Getting past the shakes

[mick59wests]

Hi all,

I have lately turned a few reasonable size 'bowls' which are not balanced (see attached camphor laurel platter - wood courtesy of smife (thanks again) which still needs its finish).

335568335569

This is approx 500mm x 200mm and was too big for me to use the tailstock for support.

Anyway, when turning both sides (faceplate to turn the bottom and chuck in expansion mode for the top) the lathe started shaking at around 300 to 400 rpm. I thought this was crap as I was never going to get anything like a clean cut so I sped up the lathe and could get around 800 with less shake than at a much slower speed and for the final part of the inside of the bowl I was up over the 1000rpm.

Does anyone have further knowledge / understanding of 'why is this so'? Everything I have ever read or seen says as soon as the lathe starts shaking to back the speed off but in this case I pushed through the shakes barrier. I am guessing if I had a big and heavy out of balance piece this would not be such a good idea :)

Any thoughts (especially on when this is safe) would be much appreciated and hopefully an education to many of us

cheers

Mick

[Hermit]

Hi all,

I have lately turned a few reasonable size 'bowls' which are not balanced (see attached camphor laurel platter - wood courtesy of smife (thanks again) which still needs its finish).

335568335569

This is approx 500mm x 200mm and was too big for me to use the tailstock for support.

Anyway, when turning both sides (faceplate to turn the bottom and chuck in expansion mode for the top) the lathe started shaking at around 300 to 400 rpm. I thought this was crap as I was never going to get anything like a clean cut so I sped up the lathe and could get around 800 with less shake than at a much slower speed and for the final part of the inside of the bowl I was up over the 1000rpm.

Does anyone have further knowledge / understanding of 'why is this so'? Everything I have ever read or seen says as soon as the lathe starts shaking to back the speed off but in this case I pushed through the shakes barrier. I am guessing if I had a big and heavy out of balance piece this would not be such a good idea :)

Any thoughts (especially on when this is safe) would be much appreciated and hopefully an education to many of us

cheers

Mick

I think that would be due to resonance, Mick. At the lower speed, the piece was probably shaking at close to the lathe's resonant frequency, exaggerating the problem.
Once you increased the speed, you were no longer in the 'resonant' range, so things settled a bit.

I had the same problem a couple of weeks ago on my mini-lathe, while having a go at the lattice turning, with the blank mounted off-centre on a faceplate. Like you, I found that 1000 to 1200 rpm was far better than my original 650rpm.

I'm sure someone will correct me if I'm wrong.

Nice looking bowl, by the way. I love CL, both the look and the smell. (And we're ridding the country of another weed.)

[smiife]

Hi mick,
I have done the same thing, speed up and you are past
the shaky stage, but just be carefull cos at a faster speed
you can, t always hear things when they go wrong:o
Nice platter, good to see you are using the CL
Is that a crack on one of the wings, might need a bit of CA!

[fletty]

Vibration is a very complicated subject and when it meets rotational imbalance as a trigger it gets to an even more bizarre level. Most bodies have a frequency at which they will resonate. We use this deliberately in musical instruments and consciously when we speed up or slow down to drive smoothly over a corrugated road. This resonating frequency is usually over a very narrow band so going faster (a higher frequency) OR going slower (a lower freqency) will take you outside of that band. The reason why we are most commonly advised to slow down is because, if you have only just entered the bottom of the resonance band, you don't know if it will get much worse before you leave the top limit!
This resonance band will alter if the lathe is bolted down, if you have added ballast (concete blocks, bags of sand etc) and even if there is some slop in the bearing, pulley and belt. One of the most amazing changes is the use of segmented linked belts instead of traditional continuous drive belts. Most rotating machines run much smoother with these linked belts and, in many cases, the resonance band moves so much that it is often outside of normal operational speeds.

fletty

[mick59wests]

Hi mick,
I have done the same thing, speed up and you are past
the shaky stage, but just be carefull cos at a faster speed
you can, t always hear things when they go wrong:o
Nice platter, good to see you are using the CL
Is that a crack on one of the wings, might need a bit of CA!
smife,
there is more than one crack but I tend to leave them as they are. All I now need is the rest of the world to like them this way :D
cheers
Mick

[mick59wests]

