Robert Sorby RS60 Eccentric Chuck
Thursday 20 January 2011
David Carley tests the RS60 Robert Sorby chuck and finds it to be well engineered with a good and robust design
The RS60 comes in a plastic form fitted case for easy and safe storage. Weighing in at a hefty 2.5kgs it's no lightweight, but this is all part of its integral design to give more stability, to counterbalance the workpiece when offset. The main body of the chuck has fitted to it an eccentric boss containing a bi-hexagonal socket, which is secured by two grub screws tipped with brass/phosphor bronze. When the grub screws are loosened the boss can be rotated from 0-35, in 5mm steps, the amount of offset being viewed through a window in the side of the chuck. When set at 0 the workpiece will run concentrically and roughing out can be done between centres if so wished. The simplicity of this means that any setting can be repeatable and with accuracy.
The chuck comes with a faceplate, screw chuck and a hexagonal ball drive for fixing the workpiece. All these fixings fit into the hexagonal socket in the eccentric boss and are secured with a locking screw from the rear of the boss. Also supplied are two spanners - one for a spindle adaptor, if required - and a C spanner for tightening the chuck onto the lathe. Two x Allen keys, a very
comprehensive CD, an instruction manual and a step-by-step booklet - everything you need to get started.
After reading the manual I discovered that the chuck was also capable of producing hexagons and polygons between centres, by using the ball joint combination and tailstock, the same again but tapering down, blending to the round at the tailstock. An endless set of combinations that can produce radical shapes, all you need is imagination. I attempted an offset stem goblet and an oyster box lid - as suggested by the booklet.
The instruction booklet and CD stresses safety, and this is paramount when turning offcentre. Special attention is also given to the safe toolrest position. It should also be taken into account that all lathe speeds have to be dramatically reduced and a much higher awareness of the rotating wood has to be observed. A face-shield is highly recommended, especially when into the offset positions.
The lathe I used for this test was a Record Nova DVR-XP. Fitting the supplied adaptor to the lathe - Patriot Insert 32mm (1 1/4in) x 8UNC - and then the chuck using the spanners provided proved no problem. Having selected a square piece of oak (Quercus robur) 69 x 69mm x 230mm (2 3/4 x 2 3/4 x 8in) long - hardwood is recommended - I marked out the centres at both ends and drilled a pilot hole at one end with an 8mm (5/16in) hole which was 25mm (1in) deep. I inserted the screw chuck into the eccentric boss and secured it with the washer and screw. I then fitted the eccentric boss into the chuck, set at the 0mm setting, and then tightened the two grub screws. The wood was then screwed onto the screw chuck and the tail stock brought up using a steb centre.
In the 0mm position the workpiece ran concentrically and I used a spindle roughing gouge to bring the wood down to the round to measure 63mm (2 1/2in). I then removed the tailstock and hollowed out the inside of the cup to an approximate depth of 38mm (1 1/2in) and then the outside of the cup using a 10mm (3/8in) spindle gouge. Having completed this I then turned a bead at the base of the cup using my three point tool and then the first cove with a 10mm (3/8in) spindle gouge, before finishing with a 10mm (3/8in) round-nosed scraper. I was surprised to see that during all this the workpiece remained stable and secure. As recommended, I then sanded the workpiece before continuing. I moved the toolrest out of the way
and offset the eccentric boss so that the figure 10 was visible in the viewing window and lined up with the mark on the chuck body - an offset of 10mm (3/8in). Tightening the screws up I then slowly rotated the workpiece by hand and carefully positioned the toolrest to ensure clearance before switching the lathe on. Setting the lathe speed to 250rpm I switched on and observed the workpiece, noticing the ghosting caused by the offset. Slowly increasing the speed I decided that at 1000rpm I was happy, the workpiece was balanced, and the lathe was man enough to handle the situation. Selecting my 10mm (3/8in) spindle gouge I very gingerly took my first very light cut to making my second cove. I have turned square and oblong workpieces but this was a different experience altogether, knowing that I had to remove a lot of wood before I was turning in the round again. This step had to be done with care, very slowly feeding the tool in. I suppose the first cuts could be likened to chipping the wood away; I felt more like a sculptor than a woodturner.
For the third cove an offset of 20mm (3/4in) is required. Ensuring the toolrest is moved well out of the way of the workpiece, and having set the chuck so that 20 was visible in the viewing window, I again turned the workpiece by hand. This is some offset and I have to admit I was more than slightly wary. Adjusting the toolrest to ensure clearance of the workpiece can be described as having a good arc. I set the lathe at its lowest speed - 100rpm - and started slowly to increase it until I felt happy at 750rpm. The chuck did everything expected of it. The workpiece was held safely and rotated in an arc with no worrying off-balance and wobble. It must be remembered always to carefully remove the cutting tool otherwise a 'dig in' on the side of your nicely executed cove could occur. It's quite a feeling of exhilaration when you look at the half finished workpiece knowing that all you have to do now is to repeat the process in reverse order.
Having finished the item and before oiling and parting off I decided to check the accuracy of the chuck. I reset the eccentric boss to 20 in the viewing window - the maximum offset I had used - repositioned the toolrest for clearance and switched the lathe on to 750rpm, before very carefully offering my spindle gouge up to the 20mm offset. Perfect - sweet as a nut!
The chuck is very well engineered with a good and robust design. The weight of the chuck is an essential part of its design and gives stability when in the offset modes, doing things you would not have thought possible. All you need is imagination and a little courage at the start. But once you have overcome the initial shock and providing you observe the safety rules, it's a joy to use and you will amaze your contemporaries. For basic turning, using the eccentric boss is simple, but for the more complex workpieces - overlapping circles, octagons and polyhedrons - it requires a little more assembly/disassembly of the chuck accessories. Having said that, it produces the goods.