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Discussion Starter #1
Sorry if my questions are incredibly noobish...

I do a lot of metal machining for various little products I make and sell at craft fairs, etc. So I decided to try making some of the parts out of wood for aesthetic reasons. I am actually doing the turning in my CNC metal lathe using highly polished carbide inserts (quite sharp).

I bought some 3x3 pieces of various exotic woods - Wenge, Purpleheart, Spalted Tamarind, Hububalli, Cobobolo, Ebony, Ziricote, Pink Ivory and some others. The idea is to make a cylinder that's 2.75 OD and 2" ID (so about 1/4" wall thickness). On Saturday, I turned the squares into cylinders... they came out great, I was thrilled!

Today I got the settings done on my lathe for the second step - hollowing out the inside. I did the hububalli first, then did the purpleheart. I noticed within 5 minutes, the purpleheart cylinder had two large cracks lengthwise down the cylinder on either side - right along the grain. The hububalli looks great. I noticed the cracks are just on the outside, they don't penetrate to the inside, so I don't think it happened from cutting out the inside.

What I am wondering is...

1) What would cause this? It looks like shrinkage and evaporation of moisture that led to the cracking. Is that likely?

2) What can I do to prevent it? The wood was sealed with wax on the ends when I got it. I turned the first step within minutes of cutting them to length... but then I let them sit for 2 days. What can I do to stop the cracking after machining? Apply the finish straight away? Or is there some trick to stop it?

3) I've only done the hububalli and purpleheart so far... but the hububalli looks great - no cracking. Is this more related to the nature of the different woods? Or would it be because the purpleheart maybe was wet and the hububalli dry? Or vise versa? Are there just some woods that crack more readily?

I'm sure I will have many more questions as time goes on - I hope to learn a lot here :)

Thanks!
 

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- If the cracks appeared only after the piece was finished then my guess is that it was from either sanding (high lathe speed increases heat which causes cracking- the difference in heat even on dry piece of wood from inside to out can cause both warping and cracking) Or there was a defect in the wood. Do the cracks appear along the annual lines or across them if right along this could be windshake? Or were there bark inclusions or other defects that the crack was emanating from?

- If the cracks were there when you came out to the shop but you did not notice them then it could be caused by excess moisture that was still trapped in the wood if you took off the layer of wax releasing it quickly out of the endgrain fibers. Then after waiting a few days they began to appear

There are some woods that crack much easier than others... cherry and cedar come to mind but I have not used the woods you have so I cant speak to that

Cracks of all kinds were either there already as in the form of windshake or were caused by a difference in moisture from one part of the wood to the next. I have never used a metal lathe on wood but I'm assuming those carbides are shaaaaarrrrp but if you had to use a good deal of pressure I suppose that could aggravate the issue
http://forestry.about.com/od/foresthealth/ss/Ring-Shake-And-Wind-Shake.htm there is an endgrain shot of ring shake does this crack go all the way down the piece and continue over on the other side or just one side? we love pictures here:thumbsup::thumbsup::thumbsup: happy turnin,
Bond
 

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Well Bond covered the probable causes but I do have one suggestion. Most of what I turn is exactly what you were working on, cylinders 3-4 inches long. Drill them out first then turn them doing them the other way as you did creates a lot of heat with the drill and you get cracks or worse it will grenade on you and go flying in pieces.

Just mark you center and drill the piece then turn on mandrel to completion. Slowing your lathe done to sand helps and helps keep from digging in with the sandpaper.
 

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I just wrote a long reply to someone else about waxed blanks but could not find it. Wax blanks are waxed when they are cut to prevent them from cracking. When we get them we have no idea how wet they are. They usually are not dry enough to hollow like your doing without moving or cracking.
Wood needs to dry evenly to not crack. Wood shrinks as it drys. If the outside shrinks and the inside doesn't it cracks. Leaving flat bottom like your doing won't allow the wood to move as it drys so it cracks.
I scrape the wax off the sides, label the wood, date it, and weight it. Leave the wax on the ends this evens out the drying. When it stops losing weight it's dry. This can take a year or more for a blank that is 2 1/2" thick. A moisture meter might help but most don't read that deep.
If a guy at the store tells you they are probably dry enough to use don't trust him. He has no way of knowing. I buy my wood from a reputable dealer (Big Monk Lumber) and Pete tells me he has no way of knowing when he gets the wood. You don't know when they were cut, when they were waxed, and how long they sat on the docks waiting to get here.
 

