Precision Turning to Eliminate the Concentricity Error

One of the typical errors in turning is concentricity, but we can resolve this point with a little more attention: working with maximum possible precision is the golden rule to get exactly what you want.

In practical and succinct terms, we must ensure that the axis of rotation of a cylindrical surface coincides perfectly - in the moment in which we dedicate ourselves to turning - with the reference axis of the piece or the spindle.

In this way we can eliminate or reduce to infinitesimal tolerances the eccentricity. Which in technical jargon is also defined as radial runout. The whole thing is fundamental for those who deal with precision mechanical machining and manages all the parameters of turning. Do you want to discover how to manage this last parameter as well and zero out the concentricity error?

Basic Elements: What is Turning?

To understand how to solve the problem it is right to remember what are the boundaries of the field in question. Why does concentricity present itself? Simple, it is a condition inherent in the turning movement, a very well-known mechanical machining operation.

This operation consists of chip removal to produce cylinders, cones or spheres. The piece to be machined rotates around its main axis and the cutting tool moves with a feed and penetration motion. In this way the excess material is removed until the desired shape is obtained. But the concentricity problem also appears.

A Definition of Concentricity Error

In a turned piece it occurs when the center of the outer diameter and the center of the internal hole do not coincide exactly at the same point. If concentricity is the condition whereby the centers of the cross-sections of two surfaces lie on the same point along the axis, the error includes the radial offset between the theoretical reference axis and the real axis. What does this condition entail? A metal element that exhibits this error can record vibrations, abnormal bearing wear, noise and premature breakage.

What are the Causes of Concentricity Error?

Even if you employ maximum care, the concentricity error is lurking around the corner for reasons related to the physics of the machine and the way the piece is fixed. This is the concrete case: if you turn the outer diameter of a piece, then remove it, flip it and clamp it again to work on the inside, it is almost impossible to reposition it with the exact same axis. Anchoring the piece again brings with it the inaccuracies of the chuck jaws. Which is anyway responsible for any errors.

If the spindle registers an error of even just one hundredth of a millimeter, it will transmit this margin to every surface machined in different holds. All this without forgetting that the heat generated by chip removal expands the metal unevenly if cooling is not perfect. And this leads to concentricity errors anyway.

How to Reduce Concentricity Error?

Let's start with the assumption that we can reduce but not eliminate 100% the error because the material is always subject to a minimum of imperfection. But we can operate in a way to eliminate problems related to imprecise machining. Proceeding with a single clamping, therefore without repositioning the piece, is the most effective method to decrease concentricity.

If the outer diameter, inner diameter and oriented shoulders are turned in a single clamping, the rotation axis of the tool simultaneously defines all geometries. The concentricity error between the surfaces machined in that operation is theoretically equal to zero, with the only limit being the rigidity of the machine but in this regard we can use chucks with 4 independent high-precision jaws.

These components are fundamental in balancing quality machining and allow manual adjustment of the piece. This is done via hundredth or thousandth comparator before turning: precision is everything in these cases. Excessive clamping on thin-walled pieces, for example, can lead to piece deformation, zeroing out the concentricity once released from the jaws.

Obviously, the quality of the cutting tool also makes a difference. This is why we always recommend using brazed tools for turning from Krino dedicated to different materials, from steel to cast iron.