Thread: tenon and recess fail !!

This is how I would chuck a bowl that I intend to put feet onto, I find it gives a better support to the tenon having the flat as seen in the video I posted earlier.

Sorry if this is common knowledge to many.chucking a bowl.jpg

Hi guys.
Finally got some shed time today , and decided to
attack that bowl that wouldn, t stay on the lathe !!
Thanks to all for there ideas and advice
So started by flattening the base level and used larger
jaws and superglue ! ! .............as i was pouring on
the glue i was thinking ,,,,,, mmmm not sure about this....
Anyway pressed on and shaped the bottom of the bowl
again , turned around and hollowed the inside , finished
In the norma way , eee and glow , it didn, t turn out to bad,,
But was not what i had pictured in the first place,!
Anyway it is what it is, no prize winner but i thought
I would show anyway, warts and all,,,,,
Just a note the glue trick work well and cleaned off easily
with a bit of sandpaper

A good photo is worth a thousand words.

Photo 3 (...121037) shows part of the problem and why you are having tenon failures.

Pay particular attention to the shadow between the face of the jaw and the "flat surface" on the rear of the bowl blank, they are not parallel. The bowl tenon should sit fully into the jaws so that there is no gap at all between the jaw face and flat, not bottom out on the base of the jaws, and so that the internal corner (tenon to flat) and the lip of the jaws meet.

For a tenon and chuck jaws to function as intended those two surface must meet and match well to form a "register." That registration surface should also be as wide as is practical. When "registered" there is little chance of movement between the two surfaces and it is possible to remove and replace the blank in the same position in the bowl jaws with minimal change in the centering of the blank.

With the tenon formed as you have the bowl is actually being supported and registered in the chuck jaws by the outer lip of the jaws and the inner dovetail surface of the jaws. As you turn the very minimal area of wood supported by the outer lip of the jaws (registration surface) crushes permitting the tenon to settle deeper into the dovetail recess and reduces the "grip" on the tenon, disturbing the registration of blank to chuck - thus permitting more vibration etc. On soft woods like Camphor Laurel it really becomes a significant issue.

Do you notice that you have to retighten the "loose" chuck jaws while turning? Does the bowl seem to be slightly off centre or have more vibration after you tighten?

The other issue is that the very nature of the dovetail jaw design is that it is a rather blunt wedge which will stress the internal corner of the tenon to flat as you tighten the chuck jaws if they are not well matched. The more it loosens the more we crank up the pressure on the jaws - leading to a crack initiating on the tenon. Add in soft wood that shears easily and you have a recipie for an unintended flying bowl object.

Stuart Batty explains this very well in the videos. They are almost mandatory viewing imo.

A poorly formed registration surface between jaw face and blank is the most significant weakness of scroll chuck design / use. Face plates offer a huge registration surface when compared to scroll chuck mounting and are a very good choice for turning soft easily sheared woods.

[QUOTE Mobyturns;1983250]A good photo is worth a thousand words.

Photo 3 (...121037) shows part of the problem and why you are having tenon failures.

Pay particular attention to the shadow between the face of the jaw and the "flat surface" on the rear of the bowl blank, they are not parallel. The bowl tenon should sit fully into the jaws so that there is no gap at all between the jaw face and flat, not bottom out on the base of the jaws, and so that the internal corner (tenon to flat) and the lip of the jaws meet.

For a tenon and chuck jaws to function as intended those two surface must meet and match well to form a "register." That registration surface should also be as wide as is practical. When "registered" there is little chance of movement between the two surfaces and it is possible to remove and replace the blank in the same position in the bowl jaws with minimal change in the centering of the blank.

With the tenon formed as you have the bowl is actually being supported and registered in the chuck jaws by the outer lip of the jaws and the inner dovetail surface of the jaws. As you turn the very minimal area of wood supported by the outer lip of the jaws (registration surface) crushes permitting the tenon to settle deeper into the dovetail recess and reduces the "grip" on the tenon, disturbing the registration of blank to chuck - thus permitting more vibration etc. On soft woods like Camphor Laurel it really becomes a significant issue.

Do you notice that you have to retighten the "loose" chuck jaws while turning? Does the bowl seem to be slightly off centre or have more vibration after you tighten?

The other issue is that the very nature of the dovetail jaw design is that it is a rather blunt wedge which will stress the internal corner of the tenon to flat as you tighten the chuck jaws if they are not well matched. The more it loosens the more we crank up the pressure on the jaws - leading to a crack initiating on the tenon. Add in soft wood that shears easily and you have a recipie for an unintended flying bowl object.

Stuart Batty explains this very well in the videos. They are almost mandatory viewing imo.

