Fundamentals of Jam and Cup Chucking
Tuesday 09 March 2010
John Lovatt explores the fundamentals of jam and cup chucking
This article aims to round off spherical turning, if that is the correct expression. Here, we will explore multiple turning and will then go on to explore the fundamentals of jam and cup chucking. These suggestions aim to provide you with many more options than available from shop-bought chucks alone. Useful for both repetition and one-off jobs, they can also save you time and money.
Batch turning spheres
As a rough guide, a row of eight is about as far as is sensible to go to with small spheres of say, 20mm (3/4in) diameter or smaller, with 6mm (1/4in) diameter shanks. Figure 1a shows a typical setup between centres. In practical terms, a string of 4 or 6 spheres might be wiser, particularly for really small sizes.
Flexing during turning can be a problem, but this can be counteracted by imparting axial stiffness to the workpiece with a rigid mount in the Morse taper of the lathe spindle, or by using jaws, preferably parallel, in a woodturning chuck - (as Figures 1b and 1c demonstrate).
Some chucks are designed with a variety of facilities or accessories to provide a choice of support in this sort of situation.
The process for producing multiple spheres is much the same as described for single ones, and this was explored in Woodturning (Issue 197). Very small spheres do present a little more difficultly because the facets are also very small and therefore, require a little more concentration and practice to produce even facets. However, a pair of really sharp 45 degree angled scrapers, ideally about 20mm (3/4in) wide - (see Figure 2a), are a real help, and provide a good use for old scrapers that have become too short for more general turning.
What is jam chucking?
Jam chucking is a method for holding a workpiece and relies on friction. The Oxford English dictionary describes friction as "the resistance, which any body meets in moving over another body", and by bodies we mean objects, such as a workpiece, of piece of wood, etc.
Jam chucks can be made out of metal or wood, but with the advent of modern chucking methods, jam chucks, particularly metal ones, are now very much less in evidence. In the case of wooden jam chucks this is surprising because the woodturning chucks make it so much easier to quickly produce a wide range of really useful jam chuck options, and at little or no real cost. However, the real benefit of such wooden chucks is more design freedom, and the ability to work very closely to them without risking a dangerous tool collision as well as the ability to machine a bit more of the chuck if it gets in the way of your turning.
Removing the workpiece
This is quite an easy task and can be acheived by incorporating knockout holes during the making of the chuck. Extraction of the workpiece is by means of a dowel with a chamfered end (as shown in Figure 4). Alternatively, lathes with hollow spindles simply require an access hole through the back
Making a lidded box
This method is not new by any means, but is useful when working with distinctively grained wood, as apart from little more than the width of a saw blade, there is virtually no loss of grain continuity. Moreover, it is a good example of the benefits of integrating jam chucking techniques within general workholding. Basically, it offers a discipline with some advantages of other methods and certainly provides more accurate, and controlled working.
The method for wood with less prominent grain marking can omit the insert component from the sequence.