large bowl help, bent knives, problems, etc

[Jeremy Pyatskowit]

Dear Sawmill Creek,
I was working on my biggest bowl to date, a 17" maple NE, got the outside done ok and started coring. It's going smoothly until about 3/4 the way when I got a catch and a bent the knife. Since I need to get the center out I take the next smaller knife with the closest curve and proceed to repeat step one. Now I have a lovely pair of matching bent knives and at this stage I'm seriously considering going upstairs and getting bent myself. I had to use the straight knife to get the center out so I save the outside bowl. Large bowl question here, I am using the stock rest on a pm 4224 and due to the NE I can't get close enough to use a gouge and have to finish roughing the inside with my homemade carbide tool. Am I going to have to invest in a longer tool rest of some sort? Also can you straighten bent knives or am I going to go through a life of buying knives on a somewhat regular basis? I have cored several bowls in smaller sizes with no major problems prior to this debacle. Once I get these problems solved I have 7 more large blanks waiting to be roughed and cored. Any and all helpful tips/comments/suggestions/jokes at my expense welcome.
Sincerely,
Bent Out of Shape in WI

[Dennis Ford]

I only core very nice wood so don't have enough practice to be really good at it. The first few I was turning too slowly and had lots of trouble, now I have less trouble but still have to widen the kerf some to get to the center of larger pieces. My only suggestion is to slightly slack the drive belt so that it will slip if you get a bad catch. Getting the belt tension just right will not be easy and slipping the belt is hard on the belt (but better than bending tooling).
On the tool rest; a curved tool rest works great inside big bowls.

[Rick Bailey]

What ya coring with?

[Bob Bergstrom]

Watch Dale Bornertz YouTube video for the Rockey Mountain Woodturners. He one of the best. He is on this forum and may chime in.

[Kyle Iwamoto]

McNaughtons? I'm not a pro, but I've noticed sometimes, when you get closer to the "center", the knives may start to self-feed, and I actually have to hold the knife from feeding itself...... Of course, it depends on the wood. Some do, most don't. Never turned fresh maple.

How fast are you turning?

Short list of things I do.
Cutting on center. Sharpen the cutter. Widen the entry kerf so the chips eject. I twist the knife in the holder (since there is always some "slop") to the side vice attempt to hold "straight" in the gate. Clear chips when I don't see them coming out. Wax (parafin) on the knife in the gate so the knives move freely.

[William Bachtel]

I can help you on the tool rest problem you have, Robust tools sells an offset toolrest it will give you more room. 608-924-1133 866-630-1122 They are great people to work with.

[Dale Bonertz]

My guess is the cutting tip is cutting below the center line the deeper you go. It is want ing to go under the cut but gets caught due to the curve of the kerf and bang, catch and bend. Are you extending the straight tool through the turret to its full extension when setting it at center height? If not that most likely is your problem. You should widen the kerf slightly but not to much or you get a wobble of the blade in the kerf and that can result in a catch. Chip ejection can be helped by widening the kerf as Kyle mentioned but you should spray WD40 into the kerf on deeper cuts to slick the side walls which really help with chip ejection (this is assuming your coring cut is not the finish cut). When coring you are cutting in the 9 o'clock position so I spray WD40 in the 12 to 3 o'clock position and try to spray it all the way to the bottom of the cut. This seems to help cool the blade as well which when they get hot they expand and by cooling them a bit they will move through the kerf easier. Also as Kyle mentioned make sure your tool is sharp and in deeper cuts pull the tool out and run your stone over the edge to sharpen it quick to finish the cut the rest of the way.

Yes you can straighten the tools depending on how badly they bend. I use a vise to hold the tool and a pipe over the end that is bent so I have leverage to straighten it. You may need to straighten it front to back on the horizontal line (viewed as if being used in the turret) as well in the vertical line which is more of a twist in the blade.

Good luck.

BTW there is a lot of good information out there. Joe Fleming has a wonderful article about the McNaughton and other coring tools. Reed Gray (aka Robo Hippie) has a good video. John Giem wrote an article in the AAW journal about the coring tools in which I helped him quite a bit with information. And finally I do have a video on youtube that may help you. I don't cover the straight tool in it much but the concepts are the same in use.

