Bowl explosion

[Randy Hogan]

I'm relatively new to bowl turning and I had a bowl explode off the chuck today and it scared the crap out of me, along with making me pretty gun shy. The blank was ash, approx 13" in diameter once I got it roughed out. Had the outside finish turned and mounted it on a 3" chuck ( I opted for the 3" over my 5" because of the way I wanted the base to look. I had over 1" of shoulder beyond the recess for the chuck). I was nearly finished on the inside, had the bottom completed along with the sides, from the rim down approx 3". All I had left was an inch or so of thickness to take out in the transition area between the side and the bottom to finish the bowl. I was taking my time making sure the tool rest was moved and placed correctly as I progressed. I made a mental note of how easy the gouge was working and I wasn't forcing anything, was in a good rhythm with the correct overhang off the tool rest. The shavings were flying out nicely. I started another pass with the gouge and the bowl just blew apart. I was out of the line of fire thankfully, because it was ferocious.

A few other things regarding the circumstances. I was turning at 1400 rpm. Was this too fast for that bowl size and chuck size/method? The bowl was seated firmly in the chuck and it was only expanded a few turns from fully closed to fit my recess. I had checked it several times and re-snugged it each time. I noticed no vibration or anything amiss regarding the chucking.

Is there a better order or sequence of turning the side, transition and bottom?

There were 2 cracks on opposite sides of the rim about 1" down each side. I debated about turning the rim down below the cracks to eliminate them but at that point, I had not. I didn't notice any other cracks or defects in the blank, especially down into the bottom of the bowl where I was working.

I'm at a loss for how this occurred. My tool positioning and technique was good and working fine, pass after pass. It sure didn't feel like it was a catch that initiated the failure. But I suppose at that rpm I wouldn't feel the catch before the bowl exploded.

I'd appreciate any advice I could get on this on how it might have occurred. Thanks.
Randy

[Roger Chandler]

Speed too fast for that size bowl, as what happens is that at that speed you get flexing in the rim, and may not notice it. You might have had what is known as "ring shake" in the blank without being able to detect it, especially if the wood was a bit green. Ring shake is induced into a tree when the wind twists and turns it, and it induces separation of the layers in the grain. Fairly easy to detect when the wood is good and dry, but when green, [wet] the moisture still has the fibers in the wood swollen, so it may not be visible.

I've had it happen, and wood hit me, but thankfully I had on my air helmet, and it absorbed the blow. Once you get hit, it will make you stop the lathe and check the progress as you go a few times when you get back to turning after you put on clean underwear! :D

A 13" bowl should be turned at about 750 rpm.....give or take 50-100, but if you know the wood is solid, you might sneak in another 100, to give you really clean cuts, but let the wood come to the tool, instead of pushing the cut, and you will be pleased at the quality of the cut off the gouge. Less sanding that way! ;)

[Dick Strauss]

A few questions come to mind. How deep was your recess? Did the outside chuck profile match the recess? How much and what shape of wood was left outside the recess? How long had you been turning (how much moisture was lost during the process causing shrinkage)?

[Dennis Ford]

Wood strength varies greatly from piece to piece, what works on one blank will fail on some other blank so troubleshooting a failure over an internet forum is not very reliable.
In general, 1400 rpm is on the high side for 13" diameter but not excessively high for all blanks that size. From your description, centrifugal force and the expansion force of the chuck combined to blow up the bowl. Clearly the speed was too fast for this piece, possibly it might have failed anyway due to unseen defects.

[Sam Force]

I am by no means an expert but look at the math, 13 inch bowl by 1400 rpm. that is 40 inch circumference by 1400 rpm. that equates to 56,000 inches going by the tool rest every minutes. Does the thought of that sound safe with a bowl that has a couple cracks?

[John K Jordan]

Randy,

I see you were using a recess instead of a tenon. (This is my preferred method.) I assume this was green wood.

I also think the speed may have been too high for that size bowl. I personally would not try to turn a bowl with cracks at the rim, but others do this.

Some more information would be helpful.

Can you describe from the pieces just how the piece came apart? Some photos might help.
Did it come apart at the cracks you mentioned?
What was the wall thickness?
Did the wood break at the chuck or did the wood around the jaws stay together. If it broke at the recess could the jaws have been overtightened?
If it didn't split, did the bowl come off the chuck jaws or did it stay firmly attached and break elsewhere?
Is there evidence in the pieces of a catch that you didn't feel?

How deep was the recess? Did you dovetail the sides of the recess to match the jaws?

