Turning a Bowl Only Once - How Thin?

[Glen Blanchard]

I'm a twice turned kinda guy. Have never attempted turning a bowl from green to thin and calling it done. As there are so many that do nothing but this kind of turning, I think I'm going to give it a try. Just how thin does one need to make the walls to avoid cracking? If it's important to designate a species for the sake of this conversation, let's talk Maple.

[Paul Williams]

Limited experience here, but I'll start the discussion. I go for 1/4 inch or less depending on size and use of bowl. I have made several 12 inch salad bowls and get just a few very small cracks at 1/4 inch. They fill easily by sanding while applying the first coat of finish. The bowls become oval which appeals to some people, including the source of my white oak. I have only done small forms out of green maple so not much help there. I would go as thin as you think will handle the use of the bowl.

[David Coburn]

Hi Glen,

I've been doing this a lot lately and I go for the same as what Paul said... 1/4" or less... The ones that have a very gradual sloping curve don't present much problem in keeping the 1/4".. Some of mine also come down pretty straight and then make a pretty hard curve toward the bottom.. These you have to be real careful to keep a consistent thickness through the turn.. Also, be careful how you handle the area that is inside your base (if you do a little lip to hold it up)... and where you will ultimately be removing the tenon from...

I posted a thread earlier this week called "a couple more from the woods" where I showed 3 spalted white oak bowls that were all three turned green and show what I'm describing above. All of them are 1/4" or less thickness.

cya,
david

[Jon McElwain]

Perhaps even more important than a target thickness is a consistent thickness. Bowls with a thin rim and a thick bottom will crack much more readily than a bowl with a consistent thickness from the rim to the base. I turn a lot of bowls in the 10" range, and I like a thickness just under a quarter inch. I have turned them thinner, but they feel better with just a little weight (in my opinion). My 20-24" bowls are usually closer to 1/4" or even 5/16". Small bowls around 6" are usually in the +/- 1/8" range. Sand lightly to avoid heat cracks (a mistake I made a lot when I first started out), and make sure that the base does not thicken out much if any and you should be able to successfully turn a green bowl once without cracks. I have had trouble on small bowls by leaving too large a tenon - this causes a large area of the base to dry much slower than the sides of the bowl, and those thin sides can dry out and crack before I turn off the tenon. The thin walls have much more centrifugal "force" than the center of the bowl. This drives the water out of the rim quicker than the base. Keep the tenon small - no more than 1/3 of the width to help avoid this problem.

Of course, if you are turning ash or another wood highly prone to cracking, it is pretty tough to make once turned bowls. I don't think I will ever attempt a once turned bowl with ash again - too many cracked bowls. If you take more than a few minutes to get the bowl to it's final shape, you will probably need to keep some water on it (spray bottle) to keep the rim from drying out while you are turning it. I have had one bowl work just fine by turning quickly and keeping water on it, but the next bowl from the same piece of wood cracked in the middle of my turning because I was going too slow and didn't keep spraying it. Keep in mind that a bowl may begin to warp while you are turning it. Once you begin hollowing the inside, be quick and sure with your cuts, and don't try to go back to the rim.

Hope that helps.

[Dennis Ford]

I like functional bowls to be 1/4" thick or more, 3/8" thick is not too much for a 10 - 12" bowl and 1/2" seems good for large bowls.

My experience is different than Jon's; I have less cracking problems with once turned bowls than twice turned. I do agree that you should not waste time while hollowing the inside and "don't try to go back to the rim".

How you dry them is important, can't put Anchor-seal on a once turned bowl and you don't want any mold or mildew either; but you do need to slow the drying rate unless the thickness is 1/8" or less. Ideally you want the "free water" to evaporate quickly to avoid mold/mildew and the water inside the cell walls to evaporate slowly to avoid cracking.

I don't have a set formula for drying technique, but use judgement and experience to tell me which bowls can set unprotected in my shop and which need a paper bag (occasionally double bagged). The factors are: wood species, evidence of stress in the wood, size & shape of bowl, temperature & humidity. Try leaving the bowl in the open until it begins to dry (an hour or two in dry weather, much longer in cool damp weather), and then put it in a paper bag for a few days.

[Thom Sturgill]

Good answers os far. The important point is that it must be thin enough to move WITHOUT cracking, and that is somewhat species dependant. They will go oval and depending on orientation, the top may end up with two points resembling the prow of a boat. A recess instead of a tenon will allow a semi-finished bottom so that it can be remounted for final sanding.

