Large Bowls splitting while turning

[Bob Cook]

I have attempted to turn 2 large bowls, 10-12" dia, and both have split while turning. When I get to finish turning the last 2-3 inches nearest the rim the bowls have begun to vibrate very badly. In both cases the bowl has split while I was turning it. What am I doing wrong?

Thank you,

Bob

 

[john lucas]

Sounds like you might be taking too long to turn or even more likely the wood had very small checks in it before you started. With practice you will be able to turn the bowls quicker which gives them less time to crack. If I'm in doubt I spray the bowl with a spray bottle to keep the outside wet and even the inside if necessary. If you stop for any reason, lunch, bathroom, etc, put a plastic bag over it. This will keep the moisture from escaping so quickly.
If the checks are there before you start the only solution is to try to get to the wood sooner before it has time to check. If I'm starting at the end of a log I cut off about 3/4" and then try to break it. If it breaks easily cut off another 3/4". When you hit solid wood then cut your bowl blank.

 

[MichaelMouse]

The words "finish turning" concern me. Most of the bowl turners I know are TDT (Turn, Dry, Turn) types. If that's true for you, then there's really no reason other than improper cutting technique that would cause a bowl to split during final turn. What was there at the beginning in the way of checks would be very evident after drying.

If you're turning warp and go types, where the bowl is turned completely while wet/green, you might run into undiscovered end checks. John's method is one good one for finding and avoiding them. I wet the end with water and look for wicking myself. Pretty reliable, but remember that low down where you'll be removing stock is less a concern than high up where the diameter will be greater. No sense throwing away diameter because what you'd have removed anyway is checked.

Stage cutting, where the bowl is turned to final thickness in stages, top to bottom, is a reasonable thing to try. Don't go back and try to cut circular, just let it happen. As you acquire skill, you'll be able to work wood in thinner section, and if you acquire a steady, you can sneak up on translucent. In the meantime, concentrate on making shavings, not chips, and make your piece in one session. If not, bag it with its own shavings to keep damp, but remember that mildew can stain it in a matter of days.

It wasn't going loose on its hold while you tried to finish, was it? And you weren't reaching around from the rear trying to re-true the outside of an ovalling piece?

 

[odie]

I have attempted to turn 2 large bowls, 10-12" dia, and both have split while turning. When I get to finish turning the last 2-3 inches nearest the rim the bowls have begun to vibrate very badly. In both cases the bowl has split while I was turning it. What am I doing wrong?

Thank you,

Bob

You've got a couple pretty good guesses by MM and JL........but, it's hard to know for sure with the limited information you've supplied.

The species of wood, height, wall thickness, grain pattern, shape, etc,etc,etc, will all have their effects. What works for one set of general characteristics, may not work at all for you when any one of those physical characteristics changes.........

Off hand, here are a few possibilities:

Tools not as sharp as they could be.

RPM too high

Failure to keep supporting wood intact until no longer needed.

Attempting to remove too much wood per pass

Improper tool height, and/or presentation technique

Failure to listen to, understand, and respond to the audible indications of oscillation

.....and, more may apply!

ooc

 

[John Beaver]

You say when you get to finish the "last 2-3 inches near the rim".

If this is on the inside, are you starting at the rim and working towards the base?

If it's on the outside, it sounds like it could be too thin to be going back and finish turning.

 

[Tony Latham]

Bob:

I am wondering about one of the basics....

You have a green "bowl blank" right? And you've cut this blank by cutting a 12" piece of log.... and then split this log in half down the center line.... the flat side where the center of the wood was is now going to be the top of the bowl and the bark side will be the base of the bowl?

I just want to make sure you are not trying to turn a green bowl where the rings of the tree will be centered on the bottom center of the bowl? That would be your problem with +90% of the species I think...

Am I making any sense here?

TL

 

[Joe Greiner]

The "last 2-3 inches near the rim" bothers me too. You might get better results by turning the FIRST 2-3 inches near the rim to final thickness, and letting it warp as much as it desires. For reinforcement, wrap the rim with filament tape.

Somewhere here (Ah, here it is: http://www.aawforum.org/vbforum/showthread.php?t=5668 ) is a bowl turned from a crotch. Three lobes near the top, and round near the bottom.