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Make and Use a Longworth Chuck
Tuesday 11 May 2010
1.Firstly, using a router fitted with a 6mm (1/4in) straight bit and a suitable circle cutting jig, cut discs from the plywood and MDF to a size appropriate to the swing of your lathe
I first became aware of the 'Longworth' chuck designed by the late Mr. Longworth of the Hunter Valley Woodturning Club, New South Wales, Australia, when I was looking for an inexpensive way of reversing bowls on the lathe without the inconvenience of removing the dovetail jaws from my chuck and replacing them with Cole jaws. You then had to adjust the buttons on the Cole jaws to fit the bowl - an operation that involves a minimum of 16 fastening or unfastening operations using an Allen key - 32 if you need to move the buttons. Of course, I could just make up jam chucks in the time honoured way, but having to make an individual jam chuck for every bowl soon becomes a bit of a chore if, like me, you never make the same sized bowl twice.
Internet research threw up a couple of articles describing the making of Longworth chucks, including one from issue three of Woodturning magazine, and another from More Woodturning in 2005. Whilst both of these articles provided instructions that would enable the manufacture of a chuck, I felt that the chucks produced fell some way short of what I wanted in a number of areas.
Although reference was made to chucks with eight slots, both of the articles resulted in four slot chucks, insufficient, in my opinion, to provide a secure hold on a large bowl.
As well as showing you how to make your own version, this article also shows tips for using your improved chuck effectively.
Design considerations
1. Options for the rubber buttons included walking stick rubbers, wine corks and doorstops, all intended to reduce cost, but none gripped as well as commercially made buttons. The buttons were held in place by bolts and wing nuts, exposing revolving metal parts which I felt could cause injury. The axle of the chucks used a woodscrew which was not an ideal solution, as later adjustment could slacken the grip of the woodscrew.
2. It is important to give thought when setting out to the pivot points for the router. When I came to cut the last slot I found I had a slot where the pivot point should be and found myself having to improvise with a wedge in the slot.
3. Apart from hand tools you will need a protractor, a large pair of compasses and a router fitted with a 6mm (1/4in) cutter, as well as a circle cutting jig.
4. Components required are 9mm (3/8in) plywood and 15mm (5/8in) MDF large enough to cut the discs - sized to suit your lathe - a backplate to attach to the lathe spindle (an old faceplate is probably the best), a set of buttons from Vicmarc, Axminster or similar, cap head bolts, washers, a Nyloc locking nut, plus knobs and woodscrews to attach the faceplate to the chuck body.
5. I made a prototype of the original design, to ascertain whether the improvements I planned would be feasible, from materials I had available in the workshop or was able to obtain very cheaply. It became clear that building a chuck with eight full length slots would result in a structure that was weakened near the hub of the chuck, so I decided to incorporate four full-length slots and four shorter ones, so that four buttons could be used for small bowls and a further four added when a larger item was to be held.
6. Using the prototype I tested buttons from Axminster and Vicmarc along with some of the cheaper options and decided that the Vicmarc buttons, which are square with a concave curve on two faces and a convex curve on the other two, afforded the best grip on the bowl.
7. I planned to resolve the safety issue of spinning wing nuts by providing guarding, but any structure to restrict access would add considerable inconvenience when tightening the buttons and would restrict maximum bowl size that could be held in the chuck. I decided instead to use small 22mm (7/8in) diameter thumbscrew knobs to replace the wing nuts. These might still give your knuckles a rap if you get in the way of them but they are less likely to take a chunk out of your finger. The bolt heads on the face of the chuck were not a problem with the Vicmarc or Axminster buttons as they were recessed into the face.
