The Geometry of Woodturning and why catches occur

[Keith Harrell]

A friend of mine askes me to explain the way catches occur and I would like to be able to direct them to a good video or book or web page to help explain why catches occur and how the tool get sucks into the wood when the edge catches right sometime. I'm still new and don't want give him some bad info.
Thanks

 

[steelguy]

Preventing Catches

Lyle Jamieson article from the journal:

https://www.lylejamieson.com/tools/documents/AAWArticle-FearFactor-Winter06.pdf

 

[hockenbery]

Kieth,
The simple explanation of geometry:
If the wood can drive onto the tool you get a catch.
This is easy to show a student by turning the wood by hand while cutting with a turning tool.
In the cutting positions wood cuts cleanly.
In the catch positions the cutting edge gets buried in the wood and you can't turn the wood.

.There is another article (earlier) on the AAW members page too:
http://woodturner.org/DownloadContent/articlesview.asp

Article Name: Five Ways to Avoid a Catch
Author Name: Jamieson, Lyle
Date Published: Spring 1996 Volume/Issue: 11.1

happy turning
-al

 

[MichaelMouse]

Al's got it. A catch happens when you let the tool get underneath the wood against the direction of rotation. When the tool gets under the wood along the direction of rotation you can still catch if you dig too deep, but it's rare.

The key is to have tool control for the entry phase, when you have to spear the wood so you can slice. That's where the A-B-C method comes in. Anchor the tool to the rest. To me that means hand over, not fingers pushing. Bevel set. Means touching the work with the bevel somewhere behind the edge where possible before swinging the handle to a cutting position. Make sure you have air over the tool up rotation before you swing to Cut across and into the wood.

Lyle's illustration showing "Bowl Gouge Inside the Bowl," if followed, explains why so many people complain about catches. "Starting a Cut" is a fine illustration, and as long as the tip contacts first, probably the best way to start and finish a cut. Problem with these dumb cylindrical gouges is they have such a narrow sweet spot, defined as the distance between the exit and entry point of the gouge in the wood. Broader radius gouges, as I prefer for finishing, extend that sweet spot, giving more support for the bevel, which by the way isn't perpendicular, but parallel to the edge.

Another problem with "bowl" gouge grinds is illustrated by the oak photo. The bevel angle isn't constant. Roll that gouge a bit counterclockwise, or when working inside a decreasing diameter bowl, fail to rotate it clockwise, and the edge becomes what you see in the first illustration, a spear into the rotation rather than a slice across. I hog that way, but hogging is just a narrowly controlled catch. If the gouge is of uniform thickness and with a uniform bevel angle it can't catch when used below centerline. The curve of the gouge keeps air between itself and the piece, and positioning the tool at/below center keeps air above it as well.

Here's my favorites in operation. As a still and as videos.

http://s108.photobucket.com/albums/n28/MichaelMouse/?action=view&current=CherryPeelOut.flv

http://s108.photobucket.com/albums/n28/MichaelMouse/?action=view&current=CherryPeelIn.flv

 

[john lucas]

Lyle's article was good. In my classes catches occur for 3 reasons. They come off the bevel. That is basically what MM is describing. Without the bevel acting as a fulcrum the point of the tool just goes into the wood really deep and throws the tool back at you.
The try to cut with an unsupported edge (see Lyle's article for drawings of this. If you think of a rough out gouge as a U shape. The bottom of the U us supported by the tool rest. If you cut with the bottom of the U your OK. If then try to cut with the upper portion it won't be supported by the tool rest. The wood will try to pull this upper portion down and to the tool rest and you get a catch.
3rd is lack of attention. I believe this is the main cause. They stick the tool into the wood without rubbing the bevel or they touch the wood with the tool not on the tool rest. Mark StLeger describes proper tool usage as ABC. Anchor, Bevel, Cut. In other words anchor the tool on the tool rest. Touch the bevel to the wood so the tool isn't cutting. Then lift the tool until it cuts. Following this will eliminate most catches.

 

[Bill Boehme]

A friend of mine askes me to explain the way catches occur and I would like to be able to direct them to a good video or book or web page to help explain why catches occur and how the tool get sucks into the wood when the edge catches right sometime. I'm still new and don't want give him some bad info.
Thanks

Once your friends understands the basics of the mechanics and kinematics involved, it is time to move on. You can't learn to dance by learning the geometry of foot positions. Thinking in terms of geometry will only get you so far, but to turn without fear of catches requires learning body positions and movements and tool handling in presenting the tool to the wood.

