Chucking?

[Bonnie Campbell]

I'm TRYING to turn a brazilian cherry bowl. Been having nothing but tenon break offs. So, going to try insetting (?) the chuck instead of using a tenon. What I'd like to know is should the hole be big enough to open the chuck about halfway? Or should I make the hole as small as possible for the chuck to fit?

Does that make any sense? I guess I'll see lol

[Stuart Johnson]

I would make it so the chuck will expand out to just about full width. First though, can you figure out why the tenons are breaking. If you are cutting so aggressive that catches are causing the problem you will probably knock the bowl out of the chuck. You might be able to resolve the issue by taking lighter cuts.

[Gordon Seto]

Bonnie,

The chuck has the best holding power when the jaws forms a full circle. The jaws have most contact area.
In rough turning, you have to allow for the wood to distort and true up the tenon or recess later.
In recess chucking, you also have to allow enough wood on the outside so it won't split.
You can also try to use thin CA to strengthen the tenon on punky wood.
Don't take too big a cut with your new Thompson gouges. LOL.

Gordon

[Jim Becker]

Bonnie, when using the tenons, are you providing a shoulder for the top of the chuck jaws to sit on firmly? It not only provides more support, but also keeps things from getting cocked, resulting in less chance of splitting off the tenon.

[Raymond Overman]

Bonnie,

You've gotten some good answers to your recess question but you haven't looked at your original problem too closely. One thing to remember when you use a tenon is more is not always better. You do not want the tenon to be longer than the jaws and seat into the bottom of the chuck. It's a matter of leverage versus mechanical hold and leverage will always win.

When you make your tenon, make it only 1/4" to 3/8" long and leave a flat lip around it for the jaws to seat against. This will give your chuck the most reliable hold and allow it to compress correctly.

Leaving the tenon longer than the chuck jaws usually ends up with the problem you are describing. Hopefully that is what you're doing because it's an easy fix.

Good luck.

[Ken Fitzgerald]

Bonnie,

As for using tenons....You have gotten some sage advice here.

If the tenon is too long it bottoms out in the chuck and that isn't a good thing. I've actually turned bottle stoppers with only about 1/8" long tenons. The ends of the jaws rest on the shoulder where the tenon meets the body of the project (wood) and bears the force. The other thing...sharp tools and light cuts.....don't try to use brute force.

I had similar problems when I first started turning and the folks here gave me the same advice. Once I started using their advice, my tenons for the most part quit breaking off. I had one recently that I just clamped down too tight and the wood had a defect in it resulting in it breaking.

Good luck!

[Bonnie Campbell]

I've had the tenon about half into the chuck. Not sure of the size exactly. But it did have a flat shoulder to sit on. Of course I'd been trying to turn it with a scraper and skew :o I'll give the recessed thing a shot and see if I still screw it up with my new chisels. About how deep should a resses be (so I don't over do that :o). This IS my bowl from hell though. I knew when my brother asked for it I should of told him no lol

[Bernie Weishapl]

That could be part of the problem is your tools. Are you getting catches and that is what is breaking the tenon? I have never used a skew on a bowl and probably wouldn't. I have always used a bowl gouge to shape plus hollow the bowl out. Then I have used a scraper to refine the surface. Now I use a shear cut with the bowl gouge on the inside and outside. Got this from watching Bill Grumbines DVD on bowl turning. Just a thought.

[Reed Gray]

Bonnie,
I use a recess almost exclusively. I think the general rules for both methods are the same.

The foot (part you are grabbing onto or expanding into) of the bowl should be 1/2 to 1/3 the diameter of the bowl. So for a 12 inch bowl, you want 4 to 6 inches. So your tenon should be minimum 4 inches wide. Your recess should fit into the 4 inch recess, and leave minimum 3/4 inch shoulder outside the recess before the sides of the bowl start to curve up. 2 inch or more shoulder for a 18 inch bowl with my big Vicmark jaws, 2 5/8 inch wide.

Depth wise, I turn and core bowls up to 18 inches with a recess about to 1/4 inch deep, and never less than 1/8 inch. This is the end result of turning a couple thousand bowls and experimenting a lot. I do not use the tailstock when turning, as I have found out what works for me. I am very agressive when roughing out.

You can overtighten. I broke off a number of tenons, and split out a number of shoulders around a recess from just plain overtightening, and or not leaving enough wood around the recess. When all else fails it is nice to be able to rely on brute strength was a motto of mine when I did concrete work. You want a snug fit. Borrowing a quote from a movie about holding a sword, "hold it like a bird, too tight and you kill it, too loose, and it flys away". If it is under a lot of stress from being too tight, and you do have a catch (only when some one is watching me of course) it will fail at the point of most stress. I think all keyed chucks have 2 places to insert the key for tightening. Use both places a couple of times, and get it snug each time, but not tight.

