Day off and Tennon Problems

Dan Mosley

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Palm Springs, Ca
Well i got called off work today .....what a shame now ill just have to stay home and turn wood and work in the garage......LOL. Anyway i have an ongoing problem. When i turn my bowls the tennon breaks off very frequently. I have pictures at the bottom to show. The Tennon is about 7/8 in depth and fits into my spigot jaws on my chuck - i hollowed this one by leaving a tennon in the middle of the bowl and hollowed down to the depth i wanted. Then turned the tennon out of it....then with the scrapper showen i started to take shear cuts off the inside (freshly sharpened) and after in a short time the tennon broke off.........luckily i was on my last few cuts so ill be able to hot glue the pc back on and finish sand it.....
Wondering what others think about why this keep happening or what i can do to make it more secure...........
Im thinking that because its green wood (hemlock) that it probably stressed cracked when i was hollowing and didnt come apart because i have the tailstock up tight into the interior tennon i left on. Then after removal and still being green....and being far off the rest and deep hollowing it just cant take the stress even in light cuts..........

Thoughts ?? thanks Dan
 

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Dan, your tenon is too long. It should not bottom out on the bottom of your chuck. In fact, the top of the chuck jaws should register against the bottom of the bowl. Try it and see if it doesn't work better for you.
 
Dan, I agree with Doug, the tenon is too long. Also if the tenon is about the size of the smallest opening of the chuck jaws, the jaws will grip better.
 
day off tennon problems

Another thing that wasn't mentioned is that even though you are making a small bottom on your piece ,you don't have to have such a small tennon.You can do as allready said,but have a bigger tennon which will give you more wood for strength,then when you part it off you should be about an inch or so away from the tennon and can make the bottom as small as you like and that should end your troubles.
 
hello, i prefer not to use tenon's for this type of work, i always use faceplates and create a waste block and part of above this.
if you do ues a tenon you can strengthen it by using a couple of drops of CA or make them larger as has already been mentioned-but ideally avoid them.
 
hello folks just to clarify any confusion i may have caused this is how i work to avoid snapping tenons.

this is a piece of spalted beech 14" long by about 7" wide

i start between centres
DSCF4043.jpg


i cut one end slightly concave for the faceplate
DSCF4045.jpg


i then mount my faceplate this one is 6" straight into end grain using sheet metal pan head self tapping screws about 1 3/4" long
DSCF4046.jpg

DSCF4047.jpg


i wont bore you with the rest of these wip pic's and complete hijack this thread so we will skip along to the waste block bit

in this pic the vase is complete except for sanding and you can see to the left is my waste block( but not always waste!!)
DSCF4052.jpg


sanded and oiled
DSCF4053.jpg


waste block shaped into a bowl
DSCF4058.jpg


i then used a 3"wide by 1/4" deep dovetail tenon to hollow it out and finally got rid of that tenon using a vacum chuck
DSCF4059.jpg


the days efforts
DSCF4063.jpg


a 3" tenon may strike some of you as excessive for such a samll bowl but splated beech isnt very strong and as i've already mentioned i prefer to have a large tenon if i'm going to use them. I do not know of any chuck jaws which would have given me such a strong hold on this vase- plus it doesnt help matters that i'm also clumsy and heavy handed so i need it to be strong :)

hope this helped
george

p.s i have quite a few wip pictures of various projects if you would like to see them???
 
Thanks for that tutorial George. I don't have a vaccum chuck, but my flatjaw can serve the same purpose. I liked the way you size the pieces to get maximum use out of your stock. And yes...pictures are always needed. Cheers!! :thumb::clap:
 
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thank you for the advcie and ill try it that way - However when you dont have a long pc of wood as the one in your pic i think you would have to chuck it differently (a pc of a log or if you cut it thru the pith and made two blanks)..........so with a pc of branch or tree where you are unable to have that much length to deal with - what i have done is to use a faceplate and turn a tennon on one end....shape the bowl as much as possible and then flip it around and secure the tennon in the spigot jaws for hollowing....when done hollowing ......jamb chuck it and turn off the tennon......this is the way i have been doing them unless anyone has another suggestion ???? thanks Dan and i will try your way when i have a longer pc...........
 
no problem dan, as i said an alternative is to use larger jaws thus giving a greater surface area and less chance of the tenon snapping. :thumb:
 
Day off and tennon problems

Don't mean to hijack the post,but while looking at the pics again,I was just wondering what material you used on the vac plate,or is that a factory plate.
Thanks ken
 
Don't mean to hijack the post,but while looking at the pics again,I was just wondering what material you used on the vac plate,or is that a factory plate.
Thanks ken

factory i'm afraid - but i do know that some home made plates use neoprene with sucess :thumb:
 
Back to the original question.

Dan,

So far you have recieved a lot of good information. After carefully looking at the photos you posted I have some observations. I don't think your tenon is too long as I beleive I see jaw stains on the shoulder above the tenon. That would indicate to me that the piece is seated properly in the jaws. The diameter of the tenon, as others have suggested, is probably too small. The biggest problem, in my opinion, is grain direction. You are turning a large piece of wet softwood that hangs a long way off the chuck, and you have it oriented so the growth rings are parallel to the chuck face. This is the absolute weakest fracture plane there is, especially in a softwood. If the wood was drier, it might stay together better but I doubt it. You most likely could twist this piece off the tenon just using your hands easily. The small tenon adds to this weakness. If this was a hardwood, this would not be as weak, but still a weak point. The piece George uses to illustrate his point was mounted so the grain is parallel to the lathe bed and eliminates the weak point. I realize you are turning a natural edge bowl ( which looks great by the way), but you need to consider grain direction when mounting your work. All this being said, when turning a regular bowl in "faceplate" orientation, we encounter the same fracture plane weaknesses, but normally use a larger tennon and don't narrow the foot as much as you have done here. And we are often using a stronger hardwood than Hemlock, which is technically a softwood. Also when turning tall pieces like George's, it is common practice to orient the wood as George did and turn it as an endgrain piece instead of side grain as yours is. It can be done sidegrain, but care must be taken.

To clarify, softwoods are catgorized as coming from trees that do not lose their leaves in winter, and hardwoods come from trees that do lose their leaves in winter relative to North American species. So there can be soft hardwoods and hard softwoods. Hemlock is actually a rather hard softwood, especially when dry. You can barely drive a nail into dry Eastern Hemlock.

Sorry for the long disertation, I just wanted to try and help
 
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