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... I noticed the cracks are just on the outside, they don't penetrate to the inside ...
Therefore highest probability IMO is cracks due to shrinkage at the surface exceeding the rate the innards shrink.

... The wood was sealed with wax on the ends ...
It could be wet, it could be dry -- whenever a piece is waxed, the safest approach is to treat it like it's wet.

... I turned the first step within minutes of cutting them to length... but then I let them sit for 2 days ...
Ah ... this is most likely what provoked the cracking.

Had you immediately hollowed the cylindrical rod to a quarter inch thick, it would have been flexible enough to survive a little shrinkage.

However -- it would most likely have warped as it dried, so if you hoped to turn out perfectly circular pieces you'd've been out of luck.

Here are three possible ways to solve this problem:

(1) only buy dried wood.

(2) dry it yourself - scrape any wax off the sides (leaving it on the ends) and weigh it, recording its weight. Put it on a shelf, weigh it again in a month and record its weight. The two numbers will be different if it lost moisture. Continue the monthly weighing, recording, comparing the numbers ... when it stops losing weight, it has stopped losing moisture, should be "safe to turn".

(3) rough turn, let it dry (using the same method to see when it's stopped losing moisture), then turn it again to final size. When making a bowl or endgrain hollowform, the general rule is to make the thickness of the walls 10% of the bowl diameter. This allows the shape to distort as it shrinks but hopefully leaves enough meat on the walls that the final cylindrical shape can be made by removing material.

HTH

(also ... it always helps if you can post photos that illustrate the problem you're having. Plus we like pix. Lots of them. Please. :laughing:)
 

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Discussion Starter #6
Thanks everyone for the replies!

I've attached some pics to illustrate what I'm experiencing.

So, first of all - one person mentioned excessive heat from sanding. I think that's what did it actually. I will sound like an idiot, but I have a smaller manual lathe - I threw the purpleheart on there and sanded it around 1,000RPM... the sand paper was getting so hot I had to let go and shake my hand. I didn't know getting the wood hot was bad, but it sounds obvious now that someone said it. I think that is probably what cracked the purpleheart.

I have some follow-up questions if you guys don't mind :)

1) This is for a product I'll be selling, so I am hoping to be able to reliably make these. Based on that, are there certain woods that are more susceptible to cracking? If so, what woods are those? Or are there some that are especially easy to turn? if so, what would those be? I notice the hububalli looks great, and I got that just as hot as the purpleheart...

2) When I am doing the CNC cutting of these - I'm guessing it would be best to sand and finish immediately to lock in the moisture and prevent any further movement/cracking - is that a good idea? I was considering either a tung oil finish, or polyurethane. I sort of like that "almost like glass" finish. Is either of those finishes better than the other to prevent cracking?

3) My cylinder is 3" long, 2" inside and 2.625" outside. Would making the walls either thinner or thicker help prevent cracking?

4) If you look at the pics of the spalted tamarind... I went to the block I originally got and saw it had a crack I didn't notice. But the finish to me looks a little rough (this is before I sanded it). I am using carbide tooling made for aluminum, it is very sharp... but maybe not as sharp as a specific wood turning tool. Based on my picture, does that finish on the tamarind look good? bad? normal? It almost looks like it was tearing at the wood a bit instead of cutting, but maybe that's normal.

5) I bought the wood from Amazon Exotic Woods. They were real nice to deal with - but I know nothing about wood. Is it normal that a piece would have a crack in it like that Tamarind did? Or should I consider switching suppliers?