 

[Keith Harrell]

Well I have learned a lot about catches and how to prevent most. I was getting a few and didn't really want to tell them don't use this cut here as it will generally catch which I had learned the hard way. In looking at the catch results in the wood I see how the gouge really got pulled in like a plow in a field now. I think I was not watching the tool rest height when I was showing him how to turn the inside of the bowl which was always my weakness.
Thanks

 

[robo hippy]

Well, I agree with a lot of this, but have a few things to add.

There are a lot of cuts, primarily roughing cuts, done with either a gouge or a scraper which are safe to a certain degree, but can be catch prone if you are hanging out too far off the tool rest. It is a leverage thing, and we all do it. You know, getting that last bit off before moving the tool rest, and having the gouge or scraper start to chatter, then dig in. The wood and torque of the lathe can be too much if you don't have a long enough lever to support the amount of wood being taken off. Mechanical advantage to the turner when the tool rest is closer to the wood, and you have a longer heavier tool.

Another is biting off more than you can chew. This seems to happen more on uneven bowl blanks when first starting to rough them out. You have to nibble away at the rough areas before starting to really rough it out. If you just plunge in, you can get one of those uneven spots, and bam, you have much more steel on the wood than any lever can handle. This is much more of a problem on chainsawn blanks than on bandsawn blanks. This also can happen in the transition area on the inside of a bowl. You go from a thin cut on the wall, to a bigger cut in the transition. This is more of a problem with a big round scraper if you have a steep transition and you end up cutting on both the bottom of the bowl and the side at the same time. This same thing can also happen with a gouge if you have the whole nose of the gouge buried in the cut and the flutes are more up rather than on the side, pointing away from the inside of the bowl. More wood on the tool than any lever can handle. Also, the up hill side of the wings of the gouge will catch.

Another situation is on the inside of the bowl, when you get to the center. If you are pushing too hard, and go through the center, you hit the up side of the rotation, the tool, in contact with the wood goes up, and back around to the proper cutting side, and bam. Explosion, and damage to gouge and some times fingers as well. This some times happens when you are through with the cut to the center, and take your eyes off the tool as you lift it off the tool rest, and out of the inside of the bowl. The tip comes into contact with the spinning wood, and bam.

The primary roughing cut on a bowl, with a gouge, to me is a scraping cut. Flutes are held level, or with the handle held lower, or for me, I prefer a scraper. The cutting edge is pretty much at 90 degrees to the rotation of the wood. This can be done with the bevel rubbing, or not rubbing. The catching risk isn't from coming off the bevel, it is when the uphill wing of the gouge comes into contact with the wood. Lower side, no problem, mechanical advantage to the turner. Uphill side of the gouge, can be used, but it does make my sphincters pucker ("When sphincter tightening exceeds chuck tightening, you have a problem."--Pats Fan on Woodnet forums). I have seen Stewart Batty use this cut across the outside bottom of a bowl, no nose contact with the wood, just the wing, and like a skew, if you come off the bevel, you can get a catch. Rolling the gouge so the flutes are at 45 to 90 degrees to the rotation (10:30 to 9:00 o'clock) is much safer.

robo hippy

 

[hockenbery]

"Another problem with "bowl" gouge grinds is .... The bevel angle isn't constant. "

I'm a big fan of the Side-ground (Ellsworth) bowl gouge.
I call the changing bevel angle a "feature" that makes the tool able to accomplish so many different cuts with great efficiency. Unfortunately this "feature" often becomes a "problem" for folks trying to learn on their own.

For example the shearing cuts and peeling cuts produce incredibly smooth surfaces. Those cuts while easy to control once you've learned them are just a tiny roll of the tool away from a big catch. gripping the tool to tightly can cause that roll. On the other hand they are so easy to control once the bevel is locked in that they can be made one handed.

A properly ground tool is essential too.
If you have dips and bumps on the cutting edge of any tool the bumps will want to dig in and will.

happy turning,
Al

 

[john lucas]

One thing not mentioned is the shape of the object being turned. How many bowls have you seen beginners do that resemble what we call a dog dish. Not meant to be a snide remark it just describes the shape. You have a fairly straight or sloping side that changes abruptly into a reasonably flat bottom. That transition from side to bottom has probably caused more catches than anything else. This happens because you can't rub the bevel all through that transition point. I see it at club meetings all the time because they will have a nice looking bowl with tearout in the transition area. A smooth slope from side to bottom is far easier and safer to turn.

