Tenon joints in section

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Tenon joints in section

Postby gavin » Sun Jan 13, 2013 9:49 pm

Following Donald Todd's comments about the stress caused by whacking over-sized tenons into mortices, I made some test pieces and then cut them in section.
Donald suggested the tenons could mushroom if struck with a metal hammer with no padding. He prefers sash cramps.

Here is the hammer-struck tenon after the rung has been driven home into the first mortice - I can't see any mushrooming.

(The method I use is laid out in Mike Abbott's Going with the Grain. )

IMG_0490 (WinCE).jpg
Tenon after driving with lump hammer
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IMG_0492 (WinCE).jpg
Close up of struck tenon
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IMG_0515 (WinCE).jpg
Cross section of hammer-struck tenon
IMG_0515 (WinCE).jpg (12.53 KiB) Viewed 4852 times


I saw no evidence of the auger lead-screw causing any splits.
IMG_0519 (WinCE).jpg
close up of mortice which had hammer-struck tenon driven into it
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And if you want better definition, here is the same image at higher resolution:
IMG_0519 (Small).jpg
IMG_0519 (Small).jpg (39.98 KiB) Viewed 4852 times


The mortice is 14 mm, the tenon was turned green at 5/8" on veritas tenon cutter. I then leave it shrink and dry. Then I whittle to something between 14 and 14.5 mm in the vertical plane and I don't worry about or measure the horizontal plane.

Here is an image of the leg section after driving it onto the 2nd tenon, Donald Todd felt the leg would be marred by the hammer blows.
IMG_0494 (Small).jpg
I can see no hammer-marks on this surface - can you?
IMG_0494 (Small).jpg (45.17 KiB) Viewed 4849 times
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Re: Tenon joints in section

Postby Donald Todd » Tue Jan 15, 2013 10:08 am

The damage to the posts from hammering isn't obvious because it is a rough surface and you will see it better with illumination along the axis of the post. Try varnishing or oiling it; that may make it more apparent. It's the weakening of the material that's the issue. "What the eye doesn't see…"

You have cut your trial post along the line that the auger induced split would occur, and as I said the newer augers have a shallower angle on the leadscrew, so you are more likely to get away with it. To test for splitting, stop the auger just as the spur(s) start to cut. Remove the auger and put a drop of dilute ink on the spur cut. The ink will track round the cut and run up the split if it is there.
Note that Mike did not shoot this theory down when we discussed it here.
I use old augers that I inherited and they have the advantage of a relatively shorter leadscrew. I also have a follow up bit which has spurs and leadscrew removed, allowing me to drill the mortise to the depth of the first bots spur cut, making the "extension" of the hole even shorter.
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Re: Tenon joints in section

Postby woodlife » Wed Feb 13, 2013 9:48 am

Hi everyone . Its been a while since I 've been on this site. At present I am making a ladder back chair with some kind of stringybark gum usually just used for firewood. I made a winsor type chair with it and it looked amazing after I put the Danish oil on. But it is very difficult to dry a seat without it splitting, so hence the ladder back woven seat chair. I pole lathed most of the bits last year and recently I split and spoke shaved the back legs. I was aiming for oval tenons to go into a 20mm mortice so I polelathed the green tenons at about 22mm or 23mm . I can't remember which. I should have written it down. Anyway, they shrunk to 20mm short dia. and 21.5mm long dia. which i thought would be just right. However, a 20mm test hole was bored and the tenon pressed in using Mike Abbotts sash clamp method and the tenon piece could be easly pulled out again. A 19.5 mortice was tried with a slightly wittled tenon and had the same result. A 19.2mm mortice was satisfactory with a hammer needed to separate the pieces. It looks as if I will have to wittle all the tenons down to 19.2mm . So Mike, if you are reading this. Your method may work for Ash, and other English woods, but not this one. I still think drying the tenons to an oval shape is a good idea. I have bourght three of your books and don't regret it. Next post, I will try to send some photographs of the chair, if I can figure out how.
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