Jumbo Jaws Overview: Many years ago when I first opened my studio, I was finishing the bottoms of my bowls and platters using time-honored fixings like jam chucks and friction chucks. These fixings worked fine, but they were just too slow in a production environment where time is money. When I used the tailstock for extra support, I would have a small nub left in the bottom that required finishing off by hand, requiring even more time and labor.
As my business began to grow rapidly, I knew I had to find a better and faster fixing for finishing off that allowed more throughput and less wasted time. I decided to invest in a set of jumbo jaws (also known as "Cole" jaws by some manufacturers) for my large Vicmarc scroll chuck. I heard good things about them from some friends and was eager to give them a go.
Front view of my wooden faced jumbo jaws. When
closed, the outside diameter measures 19"
Jumbo jaws are large aluminum wedge shaped plates that attach to a scroll chuck just like a pair of jaws. My jumbo jaws measure 19" when fully closed and are mounted onto a large 5.25" Vicmarc heavy-duty, four jaw scroll chuck. Each jaw plate is drilled with numerous holes that are threaded to accept retainer bumpers that screw into the holes. These bumpers grip the rim of bowls and platters, allowing you unfettered access to the bottom of the piece for finishing off the bottoms.
The advantage here is that since there is no tailstock needed, you can complete the sanding and finishing on the lathe, without having to complete a supplemental hand sanding operation to remove the tiny nub left by the tailstock's centre point. Great idea right? Well yes and no… As I began to use my jumbo jaws, I noticed that the retainer bumpers did not always provide as secure a grip on the project as I would have preferred.
When you're working on any project, especially one that will sell for a few thousand dollars, the last thing you want to happen is for the piece to come out of the chuck and hit the floor, or get launched into the stratosphere. Down in Texas, we call this kind of an unintended result a "Flier." I asked around and found out that many other turners also felt that the manufacturer supplied bumpers were not the best they could be. Some thought the design of the bumper was to blame, others felt that the bumpers were just too hard and would not grip curved rims very well.
One of my friends suggested I replace the original bumpers with better quality rubber bumpers, like the kind sold for lab use as a stopper for test tubes. I tried this option and while it did seem to grip the projects better, it did not give me the kind of reassurance I was looking for when working on up-market projects. What I really wanted was some type of adjustable friction, or jam chuck. One that would be easy to custom fit for each project and would be reusable. Yes, I wanted to have my cake and eat it too! I also wanted seconds and a large glass of ice cold milk to go with my cake. :-)
With the jaws fully extended, the outside
diameter measures approximately 22"
Necessity being the mother of invention forced me to experiment with different ways of securing projects onto my jumbo jaw plates. I liked the idea of having a large set of jaws on my chuck that I could use for finishing off, I just wanted a better way to secure the projects when turning the bottoms.
I experimented with using strapping tape in conjunction with the test tube bumpers. This worked better than using the bumpers alone, as the tape offered a little bit of extra protection that the project would stay in the chuck. I also experimented with using wrapping plastic (like the kind sold to wrap pallets or boxes for shipping). This type of plastic sticks to itself, but not anything else, so there was no residue left on the surface of the project when I used tape. This worked well with bowls, but not as good with thin platters, since there was not much depth for the plastic to grip.
Down but not out, I thought that cork might offer a good solution, but it would need some type of a stiffener in the core to prevent gross deformation when the project was secured in the chuck. Corian scraps were everywhere in my studio, so I turned a set of bumper cores with inset rims. Then I glued a regular wine bottle stopper cork to the turned Corian core.
The Corian and cork bumpers were drilled and counter bored to accept the manufacturer supplied bumper screws. This modification worked well on most projects, but it still took a bit of time to move the bumpers around for each project and then secure them onto the jaw plates. In addition, some projects featured rim designs that did not allow a secure fitting with bumpers. Although I had a very good set of bumpers now, they fell short of what I really wanted.
This view shows the jumbo jaws slightly
opened,ready to turn the recess
Undaunted, I kept experimenting until I decided to attach wooden faces to the front of each jaw. Ka-Ching! The wooden faces act much like an adjustable friction chuck and are a fast and easy way to fix items on the chuck for finishing off. The wooden jaws are made from a soft hardwood like Poplar. You do not want to use a dense hardwood, since this may damage the rims on softer hardwood projects when secured in the chuck.
For making the wooden faces, I use a S4S (surfaced four sides) Poplar plank that is 2.0" to 2.5" thick. This makes it easier to mount each segment on the jaw faces, since the mating surfaces are flat. You can use one of the jumbo jaw plates to make a template for cutting the four jaw face segments on the bandsaw.
Back view showing the large
aluminum jumbo jaw plates
For securing the segments onto the jaw faces, I use hardened steel woodscrews. These are secured at the extreme edges of the inner (near the mounting screws for the jaw plates) and outer rims into the existing drilled and tapped holes. Once all four faces are mounted and secured, I turn on the lathe and true up the outer rim and the face of the wooden segments, so the chuck will run true. At this point, the chuck is ready for use.
