Chucks---Pro/Con

[cwearing]

I observe that some writers almost always use the expanding dovetail jaws in their articles. Other than personal preference what are the factors in choosing either the compression or dovetail modes for a given project?

 

[john lucas]

I would say most turners use compression mode. Large vessels and bowls put quite a bit of stress on the wood and expansion mode can pry the vessel apart if too much force is applies. I use compression mode mostly to hold anything that sticks out very far. This could be a large vessel, bowl or even a Christmas ornament finial.
For expansion mode I use do platters and my hand mirrors. These don't stick out very far and have a lot of wood around the jaws so the expansion mode is less likely to crack this kind of work.
When turning bowls I've known several people who use expansion mode to hold the foot. You have to be careful and leave enough wood so the expansion mode won't crack it. You can also make the hole for the chuck go up into the bowl body which adds considerable strength but may not work for your design.
Another consideration in either mode is the size of the tenon or hole. Most jaws grip the tightest when they are almost closed because more of the jaw contacts the wood. This means it takes less force to grip the work and hold securely. That is why many of us have numerous sizes of jaws. Oneway attempted to get around this by designing their jaws to grip more sizes.

 

[MichaelMouse]

I'm an innie unless I'm dealing with long overhangs, like goblets and such. Reasons are simple. Takes less depth away from the piece, since you're already inside and holding at a diameter where you would only be beginning the shoulder for a tenon. It's also easier to keep the hold closer to the optimum size with an innie when doing TDT. The wood contracts not at all along the grain, and requires only removal of enough of the filled in cross-grain area to get to back to circular when dry, preserving the optimum contact size you had when green. With an outie you have to make the green tenon larger than optimum contact size to compensate for the contraction across the grain as the wood dries.

Other advantage is that you can finish the bottom prior to reversal for final truing of the inside. You don't have a tenon to part off or a "foot" to clean up afterward. No need to fiddle with vacuum chucks or Longworth/Cole types.

Advantage to smooth over serrated jaws is also pretty obvious. You're going to get the same hold by dividing the load over a larger smooth area as you would applying a greater "grip" than the wood could take without distortion or ugly compression marks from too little contact area. Then there's the natural wedge of the dovetail working to snug the nose of the jaws up against the trued surface inside or shoulder outside. With symmetrically serrated jaws the wood is as likely to push itself off the contact as not, so you have to pay special attention to seating the nose of the jaws and preserving them through the tightening. Overall, it's important to consider that you're not "gripping" either hold as if grabbing a nut with pliers, but "fitting" as you do with a properly sized wrench.

As to splitting along the annual rings, it's as likely to happen to a poorly positioned tenon as a mortise, so on ring-porous woods I am inclined to reinforce the area inside the mortise with some thin CA, cured and trued afterward for proper fit.

 

[almather]

Michaelmouse said he was an 'innie'. Michael, I had trouble figuring out whether this meant you like to use a tenon (compression) or you put the jaws in a mortise (expansion). I gather after reading slowly and carefully you like expansion. Your 'innie' and 'outie' aren't to me, part of the standard vocab for this type of question.

Did I interpret you correctly? And what is TDT?

Al

 

[MichaelMouse]

Innie and outie are navel terms, but they also describe points of attachment in the birth of a turning, and so seem appropriate. TDT is Turn, Dry,Turn, probably the most-used process for turnings, and the one that produces as much frustration as the skew.

 

[waltben]

I almost exclusively use the expanding method for bowls and enclosed forms, as it allows me to have a foot (I use a vacuum system to turn off extra wood and clean it up). With well over three hundred pieces done this way, I've only had one that cracked bad enough to not be able to finish. Two others cracked, but were saved by CA. If I turned hollow form urns, I'd have to rethink it all.

I also almost exclusively turn wet wood to completion - it gets dry quickly when its thin. Maybe I'm impatient, but I see no use in letting things sit for a month or so. The down side is all the rust film on things.

 

[Brian Hahn]

I use both modes:

• For platters I'll use expansion mode.
• For tall narrow forms (vases, hollow forms, goblets, etc.) I use a tenon and compression mode.
• For bowls I'll use either mode, depending on the bowl size and type of foot I want. I've broken out a couple of foot rings that were too weak and sheared off a couple of tenons, so neither mode is foolproof

I always try to use the optimum gripping configuration, which is real close to the minimum size that the jaws will handle.

In the end, it really boils down to your personal experiences and resulting personal preferences.

 

[Steve Russell]

I use compression mode about 99% of the time...

Hello,

I agree with John's post... Personally, I use compression mode about 99% of the time in my studio, for production as well as artistic work. I use expansion mode only on the rarest of occasions, like when turning dinner plates and sushi plates from 3/4" thick stock. Good luck to you and all the best to you and yours!

 

[pfduffy]

innie and outie??

Innie and outie are navel terms, but they also describe points of attachment in the birth of a turning, and so seem appropriate. TDT is Turn, Dry,Turn, probably the most-used process for turnings, and the one that produces as much frustration as the skew.

