Bowl Coring Issue

[Mike Goetzke]

Found some time to spend on the lathe yesterday afternoon. I had a sycamore log that I prepared into two blocks for bowls.

Was having so much fun following what I learned at a local woodturning club to shape the outside bowl. I'm getting pretty good as I'm starting to get little curlies all over the shop. Plus as I practice I'm getting much faster.

I have a McNaughton center saver system. Since I'm a new turner I originally had it on my tool list so I could have more blanks for bowls to get more practice. I've used it a couple of times and while not perfect I did not produce any funnels or lamp shades.

Background: I have a PM 3520b, 12" bowl, 4-1/2" tenon, wet sycamore (but it's little pnuky from being stored in bags probably too long), running at 500-600RPM.

Started coring with small radius mini knife. I had about another 3/4" to 1" to go and I got a catch! It broke the small bowl out and sheared off the tenon. Alright I'm not a quitter so chuck up the second one. Using the same knife maybe it's the wood but seemed to have more vibration yet wood still escaping. Pull the knife to see how deep I am. Start it up again and I go slow because feels sounds catchy and then ... Oh no disaster ... huge catch! This time the knife got stuck and it's so bent it's $50 down the drain! Took some time but I was actually able to get the mini small radius tool close to shape (it was twisted and bent).

So I read that you can loosen the belt on the lathe to make it slip before big catch so did that. Also sprayed some WD40. So went back at the second blank with the mini. Was very careful this time (well more careful). With lathe at about 500rpm I widened the opening a little, but, then again when I get in deep it won't cut and almost sounds like I'm hitting metal. Starts to vibrate. So after few tries I take it off and knock the small bowl out with a 5 lb sledge hammer. I remounted the bowl and brought up the knife and spun the lathe by hand and could feel the spot. Ends up there is a hard knot (but don't know if this is my issue).

Once again, not knowing when to quit, I set up to core the center bowl. Need to use the mid radius standard blade. Was watching everything carefully this time. I had curlies coming out of the slot. Then when I got about 3-4" deep I started getting vibration and noticed the curlies had turned into mush - looked like wet sawdust. This was causing the belt to chirp a bit even without cutting. It was even hard to clean out with a screwdriver.

Why would I have nice chips to start but change over to sawdust mush?

Is this user error or the wood?

Any help appreciated.

Mike





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[Dale Miner]

Found some time to spend on the lathe yesterday afternoon. I had a sycamore log that I prepared into two blocks for bowls.

Was having so much fun following what I learned at a local woodturning club to shape the outside bowl. I'm getting pretty good as I'm starting to get little curlies all over the shop. Plus as I practice I'm getting much faster.

I have a McNaughton center saver system. Since I'm a new turner I originally had it on my tool list so I could have more blanks for bowls to get more practice. I've used it a couple of times and while not perfect I did not produce any funnels or lamp shades.

Background: I have a PM 3520b, 12" bowl, 4-1/2" tenon, wet sycamore (but it's little pnuky from being stored in bags probably too long), running at 500-600RPM.

Started coring with small radius mini knife. I had about another 3/4" to 1" to go and I got a catch! It broke the small bowl out and sheared off the tenon. Alright I'm not a quitter so chuck up the second one. Using the same knife maybe it's the wood but seemed to have more vibration yet wood still escaping. Pull the knife to see how deep I am. Start it up again and I go slow because feels sounds catchy and then ... Oh no disaster ... huge catch! This time the knife got stuck and it's so bent it's $50 down the drain! Took some time but I was actually able to get the mini small radius tool close to shape (it was twisted and bent).

So I read that you can loosen the belt on the lathe to make it slip before big catch so did that. Also sprayed some WD40. So went back at the second blank with the mini. Was very careful this time (well more careful). With lathe at about 500rpm I widened the opening a little, but, then again when I get in deep it won't cut and almost sounds like I'm hitting metal. Starts to vibrate. So after few tries I take it off and knock the small bowl out with a 5 lb sledge hammer. I remounted the bowl and brought up the knife and spun the lathe by hand and could feel the spot. Ends up there is a hard knot (but don't know if this is my issue).

