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View Full Version : Why i use expansion mode



Bruce White
5th June 2013, 01:51 PM
Thought, because most on the forum seem to prefer it, that I would give contraction mode with a dovetail another go this morning.

X#%@&^!!!!.

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I finished the outside ofthe bowl - 34cm diameter - made progress and then got the catch you can see on the rim. The blasted tenon snapped clean off and the bowl went bouncing across the floor. DAMN IT! Back to expansion mode for me - it is more forgiving/sturdy as far as I am concerned.

On a lighter note, I did this one yesterday (with expansion mode). Sheoak 30cm diameter, 10cm high.

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Been busy the last couple of weeks - knocked out a few pieces.

271118

Oh well, back to it. Have to cut a recess in a part finished bowl to take a chuck in EXPANSION mode!

Tim the Timber Turner
5th June 2013, 02:59 PM
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:):

Bruce White
5th June 2013, 03:08 PM
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.

Scott
5th June 2013, 05:03 PM
Trying to turn a deep 340mm bowl on a 50mm spigot using contraction is asking for trouble.

Yup, my thoughts exactly.

orraloon
5th June 2013, 06:04 PM
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

NeilS
5th June 2013, 08:58 PM
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.

hughie
5th June 2013, 09:15 PM
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.

mat_au
5th June 2013, 09:46 PM
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

DavidG
5th June 2013, 10:03 PM
I generally use compression mode with the foot 1/3 to 1/4 the bowl dia.

Not a hard and fast rule though.

mick61
6th June 2013, 12:25 AM
Gday I have to disagree I only use spigots, I have only had trouble with expansion mode.
My 2 cents. Mic:D

Paul39
6th June 2013, 03:43 AM
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.

NeilS
6th June 2013, 09:33 AM
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

Mobyturns
7th June 2013, 10:49 PM
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.

RETIRED
7th June 2013, 11:20 PM
:aro-u:An excellent reply.

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

Big A
8th June 2013, 09:05 AM
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".

Bruce White
8th June 2013, 11:17 AM
Thanks for the advice all - Mobyturns in particular for the comprehensive analysis. I think in my case the big question is "What can I do to avoid this?". Answer: Go back to what has always worked for me - recess and expansion mode.

I mount the blank on a faceplate, finish the back (including sanding), turn around and mount in expansion mode, finish the inside, polish on the lathe and then re-mount on the Coleman chuck to tidy the recess. I rarely have any problems using this method and I commonly turn platters & bowls up to 40cm diameter.

I am sure with all your good advice I could master spigots and contraction mode, but I still struggle to see any advantage. Either way the base needs to be tidied up and from a safety point of view, expansion mode seems to be lower risk. I rarely cut a recess larger than 50 - 60mm (with a good bit of surrounding timber), even on the larger items. I started out this way as I was given to understand that the chucks work best with minimum expansion in that mode. Very, very rarely need to tighten the chuck (even though I check regularly).

As to being lucky I was not injured, yes, although in this instance and in almost all of my turning time, I try to work from a position whereby any flying objects bypass me. Using the swivel head on the lathe and being naturally ambidextrous help here. I made this a habit after having a couple of close calls when I first started turning - using spigots now that I think about it! Did a bit of spindle turning yesterday and had a piece fly off. Lucky I wear my Triton full face respirator as it hit right in front of my eyes.

Anyway, unless someone can provide a compelling reason why spigots are an advantage, I guess I am a recess man! (My Son would say Caveman)

NeilS
8th June 2013, 01:35 PM
I try to work from a position whereby any flying objects bypass me.

+1

WaggaSteve
14th April 2015, 02:38 PM
Sorry this is so long ago but just got back after long time away. The chuck on the left - is that a Leadbeatter chuck and if so is it as simple as drilling a forstner 60mm "dimple" , inserting chuck and turning to engage or is this totally wrong - I have no instructions with the chuck. TIA
Steve

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

mañana
14th April 2015, 06:01 PM
Big A, they used to call that (Nos 1,2&3) "Risk Assessment and Hazard Management" and have all these little Tables where you had to enter the degree of risk. I spent more of my time at courses that were preaching this than I spent running my work-sites. Glad I'm out of it!!
Cheers
Mick C


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".

pommyphil
14th April 2015, 06:20 PM
Mick Check the dates. This is nearly 2 years old. Phil

hughie
14th April 2015, 07:49 PM
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 still use mine, a great little chuck wouldnt be with out it. Still use my SN2's for a lot of turning.

