Bowl Chatter

[Captain Chaos]

G'day everyone,
I'm having trouble with "chatter" or harmonic vibration ( ? ) towards the rim of bowls etc. when hollowing the inside of same. The tool(s) leave rough "chatter" marks & I have a fair bit of trouble getting a good, clean finish in this area. I've tried different speeds, sharpening my tools ( gouges & scrapers ) to a very keen edge & the angle of attack, all with varying degrees of not a whole lot of success!:confused:
My best option seems to be supporting the outside of the bowl around the rim & guiding the tool ( scraper ) with my left hand. This usually gives a good finish but the noise is pretty frightening ( although no more so than if I don't support the rim ) - I have visions of the bowl shattering & the scraper embedded in my hand.:eek: BLOOD! MY BLOOD! Aaaahh, Klunk!! :eek:
I must also mention that I turn the walls of the bowls rather thin, from approx. 6mm. down to 2.5 - 3mm. for some of my smaller turnings. Also, the majority of my turnings are redgum timber - it's what I've got, sooo, it's what I turn.
Thanks in advance for any light you can shed on this little dilemma of mine.images/icons/icon9.gif
Take good care,
Barry.
P.S. I'll post some piccies of my turnings once I work out how to get them into the thread.:confused:

[Cliff Rogers]

You are on the right track by deadening the vibration with your hand.

Try not taking the whole bowl down at the same rate.
Try finishing the first 20 or 30 mm & then move on deeper.

I can usually manage a feather thin shear scrape with a thumb on the scraper & a couple of fingers on the outside rim of the bowl.

[Captain Chaos]

Thanks Cliff, I'll definitely try that. Do you aim for a final finish on each section before moving on to the next section or do you clean up with a shear scraper once you've completed hollowing the inside ?
TGC,
Barry.

[Cliff Rogers]

Depends on the timber, if you have to go the way of doing it in steps, you do all the tool work & then move on & don't come back with anything other than sandpaper.

I've had some timber warp so much when I do it in the step fashion that I couldn't go back to it with a tool if I tried.

I have had one big mango salad bowl explode on me.
I had finished the bowl & it was a bit plain so I thought I'd scrape a decorative groove in the outside wall about 10mm down from the lip.
I set up the rest just below centre & stuck a small square end scraper straight in to do a small flat scrape.
The bowl went howl DIG BANG SPLAT! :eek:
3 pieces flew out of it, the biggest bit bounced off my head.
Lucky I had soft plastic googles on at the time & they took most of the spin off it before it rolled off my forehead.
There's still a bit stuck up in the rafters as a reminder. :rolleyes:

[TimberNut]

Captain,

Cliffs right on the money there. The amount of movement will depend on the timber being turned, moisture content, and wall thickness.

Definately turn down in steps, and as Cliff said, tool finish each section before stepping down (generally about 1 inch at a time).

Once tool work is done, sand by hand only if it has moved a lot. Use the other hand on the outside to support the wall as you sand.

With practice you can get walls so thin they are semi-transparent (a good light source behind it will help a lot with getting even walls as you just take down till the new section is as 'bright' as the one before). But then, nothings as accurate as turning the lathe off, closing your eyes and use your fingers to judge wall thickness. They'll pick up bits your eyes would miss.

Having said that, if you want to go that thin, probably steer towards detail gouges and good technique. Scrapers have a bad habit of catching when you least expect and you end up with results like Cliff described.

[Captain Chaos]

G'day Cliff and Ian,
Many thanks for your responses to my query. I'll be using your methods on my next bowl, hopefully on the weekend.:D
I use bowl gouges for the bulk of hollowing out & scrapers for finishing only, mainly as shear scrapers. I try not to revert to the #80 finishing gouge if I can possibly help it as I reckon that it lacks a certain finesse, ;) plus it seems to create an excess of very fine dust that my respiratory system can well do without.:eek:
I've "headed" a couple of bowls or parts thereof off of my noggin as well. Not a whole lot of fun really eh?
Take good care,
Barry.

[Cliff Rogers]

I have some big blocks of Raintree that make magnificent big salad bowls but the end grain tears like tissue.:(

I have found it heaps faster to rough it out with a big bowl gouge & then finish the bowl by removing the torns end grain with a #60 grit disk on an angle grinder. :cool:
Geeze it makes a mess of the shed but it gets the job done quick smart. :D
I just go down through the grits to #120 disks on the angle grinder & then swap to the rotary sander. I stop at about #320 & oil it, bludy beautiful. :)

[Captain Chaos]

I wasn't knocking the #80 finishing gouge - as a matter of fact I've got a roll of it thats around 200mm. dia. that I picked up at a swap meet a few years ago for about $10.00. ;) It's good quality Aussie made sand paper that is used with floor sanding machines. Say, that's an idea - wonder how I'd go sanding a bowl with one of those??:eek:;)
I haven't turned any big bowls yet as I've not been able to source any larger pieces of timber. Timber for bowl turning is a bit thin on the ground down here in South Oz.:( ( With the possible exception of Redgum. { E. Whitepostii } )
Sounds like Raintree is a reasonably soft, fibrous wood? I haven't heard of it until now but will Google it to find out more.:)
Take good care,
Barry.

[Cliff Rogers]

Raintree is an Albizia (sp?)

Not a native but now very much a part of the landscape around settles areas in the tropics.

Beautiful big trees that look like they should be in Africa.

[Skew ChiDAMN!!]

A slight deviation from topic: don't just relegate chatter to the "should be avoided at all costs" category.

With a bit of practice, a thin scraper (thin in height, not width) can be used to create some interesting chatterwork. The patterns can be modified by adjusting the toolrest to alter the working length of tool overhang or using different thickness scrapers. ie. changing the harmonic, as you put it.

[rsser]

Some turners rough turn and then let the bowl sit for a few days to let the stresses do their stuff.

Raffan doesn't recommend using the scraper near the rim, tho as folks have said, a small shear scraper with a light touch can sometimes help.

I had built a simple bowl steady so I don't have to turn one-handed. It works if the bowl hasn't gone out of true. http://www.woodworkforums.ubeaut.com.au/showthread.php?t=11442&highlight=cheap+cheerful

[DanP]

Here's some Raintree turnings, courtesy of Little Festo.

Dan

[Captain Chaos]

G'day everyone,
Thanks for your input on my bowl "chatter" query. It is very much appreciated.:)
First to Cliff, I had an Albizia growing in my back yard until a couple of years ago. It just died off very quickly, so I donated to my neighbours for FIREWOOD, egads!!:eek::eek: It didn't look much like a good turning timber as it was pale & rather bland looking!! Oh well.
To Skew, G'day mate, How'd you go with your lathe chuck? Got it all sorted I hope. I know what you mean about the chatterwork being used to create a certain style of finish but believe me the type I was getting could in no way be described as a decorative feature!!!!:o
G'day Ern, Thanks for your idea of the bowl support. I've sort of toyed with the idea of making one myself but the Sacred God of Intellectual Inspiration shot off to the footy grand final at about that time.;) I reckon that I'll make one up if that's ok with you AND I can snaffle a couple of wheels of of some little kids "scooter boots" when (s)he isn't looking.:eek:;) Just Kidding, I reckon that one of my nephews has some old roller blades cluttering up the parents shed.
To Dan P. & Little Festo, Thank you for posting the pictures. The bowls look really good. Were they finished with Salad Bowl oil or similar?
Take good care,
Barry (AKA Captain Chaos)