Metal lathes operate on simple basic principles – a rapidly rotating workpiece and a cutting tool to shave, trim, and cut the piece into shape. But while the basic design of a lathe provides a huge amount of flexibility, it does pose some challenges when it comes to holding and cutting certain shapes.
To resolve those challenges, metal lathe operators rely on a number of specialized tools and techniques. Most of these tools aren’t particularly complex, but they provide workarounds that allow operators to cut complex or challenging pieces.
A ball-shaped problem
Most workpieces are mounted to a metal lathe “between centers,” rotating between the headstock and tailstock. With two points of contact, larger and heavier workpieces can be turned to a high level of precision.
However, mounting pieces between centers poses a challenge for certain shapes. A workpiece with two points of contact is impossible to machine into a sphere or even a hemisphere; cutting a circular shape would require cutting through one of the points of contact. Turning a decorative ball at the end of a metal post, a nose cone, or a ball bearing end requires a different solution, one that allows for a highly accurate, smooth finish on round surfaces.
Special tools for special problems
In layman’s terms, a radius is an angle or curve on a workpiece. A sphere has an external radius – the curve of the outer edge around the ball. External radii are the most obvious kind, but internal radii can be just as important. Milling or cutting an internal radius can make all the difference when it comes to the fit of a particular screw or bolt; if the radius isn’t cut correctly, the part won’t fit.
An internal radius can often be machined using a specially-shaped bit or cutting tool to achieve the desired geometry. That becomes trickier when machining an external radius, particularly a smooth sphere.
To create a smooth external radius, machinists employ a ball radius turner. This is a simple device that fixes to the tool arm of the lathe, and allows the cutting tool to pivot smoothly and evenly along a radius, removing material as it does so.
How to use a radius ball turner
Ball turners are available in a variety of shapes, sizes, and styles. Many experienced machinists make their own, customized to any particular radius they require. A quick search will show a number of options available commercially as well.
When machining an external radius, the workpiece is mounted to the headstock of the lathe with a chuck. Once secure, the radius turner is attached to the tool arm or (depending on the exact tool in use) attached to the slide rest. It is positioned on the center point of the workpiece.
In the cutting process, the machinist uses the handle on the radius turner to swivel the cutting tool along the radius of the workpiece. Each pass removes more material until the desired dimensions and finish are achieved.
Despite being somewhat unusual in appearance, the radius ball turner is an essential tool for most professional machinists and hobbyists alike, making it an excellent tool for your toolroom.