Vibration is a very complicated subject and when it meets rotational imbalance as a trigger it gets to an even more bizarre level. Most bodies have a frequency at which they will resonate. We use this deliberately in musical instruments and consciously when we speed up or slow down to drive smoothly over a corrugated road. This resonating frequency is usually over a very narrow band so going faster (a higher frequency) OR going slower (a lower freqency) will take you outside of that band. The reason why we are most commonly advised to slow down is because, if you have only just entered the bottom of the resonance band, you don't know if it will get much worse before you leave the top limit!
This resonance band will alter if the lathe is bolted down, if you have added ballast (concete blocks, bags of sand etc) and even if there is some slop in the bearing, pulley and belt. One of the most amazing changes is the use of segmented linked belts instead of traditional continuous drive belts. Most rotating machines run much smoother with these linked belts and, in many cases, the resonance band moves so much that it is often outside of normal operational speeds.

fletty
fletty,
much appreciated with the detailed reply. Now I think my lathe is in its permanent position I'll look at bolting the stand to the concrete or at least adding some heavy weight.
cheers
Mick

[Goggomobile]

Mick
A few things which might help. I think I've made most mistakes but fortunately I learnt quickly and live to tell the tale.

The heavier the lathe and mounting eg bolted down, heavy stand, sand bagged etc .. The better. I have owned a small lathe sand have a heavy lathe. Harmonics can change from machine to machine and from mounting to mounting and frame used for stands.

Another is how well your lathe is balanced. This depends on bearings and adjustment, straighness of drives and shafts, direct drive or belt drive. Anything that rotates even the motor...

Ive found in general pulleys, shafts etc all have some small cumulative out of balance or don't run true. This results in vibrations.

Ruling Out Machine and Attachment Out of Balance
Test - Run lathe with nothing Mounted on the head stock. Run at different speeds to see what vibrations you get. Then progressively add different attachments eg chucks, etc ... No wood .. Just the attachments.... If you suddenly start finding a particular attachment or combination,is worse then note and which speed. If you can isolate to one In particular then get it balanced or supplier check it out. Remember safety first at all times.

This is were VS drives can be very handy.

Out of balance can also be long wise down the lathe bed .. This can occur when you have a long large timber which you are roughing out .. Im always amazed how quickly vibrations disappear when roughing out comes to size.

The bigger the diameter where the out of balance is furtherest away from the centre the higher the out,of,balance. A small amount of out of balance weight at a large diameter can creat a lot of vibration.

Also, remember that timber doesn't always have uniform density and can be out of balance even though it is round. Eg internal hollows, sap pockets, rot etc ...

Once you have bolted down the lathe and are happy the lathe and attachments are balanced. You might like to look at a counteracter balance attachements. .?

Randal

[chuck1]

I have found speeding up lathe will reduce vibration when turning balusters 960 long at 1800 rpm, I get the odd couple that have grain tension, I can rough the cylinder and turn all the beads and slight curves and start the hollow/ coves and boom the balsuster starts running out terribly
so it's time to apply steady and speed game. Or give up with it and start a new one! ( throwing timber out depends on species, if it's pine it's firewood and glue blocks it's rare I'll do my best to save it.)
I don't like going to 2400 rpm on 960 long bals as if it snaps with the steady on it comes up and out!

[Mobyturns]

Please do not read this as me being alarmist, just my views on helping to improve turners safety as severe head injuries do occur in wood turning.


Vibration is always a potential hazard in wood turning & part of our craft is to learn how to control or minimize the effects of vibration and its potential to create other hazards. Vibration can come from a number of sources, the lathe, how the lathe is mounted, the turning methods used (technique, sequence of cuts, steadies), and the blank itself.


Wood is not a homogeneous material, so its density can vary considerably along its length and across its width. Deliberately using unbalanced blanks such as winged bowls etc simply introduces more potential hazards that we must deal with.


As others have stated particular combinations of lathe speed, blank dimensions, tool pressure, & changes in the blank profile as we turn it away can set natural harmonics into play.


There are a number of safety issues to consider with work pieces that are unsound, unbalanced or that can potentially become unbalanced. Unbalanced blanks generate unbalanced rotational forces that cause vibration, just think of vehicle wheel balancing & why it is performed.


Prolonged severe vibration can cause damage to any lathe because it stresses bearings, work holding methods etc & it can considerably stress the castings or fabricated parts and stands on lightly constructed lathes, no matter how much ballast is used.


