Turning Archive 2005

Subject:
DIY Scrollchuck with Cole Jaws (Long & Many Pics)

Garrett in Victoria
>A few years ago I came across an article on something called the “Longworth chuck” – background info at the end of this post - and resolved to make one because the idea of a self contained scrolling set of Cole Jaws was just to appealing to resist. I finally got around to it today, and was surprised at how simple it is. The cost was a couple of pieces of scrap and $5 for 4 crutch tips - buy rubber not vinyl -and 4 ¼” x 2 ½” machine screws and a couple of hours in the shop.

However, it’s important to do things in the right order. Believe me.

The first step is to measure the distance between the centre of the headstock and the bed. Obviously, that is the largest radius your lathe can manage unless you turn outboard. (NOTE: for an outboard rig the routed curves must go in the OPPOSITE direction from those described here.)

In my case, the lathe swings 8”, so my chuck is 16” in diameter. I found a couple of suitable pieces, ¾” MDF for the back, and ¼” Baltic Birch for the front,

and the bandsaw circle jig quickly turned them into disks.

Time to build. First, mount a faceplate on the back disk. It’s important to do this now, because everything after this revolves around the centrepoint, literally and figuratively. You will notice 2 things in the following photo. First, that I mounted my “faceplate” after routing, a mistake that wasted a lot of time to get it centred exactly. Second, that it’s obviously a nut rather than a faceplate. Some time ago, I found a source for some 1 1/4” x 8 TPI nuts and bought a bunch, because although it’s a standard headstock thread, it’s an unusual size in fasteners. (Pipe thread is 1 ¼ x 7.) I’ve been using them in various ways to make faceplates and vacuum chucks, and it works very nicely in this application, too.

Choose the poorer face of the front disk, and find or make a big compass. Draw three circles. The smallest is the diameter of your faceplate plus a bit. The largest is about 1” in from the outer edge of the disk, and the middle is, in fact, centred between the first two. Draw perpendicular lines through the centre.

Carefully dimple the intersections of the perpendiculars with the outer ring and use the dimples to centre your compass to draw arcs from the outer ring to the tangent of the inner ring. (These arcs will ensure you make no mistakes while routing.)

Tack the two disks together, being careful to stay away from the arcs, and mount them in a vise. If you haven’t already got one, make a circle cutting base for your router. Drill the dimpled centre points to accept whatever you use as a pin - mine is just a small finishing nail - and carefully rout the 4 arcs through both disks. (MDF makes a LOT of dust!) I made a circle-cutting base out of another scrap of ¼” Baltic Birch, and made the interior hole 3” in diameter to ensure good vision since all start and stop points are done by eye. The best way to start a cut is to be an inch or so away from an end point, plunge the bit into the work, back up to the start point and then move forward to the end point. With a solid carbide up spiral bit, only 2 passes were required to go through both disks (1”).

The result will look something like this:

Now drill some ¾” finger holes around the perimeter of the disk sandwich.

Before separating the disks, mount the sandwich on the lathe, true up the edge, and use some sandpaper to ease the sharp corners. Then draw a registration line across the edges of both disks so you can line them up again later.

Use a drill chuck in the tailstock to drill a small hole through the exact centre of both disks. Separate the disks and remove the front disk. If necessary, this is a good time to true up the face of the back disk.

Take a #10 1” round head screw, preferably with an unthreaded shoulder, and enlarge the centre hole in the front disk so that the screw can just pass through and the disk can rotate on the shoulder of the screw. Enlarge the centre hole in the back disk to the inside diameter of the screw’s thread.

Now, REVERSE the front disk so that the routed arcs cross each other and drive the screw into the centre holes. Back off just enough to allow the front disk to rotate.

Make the holders. I turned a piece of dowel to 5/8” d, cut it to length to fit inside each cane tip, and drilled ¼” holes lengthwise through the top of the tip and the dowel. Wing nuts and washers on the back make for quick and easy adjustment.

In the photo below, I’ve inserted the bowl holders. When the nuts are loose, the front disk rotates very easily and synchronizes the movement of the 4 holders from the innermost position to the outermost with no effort at all, and can be snugged up at any point to make the whole unit very secure.

In use, it’s quite flexible for big (12” bowl) and small (3.5” hollow form).

And, no more having to mount and dismount 8 jaws on my Super Nova 2 every time I want to reverse a bowl that I cannot vacuum chuck.

A couple of closing comments:

1) Be very, very careful of the spinning bowl holders on the front and the wing nuts on the back. They hurt. A lot.

2) The chuck works quite well, but with only 4 holders it isn’t designed for heavy side pressure, so take it easy.

3) Having made this one, I intend to make two more, one about 10” in diameter for small work, and another 16” the same size as this one. The smaller will just be a miniature of the one displayed here except that I’ll use rubber buttons as holders rather than the cane tips fro much better access to the workpiece. For the larger chuck, however, I’ll make the smallest circle about 5” or 6” in diameter, because a larger inner circle will enable me to have 6 or even 8 arcs and holders offering far better security. And anyway, as the photo shows, providing for such small bowls on so big a disk is awkward if not plain silly. and, finally,

4) Mr. Leslie Longworth of Australia invented this chuck in the late 1980's and published only the first of a two part an article on it before he died soon after. More info is available at http://www.fholder.com/Woodturning/chuck.htm, including that from which I made the version above.

Cheers, Garrett

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