Thread: Why i use expansion mode

Hi Bruce

Trying to turn a deep 340mm bowl on a 50mm spigot using contraction is asking for trouble.

When turning near the outer diameter there is vibration set up because of the small area holding the bowl.

This can lead to having a catch which in turn can snap the spigot off.

The ideal minimum size jaws for this large bowl would be 100mm.

A larger chuck would also make for a smoother running.

I have found that she oak is very prone to splitting like this.

Hope this helps.

Cheers

Tim:

Thanks Tim. I have both larger jaws and larger chuck so may give that a try. Having said that, I don't have any problems using 50ml jaws in expansion mode - probably because there is more meat in that mode.

Originally Posted by Tim the Timber Turner

Trying to turn a deep 340mm bowl on a 50mm spigot using contraction is asking for trouble.

Yup, my thoughts exactly.

I mostly go expansion mode where I can. If there is enough wood and the grain looks like it can take it. I only have the 50mm jaws for my Nova and may feel different if I had something else for larger work. The other chuck I use(more than the nova) only works on expansion anyhow.
Regards
John

Originally Posted by orraloon

The other chuck I use (more than the nova) only works on expansion anyhow.

John, that one's a blast from the past. Surprised to see it still in use, but then it did a great job before other options became available. Only pensioned mine off after I got a few Novas.

It is probably the reason why I developed my techniques around expansion mode and rarely resort to spigots and contraction mode.

I use contraction and rarely if ever go as small as 50mm tenon, unless its one of the smallish HV around 100-120mm dia otherwise its around 75-100mm.

yes i had the same problem with the smaller tenons. I just got bigger jaws 100mm ish havent had a problem so far been turning 12in plus bowls

I generally use compression mode with the foot 1/3 to 1/4 the bowl dia.

Not a hard and fast rule though.

Gday I have to disagree I only use spigots, I have only had trouble with expansion mode.
My 2 cents. Mic

For big bowls I use a big chuck with #3 jaws which grabs a 5 inch - 127mm spigot.

I use lots of strange spalted, bug eaten timber that I often have to go pretty deep to find something firm to grab. Expansion would likely just split the blank.

Each of us needs to use the technique and tools to produce the end result we want.

When possible I make a recess and finish the bottom before doing the inside, as that is faster than cutting off the spigot and finishing a flat bottom.

Originally Posted by Paul39

Each of us needs to use the technique and tools to produce the end result we want.

When possible I make a recess and finish the bottom before doing the inside, as that is faster than cutting off the spigot and finishing a flat bottom.

+1

Bruce, first off it’s heartening to see both you and the bowl escaped relatively unscathed. The bowl is still recoverable. This is a great example of how several issues have combined to create a failure.

I apologize for the long response in advance – but another US turner has just very recently suffered severe head injuries turning a bowl – even reportedly wearing a “helmet.”

When choosing a holding method for any turning project we have to consider the actual physical properties of the raw material, is it wet, dry, open grained, grain direction (quarter cut, rift sawn?) etc; the shape & size of the chuck jaws; the speed at which we will turn; the shape of the proposed project; how thin we intend to turn it; how the timber will perform with our preferred holding method; and the strains we will place upon the timber & our equipment both intentionally and unintentionally i.e. a catch, when we turn it.

Then we should consider “What if?” What potentially will happen if the bowl or a large part of it comes off the lathe? Will I walk away or will I be spending time in hospital or worse? What will be the potential outcome? No injury, a light graze or a severe head injury? Would you let me throw a cricket ball sized (or larger) chunk of wood at your head at 100kph or more? That is the sort of impacts we are talking about when a bowl comes off or apart at speed.

I see a few potential hazards with this bowl out of a relatively open grained timber with strong medullary grain,

1. is the grain orientation - it appears to be aligned with the weakest grain direction parallel to the face of the chuck as evidenced by the medullary flecks in the sheared off tenon.
2. you have a relatively small tenon for the bowl diameter & size.
3. the design of the Nova jaw set has a distinct ridge which when clamped tight crushes the timber in the V of the tenon – promoting the start of a crack if the tenon & jaw profile aren’t matched & the timber is relatively soft.
4. the second photo shows a slight compression of the face of the timber which mates to the front of the chuck jaws - which to me indicates the blank was “wobbling” on the face of the jaws for at least a while before failure; or that the tenon profile was not well matched to the jaw profile.
5. what changed? – did you vary the lathe speed or some other factor – or was it just the natural tensions in the timber changing as material was removed. Did you have to retighten the jaws during the turning process?
6. you then experienced a catch.

All of the above combined to create a failure. Now if you had been turning at high speed & were in the firing line without any personal protective equipment when this happened you may not have been telling the story. Your partner, family & friends or a police officer may have been trying to work out what happened to cause your demise.

I would suggest the catch was inevitable & that the catch didn’t cause the failure but that it’s most likely a developing failure of the tenon that contributed to the catch. I have experienced similar failures in similar grained timbers when I knew the tool presentation was good and I was producing “angel hair” like shavings with a very light finishing cut when the failure occurred. I changed something – I increased the lathe speed to “get a better cut” which then exceed the physical strength and properties of the timber to resist failure using my preffered work holding method!

Now you could eliminate or at least reduce some of the potential hazards and reduce the risk to you and the bowl by introducing a few controls like;

A. Using a larger jaw set like the 100mm dia bowl jaws suggested which will give you at least four times the cross sectional area of the tenon.

B. Consider how the profile of the jaws will affect the timber when tightened

C. Select another holding method say a denser grained sacrificial block glued to the sheoak with the tenon cut in the waste block or fastened to a face plate – similar to how Vaughn Richmond prefers to turn.

D. Modify your turning process so that you are not removing bulk material off a small tenon. Use a larger tenon for bulk shaping then refine later in the process.

E. making sure you use suitable profile bowl gouges, keep tools sharp & use good tool presentation & stay alert.

F. maintain tailstock support for as long as is practical.
G. turn within safe lathe speed limits – rpm x bowl dia in inches should not exceed 6000 to 9000 as a guide.

The next thing to consider is if you used your chuck & jaws in expansion mode into the foot design you have chosen for your bowl would the outcome be any different? How much cross sectional area of timber is there to resist shear? Is it stronger than the tenon? Will the jaw profile potentially start a developing crack in the corner of the recess?

Lots to think about & sincerely hope it helps to keep you & others safe.

An excellent reply.

I play the "what if" game from the time I decide to start turning. On everything!

In my job, I have to go onto a mining site, therefore I have had to do inductions in a whole range of of things. The generic mine induction (now called something else) is one of these. There is a lot of information given out and not a lot of time to actually learn anything. But one piece of wisdom I have absorbed, because the instructor emphasised it:

1. What am I about to do?
2. What can go wrong?
3. What can I do to avoid this?

Which sums up what Mobyturns said and what does.

You all know about the "Hierarchy of Controls" don't you?

"A".