[Bob Bergstrom]

My guess is the cutting tip is cutting below the center line the deeper you go. It is want ing to go under the cut but gets caught due to the curve of the kerf and bang, catch and bend. Are you extending the straight tool through the turret to its full extension when setting it at center height? If not that most likely is your problem. You should widen the kerf slightly but not to much or you get a wobble of the blade in the kerf and that can result in a catch. Chip ejection can be helped by widening the kerf as Kyle mentioned but you should spray WD40 into the kerf on deeper cuts to slick the side walls which really help with chip ejection (this is assuming your coring cut is not the finish cut). When coring you are cutting in the 9 o'clock position so I spray WD40 in the 12 to 3 o'clock position and try to spray it all the way to the bottom of the cut. This seems to help cool the blade as well which when they get hot they expand and by cooling them a bit they will move through the kerf easier. Also as Kyle mentioned make sure your tool is sharp and in deeper cuts pull the tool out and run your stone over the edge to sharpen it quick to finish the cut the rest of the way.

Yes you can straighten the tools depending on how badly they bend. I use a vise to hold the tool and a pipe over the end that is bent so I have leverage to straighten it. You may need to straighten it front to back on the horizontal line (viewed as if being used in the turret) as well in the vertical line which is more of a twist in the blade.

Good luck.

BTW there is a lot of good information out there. Joe Fleming has a wonderful article about the McNaughton and other coring tools. Reed Gray (aka Robo Hippie) has a good video. John Giem wrote an article in the AAW journal about the coring tools in which I helped him quite a bit with information. And finally I do have a video on youtube that may help you. I don't cover the straight tool in it much but the concepts are the same in use.

I buy WD-40 by the gallon an use a quart spray bottle with an adjustable spray nozzle to shoot it into the kerf in the same area. A can with the straw on it will go deep into the kerf also. I think of coring with the McNaughton is like throwing a curve with a baseball. The more you do it the more you can predict the outcome. Of course there is always a wild pitch. Which results in a funnel!!:confused:

[Jeremy Pyatskowit]

Thanks for all the info everyone. Judging by the responses it would appear my kerf was too wide since I readjusted the angle a couple times. I do wax the blades frequently and made sure I was not below center. It looks like I could sharpen before finishing out. For the interior curved rests is the robust 12" adequate for large bowls or do I need to go to the 14" j shape rest?

[Reed Gray]

I do hope you had a bunch of practice with the smaller blades before taking on a bigger bowl. Really important to get the feel of the tool.

Two probably causes of the catch, and they usually combine. One is the blades drift as you go down inside a bowl, and always to the outside. This is why you need to open up the kerf. Figuring out how much and where to widen it is difficult. As the blade drifts, you have the blade binding up in the kerf. This adds to pressure. The getting below center is another problem here. There is a lot of flex built into the system. If you have ever tried to take off the knob at the bottom with your coring blade, you can see just how much there is. All the pressure can really push the blade tip way down below center. Being experienced with this, I (professional driver on closed course, do not attempt) I can feel when it is below center, and most of the time I will drop the handle a bit to raise the tip, and this is with the full length handle. You can also raise the tool rest. The way you are told to figure how high to set the tool rest is under a no load situation. This is fine near the rim and for smaller bowls. With bowls in the 12 inch diameter range and bigger, this does not work. I usually have the tool rest at least 1/4 inch higher than the center position. The tool is a scraper, and on the inside of a bowl, you always keep scrapers at or slightly above center.

The biggest problem I have had with a wider kerf is that shavings don't eject as well, and I get more plug ups in the kerf. Stalled lathe, squealing belt, and even broke a bowl once.

I so show bending a blade back to straight on my coring video. Vice, big pipe wrench, and a grunt or three.

robo hippy

[Jeremy Pyatskowit]

great info Reed, I bent them back on my half snow day from work but there is a wrinkle on the bottom edge. I'm guessing they might need to be replaced. I can post a pic later when I get home from work for some input from you folks.

[Dale Bonertz]

That is so funny and true Bob. Made me laugh out loud. Thanks.

[Kyle Iwamoto]

Funnel? I'm sure you mean a stand for a bowl....... I've made them too......

[Dale Bonertz]

Dennis a friend of mine said that to me when I told him that I was going to core my bowls from salvaged wood. He said "Coring is for people who buy expensive wood". Although I can't argue with that train of thought I told him "You may be right but those who buy expensive wood have no business coring it if they haven't practiced on the ordinary stuff". This can be applied to nice wood as well. Not trying to be difficult but practicing on ordinary stuff insures better success on the nice or expensive stuff.

[Jeremy Pyatskowit]

Just an update to the thread, I managed to get the knives straightened and successfully used on of the straightened knives using some of the suggestions in the thread, thanks for all the input. My wife thanks you for me not spending more $$(other than a new tool rest).

[Jeramie Johnson]

With the great advice, sounds like you have handle on the issue. Nice work!
If you want to make matching bowl sets, coring is the only way to go. I'm south of you, but have a Woodcut Max.
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