured with the blade at the start of travel, and at the end, it was fully 5/8" lower at the fully extended position. I really am scratching my head, because as my memory serves, the turret and lock bar were always together, but I may be wrong here. I don't know what has caused my problem; bending of the over the blade lock bar is next to impossible. After measuring everything, and trying to put a spacer between the guide fingers, I realized the solution was to put a thin slice of PVC in between the guide finger "turret" piece and the pivoting over the blade lock bar. The effect of this is to bring the over the blade lock guard a bit lower, making the blade move straighter in its arc. I had to use a small sledgehammer--a 2lb rock splitting hammer, really, to loosen the pieces off the tool post after loosening the grub screw fully. I can't find my camera cord to upload pics, maybe later. I did put loc-tite on the threads to make sure the grub screw doesn't loosen up over time. I like it tight as well; I didn't at first, but I seem to be able to keep the cut straighter with it locked.
After putting the PVC spacer in, it moves only 1/8" from start to full extension with the largest of the standard length cutters. I don't have the jumbo set. I did not take the time to re-measure the other cutters in the set, but I imagine that the results are similar. I haven't actually tried it yet, as I don't have time today, but I will in the next few days to a week for certain. I'm a little worried that this slope was intentional to keep the cutter moving in the cut, but right now it sucks it in too fast, and catches too much.I will definitely be posting a follow-up with before & after info.
Let me say as a disclaimer, that I do not suggest anyone go out and do what I did; I have not tested it yet, and take no responsibility for your outcome.....;)


Thanks again everyone. I'll post an update soon.