With a recess, you want the the recess to fit your jaws as closely as possible. I have a dedicated compass that I use for each chuck, and they are super glued into place that is the exact measure of the width of my chuck jaws. Mark the line, take a first cut just inside the line, then just barely take the line, like 1/16 past the line. The more metal on the wood, the better the grip. A chuck with jaws that are 2 inch diameter won't hold well in a 3 inch wide recess. You may be able to get away with it on a smaller bowl, but not on anything of any size, and you can't be agressive with it. I would imagine that it is the same thing with a tenon. If you have a 4 inch wide jaws, you want a 4 1/4 inch wide tenon.

One other thing, I think that dovetailed jaws hold a lot better than straight jaws. You do get a tiny bit more metal on the wood, but the angle serves to suck the wood down tighter onto the jaws. You do want the angle of the dovetail to match the angle of the jaws as exactly as possible. This is easily accomplished by having one of the dovetailed scrapers that you can grind to the proper angle when the scraper is parallel to the ways of the lathe. Use a plunge cut goes in at the proper angle almost all by itself, and then remove the rest of the center. Also, the last thing I do before reversing is to do a "whisper" cut on the outer edge of the recess. Put the scraper in at the outside of the recess, and let it just barely contact the wood, making the lightest cut possible. This takes out some of the uneveness that you get with the tool as it cuts long grain, then cross grain. Using this method, I get a lot less run out when I reverse the bowl.

robo hippy

[Nathan Camp]

I'm TRYING to turn a brazilian cherry bowl. Been having nothing but tenon break offs. So, going to try insetting (?) the chuck instead of using a tenon. What I'd like to know is should the hole be big enough to open the chuck about halfway? Or should I make the hole as small as possible for the chuck to fit?

Does that make any sense? I guess I'll see lol

Bonnie,

The other thing to remember is that brazilian cherry is hard and brittle. If I use a brad nailer to put a brad into a piece, it will usually split for several inches on either side. This may be the reasons your tennon breaks so easily.

I believe the recess will work if you leave enough wood on the outside so that expanding the jaws will not split the piece.

Nathan

[Steven Wilson]

Make sure you're using the correct jaws for the diameter of the tennon. I use Oneway chucks and if I have problems I end up looking at the manual to make sure I'm using the correct jaws - most of the time it's pilot error. I'm not sure how much excess stock you have but you might want to consider a screw chuck. I've had good success with the Glaser screw chuck when others seem not to work. Finally, lighten up on your tools and make sure they are extreemly sharp - you may have to go slowly.

[Bonnie Campbell]

Not much in excess stock after breaking a tenon. Now I've got a piece of wood glued to the bottom and figured, once I broke the tenon off of it, that now I'll try for a recessed chucking. It could be I was horsing the chuck to tight on the tenon. What's 'tight' for a woman ain't necessarily so for a male, so I'm not sure. I'll try easing up on that and try turning it again in the next day or two.... don't it figure people decide at the last minute they want you making something for their gift giving :rolleyes:

[Jim Becker]

Bonnie, there are two issues with using a recess for chucking bowls. 1) For many bowl forms, a recess leaves very little support for the pressure you are applying, unlike something flatter like a platter. 2) The size of the recess will limit the creativity you can have with some forms. I'm only bringing this up as you need to consider it while you decide what method to use for each turning.

[Bonnie Campbell]

Here's a 2" diameter smaller version of what my brother wants..... I told him the shape was ugly and clunky, but it's what he wants :rolleyes:

http://img.photobucket.com/albums/v630/annieoakley/Crafts/ResizeofBrazilianCherry.jpg

[Jim Becker]

Bonnie, that form should pose no problem for a recess method...the base is very wide and therefore provides a lot of wood fiber for supporting the chuck pressure in the recess.

[Dick Strauss]

Bonnie,
Beyond the other good suggestions like using a bowl gouge instead of a skew...

Which chuck do you have? If it uses a dovetail jaw and you overtighten, it acts like a wedge trying to separate the tenon from the bowl. Make sure you have a good square shoulder if it is a std jaw or a proper dovetail as needed.

If you go the recessed route by expanding the jaws, make the recess as deep as will look good (don't forget you need to leave the bowl inside bottom extra thick to account for the recess). The jaws will only grab at the top rim if they are like the Strongholds. However, make up a donut of wood slightly larger than the recess diameter that is thick enough to take up the gap between the jaw edges and face. If you have a recess in your blank that is 1/8" and the jaws are 3/8" high, make the donut >1/4" thick (so that there is pressure on the donut if its hole is slightly larger than the recess diameter). In other words the donut would be flat on the jaw faces followed by the bowl blank with recess and flat bottom.