 

[MichaelMouse]

"Another problem with "bowl" gouge grinds is .... The bevel angle isn't constant. "

I'm a big fan of the Side-ground (Ellsworth) bowl gouge.
I call the changing bevel angle a "feature" that makes the tool able to accomplish so many different cuts with great efficiency. Unfortunately this "feature" often becomes a "problem" for folks trying to learn on their own.

Or, in this case, the tool itself is inherently unstable. Sort of like an aircraft, the more maneuverable, the more unstable. Since I have a choice of gouges, I choose the one appropriate to the task. Not that I can't cut with any other grind at hand, since I've worked wood with many edged tools. See the pretty picture below.

I see a lot of people talking about making this grind or that do all things. I can only ask why they they don't study the history of turning and learn that there was, and still is another way. If you can't turn the corner on the newbie bowl John mentions with a "bowl" gouge, use a different tool, even if some silly authority calls it a "spindle" gouge.

 

[Bill Grumbine]

A friend of mine askes me to explain the way catches occur and I would like to be able to direct them to a good video or book or web page to help explain why catches occur and how the tool get sucks into the wood when the edge catches right sometime. I'm still new and don't want give him some bad info.
Thanks

It's been my experience that new people are the best at showing catches. I know when I was new, I knew all of them well. I have taught many hundreds of people to turn, and I think I have seen just about every catch there is. Some are more common than others. I don't want to repeat all that has been written here already, but there is one thing I teach in my classes that I call forensic woodturning.

When a catch occurs, it helps to stop the lathe, and place the tool into the hole that it made in the wood. This shows the turner how the tool was being held at the time of the catch. This helps the turner to understand what was happening with the tool when the catch happened. One cannot stop doing the wrong thing until one knows what it is he (or she) is doing. Our problem is that many times when a catch occurs, we grab onto the tool tighter, and jam it into the wood harder in order to show who is boss. And the lathe always wins. After two or three of these catches, the shop is closed, bad words are said, and many beers are consumed. But if we stop and examine the scene of the crime, we can see what we were doing wrong, and that is the first step in fixing it.

 

[Bill Boehme]

Our problem is that many times when a catch occurs, we grab onto the tool tighter, and ....... bad words are said ......

I have a couple tools with permanent hand impressions in he wood from gripping it so tightly.

I won't say that bad words were said -- at the time, they seemed to fit the occasion and expressed my feelings quite well.

 

[MichaelMouse]

When a catch occurs, it helps to stop the lathe, and place the tool into the hole that it made in the wood. This shows the turner how the tool was being held at the time of the catch. This helps the turner to understand what was happening with the tool when the catch happened. One cannot stop doing the wrong thing until one knows what it is he (or she) is doing.

One of the best ways to avoid getting there in the first place is to "dry fire" the prospective cut with the prospective tool. Instead of finding yourself inside and upside down trying to swing some "Texas Toothpick" on your JET mini, you may find that a fingernail ground shallow/short gouge will swing down the wall of John's dog bowl and scrape up to it on the bottom. Of course, the advice in my tag line is always valid.

Our problem is that many times when a catch occurs, we grab onto the tool tighter, and jam it into the wood harder in order to show who is boss. And the lathe always wins. After two or three of these catches, the shop is closed, bad words are said, and many beers are consumed. But if we stop and examine the scene of the crime, we can see what we were doing wrong, and that is the first step in fixing it.

As old Frank Pain says, the wood will teach you how it wishes to be cut. What loads you least cuts the best. Some folks can't get it through their heads that slowing/stalling a full HP electric motor is as positive proof of poor technique as a catch. What's their answer? Get a bigger motor or turn up the speed and become more of a danger to themselves.

Lots of good technique in those pole/bow/treadle lathe users, especially when they, not the apprentice, were providing power.

 

[Keith Harrell]

Thanks for the suggestions.
I still find myself gripping the handle like my life depended on it so I needed to just relax more I guess or put gun grips on it.

 

[Bill Grumbine]

Thanks for the suggestions.
I still find myself gripping the handle like my life depended on it so I needed to just relax more I guess or put gun grips on it.

You're not supposed to grip guns like that either. I am a certified NRA instructor too. But that is a big point. The harder you hang on, the more likely you are going to lose control. Good luck with it.