Jaw Types, Screw Chucks and Tailstocks:
Philip Greenwood looks at the different jaw types and sizes
How many different jaws are there?
There are many but here we will look at the four most common ones I use. Why are there all the different types and sizes? The size is easy, the larger your item the larger the recess or spigot needs to be for the extra surface contact area between the work and the chuck jaws, also the weight and length from the chuck. If the contact area is too small your work will not be held securely. The types can be chosen to suit the type of item you are making from holding pen blanks to drill, to remounting jaws for bowls, etc. Do remember to buy jaws for your particular chucks; most jaws are not interchangeable for other manufacturer’s chucks. We will also look at the use of the tailstock for supporting your work.
Which type to use?
Most chucks come with 50mm jaws as standard; these jaws will hold most of the items you make, from a spigot on a bowl to holding a trinket box. If I had to put a figure on how much I use these jaws it would be around 97%. Pin jaws can be used to hold in a very small recess on a small item. They can be used to hold small round items as well as something like a small piece of timber for a finial. The last use is for holding an item that has a drill hole, this needs to be around 30mm diameter by the depth of the jaws and expanding into the hole. This is a way to hold a natural edge top bowl with tailstock support.
Shark jaws are the jaws I use for turning vases above 150mm in length; they have a very good grip due to the serrated jaws. They do need a long straight spigot, so do remember the extra waste when cutting your item to length. Check with the jaws manufacturers for guidance and specification for the jaws. The last type is the O’Donnell jaws set; this type gives me three jaw sizes with only four screws to change the jaws, a lot quicker than the eight normal ones. These give you spigot sizes of 25mm, 38mm and 50mm with recess sizes of 34mm, 50mm and 62mm.
A screw chuck can be a separate item which screws onto the lathe spindle or a screw that fixes in the chuck jaws. This is one method I use for mounting bowl blanks to turn the outside. It’s a quick method that holds well. Check what size hole you need to drill. This type of holding is best on cross grain timber, if the grain is running parallel to the lathe bed the screw can pull out. Do make sure that the item is flat against the jaw face. On larger blanks I would use a faceplate, as in a previous article. I have made a small version that fits in the chuck jaws for small items.
Do remember to check the tightness of the chuck jaws after a few minutes if using wet timber, the fibres will compress meaning the item will no longer be held tight in the chuck. I can sometimes turn the chuck key a further ¼ turn. Do this as many times as is needed. Always check the tightness of the jaws if you have left the item in the chuck for several hours before turning on the lathe and recommencing turning.
Pros and cons of using the tailstock
The tailstock can be used to give additional support to your work while using the chuck, unless you are turning a large item outboard when tailstock support is not available. The tailstock serves to reduce vibration while turning an item that is a long way from the chuck and it also takes some of the strain off the headstock bearings due to a heavy weight just being mounted on the chuck, but don’t over-tighten the tailstock as this will place lateral forces on the headstock bearings, just tighten enough to give support to your work. If you have a headstock that swivels be sure to realign the headstock with the tailstock, or you will have misalignment when you place an item in the lathe. This will show up when an item is placed in a chuck.
If an item is placed between centres you will have vibration and turning problems. Leave the tailstock in place as long as you can before removing from the work, then take light cuts only to reduce vibration. Don’t forget to remove the revolving centre from the tailstock when you have finished, if you catch your elbow on this it will hurt.