Once again, not knowing when to quit, I set up to core the center bowl. Need to use the mid radius standard blade. Was watching everything carefully this time. I had curlies coming out of the slot. Then when I got about 3-4" deep I started getting vibration and noticed the curlies had turned into mush - looked like wet sawdust. This was causing the belt to chirp a bit even without cutting. It was even hard to clean out with a screwdriver.

Why would I have nice chips to start but change over to sawdust mush?

Is this user error or the wood?

Any help appreciated.

Mike





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It sounds like the knife was going below center as it extended and was being subjected to the down force of cutting. Also, a build up of shavings can cause the knife to flex down. When the knife goes below center, the cutting action has a tendency to pull the knife in. As it is pulled in the cutting force increases and pushes it lower that causes it to be pulled in, etc. It all happens in less than one rotation.

Before starting to core, make sure the knife is about 1/8 to 1/4 inch above center with the knife fully extended while lifting up on the handle end. Then as cutting progresses the knife will remain above or at center.

The second time in with the catch gouge already in the bottom of the cut takes a real steady hand and sloooow feed to avoid another big time catch. Get shed of the large handle that comes with the set and make a handle the size of a file handle. A large handle is not needed to apply cutting force and only makes fine manipulation of the knives more difficult.

As to the knife being bent beyond use; Nah, put it in a vice and work it back straight with a Cresent wrench being careful to not damage the cutting edge.

Shavings turn to mush. When the cutting starts the cutting action is cutting lengthwise with the grain. When getting near the center, the cutting action is cutting into the end and the shavings are breaking across the grain and become smaller and smaller.

[Mike Goetzke]

It sounds like the knife was going below center as it extended and was being subjected to the down force of cutting. Also, a build up of shavings can cause the knife to flex down. When the knife goes below center, the cutting action has a tendency to pull the knife in. As it is pulled in the cutting force increases and pushes it lower that causes it to be pulled in, etc. It all happens in less than one rotation.

Before starting to core, make sure the knife is about 1/8 to 1/4 inch above center with the knife fully extended while lifting up on the handle end. Then as cutting progresses the knife will remain above or at center.

The second time in with the catch gouge already in the bottom of the cut takes a real steady hand and sloooow feed to avoid another big time catch. Get shed of the large handle that comes with the set and make a handle the size of a file handle. A large handle is not needed to apply cutting force and only makes fine manipulation of the knives more difficult.

As to the knife being bent beyond use; Nah, put it in a vice and work it back straight with a Cresent wrench being careful to not damage the cutting edge.

OK - will set the knife higher and give it a try. I read to put the large straight knife in and set it at the center and the others should be ok.

Mike

[Bob Bouis]

I don't have experience with the McNaughten, but I will add that catches like you're describing can be caused by a dull cutter. Coring knives don't stay sharp long and need to be resharpened after literally every few cores.

I'd also suggest moving the knife in and out after making the cuts to widen the kerf a little. At least on the oneway system, what will happen is that the knife bends a little, usually toward the headstock, as it goes in and this compounds itself as the cut channel pushes the knife further and further toward the headstock.

Oh, and use the tailstock if you can. It makes a big difference.

[Reed Gray]

Well, if we had the Star Trek transporters, I would beam on over for a play date.

First, the height of the blade... I always have it at least 1/4 inch above the center. When you set the height according to a blade that is not cutting, this does not take into account the amount of pressure on the cutting tip when it is in action. There is a lot of flex in the system from the tool rest and the blades. If you have the medium blade on, as far out as it will reach, and try to cut off the nub from the core you took out, when you touch the nub, the blade tip can drop 1/2 inch or more. For me, I can 'feel' when the tip is too low as you get near the center. Some times I raise the tool rest. If I have the long handles on, some times I just drop the handle, but warning note here, this is professional skill level only. You can't start out too high because the vertical part of the bar won't fit into the round part of the kerf. All coring blades are scrapers. Scrapers should always be at, or slightly above center when cutting on the inside of a bowl.