Assuming you had turned a nice flat rim inside the recess for the jaws to bear tightly against. One thing that can help for a more secure fit is to slope the bottom of the recess slightly so it is a bit deeper at the outer diameter than it is towards the center. Any wood fibers left in the "corners" can prevent the jaws from expanding tightly and the slight slope on the bottom can prevent this.

I prefer to turn with a recess but usually have more than 1" of wood outside for something that size. A very safe option is to glue a waste block on the bottom and hold with a fairly large tenon (or a face plate).

JKJ

[Grant Wilkinson]

John's questions are spot on. I would be willing to bet, since you say that the bowl "exploded" that you opened the jaws too much into the recess. It's all too easy to introduce a lot of stress and even cracks into a bowl expanding into a recess.

[Bert McMahan]

I'm no expert on bowls but I do know that a 13" bowl spinning at 1400 RPM has some *incredible* stresses in that hoop that'll try to rip cracks apart.

Stress in the hoop is linearly proportional with diameter, but squared with rotational velocity. Your bowl was experiencing about 360x the force of gravity in centripetal force. Another way to look at it, if 750 RPM is the "correct" speed (what Roger said), 1400 RPM puts about 3.5 times more force on it. Add cracks to the mix, and I bet that's your issue right there.

[Reed Gray]

If there was no catch involved, most likely it was the wood coming apart. Perhaps that is the best reason for taking a lot of care with turning anything with cracks in it. I do turn bowls that size in that speed range, but I did production work, so kind of a 'professional driver on closed course, do not attempt' type of thing.... If there are any cracks, the speed goes way down till the cracks are turned away, completely. I had one ash bowl years ago that lasted 6 or more years before ring shake caused a crack in the wood (wood cracks along the growth rings rather than on center (both from inside and outside..). So, which type of cracks were in the bowl? With ring shake, you can lose the center section of the bowl. With a radial crack, it can split into pieces, so maybe 2 halves of the bowl were left... If you got a bunch of pieces of all sorts of sizes, then most likely there were a lot of hair line cracks that were invisible, the type that show up when you apply finish. For a bowl that size, I do use my bigger chuck with jaws that are 2 5/8 inch diameter and about a 1/8 inch deep recess, though I never measure. Glad you were standing out of the line of fire.

robo hippy

[Randy Hogan]

The bowl split into 3 main pieces. It split from crack to crack. I do see there was less than 3/8" of wood between the bottom of the bowl and the recess. I did notice a vibration between 800-900 rpm that 'mostly' disappeared over 1000 rpm. I could not get the vibration out after doing my best to true the bottom, side and top of the blank.

I may have over tightened the chuck but I didn't hear any audible cracking. I feel there was sufficient shoulder around the recess.

Everything came off the chuck.

The wood was from a 2 yr. dead ash tree (an epidemic here in the midwest) that had been recently felled and I cut a log off the end 5 days ago.

Good stuff gentlemen. I appreciate the feedback.

[John Beaver]

I'm going to guess that the force of the expanding chuck was too much force as the bowl was turned thinner. When the bowl was thick there was enough meat to hold it together, but as you went thin the force of the chuck overwhelmed the thickness of the wood. Add to that some cracks, and the high speed. Most accidents are a combination of factors - usually three. In this case you had the perfect storm.

[Alex Zeller]

I have plenty of green ash around here and it's a very strong wood but it takes very little to split it for firewood. I'm no expert but all I would suggest is to leave the bottom until last. I like to leave lots of meat on the base and then use my cole jaws to finish off the bottom.

[Bill Boehme]

The best way to view this experience is that you learned some things and didn't get injured.

Personally, I prefer using a tenon, but when using a mortise there isn't any reason to do a white knuckle tightening of the chuck. Just go for snugging it up and then just a little bit more. Then try to wiggle the wood to make sure that it is being held securely. When the wood is dry and you need to stop and tighten the chuck that's a red flag. Another red flag is when a vibration starts or you hear a change in sound. That's the time to stop and do a thorough check of things. Tightening the chuck at this point would make me worry about the wood being cracked. The split across the bottom sure does look like it was caused by overtightening the chuck. And, of course, turning too fast just means that you will get in trouble sooner.

[Pat Scott]

I prefer using a tenon. Not that you have to, but if you tighten down on a tenon too much the jaws will bite into the wood deeper instead of pushing harder against the side walls of a recess. Biting deeper can cause the bowl to not run perfectly true, but that's about it. I figure there must be a reason that all the pro's use a tenon instead of recess.