Varying speed to move outside this harmonic range to minimize the vibration is one commonly used control. However higher lathe speeds typically introduce higher risks, much higher probabilities of blank & tooling (chucks etc) failure and potentially more severe injuries if you should be hit by high speed flying objects of any significant mass.

The iconic American turner Dale Nish came up with a widely accepted formula for recommended lathe speeds for typical sound & balanced bowl blanks -

Bowl dia (in inches) x lathe RPM = > 6000 & < 9000.

Remember these are Dale's general recommendations and not a rigid rule. He also advocated the lower end speeds.

So a typical round bowl of 500mm dia or 18" should be turned at 500 rpm max. At 1000 rpm you are near twice his recommended upper speed and near three times his lower recommended speed. Now we all know many turners turn at far higher speeds than this formula recommends & they get away with it, but it presents higher risks.

Change the project to a thin flat unbalanced winged bowl and Dales recommendations go out the window. What is a safe speed?? What is an optimal "cutting" speed for the tools to increase performance and reduce wear?

Consider the rim speed of the tips of the wings (linear or tangential speed) - what speed are they actually doing? A 500mm dia bowl @ 1000 r pm its about 95kph, at 500 rpm it was 47kph, now at 1500 rpm it becomes an alarming 140 kph. Remember the old high school physics, basically double the speed means four times the force, triple the speed means nine times the force. So an increase in speed has a very significant influence on the amount of energy ie damage a flying object can cause to the shed or us.

Coming up with a reliable "safe" speed for these projects is not easy and is dependent upon a number of factors some of which are highly variable - soundness of the blank (cracks, knots etc); "centering" (or offset of the bowl from the blanks natural axis) of the blank; vibration; tool sharpness; operator skill & experience; hazard controls in place; mass of the lathe; desired thickness of the wings (potential to flex); the chucking technique (contraction vs expansion, cross sectional area of tenon / recess etc); and even the design profile of the bowl and wings.

There is no right answer, and it may change as the project progresses, however the lowest practical turning speed definitely does lower the risk of severe injury to the turner should the project fail.

We all have to balance the benefits of speed to overcome vibration and "to get a better cut" against the risk of turning at higher speeds and the potential damage that can be caused if/when things go wrong. Unfortunately most turners underestimate the risks involved and the probability of failure. We truthfully must think about how many near misses or bowl failures we have had.

Generally lathe injuries are not "accidents" but are the result of a number of factors in combination. Several factors like high lathe speed, unstable blank (cracks), perhaps an inappropriate chucking method causing stress to the blank and increasing the probability of failure as material is removed (ie expansion mode), maybe the wings flexing, perhaps a minor catch from a dull tool being forced into the cut ..... all can combine to cause a catastrophic failure.

In your photos there is evidence of loose bark and some concerning structural cracks so you have introduced a higher risk level than for a sound blank. Now consider what happens to the balance of the bowl if a portion of the rim of the wing or the bark comes off and how it may lead to further vibration perhaps almost instantly causing the total failure (splitting) of the bowl.

In your case it remained together but reduce the wing or bowl wall thickness a little more and it may (will?) fail. We all like to test out limits but we have to learn to do it with the odds in our favor so we remain uninjured.

In some instances where the bowl is not centered on the natural axis of the blank it may never be "balanced' through speed adjustment and may require some form of counter balance system which then introduces new risks / hazards.

There are many factors which will improve your success rate and your personal safety so hopefully others will offer additional practical advice on how they turn winged bowls.

One thing all turners should make the time to read is Lynne Yamaguchi's blog http://www.lynneyamaguchi.com/wordpress/2012/09/28/an-accident/ . Two years on after her injury from a bowl failure she is still dealing with the injury from what is a relatively common practice in bowl turning and a rather typical turning scenario. She was lucky she survived but several turners in the past several years were not so fortunate.

[mick59wests]

mobyturns,

that blog is scarey reading but a good eye opener for me as alot of my wood seems to have cracks somewhere along the line.

thanks

Mick

[Mobyturns]

mobyturns,

that blog is scarey reading but a good eye opener for me as alot of my wood seems to have cracks somewhere along the line.

thanks

Mick

Yes it is and it could easily happen to any one of us. Often we are exposed to far higher risk than we think but we should not panic about risk.

Risk and hazards can be managed with a sensible and practical approach. Sharing knowledge and experience is what post like yours are all about.