Expansion Mode for your Chuck

[brendan stemp]

Well this one might settle (or even start) a few debates. I hope you get something out of it.

Using your Chuck in Expansion Mode - YouTube (http://www.youtube.com/watch?v=PzG70rAJRIQ) looks at the pros and cons of the scroll chuck being used in expansion mode to hold a bowl onto the lathe.

[jay h]

Clear concise easy to follow. Well done as usual. Jay

[DaveTTC]

Thanks Brendan

Nothing like an Aussie tutorial

Your video leads me to a design 'flaw'.

Perhaps dovetail jaws should be cut smaller (the 1/4 sections) so that when compressed inner and outer are diameter is smaller then what the radius of the arc should be. This way they can be expanded (allowing for the slack taken up due to the dovetail) to 'truley round'

Jaws could be supplied with the actual grip range, inner and outer. Instead of 8 or 4 point contact we could get closer to complete contact

regards

Dave

ps - I used to live in Horsham, shame I did not get into turning in those days

[Mulgabill]

Good YouTube video Brendan. I am sure it will evoke some discussion.
BTW Carl Jacobson gave you a good mention in his latest video.

[turnerted]

A good video on the pros and cons.Brendan
I hardly ever use expansion mode because of the risk of cracking the dovetail.I never thought about the 4 vs 8 contact points.
On the few occasions I have used it, I would still tidy up the dovetail when the bowl is finished but I am not trying to make a living from woodturning and time is not important to me.
Using a tennon also makes it easier to apply the finish.I know you like to spray your bowls, but I like to use DO as a finish and apply it while the bowl is on the lathe. With a tennon, you can grab hold of it and don't end up leaving fingerprints on the wet DO or having to leave the bowl in the chuck overnight.
After finishing the outside of a bowl,I will often hotmelt glue another tennon on top of the original tennon. This gives me even more to grab hold of when applying the finish.
If I am using my bowl saver, I will often cut a 100mmx10mm tennon on my blank then after coreing, glue on a smaller diameter tennon and then I can incorparate the original tennon into the foot of my bowl.
Just a few thoughts.
Ted

[hughie]

I rarely use expansion and go with contraction most of the time. I use a lot of brittle hard woods and find contraction even though I waste more timber, works well for me. I also turn some fairly chunky pieces up 250mm deep through smallish holes. So I can at time have 250+ hanging out of the chuck which is a bit of leverage and on occasion I lose them.

But never the less a good video and we all must work as safe as we can and that can mean different things depending our current level of experience.

For those who are just starting out this video should be a must as it raises very good points on jaw contact.

[mick59wests]

Brendan,

thanks for another great video. I am very grateful for your videos. It is so much easier to learn from watching than reading. Personally I prefer the look of the bowl with expansion mode so I use it when I can.

My questions to all you experienced (and you too Brendan :), as you mentioned it) is the depth of the recess. My schoolboy physics would seem to think that the deeper the recess, the more contact and less chance of breaking the wood, but even the instructions for the Teknatool scroll chuck say a maximum of 6mm while there could be a maximum depth of around 12mm before the wood hit the 'bottom' of the chuck. I'd love someone to tell me if I have this correct or why I don't understand. I understand for cosmetic reasons why you want a smaller recess but if you need strength why not go for deeper?

cheers

Mick

[brendan stemp]

Brendan,

thanks for another great video. I am very grateful for your videos. It is so much easier to learn from watching than reading. Personally I prefer the look of the bowl with expansion mode so I use it when I can.

My questions to all you experienced (and you too Brendan :), as you mentioned it) is the depth of the recess. My schoolboy physics would seem to think that the deeper the recess, the more contact and less chance of breaking the wood, but even the instructions for the Teknatool scroll chuck say a maximum of 6mm while there could be a maximum depth of around 12mm before the wood hit the 'bottom' of the chuck. I'd love someone to tell me if I have this correct or why I don't understand. I understand for cosmetic reasons why you want a smaller recess but if you need strength why not go for deeper?

cheers

Mick

You're absolutely right Mick. The deeper the better for strength but I go for shallower recesses because of the aesthetics, as you mentioned. Also, the deeper it is the more the jaws will scroll out.

[NCPaladin]

Great video Brendon.
I had never really thought about the 4/8 points.

An additional point to me is wood area. Nova suggests 1” of wood surrounding a recess and I do this on green wood. A 2” spigot has 3.14 sq inch of wood surface to break away. A 2” recess surrounded by 1” of wood has 12.56 sq inches; minus the interior 3.14 (which is holding nothing) you still have three times the area of wood holding the item on.

I usually go at least 3/8” deep with green wood but with dry maybe only 1/8” depending on size and wood strength. If it is too deep I just trim a little off while cleaning up the foot; sometimes I trim it all off and just leave a little concave.

I do the same cutting the recess diameter. According to Nova the kerf made by cutting the jaws apart is 2mm so I cut the recess so the jaws will just slide in when fully closed. When expanded in the dovetail it is as close to a perfect circle as you can get.
Sketch is how I do it.

[DaveTTC]

NCPaladin - thx for the info on the jaws. Answered my concern

[hughie]

Brendan,

thanks for another great video. I am very grateful for your videos. It is so much easier to learn from watching than reading. Personally I prefer the look of the bowl with expansion mode so I use it when I can.

