Skating- a different explanation?

[Lars Hansen]

Hi.
Just saw a video that gives an explanation of the physics behind the skating of bowl gouges. (Short: It's the alternation between side grain and end grain that forces the gouge backwards)

View: https://m.youtube.com/watch?v=IbjjbKcQmSU
(from around 3:30)

The explanation is new to me and quite different from my own explanation (and experiments) but.. live and learn!

Would someone with a solid grip on physics/mechanics care to watch and comment?

TIA,
Lars

 

[Bill Boehme]

He is all wrong. The problem is caused by an unsupported cut which means that the gouge's contact point with the wood isn't directly above where the gouge contacts the tool rest. When that happens there is a torque that rolls the gouge to the 12 o'clock fully open position and the tool catches. If his theory were correct there would be no way to avoid the problem.

You can also get a catch if there isn't bevel contact.

 

[Lars Hansen]

@Bill: Thanks, that explains a catch. To me it seems logical that skating appears when you present the tool (i.e. the lower part of the flute) at an angle to the vertical force from the wood without bevel support to stop it.
If his theory was right, skating wouldn't appear on spindle turnings or on pure end grain.

 

[Dennis Weiner]

I agree with Bill, he is way off. If I were turning a pure end-grain bowl, the same skating can occur. Grain orientation has little or no effect. At the described entry point, The centrifugal force of the lathe is pushing back at the gouge at this entry position. Normally, The toolrest absorbs/ neutralizes the force at 90 degrees but entering the wood from the side, there are forces that push your tool to the side. It's about physics, not alternating grain orientation.
This is why I recommend to all beginners to look at AAW previewed videos before watching all these self-proclaimed experts on You Tube.

 

[john lucas]

I agree it's not about the grain. It's about flute orientation a d starting gently. As john jordan once told a class I was in. If you start the tool forcefully it will.be rejected forcefully. If you start gently it will be rejected gently. Its all.about starting g the cut with the flute at 3 o'clock and then starting gentlybenough to establish that shoulder for the bevel. I have found that you can start with the flute at other positions if you start very gently and have a thumb against the back of the bevel.to.prevent kickback until you get the shoulder established. This is not as good as starting with the flute at 3 it was just an experiment to learn more.

 

[robo hippy]

I also agree that it has nothing to do with grain orientation, and every thing to do with bevel contact. It is true that the wood, in bowl/side grain orientation cuts differently when you are pushing straight across the bowl when compared to cutting down through the grain. Another explanation of that is if you cut straight across the top plane of the bowl, and when you are trying to go across the flat bottom of the bowl, when the end grain comes around, you are pretty much head butting it, and that can make your tool bounce, especially if you are pushing hard, which is common for heavy roughing cuts. When you cut from the rim to the bottom of the bowl, you are going down through the grain rather than head butting it, so the bounce is much less.

The kick back happens because you are not rubbing the bevel. So, straight across the top plane of the bowl your bevel angle should be parallel to that plane, and I will call that 0 degrees. If you come off the bevel to about 10 degrees, you will get minimal skating. If you come off to 30 to 45 degrees, you can get big kick back. When you go past the 45 degree range, the kick back starts to slack off. If your bevel is 90 degrees to the plane of the top of the bowl there is no kick back.

Flute orientation can play a bit into this. With flutes straight up, if you are not rubbing the wing bevel, you can get a catch. With the flutes straight up, you are coming very close to doing a scraping cut, or are actually doing a scraping cut. I always cut with my flutes on the side. Never related to the concept of open or closed flutes. Open flute to me means spindle detain gouge, which is ) shape. Closed flute is V shape..... Yes, I am more than a little different....

As for the rings in the bottom of the bowl, those come from a couple of things. One is the heel of your bevel. It is easier to control your cut by having the cutting edge as close as possible to the cutting edge. No problem on the outside of a bowl, which is generally convex shape. On the inside of a bowl, where the shape is concave, if you have a long/full length bevel, your rub spot is farther away from the bevel. This is why most bowl turners grind off at least half of the heel of their gouges. I prefer to grind off 3/4 or more of it. I also make sure it is rounded rather than a straight 'secondary' bevel. The other thing that contributes to the rings in the bottom of the bowl is pushing too hard. Your feet/minute speed goes down to almost nothing when you get near the center of the bowl, yet we all want to continue cutting at the same speed we do when we are on the rim. I have to bite my lip to make myself slow down and let the wood cut rather than cut the wood the way I want to. Again, the head butting that end grain contributes to the bounce, which contributes to the rings.

robo hippy

 

[Don Wattenhofer]

I agree the grain orientation has little to do with the problem. The bevel contact has nothing to do with starting the cut because there is no place for the bevel to contact until the cut has been started. The very point of the gouge must be in line with the desired path of the cut, which is usually with the flutes at 90 degrees to the tool rest, and brought in gently otherwise the screw effect will pull it out or in depending on the angle relative to the desired cut path. Once the cut is started you can you can gradually rotate CCW to get a bevel riding chip clearing cut. The kind of grind shown in the video would be extremely difficult to start a cut but a 40/40 grind with the heel rounded would be best, but that is just my opinion.