[George Overpeck]

About crack control and spritzing with water - reflect on how that moisture is going be lost from different areas and at what rate. You can even do an experiment with a chunk of sacrificial wood - turn it and don't bag it and see what cracks first, and is it checking or cracking or both. The areas that check or crack first need a little spray of water.

Clearly the end grain is going to give up it's moisture first, so it might benefit from a little extra moisture. If you turned it at high speed or if it heated up during turning you might need to get some water on it just to bring it back into balance with the side grain. Also, in my experience the outside has more surface area and more opportunity for the moisture in the air to be circulated away. Maybe a little more spray on the exterior, if the shape calls for it.

Also, as you go into the bag - is it a small thin bowl going into a full size paper bag that's been sitting in a sunny spot? Any chance the bag's wicking ability could pull moisture out of the piece too quickly? Maybe spritz the bag. Sometimes the mass of the piece calls for double bagging, or using shavings strategically positioned in the bag.

I believe the number one cause of cracks for me at this point comes from opening the bag too soon. Lots of times I'll open a bag to inspect, judge the piece not quite ready to come out (but still uncracked) and then when I look a second time it has cracked. So stay cool.

[Reed Gray]

My specialty. If for no other reason, using your best Jeff Foxworthy voice, "You couldn't have done that on a lathe cause it ain't round, so how'd you carve it out like that?" When you explain, they go "It's so organic!"

How thick you can leave it depends a lot on the wood. With madrone (it starts to split when you take the chainsaw out of the truck. Well, maybe when you load the chainsaw up into the truck....) I can get away with 5/16, but not more than that. With big leaf maple, I can do 1/2 inch. I don't particularly like ones that are thinner than 1/4 inch for daily use bowls, they just feel too light weight.

Protecting the rim is probably most important as that is the most vulnerable spot. Kind of difficult to maintain even thickness when the wall ends. Very important to round over the rims (also helps to keep you from getting sliced by the rim). When I take the finish turned bowl off the lathe, I wrap the rim with stretch plastic film (5 inch roll from any office supply or shipping supply place). I go around several times, and really stretch it out. About 1 inch on the inside, and the rest on the outside. This puts compression on the rim which helps prevent cracking. When drying, I am now putting them bottom down on the concrete floor in my shop, where before I would stack them at an angle like in a dish rack. This is really necessary for the madrone, but other easier to dry woods don't really need it. Mostly it keeps the bottom which is generally thicker in the transition area protected. After a day or three, they generally go up on a wire rack. I do let them dry before sanding 7 to 10 days max. They can be dry enough to sand out in a day or two. It takes more time to wet sand than it does to dry sand, and wet sanding is messier. Having a lathe that will go down to 15 to 20 rpm helps a lot too because any speed over that and you can not keep any abrasive on the wood. Any 3 phase converter can do this, but PM and Jet don't do it any more. They tell you the motor will over heat and you will fry it and the electronics. I have sanded out thousands of bowls at those speeds, and the motor runs a lot cooler than when I am turning. I have a Robust and Brent walked me through the steps of lowering the minimum rpm speed. Most turn off at 50 rpm. If you don't have that, then you need a 2 or 4 stop spindle lock for power sanding. Too much work to hold the bowl with one hand, and the sander/drill with the other. The bagging never seemed to help my once turned bowls. For remounting for sanding, I use a recess. I have a set of extended jaws which are narrower than my standard jaws, and if I wiggle it around a bit I can get good 4 point contact which is strong enough for slow speed sanding, but not strong enough for turning. I never worry about returning the bottom so they sit flat. Again, it is that 'organic' thing. I do have a bowl sanding clip up on You Tube. Type in robo hippy.

robo hippy

[Leo Van Der Loo]

I'm a twice turned kinda guy. Have never attempted turning a bowl from green to thin and calling it done. As there are so many that do nothing but this kind of turning, I think I'm going to give it a try. Just how thin does one need to make the walls to avoid cracking? If it's important to designate a species for the sake of this conversation, let's talk Maple.

Glen do your twice turned bowls crack/split ??, I assume not, so what is the problem turning them thinner ??, should make the splitting even less of a problem, in my experience keeping the wall and bottom evenly thick is better than uneven walls and bottoms, dry them as usual.

[robert baccus]

Kinda weird but I often wetsand once turned bowls right after turning. A bit messy yeah. The stuff sands very easy as I keep it wet while cutting and sanding and it stays round. I like to put it in a loose plastic bag with several finger holes for slow drying--depending on the weather like Ford says.