Recessed Dovetail

[Baxter Smith]

This morning I wanted to take Leo Van Der Loo's suggestion "The other thing I do is to use a recess for holding my blank, as I found the thick tenon to be a cause for splitting".

When things didn't go well I decided to go back in the house and get my camera. I have learned a lot from the "how do you do it - or not do it" sequences". I thought I might have one of the latter and that it might help another new bowl turner!

I have a Nova Chuck with two sets of jaws. I put on the small ones first because I wanted to have a small recess/foot. Here is the bowl after it came off for the second time with a another chip taken out.
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I decided I would try the large jaws even though they would require a wider recess. I screwed the face plate back on but since the surface wasn't flat the bowl got quite a bit smaller before I could do a new recess. Here is the chuck and bowl with its larger jaws. Sorry I forgot to take a picture with the smaller jaws in. When expanded, they only made contact with about half of the circumference of the recess.
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They worked fine and I was able to rough out the inside.
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Tried it again on another piece that had a couple of large knots in it.
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Is there going to be a checking or splitting problem when you put a knot that size in your bowl? I suppose the pink will change to brown with time but it was pretty while turning!
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For those of you that think you might want to try a green bowl. I spent several months this fall and a couple thousand dollars putting in a dust collection system that would keep my shop neat. I was also trying to be fastidious about vaccuming up any stray sawdust. Now look where my latest addiction has gotten me. And thats with it running and sucking up a lot of it! Softer to stand on though.:D

[Steve Schlumpf]

Baxter - the results you got with your recessed dovetail are why I use a tenon. Had way to many blow out on me. The one thing Leo did mention was to make sure you had plenty of wood for support - in this case you did not. Most of the time a recess makes sense when turning a platter or flatter bowl - where there is less of a curve away from the recess. Course, like everything in turning - it comes with time and doing and you will find a process that works well for you.

When it comes to knots - especially in green wood - can almost guarantee they will crack while drying. Have had some luck with CA on the smaller ones but usually wait for the cracks to stop and then fill in with some other material.

Bowls look really good!

[Bernie Weishapl]

Bowls look good Baxter. I to have had to much trouble with recesses and don't use them anymore. I use a tenon on all of my bowls except for platters. That is the only time I will use a recess. I have never had a tenon split in all the bowls, lidded boxes or HF's I have did.

[Jim Silva]

I use recessed dovetails nearly all the time on my bowls and have never had an issue unless I deserved it.:rolleyes:

Usually that's trying to core very hard, wet maple or oak and my poor technique with my McNaughton bowl saver thingy. (sorry to bog this down with technical terms...)

When rough turning a bowl I'll make the recess a good deal deeper than I would with dry stock as I know I'm going to lose some depth through truing the rough when it's dry. Shallow recesses with unstable wood during aggressive wood removal are far more likely to fail than deeper ones. (I'm talking 3/8" deep as opposed to 3/16" on really delicate pieces.)

You need a fair bit of wood to support a recess but it's very dependable if you use it knowing it's caveats. FWIW, I've had more tenons snap on me than issues with recesses.

Your mileage may vary.
J.

[Allen Neighbors]

Ditto what Steve says.
A friend of mine from Georgia uses recesses quite a bit, and leaves it quite a bit heavier outside the recess, until he finishes the bottom, then takes the heaviness away. He leaves good thickness (thicker than the intended finished thickness), about 3/4" out from the recess, which makes the shoulders of the recess a little beefier for the grip.
Make sense?

[Mike Peace]

I am also tend to avoid expansion chucking in favor of using tenons on bowls. I think both work fine if done right but it is harder to do the expansion chucking right.

You said that when expanded, your jaws only made contact with about half of the circumference of the recess. That is a problem. This will reduce the strength of your hold if you don't have the recess sized to fit with the jaws almost closed. In either method of chucking you want jaws almost closed for the maximum grip. I have found doing the correct cut on the dovetail to also be a little tricky. If you make the dove tail too steep or too deep you don't have as much steel on the wood and you can increase the chances of a a blow out.

I learn better from my mistakes than my successes!

[Bob Hamilton]

It would appear to me that you may have undercut the sides of the first recess a bit too much. It seems to me that I have read in my chuck manual or on Oneway's website that it is better to err on the side of too little angle than too much. They say the dovetail jaws in expansion mode will grip better on a straight sided recess than on a recess with too much undercut. If the sides are undercut too much then the jaws are contacting the wood right at the wood surface and it can easily crumble. If the sides have too little undercut the jaws are contacting the wood right down at the bottom of the recess where there is the most support. In this diagram the upper picture shows too much undercut and the bottom too little:

http://www.bobhamswwing.com/justpics/2010/Jan/dovetail%20jaw.JPG

Of course, the ideal is to get the angle exact.... :)

Good Luck!
Bob

[Dave Ogren]

Baxter,
I am the new guy here with very little experience. Just over 100++ bowls. All have had a dovetail recess, never done it any other way. I spent a lot of time reading Leo Van Der Loo's and Reed Grey's ( Robo Hippy) posts before I got my lathe and got started. Never lost a bowl or had a breakout. I believe that Bob Hamilton is right about the angle. I find myself slowly going shallower all the time. I add about 1/16" in dia. over the min. jaw dia. to account for warpage. Have turned to just over 20" dia. bowls. Now what might make a difference is I totally love my Big Vicmarc chucks. With the standard jaws measure 2 5/8" dia. I know for sure no problems with a 20" bowl with about a 3/16" deep recess and turning very agressively. Good Luck.