Using Wood Faced Jumbo Jaws
This 13" white ash salad bowl is mounted in
the jumbo jaws to complete the bottom
To attach a project like a bowl or a platter to the wood jumbo jaw faces, open the jaws up slightly, say ¼" to 3/8" and turn a recess that will fit the outside rim of the project. If the bowl features a beaded or other decorative rim, the recess should accurately match the design of the rim. This will insure that the wooden jaw faces grip the rim of the project evenly.
If I prefer a jam chuck type of fixing (a scrap piece of wood mounted onto a faceplate or chuck, turned with a protruding spigot to fit the inside wall of the project), I simply turn a spigot on the face of the jaws. Since the jaws open up in four pieces, the spigot acts like an adjustable jam chuck and allows for slight variations in the size of the spigot.
Platters can also be mounted in
the jumbo jaws for finishing off
When used with a recess as in the earlier example, the turned recess also opens up, allowing for full contact on the outside of the rim just like you would have if you turned a traditional jam chuck (scrap wood mounted on a faceplate or chuck and turned with a recess to match the rim of the project).
To prevent any marks on the rims of your projects, simply line the inside of the recess with paper towels before inserting the project and tightening the jaws. Think of this as a faster way to make friction or jam chucks. When the chuck jaws are secured, the jaws close slightly for a friction mounting, or expand slightly for a jam chuck type of mounting and grip the project very securely.
Side view of Pecan crotch
platter ready for finishing off
If desired, the tailstock can be used for the main finishing cuts for extra safety. Once the bottom is nearly complete, you can remove the tailstock and finish off the nub left by the tailstock live centre. At some point, you will need to replace the wood faces on your jumbo jaws, as the wood faces will eventually get too thin to make new recesses, or spigots. Replacing the wood jaws only takes a few minutes and you're good to go again.
Limitations In Use
Side view of jumbo jaws
showing 2.5" thick wooden faces
Since the wooden faces are attached to the jumbo jaw plates with wood screws, you lose some capacity with the jaws since the area around the screws must be avoided. This however, is a small price to pay for the versatility of using this fixing. If the jaws are over tightened, you may damage the rim on your project. If the jaws are not tight enough, your project may come out of the chuck and become damaged.
While this fixing is not perfectly suited to every situation or project, it does offer a fast and easy way to mount some projects on the lathe for finishing off. It has worked so well in fact, that I have used this method for finishing off my production bowl and platter bottoms for the last twelve years. During that time, I also experimented with several other fixings for finishing off, but I always preferred using my wooden jumbo jaws to any other method.
Small bowls like this 7" Elm bowl can also
be finished using the wooden faced jumbo jaws
As with any one-sided fixing (jam chuck, vacuum chuck, friction chuck etc.), there is always a danger that you may get a catch and pull the piece from the chuck/jaws. I have not found this to be a problem, since you should always take light cuts with sharp tools when finishing off. In addition, you should always reduce your lathe speed down when working with one-sided fixings.
Close-up view of the small 7" Elm bowl
mounted and ready for finishing off
For extra security, you can easily add some strapping tape, or shrink wrap around each jaw and across the project. I have used this technique on a few pieces that featured lots of negative space around the rim and on a few whose profile required a supplemental fixing due to the shape of the rim. It worked very well and offered an extra measure of confidence when working with these special situation projects.
Recently, I have begun experimenting with using a vacuum chuck on some projects, instead of using my wooden jumbo jaws. While the vacuum chuck excels at some fixing situations, it is not the best for every situation or need. Like everything else in woodturning, you need several tools in your toolbox so you can choose the one that best meets your needs on the specific project at hand.
For me, that means that my trusty set of wooden jumbo jaws will be getting new Poplar faces soon, and I will continue to use them for the bulk of my finishing off needs when working with bowls and platters. When my jumbo jaws do not offer the best solution for the project, I can always use my vacuum chuck, or perhaps go old school and use a good old fashioned jam chuck, friction chuck, glue chuck, tape chuck, or maybe even a pressure chuck. You can never have too many options in woodturning!
Safety Note: Always follow all manufacturers safety instructions before working with your lathe, or any of the tools or products you may use. If you are unsure about any operation, obtain competent professional instruction before proceeding. Use and wear all necessary safety devices during turning and observe safe woodturning practices to prevent accident or injury.
Steven D. Russell is a professional studio woodturner, teacher and writer. He has written numerous articles for international woodturning magazines, which have been published in more than 78 countries around the world. Steve has demonstrated in numerous cities across the United States. His studio, Eurowood Werks, specializes in bowls, platters and hollow forms with unique visual and tactile treatments.
Steve is also the current and founding President of the Lone Star Woodturners Association, Inc., an AAW member chapter. The LSWA is a 501(c)3 non-profit educational organization dedicated to teaching and demonstrating the art and craft of woodturning.
Steve is also a featured writer for the Guild of Master Craftsman's "Woodturning" magazine, published in London England. Woodturning magazine is the world's leading magazine for woodturners. Look for his articles covering technical topics, or project based articles in an upcoming issue.