My questions to all you experienced (and you too Brendan :), as you mentioned it) is the depth of the recess. My schoolboy physics would seem to think that the deeper the recess, the more contact and less chance of breaking the wood, but even the instructions for the Teknatool scroll chuck say a maximum of 6mm while there could be a maximum depth of around 12mm before the wood hit the 'bottom' of the chuck. I'd love someone to tell me if I have this correct or why I don't understand. I understand for cosmetic reasons why you want a smaller recess but if you need strength why not go for deeper?

cheers

Mick


Mick I often go for a full 10mm and trim the base later. Quite often I dont even bother with a spigot, just simply glue a block on the bottom. The depth is generally the same as my power grip jaws. It really depends on the timber and what I am planning, ie If its a small bowl I will completely finish the outside with a bead for a foot and give its primary coating of finish. Then flip it around and hollow it out, my foot beads are around 6mm.

[Cliff Rogers]

I rarely use expansion and go with contraction most of the time. ...

I use shark jaws expanded into a hole cut with a bit forstner bit to hold my bowl blanks for roughing the outside & turning a foot with a bevel for then gripping in contraction.

[NeilS]

Another excellent video, Brendan.

Like you, I use expansion mode in the same way for the same reasons.

Because I use a wax finish, I do that on the lathe, so the piece is finished when it comes off the lathe after only being flipped once.

A double layer of Gladwrap (food wrap film) between the jaws and dovetail protects the wax finish while turning and polishing the inside.

Another downside of expansion mode, and matching the dovetail diameter to the jaw size, is the number of jaw sizes you need to have to keep the foot size in proportion to the bowl diameter. At last count I had 13 different jaw sizes, which gets a tad expensive when you hate changing jaws and like to have a chuck for each set of jaws...:C

A chart of previously worked out Blank sizes: to Foot diameters: to Jaw sizes saves a lot of unnecessary splinters under the fingernails...:U

The exception to using expansion mode is when I'm coring, which involves a lot more torque, in which case I use contraction to clamp on hard. But I do turn a dovetail on the inside of the cored bowl with which to grip it when I come back to turn the outside of the bowl when dry, at which stage I revert back to expansion mode.

[DaveTTC]

A double layer of Gladwrap (food wrap film) between the jaws and dovetail protects the wax finish while turning and polishing the inside.



Neil - another good tip

Thank you

[Bluegum]

Excellent video Brendan. Thanks for the tips. Quite a few things there that I hadn't thought about.

[robo hippy]

I think I started using the recess after reading a Richard Raffen book. It is argued a lot on which holds better, a recess or a spigot. I always say that if they are made correctly, they both work excellently. I hadn't thought of the 4 vs 8 points of contact. I do make my recess to size, using a two dedicated sets of dividers that are super glued into place, so they never change size. Other than that, matching the dove tail angle as exactly as possible is necessary. There are specialty dove tail scrapers for that. If the angle matches your jaws (I think 7 degrees, which is what is used on dove tail joinery for drawers and such), you plunge straight in, and eyeball the tool shaft with your lathe bed. The scraper moves to the side all by itself. Depth wise, I never go deeper than about 1/4 inch, which I think is around 5mm to you metric people. With dry wood, I have seen Mike Mahoney do an 18 inch platter with a recess about 1/16 inch deep, so, maybe 1+mm. With wet wood, I am usually in the 1/8 inch depth range, and this works fine with my big Vicmark with 2 5/8 inch wide jaws on bowls that are up to about 16 inches, and I core and do heavy roughing on them. I do leave a bit more timber on the shoulder to push into. If you go deep, you need to make sure the angle is almost perfect, otherwise, there is unequal pressure on one part of the shoulder, and that can cause breaking problems. Like stated, you can over tighten, and if you don't break it outright, any extra little bit of stress in turning, like a small catch, can cause failure. With the spigot, you can trim the spigot up to be part of the foot rather than having to remove it.

robo hippy

[Cliff Rogers]

I've had more break outs/stuff ups/looses using a recess than a spigot.

[Mobyturns]

Both expansion & contraction modes are safe IF you understand the strength of your materials and how much "meat" you require to hold the size of project you have. A simple calculation of the surface area (pi x r x r ) of the tennon or donut ((pi x R x R ) -(pi x r x r )) of material around the recess is a worthwhile exercise.

The grain, its direction and the shear strength of the timber in the grain orientation (face or spindle) you use has a significant influence on your choice. Next consideration is the shape and accuracy of the tennon or the recess to match your jaw set. Then how much pressure you use to tightening the chuck jaws. The 4 or 8 point holding is only part of the story, chucks perform in an ideal manner in only a very small part of their jaw movement range, every other jaw position is a compromise.

Mismatched or incorrectly shaped tennons / recesses are mostly the problem as they encourage turners to overtighten chucks to compensate or to improve the hold which then creates micro cracks in the timber from the edges or corners of the jaw sets crushing the timber, which starts a cycle of the jaws slackening as the timber crushes more, so the turner retightens etc. This is a significant cause of failures in face grain orientations for both tennons & recesses. How often do you retighten a chuck whilst turning? Have you ever noticed how the bowl may start a small vibration before it fails?

Raffan and others have used very small detail on the inside of pieces to hold and reverse turn, but the key is understanding the whole process & to use light cuts that employ the tool to also hold the piece against or cut "towards" the chuck and not "away" from it.

I much prefer contraction mode AND large jaw sets & mostly use the 90mm Vicmarc bowl jaws with glued on waste blocks and a tennon (Titebond, AVXL Plus PVA's or Bostick hot melt glue), only because it gives me plenty of flexibility and I prefer bowls with a minimal or no foot at all.

[countstefan]

Great video Brendan. I think you point about the diameter of the recess is really important in expansion mode. I always undercut my recesses, but that's the way I was taught to do it.
Any thanks for taking the time and I subscribed to your YouTube channel

[Bruce White]

:2tsup::2tsup::2tsup:

Excellent Brendon! I do not feel the need for spigots at all the way I turn.