Turning Oval Profile Tool Handles
Ornamental and other special purpose lathes can turn pieces with an oval profile whereas a conventional wood lathe is primarily designed to produce items with a circular profile. Nevertheless, anyone who has the capacity to use an ordinary lathe can use it to produce quite satisfactorily items like oval shaped tool handles.
Hammer handles and many other tool handles are conventionally elliptical in cross section as this oval shape seems best to guide the hand and avoid the occurrence of glancing blows. From time to time handles have to be replaced and while replacements can usually be found at hardware stores, it is not difficult to make replacement handles using a lathe.
The most basic way to turn an approximately oval profile is to turn the work on three centres. Initially the work is turned to a cylindrical profile in the normal way. The work is then moved to a supplementary centre and turned again before being moved again to a second supplementary centre and turned a third time. To achieve an approximately oval profile in the finished job the supplementary centres must be equidistant from the original centre and directly opposite each other (see Figure 1).
When using supplementary centres to achieve an ‘oval-ish’ profile, the most important issue is determining the distance from the original centre for positioning of the supplementary centres. This distance will depend on the ratio between the intended major and minor diameters of the required oval.
If the oval profile is to be used as a tool handle the ratio of the diameters needs to fall within a narrow range in order for the handle to ‘feel’ right. In looking at ovals with different ratios (see Figure 2) a ratio of 10:6.5 appears rather exaggerated or ‘pointy’. A ratio of 10:8 however gives an oval that seems not far enough removed from a circle. Ratios in between—such as 10:7 or 10:7.5—appear to be quintessentially elliptical. Nevertheless, appearances are sometimes deceptive and examination of a selection of hand tools suggests that most handles that ’feel’ right have a ratio of about 10:8 with a few getting close to a ratio of 10:7.5.
If a ratio between diameters of 10:8 is required for a tool handle, then the supplementary centres need to be placed away from the original centre by a distance of about 15% to 17% of the major diameter. For example, if the major diameter is 40mm and the minor is 32mm, then the set-over to each supplementary centre is 6 to 7mm (15% or 17% of 40mm). A set-over of more than about 17% will produce a shape that is too rectangular.
If the ratio between diameters is 10:7, then a set-over of 15% will produce a shape that is too ‘pointy’. In this instance the best set-over will be about 20% to 25% of the major diameter. (A set-over of 30% will be too rectangular).
Most tool handles (or pick-handles for that matter) are tapered from one end to the other or vary in width along their length. This means that the major and minor diameters may be different at the top from the sizes at the bottom. To maintain a consistent oval profile the set over of the supplementary centres must be a consistent percentage of the major diameter at that end of the job. For example, the diameters of a hammer handle might measure 42mm and 33mm at the grip end and 32 and 25 at the end where the hammer head will be fitted. Both of these sets of diameters have a ratio of just under 10:8. If a set-over of 17% of the major diameter is selected then the set-over at the grip end will be 17% of 42mm (equals say 7mm) while the set-over at the hammer head end will be 17% of 32 (equals about 5.5mm). In practice of course it may be difficult to lay out offsets to 5.5mm exactly especially in timber that contains bands of varying hardness. Nevertheless, it is important to get as close as possible to the correct set-over so as to minimise the hand work needed later to true-up the profile.
Obviously, the ‘three centres’ method will only give an approximation of an elliptical shape but if the set-over is carefully calculated and marked out then the shape produced will be quite close to an oval. Small adjustments (by sanding, or by using a hand scraper or spoke-shave plane) will then give a profile that is very close indeed to a true ellipse. In the best situations only a small percentage of the cross section will need to be pared away by hand to give a very satisfactory result.