Sorry for blasting you gents with questions, I'm trying to leech as much knowledge out of y'all as I can :)
 

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Discussion Starter #7
I forgot to mention - one of the things that got me interested in wood was watching that TV show "Wood Works" with (I think his name was) David Marks. One of his tricks was to spray some water on the piece of wood, to give you an idea how it would look when you applied the finish. Well, I've sprayed each of these a few times without regard to what I was doing next. I am pretty sure I wet down the purpleheart before I sanded it. Maybe my putting water on them made the cracks happen faster/worse? Is it a bad idea to let the woods get wet at all?
 

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1000 rpm is way too fast, slow it down if you can. Heat and excess moisture were the most likely problem. Spalted wood is going to have more defects just from the nature of what it is, a wood that is beginning to decay. You will find cracks and punky (soft) areas in it more than non spalted pieces. But any wood can and will split just part of what we do.

No you can't say change suppliers from just this batch especially as you are still learning, we all are btw. Just keep trying. If you can let wood sit for a period of time from weeks to months it will help ensure the moisture has had time to get out.
 

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Corp666 said:
1) This is for a product I'll be selling, so I am hoping to be able to reliably make these. Based on that, are there certain woods that are more susceptible to cracking? If so, what woods are those? Or are there some that are especially easy to turn? if so, what would those be? I notice the hububalli looks great, and I got that just as hot as the purpleheart...

2) When I am doing the CNC cutting of these - I'm guessing it would be best to sand and finish immediately to lock in the moisture and prevent any further movement/cracking - is that a good idea? I was considering either a tung oil finish, or polyurethane. I sort of like that "almost like glass" finish. Is either of those finishes better than the other to prevent cracking?

3) My cylinder is 3" long, 2" inside and 2.625" outside. Would making the walls either thinner or thicker help prevent cracking?

4) If you look at the pics of the spalted tamarind... I went to the block I originally got and saw it had a crack I didn't notice. But the finish to me looks a little rough (this is before I sanded it). I am using carbide tooling made for aluminum, it is very sharp... but maybe not as sharp as a specific wood turning tool. Based on my picture, does that finish on the tamarind look good? bad? normal? It almost looks like it was tearing at the wood a bit instead of cutting, but maybe that's normal.

5) I bought the wood from Amazon Exotic Woods. They were real nice to deal with - but I know nothing about wood. Is it normal that a piece would have a crack in it like that Tamarind did? Or should I consider switching suppliers?

Sorry for blasting you gents with questions, I'm trying to leech as much knowledge out of y'all as I can :)
1) Yes some woods are more prone to cracking but most woods should be manageable in the size project you are doing. Are you needing these pieces to stay round or is some warping and shrinking acceptable?

2) most finishes will slow the drying process to some degree which will help prevent cracking but movement is tough to avoid. It's just a matter of time if the moisture content is above equilibrium to start with.

3) uniform thickness is more important than the actual measurement. Thinner walls can be allowed to dry more quickly but you'll still get movement.

4). Spalted wood often has soft spots which are tricky to cut cleanly. Without biting off a huge chunk of woodturning tool craziness, you may have to accept some less than gallery quality surfaces and just keep your cutters sharp and take light cuts. You may get better results with denser woods.

5) I'm not sure about purchasing wood. I get mine straight from the tree.

Good luck with your product.
 

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Just a couple of additions to other comments.
Spraying with water. Will work OK but raises the grain quite a bit. I would give it a quick wipe with mineral spirits; it will be dry enough to turn in a couple of minutes.
On spalted (punky) items like your second you can make a mix of 1 part shellac to 2-3 parts of DNA. Just paint it on and again it will dry quickly. It isn't a finish so don't worry about lap marks or runs. The grain is then hardened so that it can be cut rather than pulled out. It only penetrates about 1/16 inch so you may have to make several applications as you get near the final diameter. Almost any other finish will go over shellac.
If pits remain you can sand with and oil and let the slurry fill the pits then buff with 4-0 steel wool. You will have to wait for the oil to cure for your final finish.
They are about the same size as boxes and for boxes I drill them (if a 2” inside I drill 1.5”). Rough turn the outside and seal all of the end grain. Place in a sealed paper bag and allow to dry. If you also use DNA, soak and then coat the end grain; they should be dry enough to finish turn in 2-3 weeks.
 
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