Monday 9 July 2018
This chuck sleeve is extremely easy to make. You simply turn a 'bowl' with a stepped inside. The smaller diameter is the same as the outside diameter of the chuck. The larger diameter is the distance between two opposite jaws when extended to its safe maximum travel. For the chucks I use, the measurements are 98mm and 110mm, respectively. The outside diameter of the 'bowl' is 126mm. The bowl is just deep enough to contain the chuck and the diameter of the opening in the base and should be a good fit over the hub of the back plate. The thickness of the base is very critical. When we mount the chuck on the lathe spindle, the back of the back plate hub screws up against the shoulder of the main spindle. The two surfaces should meet perfectly – this makes the chuck run true. If the bottom of the sleeve is too thick, it will stop the chuck being mounted safely and truly. When the whole assembly is satisfactory, you drill two holes in the side of the sleeve for the chuck key. The drawing opposite gives an idea of the general arrangement.
True up face and sides, then, either drill or bore the end hole suitable for the backing plate of your chuck
Reverse the wood and hold it in the chuck jaws in the hole. Just cut and open up the inside, first creating the widest area of the hollow, which must be of a width that stops the jaws from extending out of the chuck too far
Then hollow out the narrower section of the sleeve. This should only be about 3-4mm wider than the main body of the chuck. Remove from the lathe, check for fit and then measure and mark the hole positions for the chuck key operation
Once marked, drill and clean up the holes. Drill three equidistant holes for machine screws around the outer edge, tap the wood and fit the screws. Fit the whole assembly on your chuck and lock the shield in place, using the three screws. Check everything works properly before you mount the work
Background of the four-jaw chuck
When I bought my first lathe in 1959, woodturners did not use chucks. Chuck design for woodturners evolved over the last 30 years. It started with numerous variations on expanding collets, using rubber bands and spiral springs, and interestingly, it finished with the adaptation of the well-established engineering chuck. Scroll chucks, on the other hand, have been around for over a hundred years.
Woodturners often start their turning with a square piece of wood, hence four-jaw chucks became the industry standard and came up against the three-jaw version, which is common in metal turning.
Lathe operators are all too familiar with the possible problems associated with the over-opening of jaws. Most manuals warn the user to not open the jaws too wide. If you do, the jaws projecting outside the chuck body can cause an accident or can even fly out of the body, thus causing serious injuries.
0This problem is as old as the use of chucks. The magazine Popular Mechanics published an article in 1959 on machining and stated that: 'A wide rubber band cut from an inner tube – of a car – and snapped over the extended jaws reduces accident potential.'
And 40 years later they spoke abut the following innovation: 'The chuck has an inbuilt safety feature which prevents the jaws from becoming dislodged from the chuck. This feature is first in the industry.'
Chucks can be very dangerous if misused. I have seen the cast aluminium nameplate from a Graduate lathe, which was broken into two pieces by one segment of a Cole jaw, which flew across the workshop.
Most new chuck designs limit the amount of travel within safe limits to prevent the jaws from coming out of the chuck body if extended too far, but will not necessarily stop the jaws, which although stopped in their travel still extend from the chuck body and can do damage to hands and knuckles if you aren't paying attention. If you are concerned about this issue with your older type four-jaw engineering chuck, you might like to consider turning a wooden sleeve which will render your device safer without compromising its use.