Dave

[Ryan Baker]

Like Bob, my first reaction to your picture was that the recess looks undercut too much. Make sure the angle matches the jaws closely or you will definitely break the piece. Matching the diameter as closely as possible to the jaws-closed diameter helps a lot too. Make sure there aren't any cracks or defects in the rim around the recess before you start too.

That said, like many others, I almost never use a recess. It is just too easy to break a recess. I have never broken a tenon. Especially on something that will require a lot of torque or is catch-prone, a proper tenon is much better in my book. (It fits my process better anyway -- IMO, there's not much uglier than a dovetail recess left in the bottom of a bowl.)

[Chris Stolicky]

For those of you that think you might want to try a green bowl. I spent several months this fall and a couple thousand dollars putting in a dust collection system that would keep my shop neat. I was also trying to be fastidious about vaccuming up any stray sawdust. Now look where my latest addiction has gotten me. And thats with it running and sucking up a lot of it! Softer to stand on though.:D

I can't help with the recess conversation, but I can recommend putting some shower curtains around the lathe. They do help control some of the flying debris....

[Leo Van Der Loo]

Good job Baxter, and you learned something also.
Yes your jaws have to fit the tenon or recess for the best hold, also with the dovetail jaws like Bob pointed out the angle of the undercut is important.
It is one of the things I like on my Oneway jaws, it has a double ridge at the jaws edge, and need only a straight recess, with those jaws the pressure on the wood is at the deepest point of the recess.
As I'm not home, still enjoying the holidays at my sons, with my grand-kids :D, I am not able to get at all my pictures, still have a few that show some recesses that I work with.

Like with this bowl where I wanted to keep as much wood as I could the recess was pretty shallow, glueing a tenon on a wet blank to start and then the bowl drying rapidly while turning and changing shape is not a very safe thing to do, as you can tell there's very little sapwood removed for removing most of the recess.
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Here's a dry rough bowl with the recess as used to turn the bowl, the recess is slightly deeper than the outside of the bowls bottom, if you look closely, you can see the marks that the jaw ridges made in the wet sapwood when the bowl was turned, those marks are always removed when finish turning the bottom.

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This next picture was made to show the way a recess could be turned true, but I very seldom need to do this as the jaws will hold quite well when positioned diagonally.

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Here's a picture of a clamped bowl, it was made for a post to show another way of holding a bowl to finish turn the bottom of a bowl

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This is the leftover from a recess that has to be removed as the bowl is finished but for the recess removal.

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I hope the pictures do help some, for those turners that like to use a recess when called for.

Baxter what I do with the knots works pretty good, but it isn't perfect for me yet.

It would be easier if I could show some pictures, but alas, I'll try to explain, the knots being like endgrain shrink and get smaller, but the wood around doesn't shrink that way, so unless we do something the knot will split, for this reason have I used thin CA and try to soak the knots full of CA, if enough gets in and hardens up the knot will not shrink and split.
I have a few bowl that had 2 or 3 knots in them and none split, though sometimes I stil get one to split, it is when like in your bowl there's that tension wood running up the side and the knot next to it, still I would try to soak the knot and hope it will not split, and if it does, it will be less than otherwise.
Just try it, all you have to loose is a little CA glue, good luck and am looking for ward to a good end-result to post/show in a couple of months :)

[Leo Van Der Loo]

Baxter I just went back to the previous posting from me to your earlier post regarding the Applewood you were turning.
Just Like I thought to remember I did mention all the required items that are important to making and using the recess successfully, like the correct size and proper angle, also the need for enough wood around the recess.

This is even more important for those new turners that tend to get bad catches and use not the best approach and angle of their turning tool while hollowing a bowl blank or those that are tooo aggressive with hollowing.
I've used 1/8" deep recesses with success, but I'm not an aggressive turner, I also just tighten my chuck with one hand on the key and rotate the lathe around with the other to just snug the chuck up going around from side to side, I never use two hands to tighten my chuck, mind you I have some experience under my belt, I know that does help :rolleyes: ;)


http://www.sawmillcreek.org/showthread.php?p=1299312#poststop

[Baxter Smith]

That must have been a good question!:)

Thank you all for taking the time to add your thoughts, pictures and diagrams. I am sure lots of other new turners will benefit as well!

I am almost glad I forgot to take a picture of the first set of jaws I was using. You probably wouldn't have brought up all these great points!
I did go back out to take some.

Sorry I don't have the expertise to highlight and then respond to all your individual comments. I am not going to try and figure that out right now either. Not after just hitting the back arrow and having everything I had written to each of you vanish!:eek: Will try again.