A poorly formed registration surface between jaw face and blank is the most significant weakness of scroll chuck design / use. Face plates offer a huge registration surface when compared to scroll chuck mounting and are a very good choice for turning soft easily sheared woods.[/QUOTE]


Hi moby,
Wow, I never saw that the jaws were not flat on surface
of the bottom of the bowl..............
I thought I had the bottom fairly flat, thanks for pointing
that out to me, I can see now why It is so important to
get the tenon perfect , I must admit I have never bothered
about It before , pity i didn, t cos i now have a nice scar on
my arm for my troubles.
I didn, t have to tighten the bowl at all , but then I did put
the superglue on chuck jaws too!
Glad you like my photos too!

Smiife, in my experience you are not going to have the break out shown in your photos if you make the recess about 3mm deeper than the actual foot, because then you have the full strength of the bowl blank thickness bearing against the chuck jaws in expansion mode, and not just the narrow foot to take the strain. If the foot is 4mm deep, just make the recess 7mm deep, and if you don't like the look of this design foot, don't make it flat bottomed, but make it convex to follow the approximate curve of the outside of the bowl. Just make sure it is below the level of the foot, so you don't end up with a giant spinning top! Then you can have two beers!

My two bob's worth, and sorry I haven't been on line for a while, as I have just had a total knee replacement operation. Haven't been to my local woodturners group nor even to my own lathe for about 6-8 weeks and have now got sawdust/shavings withdrawal. Doug.

Originally Posted by dougturner

Smiife, in my experience you are not going to have the break out shown in your photos if you make the recess about 3mm deeper than the actual foot, because then you have the full strength of the bowl blank thickness bearing against the chuck jaws in expansion mode, and not just the narrow foot to take the strain. If the foot is 4mm deep, just make the recess 7mm deep, and if you don't like the look of this design foot, don't make it flat bottomed, but make it convex to follow the approximate curve of the outside of the bowl. Just make sure it is below the level of the foot, so you don't end up with a giant spinning top! Then you can have two beers!

My two bob's worth, and sorry I haven't been on line for a while, as I have just had a total knee replacement operation. Haven't been to my local woodturners group nor even to my own lathe for about 6-8 weeks and have now got sawdust/shavings withdrawal. Doug.

Doug makes a good point about making the recess deeper as it very significantly increases the area of wood that supports the chuck jaws. However you must consider that extra depth when turning the inside of the bowl. I have seen a lot of very attractive bowls with papery thin bottoms. Personally I like to create the outside profile but leave the wood supporting the recess then refine it later when reverse chucking in cole jaws. Its an extra step but often the difference between a fail and a very nice bowl.

Thank you for this discussion, gents. It is very timely, as I had just pulled a roughed out CL bowl out of the sawdust. It has about a 58mm tenon on a 180mm bowl and weighs 854 gm.

There is still 20mm thickness on the bottom, so I will use a bit of that for a bigger tenon and reshape as well. I am thinking that diameter of tenon is more important than depth (within reason of course), so I will cut it about 7-8mm and see how that looks.

Alister.

Originally Posted by Dalboy

This is how I would chuck a bowl that I intend to put feet onto, I find it gives a better support to the tenon having the flat as seen in the video I posted earlier.

Sorry if this is common knowledge to many.chucking a bowl.jpg

Hi derek, sorry for the late reply , must have missed
It before, so what you do Is a recess and a tenon ?

Originally Posted by dougturner

Smiife, in my experience you are not going to have the break out shown in your photos if you make the recess about 3mm deeper than the actual foot, because then you have the full strength of the bowl blank thickness bearing against the chuck jaws in expansion mode, and not just the narrow foot to take the strain. If the foot is 4mm deep, just make the recess 7mm deep, and if you don't like the look of this design foot, don't make it flat bottomed, but make it convex to follow the approximate curve of the outside of the bowl. Just make sure it is below the level of the foot, so you don't end up with a giant spinning top! Then you can have two beers!

My two bob's worth, and sorry I haven't been on line for a while, as I have just had a total knee replacement operation. Haven't been to my local woodturners group nor even to my own lathe for about 6-8 weeks and have now got sawdust/shavings withdrawal. Doug.

Hi doug, good to see you back , some good Ideas there
are you saying the same as derek , make a tenon and recess ?

Originally Posted by Big A

Thank you for this discussion, gents. It is very timely, as I had just pulled a roughed out CL bowl out of the sawdust. It has about a 58mm tenon on a 180mm bowl and weighs 854 gm.

There is still 20mm thickness on the bottom, so I will use a bit of that for a bigger tenon and reshape as well. I am thinking that diameter of tenon is more important than depth (within reason of course), so I will cut it about 7-8mm and see how that looks.

Alister.

Hi Alister, i think that size tenon should be ok , for that
size bowl , but hey as I have proved I ain, t no expert
hope you have more sucess than I did !

