This fits straight onto the cross slide of your lathe machine for Quick Turning of the Concave Balls and Convex Curvature, not really ball (*refer to the image provided). The Head can be adjusted for different Ball Sizes. Maximum Concave Ball Diameter possible is 55 mm. Even a larger diameter than claimed could be more depending on the skills and expertise of the user. The much demanded graduations in imperial and metric measurements are now being provided as a reference scale for turning a approx ball diameter calculations. Its is important to note that the scale provided is for a close idea of the diameter of the desired ball size. It should not be referred for an exact measurement by the users. The insert tip should be adjusted to the given reading for achieving the desired size of the ball. Pls note the scale is a radius reading, which means that for any set parameter on this scale, you will get double of it as the ball diameter. Example- If you set the scale at a measurement of 15 mm, you will get a ball size of diameter 30 mm. The attachment has both Imperial as well as Metric graduations on either size of the attachment.