Steve - I think my mistake the first time was choosing jaws that were not up to the task of holding an 8 inch diameter 4 inch deep bowl to be roughed out. At least by me!

Bernie - I have not broken any tenons on what bowls I have done but almost all have loosened or come out of the chuck multiple times. It has seemed too only hold when I have applied a lot of pressure using one hand on each bar and thinking about whats going to happen to my knuckles if something slips!

Jim & Allen - I was/still am somewhat unsure on how thick to make the bottom to allow for finish turning. The original intent to begin with was to avoid a thicker bottom to allow for more even drying of the roughout.

Mike & Bob - Great point about the angle of the dovetail. Diagram highlights that well. I think my angle was okay since I used the same tool to cut both the one that failed and the one that held. (ground at an angle that matchedthe dovetail)

Dave - The second dovetail recess did feel more solid than any tenon I have tried. Never loosened at all.

Ryan - I wonder if my first set of jaws would have held if the hole had been smaller. Same amount of metal applying pressure to wood. Hopefuly you will see what I mean when you look at my pictures.

Chris - I used to chuckle at the shower curtain idea. But now....

Leo - Thanks for all the pictures to help illustrate your thoughts. I will have to try and pick up some CA glue and try that on the knots. I put the two bowls I did this morning into bags without the anchorseal. I think I read somewhere about packing them in shavings as well so did that too. Wasn't quite ready to go cold turkey and just stuff them into the bag!

Thanks again for all your help! It is great to have it!

Now I will share the pictures of the first and second set of jaws I used and my best guess as to why one was more successful than the other.

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The total circumference/contact area of metal to wood in the first set of jaws was about 2.5 inches. In the second set of jaws I used, the total metal to wood contact area was about 6.25 inches. So the same amount of pressure could be spread out over almost 3 times as much wood. I don't if this is exactly true but it seems like it could then take 3 times as much torque from hollowing before failing.

[Reed Gray]

I was gone on vacation for a week. I use a recess exclusively, except on hollow forms, boxes, and any other end grain turnings. Just works better for me. There are several keys to making it work.

Size: If your jaws closed are 2 inches across, then you want a recess about 2 1/8 inch wide. This puts more steel on the wood. I have a dedicated compass/ dividers that are super glued into place to mark it.

Shoulder: You need enough wood on the outside of the recess to take the stresses of turning and coring. For an 8 by 4 inch bowl, 1/2 inch is kind of minimal, but it can work if you are not turning aggressively. 3/4 inch would be better.

Depth: I never go beyond 3/16 inch deep, and most of the time, I am about 1/8 inch. Going deeper doesn't really make it any stronger. Going too shallow, on green wood turned to final thickness like I do, can result in a recess that you can't remount on a chuck, because the bowl has warped too much. Similarly, a tenon that is too deep, as in over 1/4 inch won't be as secure as a shallower one. Learned that from Stewart Batty.

Angle: You want the angle of the recess to match as closely as possible the angle of the jaws. I use a dedicated scraper for this that is available in the catalogs.

Wood: You have some knots in the area of the recess. This will have grain going in several directions, and that can cause weakness in the grip even if all other conditions are met. Same with bark inclusions and punky wood. Burl is some thing entirely different, and not really having a definite grain, will not take the abuse that straight grain will.

Chuck: I only had one experience with a chuck with the tommy bars for tightening. Didn't like it at all. The gear ratio is better on the keyed chucks, so you can get a firmer grip. You can over tighten to the point that you blow out the recess. You can also tighten to the point that it is almost ready to blow, and then any tiny catch can cause it to come off.

Metal stains: A little bit of concentrated lemon juice will remove it in about 5 minutes, especially when the wood is still green. I don't remove my recesses, they are part of the design. You can fancy them up if you want, but a disc sander will smooth it out just fine.

robo hippy

[Baxter Smith]

Thank you Reed for taking the time to write all the suggestions.

I am going to follow your one about gluing some dividers together to mark the recess width. I have plenty of old dividers and calipers I could sacrifice for this.

I turned a bowl today and kept it under 1/4 inch deep.

I have a scraper that I think matches the angle of the dovetail correctly that I have been using.

I have read that the Tommy bars are not particulary liked and I find them a little awkward at times - but they came with the lathe and it will be a long time before I can exceed what they are capable of doing.:)

Didn't really pay attention to the metal stain on the wood. The least of my worries at this point but I will remember the cure.

I feel very fortunate to have so many able and willing teachers here.

Baxter

[Reed Gray]

If you don't glue the dividers together, they will vibrate open wider than you want. Also, only touch the down hill side, or the side where your turning tool will rest. If you touch both sides, the uphill side will rotate back towards you, bending compass, scratching the wood, make you jump, bad words will be said, etc. I usually make the first mark narrower than I want, then move it out a bit at a time till it is perfect. First cut with the scraper is inside the line, then just barely take the line on the next cut, then clean up the inside. Do make the last part of the cut on the outside edge of the recess a finish cut, just barely touching the wood. This will get you a better surface for remounting.

robo hippy