Expansion v. contraction mode

[jefferson]

Nine Fingers just posted an interesting (well somewhat scary) thread about his little incident. And I didn't want to hijack his thread.

There were a few views about whether a bowl should be held in contraction rather than expansion mode, so I am very interested in what the common view is.

I tend to use expansion mode on my redgum and red box work, as there are less (if not invisible) jaw marks on the recess. Not so with contraction jaws, even if I try to get the jaws to almost perfect round.

I also tend not to go for a super-thin bowl or platter, so explosion is probably not a worry. But my R. Raffan books do however caution me on what I am doing!

So now I am thinking that I probably should be cutting an over-size tenon for the base, then turning the inside, then reverse chucking with the bowl jaws and reducing the tenon to a smaller size, thus cutting out any jaw marks.

Any thoughts or other suggestions out there?

No accidents here yet involving stitches, but I have stripped a few threads with catches and knocked quite a few pieces out of the chuck. Not lately though.

Jeff

[Skew ChiDAMN!!]

So now I am thinking that I probably should be cutting an over-size tenon for the base, then turning the inside, then reverse chucking with the bowl jaws and reducing the tenon to a smaller size, thus cutting out any jaw marks.

I'd say that's probably the "standard" way of doing it. However, because it removes the means for mounting, it also means you can't easily remount the piece at a later date. (Good for competition pieces, though.)

Your way has it's risks, but it means that if it needs a repolish or a touch-up at a later date, well... whack it back on the lathe and Bob's yer uncle.

Don't get stuck on "I should do this" or "I should do that." Practice both methods (and others) so you can use the one best suited to whatever you're turning. A few this way, a few that way. A bit of variety keeps it interesting. :)

[Cliff Rogers]

I have had more 'bust-outs' with expansion that I have had looses with contraction.

I use contraction for small dovetails now & only use expansion with shark jaws in a deep hole.

[hughie]

I have had more 'bust-outs' with expansion that I have had looses with contraction.
I use contraction for small dovetails now & only use expansion with shark jaws in a deep hole.


Thats pretty much my findings as well. Although I have a friend who uses the Leady chuck [expansion] and swears by it.

Anybody else out there use the Leady chuck?

[artme]

This, no doubt, Will be a never ending topic for debate so- being a keen debater, I will stick my oar in.

#Richard Raffan has written about the problems of re-mounting at a later date
if you have no recess.

#Modern judges seem to demand no "visible means of mounting" on finished items for competition. I am sure that this attitude has permeated itself right through the turning community. This also adds to the turner's costs as they feel the need to buy Coles Jaws, vacuum chucks and other devices in order to be a proper modern turner.

#I learnt from experience that some timbers - Mango to name one- are prone to tearing away from the tenon if that method is used. Not all the time but often enough to be a PITA.

#The cutting of a dovetail recess for expansion mode presents its own set of problems and getting the correct angle is the most obvious. Getting the depth right is another problem as this leads to the problem of the size at the top of the recess in relation to the bottom

# I have been taught to cut a recess with vertical sides. This has only failed me once. That was when I failed to make the recess deep enough in a piece of Southern Silky. The result was crushed end grain and therefore an inability to mount the piece.
Some may ask why I don't use a dovetail. The answer lies in the mechanics and the application of forces, The recess with vertical walls needs to be cut to exactly the size of the circle formed by the jaws of the chuck when they are fully closed. The fit if the jaws into the hole w must be a snug as possible and the jaws must, logically, be flush on the bottom of the recess.
When the jaws are then tightened in expansion mode the force is applied into the 90 degree corner and you have a truly secure grip enhanced by the fact that you have more surface to surface contact-metal to timber- than you can ever achieve with a dovetail.

[Ed Reiss]

Contraction gets my vote, not only because of no chance of having a "bust out', but it also gives you more options in finishing the bottom other than just a mortise.

[Alastair]

Majority of my turning is done with standard (old lever nova) chuck, used in contraction around Glaser screw for outside, and expansion into d/t recess when turning inside of bowls.

For 98% of pieces, I tend to use the 50mm std jaws.

Depth of recess is shallow, and varies from ~2.5 mm, at 6", up to ~ 4.5mm at the largest sizes, (probably 20-22" on my old lathe).

I seldom "sanitise" the bottom of bowls, instead treat shallow recess as a feature, and use it to sign the piece. Haven't had to pander to judges yet, so cannot comment on that.

In 8 years, I have probably lost at most a handfull of pieces due to expansion blowout, about evenly split between VERY heavily spalted timber, and crap technique, resulting in heavy catches.

Seldom grip foot in contraction, but on the few occasions I have tended to get marking.

One point where contraction mode is a problem, is when it is being used in conjunction with intermittent cutting. i.e, when doing square edge, or natural edge work, or similar. The repeated "rapping" of the interrupted cut acts like a rattle gun on the chuck spiral, and tends to loosen it. Expansion mode is self tightening in this case.

In general this only affects me when roughing out the outside of an irregular log blank, mounted on a woodworm screw, and held in contraction. I have learned from experience to keep checking, as frequently the inexplicable inability to get a clean cut down the outside of the bowl, is because it is moving on a subtly loosened ww screw.

regards

[NeilS]

I rarely use contraction on the feet of my bowls. Very occasionally I will do so on an exhibition pieces that is best done that way for aesthetic reasons, e.g. attached.

The thought of having to remount bowls in cole jaws having already turned the bottom seems like just one extra step too many to me. I would prefer to get it right the first time...:) There is also aesthetic reason why I complete the outside of a bow in one operation, but that is a long discussion for another time.

Occasionally I will lose a piece off the chuck that has been dovetail mounted, but have also been frustrated at times by trying to accurately remount a piece in contraction mode.

I find contraction on internal tenons left for remounting green roughed bowls effective as that wood has to be removed anyway, so it's not an unnecessary extra operation.

[STRONG OPINION ON] I disagree with this obsession of 'modern judges' about removing the chucking recesses from wood turning pieces, referred to by artme. It does not have a design or aesthetic basis. The result has in my opinion been an epidemic of very boring feet!

It seems to date back to the time when faceplates were used to hold blanks (the bowls were completed inside and out in the one operation) that left an ill-defined wide flat foot and unsightly screw holes. The genesis of this legitimate concern can be traced over the last twenty years through the early issues of the National Association of Woodturners' (of NZ) publication, Faceplate. Somehow it became doctrinaire, despite the evolution of chucking that gave the capacity to produce a very well designed and aesthetic foot that can retain the chucking recess.

Having said that, ugly jaw marks and recesses detract from the finish on a piece and should be carefully avoided. [STRONG OPINION OFF]


.

[Alastair]

:whs::whs::wtg::wtg::iagree::iagree::iagree:

[rsser]

I've not seen any shortage of pieces entered for judging that had recesses in them, incl. from pro turners.

As long as the recess wall angle matches the dovetail, the piece is wedged onto the jaws and can only come off with wood failure.

[Farnk]

Whichever way works for the blank in question!
I use both methods, but as my bowls are shaped more from a perspective of IFETH than the pure asthetic, having a recess is generally not an issue.

IFETH -> It Falls Easily To Hand or just feels good to hold!