Because you have to leave enough wood around the recess, this affects how big or small of foot the bowl has. You wanted a smaller foot so you used a small recess, but the jaws were too small for the size of bowl you were turning. With a tenon you can have a bigger holding area and then turn the tenon away or incorporate it into the foot design when you finish the bottom. Strength is in the diameter of the tenon or recess, not the depth.

I agree with others that you were turning too fast. Dale Nish came up with a formula to help new turners that is Diameter x RPM = 8,000 to 9,000. Note that 8,00 to 9,000 is just a number, not the RPM! So using the speed you were turning at: 13" x 1400 rpm = 18,200 which is twice as fast as you should have been. I agree with Roger that: "A 13" bowl should be turned at about 750 rpm.....give or take 50-100", which is the speed I use (700 to 750 rpm).

Also even though the tree had been dead for 2 years, you'd be surprised how much moisture it can retain and for how long even when dead. I bet if you checked it with a moisture meter you would have found it was not dry. You're asking for trouble with cracks in the rim, next time turn them away or fix them before hollowing out the center.

Where in the mid-West are you located?

[Reed Gray]

The size of your recess and chuck are fine for that size of bowl. I use the same tightening on both tenon or recess (though I prefer a recess) for my bowls. Good and snug, but not white knuckle. If you get it white knuckle tight, you can cause it to split, or there can be enough pressure that any slight variation, like the crack, or a catch, small or large catch, can be the straw that breaks the camel's back. I have found that trees that are down for a year or three, or dead standing trees are more prone to cracking once you open them up, even cutting into lengths. No idea why. Lesson learned here, and again, glad you have learned to stand out of the line of fire...

robo hippy

[Randy Hogan]

Good stuff guys! Yes, It was a lesson well learned. Like someone said, it prob was a combination of factors. I'll definitely be more particular regarding cracks and the speed. Much of my initial learning was from the robo hippy you tubes so I thought I was ok with my chucking, but I'm pretty sure I white knuckle tightened it . My caution level has gone way up regarding all factors and subtleties of the whole process. This forum is just great to get the input from all you guys. Thanks again.
I wish I could run out and wack a chunk off a tree when I want to turn a bowl but obviously that's not hardly ever possible. I'm experiencing the cracking in the dead trees just as you described it, robo. Cherry is horrible. The ash isn't as bad but it's certainly an issue.
Pat Scott- northern Illinois

[John K Jordan]

Good stuff guys! Yes, It was a lesson well learned. Like someone said, it prob was a combination of factors. I'll definitely be more particular regarding cracks and the speed. Much of my initial learning was from the robo hippy you tubes so I thought I was ok with my chucking, but I'm pretty sure I white knuckle tightened it . My caution level has gone way up regarding all factors and subtleties of the whole process. This forum is just great to get the input from all you guys. Thanks again.
I wish I could run out and wack a chunk off a tree when I want to turn a bowl but obviously that's not hardly ever possible. I'm experiencing the cracking in the dead trees just as you described it, robo. Cherry is horrible. The ash isn't as bad but it's certainly an issue.
Pat Scott- northern Illinois


With the pictures and further description I can understand why it likely came apart.

Although I mentioned earlier that I would not turn a piece with cracks like that, if the desire ever strikes again there is a way to do it without the high risk of losing parts of your face.

A friend has turned a number of much larger pieces from burls and such with so many voids the bowls would certainly have been in pieces otherwise. What he did was mount and turn and finish the outside completely, assuming there was enough integrity of the wood inside to hold it together up to that point. Then he wrapped the entire outside with numerous overlapping layers of the strapping tape that used to be common for taping boxes for shipping. This is the tape with embedded fiberglass filaments that give it tremendous strength. Then he would turn the inside. The tape kept the piece from flying apart. From your description of how it failed from the cracks down I suspect this method would have kept yours together to the end.

I'm not recommending anyone do this, just relaying a method that has worked. In his case, the the integrity problem was more because of voids in the wood, not cracks. In your case it would be safer but you would still end up with a bowl with cracks. I know some people embrace cracks and fill and enhance them but for me wood like that is firewood, or a much smaller blank after some quality time with the bandsaw.

I personally turn mostly dry wood and carefully cut away any cracks before start.

For green wood bowl turning, the best advice I've read is to start with sound wood, store in the shade or underwater, and turn it very soon after cutting, best within a few weeks. If that's not possible, cut the log as long as possible and when ready to turn cut some off the end of the log to get rid of endgrain drying defects, cut a blank, then turn immediately. (This last advice comes from the other John Jordan, the famous one. He said if possible he gets the entire log delivered and stores it off the ground in the shade, then when he wants a blank he cuts off about 6" from the end and throws that away, then cuts his blank and turns it that day.)

JKJ