Sorry, MM, but in my 31 years I never heard either of those terms used. Philip

 

[MichaelMouse]

Sorry, MM, but in my 31 years I never heard either of those terms used. Philip

Is it the spelling of navel versus naval?

Head down to the beach and check out the developmental attachments on the 2-piece types to see if you prefer an innie or an outie.

Faint recollections of heresy charges against Michaelangelo for putting a navel on Adam, answered by placing one on God.

 

[pfduffy]

irrlevant comments

Sorry, but it is Naval, and that term reflects the combined combat capability of the Navy- Marine Corps team. Most all folks get it wrong. It accounts for the SecNav being the senior civilian of both Services. P.

 

[MichaelMouse]

Wow! I still can't figure that out, and I did some looking.

I will only accept responsiblity for the correctness of what I say. http://en.wikipedia.org/wiki/Navel So try it the way I spelled it.

I think a navel analogy is appropriate when speaking of how we give birth to our "babies," sorry if it doesn't work for you.

 

[john lucas]

Another reason I use compression mode, especially on bowls is that I can change the shape of the foot when I reverse the bowl if necessary. I usually start my bowls and hollow vessels between centers. Sometimes when I have finished the outside and inside I look at the piece and decide that the foot needs to be smaller. If I had used an "inny" or rebate I can't change it. Another advantage to Tenons is that I can leave the tenon larger than I would the foot which reduces vibration. Then when I reverse turn it I can reduce the size of the foot to the appropriate size for that bowl.
There are obviously good reasons for using the rebate method as well.

 

[waltben]

Beg to differ . . .

"If I had used an "inny" or rebate I can't change it." I use 'inny's' all the time and change the foot size to fit the form I'm making. The socket must have enough wood around it to maintain integrity in the first place, otherwise one ends up with things flying around the shop! I try using the smallest jaw set I feel comfortable with for each piece. That extra wood lets me make a foot any size I want when reverse chucked. When making large platters (18" to 25") I've made two and sometimes three feet (rings) to support whatever the owner might put on the top. Every piece on my page of our website was made with in "inny", including "New Orleans Memory" that started out as an 80 lb pound chunk: http://www.abhats.com/Bowls.htm

 

[Aaron G+11]

If you can put your finger in it? It's an iinnie. If you can't it's an "audi"? It's an umbilical term.

 

[hockenbery]

Outside grip has less waste

I use the tenon/compression grip most of the time.

It actually allows less waste. Makes small bases easier.
Every production bowl turner I know uses this grip.

Mike Mahoney, Al Stirt and others often make a small chuck grip on the outside surface of the bowl.
this holds with only a small groove to be turned away for the finishing.

When Mike Mahoney turns a dozen and half nested bowls from a single block he uses the outside grip. You better believe he isn't wasting any wood.

I've seen Al Stirt use a similar outside grip on bowl that have crotch flame in the bottom. The crotch flame is thin and doesn't allow room for a tenon unless you waste the other half of the flame.

I also saw Sherry Hockenbery use this grip to return a finished piece she wanted to refine. I've used it a few time too.

These bowls have either no foot or a petite one. A groove is turned on the outside and the chuck jaws grip. These guys never get catches and they know when to take light cuts.

Also for end grain turning it is the only way to go.

Happy Turning,
Al

 

[Ken Grunke]

Mike Mahoney, Al Stirt and others often make a small chuck grip on the outside surface of the bowl.
this holds with only a small groove to be turned away for the finishing.

Stuart Batty showed our club that trick this last May, pretty neat. All the gripping power is at the tips of the jaws, and the wood behind that on a full-size tenon isn't doing much of anything.

I can't remember the last time I used expansion mode, except to lightly hold a bracelet, or the body of a lidded box for finishing the bottom. But never on a bowl or the initial turning of any end-grain piece.

 

[MichaelMouse]

Outside Less Waste?

Hard for me to understand where that comes from. As the log is cut, you have either a curve or a flat available. A curve is illustrated, and it looks to me like it starts out deeper and can stay deeper. The scale drawing illustrates a 2" mortise and a 1.75" tenon, which would actually have to be larger, as mentioned, to allow returning to circularity on a piece cut green with intent to re-turn. About an eighth if things are perfect, though smart money would go the full two to allow for odd warp. The 2" shrinks across the grain, so it will stay pretty much as it is when dry. Don't even have to go a full quarter inch deep if you don't want to, though a green 1/8 tenon would certainly be a challenge.

Obvious which gives greater depth if you make things flat, so that's not illustrated.

Another advantage of the mortise, cut between bedroom painting stints this morning. Had a 14x3 bowl which took a digger on a sharp object, denting the surface pretty severely. Rather than pitch it I chucked it up in the partially finished bottom and turned away the first coats and the ugly dig. Interior has some modest curl, so it was worth saving. Don't imagine it would have been that easy to take away a bit over an eighth on a vacuum chuck, especially if the bowl had been elm or oak. It would also have been somewhat challenging to turn the outside to reasonably approximate the inside if the inside were on a jam or vacuum chuck. Fingers sufficed to gage when chuck mounted.