Originally Posted by smiife

Hi derek, sorry for the late reply , must have missed
It before, so what you do Is a recess and a tenon ?

In the diagram I drew it is a tenon but the same would apply if you decide to use a recess what I was trying to get at was having a flat area for the jaws to register against as in the vidoe I linked to earlier in the thread.

The bit about the feet was just to show the area that needs to be removed after reverse chucking leaving enough to make the feet for the bowl as you mentioned in your first post.

I am not the best person at putting things into the written word so hope none of this confuses you.

Hi smiife,

"Hi Alister, i think that size tenon should be ok , for that
size bowl , but hey as I have proved I ain, t no expert
hope you have more sucess than I did !"

Be that as it may, I am no stranger to catches, so I might as well take precautions!

A.

Just a few ideas you may want to consider...

It appears the first pics are with the 50mm jaws. If you draw circles the tenon will be about 2" diameter and the wood around a recess will be about 4" diameter. If you do your math for area you will find the recess has 3X more wood to break off than the tenon (assuming you have 1" around the recess).

It has been said that a recess is much easier to break while tightening the chuck than a tenon. That is true. If both are the correct size you can break the wood around the recess, to break a tenon you must also break/crush the steel jaws. With either you need to stop and check tightness fairly often with green wood. For the recess just snug it up good.

I assume your are turning, drying, and then re-turning. In your later pic it looks like the 130mm jaws. There is no reason you can't change jaw size when returning if you are using a tenon. So on the green you can use the 130's and when dry and returned you can drop down to the 100's if it fits your project better. Or go from the 100's down to the 70's or the 70's down to the 50's. I typically use a recess but with a tenon I almost always oversize for the project to start with and drop down when I re-turn.

I have been reading this thread with a great deal of interest and although I have been turning for several years a number of points made are new to me.

When I started turning bowls I used a recess and from time to time one would break away, as per the initial post. Subsequently I was encouraged by my fellow Woodturners of the Hunter club members to move to a spigot to improve the design elements, particularly when I was making thinner walled bowls.

Initially I used standard jaws with small spigots and had a couple break off which I assumed at the time was because they were just too small for the force involved. Now I am thinking that it may well have been because of improperly formed spigot and registration surface.

More recently I have been using a set of Vicmarc step jaws in a VM100 chuck without losing a single bowl, including some quite large specimens. I have put this down to the larger diameter alone. After reading previous posts in this thread I am now thinking/wondering whether the fact that if the spigot is more than 5mm high the step jaws can only grip a maximum 5mm of the spigot and the end of the spigot sits on the bottom of the step which then becomes the registration surface, and there is a gap between the face of the jaws and the base of the bowl so no shear force generated to start a crack through the point where the spigot joins the base of the bowl blank?

image.jpg image.jpg

It may not be completely clear in the photos but the spigot is bottoming out on the step and there is a small (1mm) gap between the face of the jaws and the base of the platter.

BTW where can I find the info on Stuart Batty?

Thanks,

Steve

If I am reading it correctly I think you will be better off with a shorter spigot.
With the spigot sitting on top of the inside step and a gap between the top of the jaws and the work piece it seems to me that it could rack more. I don't have a Vicmarc chuck but I assume they are similar to Nova step jaws.

Here is a link to Stuart Batty's videos on Vimeo. He has 30+ with three on chuck, recesses, tenons with each about 12-15 minutes.
https://vimeo.com/woodturning/videos...rmat:thumbnail

Originally Posted by NCPaladin

If I am reading it correctly I think you will be better off with a shorter spigot.
With the spigot sitting on top of the inside step and a gap between the top of the jaws and the work piece it seems to me that it could rack more. I don't have a Vicmarc chuck but I assume they are similar to Nova step jaws.

Here is a link to Stuart Batty's videos on Vimeo. He has 30+ with three on chuck, recesses, tenons with each about 12-15 minutes.
https://vimeo.com/woodturning/videos...rmat:thumbnail

Stuart's videos on Vimeo explain the why's & why nots of tenon & recess design. Generally it is not recommended to have the base of the tenon register against the inside surface (base) of the chuck jaws - however IF your tenon base is formed truly square and flat it will create a good registration surface.

The forces generated while turning and especially in a catch event create some difficult to understand leverage forces that simply rip bowl tenons out of chuck jaws.

Most of the problems with flying bowls, tenon failures or problems with vibration etc can be traced back to poorly formed tenons. So it is really worth the time to true up and square off the base of the tenon, match the jaw profile, create a nice crisp internal corner etc.

Usually there is more at play than just a poorly formed tenon, blunt tools, poor technique, undersized tenon / recess for the mass of the blank, physical properties of the wood etc all contribute to failures