 

[Richard Coers]

You have to be careful taking advice from youtube. Often the person is more interested in subscribers than their own skill development.

 

[Dennis Weiner]

I love the 40/40 but the gouge grind angle does not come into play If you enter the wood with the outside gouge angle at ninety degrees to the wood with the gouge in the neutral position(not the tool at ninety degrees). At this angle, The centrifugal force of the rotating wood is pushing directly down on the tool rest. Using this entry method, There is little of this side force being applied to the gouge outward which is the cause of the "skating". Restating, you do not want the bevel engaging with the wood only the tip of the gouge straight into the wood.
I practice this making the entry cut usually with one hand. This works with many gouges but some configurations have extra metal engaged that will cause some side forces to be created.
Of course, there are forms that dictate an angular entry in which case I neutralize the "skating" with my thumb as mentioned by others previously in this thread.

 

[john lucas]

If you start the tool with the flute at 3 oclock and you have the cutting edge any direction other than straight in it will try to skate. To experiment try angling the tool handle more toward the lathe bed. It will now skate toward the inside. angled out it will try to skate toward the outside. If you start the cut very lightly these forces are easily countered with pressure from your left hand which is why you can start the cut at the angle you want. It's easier to see this when using the toe of a skew. Straight it it will push right straight in. If you angle it left or right it will skate in that direction.

 

[Don Wattenhofer]

I love the 40/40 but the gouge grind angle does not come into play If you enter the wood with the outside gouge angle at ninety degrees to the wood with the gouge in the neutral position(not the tool at ninety degrees). At this angle,

The advantage of the 40/40 is the more prominent point, which if positioned correctly has little tendency to skate, verses the blunt point on the Ellsworth grind. Review Johns post.

If you start the tool with the flute at 3 oclock and you have the cutting edge any direction other than straight in it will try to skate. To experiment try angling the tool handle more toward the lathe bed. It will now skate toward the inside. angled out it will try to skate toward the outside. If you start the cut very lightly these forces are easily countered with pressure from your left hand which is why you can start the cut at the angle you want. It's easier to see this when using the toe of a skew. Straight it it will push right straight in. If you angle it left or right it will skate in that direction.

 

[Leo Van Der Loo]

My take is that similar as to what John L. says, the tip of the gouge should be vertical and at centre level squarely at the wood, just entering the wood and making the shoulder that will keep the tool from skating out of the circle you just made, a undulating edge and rough spots makes it harder to establish that shoulder.

You can try this and see what happens with a skew that is held horizontally with the blade vertical on the toolrest with the long point down enter the wood, when the skew is no vertical, depending the angle you hold the skew at, it will go left or right or stay in the groove you make.

 

[Timothy White]

I took a look at the picture of the bowl gouge in the opening post, that told me all I needed to know about the content of the video I didn’t waste any time viewing it.
The bowl gouge is not sharpened properly, it is flat across the end, it doesn’t have a point. It has two.
If you use the techniques the pros here are telling you and a bowl gouge sharpened like this one you have almost no chance of success. Also a bowl gouge sharpened in this fashion will be catchy and grabby and hard to control.
To start a cut:
1) make sure the gouge is properly sharpened properly
2) set tool rest so that the tip of the gouge is on center the handle must be parallel to the bed ways and the bevel rotated to 3:00 am or 3:00 pm.
3). Use left thumb to anchor gouge until you get a place to rub bevel.

In another post (Robo Rest) Odie talks about marking his new tool rest. I believe these marks allow him to quickly adjust his tool rest to different size bowl gouges (Odie please correct me if I am wrong about this.
IMNSHO Developing this skill Becomes very important when turning a natural edge vessel.
Practice does not make perfect.
PERFECT practice makes perfect. Unfortunately I have yet to reach this lofty goal. But I keep trying.

 

[Bill Boehme]

Timothy, you missed one of the highlights of his video where he announced that he also has a bowl gouge sharpening video ... so be sure to watch it. I decided to pass by this opportunity to further my education.

 

[Lars Hansen]

@Tim: If you look beyond the picture, I think you will find that it doesn't show the grind that he uses - some kind of photoshop error, I guess.
My opening question didn't reflect having trouble making an entry cut in side grain - I use and teach the same techniques as described above.
Students often ask for links to good videos or want an opinion on what they find, so I do watch other stuff the the classics from Reed, Lyle, Hockenbery, John Lucas et al.
This one will definately not be added to my list of favourites but I keep trying to challange my own fixed ideas of hows and whys and trying to find better ways to explain turning techniques better than the ones I have come up with.
I learned a few useful explanations from this thread.

 

[john lucas]

I'm with you Lars. I learn something from everyone. Maybe it's just a small thing that they may not even know they are teaching. Sometimes of course it's how not to teach.

 

[Clifton C]

Amen to that John.
c

 

[Glenn Lefley]

Haha I didn’t even notice he had a square tipped bowl gouge!