Shavings, and mush.. You get more shavings when you go down the sides. As you go through the transition and across the bottom, since you are going into the end grain rather than down through it, you get more mush. There is a noticeable difference in the 'feel' of the tool as it cuts.

Bent blades, I do show this in my video which is up on You Tube. Clamp the handle in a vice, and use an adjustable or pipe wrench to twist it back into shape. Some times it is necessary to lay the blades on a circle template and correct that curve as well. Most of the time, the tips tend to go straight rather than follow the curve/arc of the circle. This makes the blade drift off to the outside of the cut rather than follow the intended arc, which in turn makes the blade bind in the cut.

I did use Pam, and WD40 on my blades to reduce friction. Not any more. After fine tuning my arcs, they bind a lot less than they used to.

You do need a good burr for coring. Most of the time I just take it for a quick touch up in the CBN wheel rather than hone. If you hone (face of the bevel only, not the top), then the coarser the hone, the better.

Hope this helps some.

robo hippy

[Mike Goetzke]

Thanks for all the help. My bowl blank was still chucked in the lathe so went at it again. I raised the blade as suggested and YES it started feeding again! Still mush and lots of noise at the end but once I got the knife is as far as it would go I was able to knock the center bowl out easily. I was within 1/4-3/8" of hitting my mark too.

Yes, I need to sharpen. So involved in cutting I forgot to check the cutter edge.

And I did watch your video Robo on straightening the knives. I was ready throw out that mangled mini till I saw your video.


Mike

[Mike Goetzke]

Just as a follow-up. I was coring again today and had a metallic like intermittent noise again and more vibration. This time I looked and discovered my banjo was loose. Guess I really need to tighten this up harder than I've been doing. I'm thinking this is what caused my original bent knife.


Mike

[Reed Gray]

Yea, every thing will vibrate loose when you core....

robo hippy

[Mike Goetzke]

After twisting up my mini knife I prepared a bowl blank for coring at the local turning club. Bob Bergstrom is a member there and helped me with the 16" diameter blank. We got 3 bowls from it. The spalting on the sycamore was really nice. I had the blanks brother at home and Bob suggested to hog the bowl out of the bark side. The wood had quite a bit of rot so I had to part off about 3" but here is the result:


347960347961


Dog gone when I went to use my new large straight knife for parting I found this:

347963

The standard size knife on the right (must have been last one ground on a Friday after a trip to the pub). Hopefully I can get a replacement at Lee Valley.



Mike

[Jeramie Johnson]

I am using a different rig, but I will attest to constantly pull out the blade to remove shavings, especially punky and wet wood. When nearing the center, make sure none of the shavings are getting in between your tip and the coring candidate. At some point I will rig up something to blow the shavings down as I am coring, but for now, just lots of clearing.

[David Delo]

Looks like your making progress Mike. Nice looking timber and should make some great gifts next year.

[Dale Miner]

After twisting up my mini knife I prepared a bowl blank for coring at the local turning club. Bob Bergstrom is a member there and helped me with the 16" diameter blank. We got 3 bowls from it. The spalting on the sycamore was really nice. I had the blanks brother at home and Bob suggested to hog the bowl out of the bark side. The wood had quite a bit of rot so I had to part off about 3" but here is the result:

.

347960347961


Dog gone when I went to use my new large straight knife for parting I found this:

347963

The standard size knife on the right (must have been last one ground on a Friday after a trip to the pub). Hopefully I can get a replacement at Lee Valley.



Mike

The offset point won't affect anything. Actually, looking at the picture it appears the tip is nice and wide. The width gets less as the knife is sharpened so the extra width means longer life.

A straight knife also works well if the tip is straight across