The larger the flat bottom on the bowl blank that matches the flat face on the donut, the more off-axis force it will take to dislodge the blank and the more stable it will be. If your bowl bottom is 4" in diameter, you have a few options. You can make the donut 5" in diameter and have lots of support. You can also make the donut 3.75" in diameter and have access to the corner of the bowl as needed.

I hope this makes sense...its late!


Good luck,
Dick

[Dean Thomas]

Hi Bonnie,

I'm a large man, so it's easy for me to weigh in on almost anything just by showing up. :rolleyes: Here goes on your tenon issues.

I have some fresh experience today, as well as exposure to a flying bowl from another turner last Saturday. There are at least two issues that I don't think have been addressed. The tools thing and making sure that they are deadly sharp and touched up often during the turning, unless you have one of Doug Thompson's bowl gouges, and then you only sharpen on New Years Day after the parades. Kidding, but just barely. Dull tools make you push if you're not aware that they're dull. Happened to me tonight it was cutting fine and then, I could not pick up a thread to save my sorry hide and I started to push when that little voice deep in the dust collector said "s-h-a-r-p-e-n-n-n-n". I did and magically, the gouge began to cut again!

Okay, on to my two cents worth. What kind of chuck are you using? You may have said somethere else or others may already know this, but I could not find it in this thread. And what jaws? All jaws are not created equal! :) My main chuck is a Oneway Talon and the primary jaws I use are the standard #2 jaws. The inside of those jaws looks like the outside of a screw in that there are ridges on the inside. On the outside, the jaws are mostly smooth except for a nice little bead at the top outer edge of the jaws. Oneway makes another style of #2 jaws; instead of teeth, there is a solid dovetail profile. With those jaws, I'd have to cut a tapered tenon in order for the jaws to get a good grip. Oneway says that these jaws are really for projects that can be done with a light cut. One of the Nova guys will correct me if I'm wrong here, but I think that the Nova standard jaws are smooth with a sharpish bead at the top inside and outside. Nova says their jaws are designed to cut into the wood, but I think their instructions tell you to cut a relief under the shoulder so that the inside bead gets a good grab and so that the nice flat shoulder of the project sits flat on the tops of the jaws.

As already noted, if you're cutting into brazilan cherry (jatoba) with any of the jaws, you may be starting a crack that gets finished by pressure in cutting. Jatoba is lovely stuff, but it's a pain to turn cuz ya have to go SLOOOOOOOOOOOOW! Not for the impatient. Kind of like the nasty maple I was turning tonight. Man it took forever to get it round. So hard...

What is the grain orientation of your bowl? If you're turning a facegrain or sidegrain bowl, where the grain pattern runs parallel to the tops of the jaws, it's really, really easy to bust a bowl right off at the junction of tenon and bowl bottom. That's what happened last Saturday when someone was turning a facegrain maple bowl. The wood was just a tad past it, I think, but he was trying to save it. Lovely stuff, but I had to duck when a 14" bowl came off the lathe without its tenon and tried really hard to hurt me! Fortunately it missed and did not break. He'll be able to figure something out to save it, even if he has to use a glue block.

One other possibility on your jatoba bowl project. The other thing you could do is to use a maple waste block and turn a socket/recess into the block and then fit a tenon on the jatoba bowl to fit inside the maple block. Glue it in and turn it. The glue will not split the wood like toothy jaws will. And you can use your jaws on the maple without issue.

Random thoughts from the midnight turner. :cool: Gotta get my Christas ornament done so it can go on my partner's tree before the big day!

[Bonnie Campbell]

Well, bad part is I've just a cheap chuck from PSI (next thing on my 'gotta get' list, a decent chuck!). It's their Utility chuck with the #2 jaws.

Patience isn't always a strong point with me. And I most likely WAS pushing the chisels beyond normal. My last try at this bowl I didn't have a sharpener yet. So I'm sure, with my hand sharpening abilities, the chisels were dull. I do now.... amazing how nice sharp tools can work :o AND I got two Thompson bowl chisels now (happy dance! :D) So I'm trying to get all my ducks in a row before attempting to defeat my nemesis.

The bowl is face grain. Something I hadn't had any problem with before. I do have a waste block of maple mounted on it now.

It's almost to early in the morning to read your suggestion Dick LOL But I think I've caught your point with the donut. Good suggestion and I'll give it a shot. I'm not beyond trying every suggestion given! This bowl is NOT going to win!!!!