Step-by-step
Step 1
Firstly, using a router fitted with a 6mm (1/4in) straight bit and a suitable circle cutting jig, cut discs from the plywood and MDF to a size appropriate to the swing of your lathe. This can also be done using a bandsaw, if you prefer this method
Step 2
The next step is to pin the two discs together using panel pins - within 20mm (3/4in) of the edges to avoid getting in the way when cutting the slots - find the centre and mount the faceplate that will be used to attach the chuck to the lathe
Step 3
Mount the assembly on the lathe and true up the edge of the discs
Step 4
With a chuck mounted in the tailstock drill a small 1.5mm-2mm pilot hole to mark the centre
Step 5
With a pencil and ruler mark a line across the diameter of the discs
Step 6
And mark diameters at 45 and 90 degrees to the first using a protractor
Step 7
Draw three circles, the first slightly greater than the size of the faceplate, the second 20mm (3/4in) from the edge of the disc, and the third halfway between
Step 8
With the point of the compasses centred at the points where the middle circle and the diameters intersect, draw arcs tangent to the inner circle to a point where they reach the outer circle
Step 9
With the pivot point of the router centred on the same points as used to mark out, cut a slot the full length of the drawn arc. Take shallow cuts to avoid putting excessive stress on the 6mm (1/4in) bit
Step 10
The next slot should stop halfway between the middle and centre circles. Marking a fourth circle will make this point easier to judge
Step 11
Continue in this way until all the slots have been cut
Step 12
Remount the assembly on the lathe and drill a 6mm (1/4in) hole right through for the axle bolt
Step 13
With a pillar drill and Forstner bit drill finger holes through both discs 20mm (3/4in) from the edge at four points around the perimeter
Step 14
Separate the two discs and remove the panel pins. Reverse the front disc and place it against the back disc so that the routed arcs cross each other. Fit the axle bolt and adjust so the discs move easily
Step 15
Fit the rubber buttons using the 6 x 50mm (1/4 x 2in) cap head bolts, thumb screws and washers. Placing a washer between each rubber and the face of the chuck will enable them to slide more easily
Step 16
The improved Longworth chuck is now complete
Alternative chuck designs
Step 17
Using four buttons to hold a smaller bowl in the chuck
If required, an Allen key can be used to hold the cap head bolts whilst the thumb screws are tightened. The cheaper prototype version, which has been made in a smaller size to fit my Axminster M300 lathe, is made using a spare Axminster chuck insert instead of a faceplate as a mounting, and uses wing nuts instead of the safer thumbscrew knobs. The buttons are made from cut down demijohn bungs recessed to house the cap head screws
10 tips for using the improved Longworth chuck
Tip 1
As with any chuck, keep your fingers out of the way of projecting parts, especially the fastenings on the back of the chuck. Even with thumbscrew knobs, getting hit with them hurts. If you have used wing nuts they can cause serious injury
Tip 2
Always let the chuck come to a stop naturally, do not use your hand to slow it down
Tip 3
When sanding or refinishing the bottom of the workpiece it is very important that you ensure to keep your fingers and any polishing cloths, etc. well clear of the rubbers
Tip 4
The Longworth chuck is intended for reverse-mounting bowls so that chucking mounts, etc. can be removed. Do not be tempted to use it for purposes for which it was not intended, such as hollowing the inside of bowls
Tip 5
Do not use excessive lathe speeds. Remember the chuck is large and its peripheral velocity will be high. Also, high speeds will increase the likelihood of damage or injury in the event of the work detaching itself from the chuck
Tip 6
The more fixing rubbers there are in place the better the chuck will grip. Always use all eight fixing rubbers whenever possible, only removing the outer four when the bowl is too small to be gripped with them in place
Tip 7
Make sure that the rubbers are butted tight up against the workpiece before tightening the thumbscrews or wing nuts in place
Tip 8
Do not over-tighten the rubbers against the workpiece as you may cause it to crack or split
Tip 9
Make sure your gouges and chisels are sharp
Tip 10
Take only light cuts. A Longworth chuck does not hold the workpiece with as firm a grip as the dovetail jaws of a scroll chuck, so over-enthusiastic cuts may cause the work to detach itself from the chuck.