For me it has to do with what I'm turning and the species being used. I routinely use my Leady chuck in expansion and all my SN2 chucks in contraction and rarely have a problem. Usually its when I push my luck and give my reflexes a work out :U

hughie
14th April 2015, 07:50 PM
me 2



+1

powderpost
14th April 2015, 09:06 PM
I use a wide variety of chucks, each one has a "best suited" application, and have done so for a long time. The first thing I do before staring a bowl is to sharpen all the tools. My biggest problem with chucks is that they dictate the size of the base, I don't like that.

If I am making a bowl I may fix it to a face plate with screws from the top and turn the back and foot. I will screw a waste or sacrificial block to the face plate, turn it to match the bowl foot and glue the two together. Face plates are a hell of a lot cheaper and more reliable than a chuck. Granted, not as fast as using a chuck, you will have to wait for glue to dry. That is called job planning.

I use a hand saw to remove the bowl from the waste block and clean up the foot. This method eliminates breaking tenons and splitting of wood. Don't get me wrong, I still and will use a chuck where appropriate.

Jim

Mobyturns
16th April 2015, 10:05 AM
....

I think in my case the big question is "What can I do to avoid this?". Answer: Go back to what has always worked for me - recess and expansion mode.

I try to work from a position whereby any flying objects bypass me.

I guess I am a recess man! (My Son would say Caveman)

Wow this one is like Lazarus - back from the dead.

Bruce raises some good points - use what you feel comfortable with - its not a matter of being a caveman, it simply works for you & your style of turning.

Work stance at the lathe is a significant part of staying safe however it is risky to heavily rely upon where you position your body if you still use risky work holding methods or blanks that have voids & cracks etc & pose a higher level of risk than sound blanks. Bowls and parts of them can easily ricochet in small workshops. "Safe" turning is simply employing all the tricks & techniques you have available to you to minimize risk.

Waggasteve,

Powderpost points out tried & proven techniques that worked for all turners before four jaw scroll chucks became commonplace. They are still very good techniques, very safe & low risk techniques. It pays to learn a range of work holding methods as each has particular advantages for certain turning applications.

Be careful simply boring a recess with a forstner bit to create a recess for chuck jaws. Forstner bits drill nice holes with parallel sides. Most jaw sets rely upon some form of jaw lip or more commonly a taper to securely grip the blank. Straight sided recesses can cause wobble.

NeilS
16th April 2015, 09:17 PM
Be careful simply boring a recess with a forstner bit to create a recess for chuck jaws. Forstner bits drill nice holes with parallel sides. Most jaw sets rely upon some form of jaw lip or more commonly a taper to securely grip the blank. Straight sided recesses can cause wobble.

I used the forstner bit method for awhile, then went back to faceplates and/or screw, which was a quicker method for me. When using the forstner method I found it was worth quickly forming a foot (recess or spigot) re-gripping the blank with that to reform a dovetail inside the forstner recess and re-gripping the blank again on that end and proceeding from there. Not necessary on small bowls, but worth it on larger and irregular shaped blanks.

Don't be tempted to make the forstner recess too deep and certainly no deeper than the depth of the dovetail section of the jaw. The jaws grip better when they bottom out in the recess.

orraloon
17th April 2015, 05:11 PM
Sorry this is so long ago but just got back after long time away. The chuck on the left - is that a Leadbeatter chuck and if so is it as simple as drilling a forstner 60mm "dimple" , inserting chuck and turning to engage or is this totally wrong - I have no instructions with the chuck. TIA
Steve

It is a Leadbetter chuck and I have a Leadbetter cutter to cut the recess. I guess a forstener bit would do the same job. I make the recess deep enough for the shoulder to sit nice and flat then turn the work clockwise to tighten.


As to concerns about no dovetail side to the recess it is not required for this sort of chuck that keeps tightening with the cutting action on the lathe. The jaw edges also bite into the recess wall. I use this chuck a lot due to ease of use and for some tasks prefer it to my nova.

Regards
John

Mobyturns
18th April 2015, 09:17 AM
It is a Leadbetter chuck and I have a Leadbetter cutter to cut the recess. I guess a forstener bit would do the same job. I make the recess deep enough for the shoulder to sit nice and flat then turn the work clockwise to tighten.


As to concerns about no dovetail side to the recess it is not required for this sort of chuck that keeps tightening with the cutting action on the lathe. The jaw edges also bite into the recess wall. I use this chuck a lot due to ease of use and for some tasks prefer it to my nova.

Regards
John

I haven't used the Leadbetter chuck. As you say it is important to follow the designers recommendations. My comments were about the now more common jaw sets using dovetail jaw design.

NeilS
18th April 2015, 05:31 PM
My comments were about the now more common jaw sets using dovetail jaw design.

Me too.