How to reduce the reject rate in machining finished parts

Rejects are unfortunately part of production and cannot be completely eliminated. Therefore, it is important to achieve the lowest possible reject rate. In this blog post you will learn a way to reduce your reject rate.

In the production of machined parts there will always remain a small difference to the theoretical (or nominal) dimension. Therefore, it is necessary to set certain tolerances ahead of the production. Accordingly, for each part a nominal size as well as a maximum and minimum size are defined. The manufactured part must then lay within this range. Parts, that do not fit those tolerances are called rejects and can definitely not be used for the intended purpose.

What causes a high reject rate?

Various causes can lead to an increased reject rate. For example, faulty delivery by material and parts suppliers can increase the amount of rejects produced. Furthermore, errors in production, such as faulty processing of the material, can also cause significantly more rejects. It is therefore not only crucial to choose the right material for your finished parts but also to know how to machine the chosen material.

Another cause of rejects are design and drawing errors. An accurate drawing, that lists all the information the technician needs, therefore reduces the reject rate. For that reason in mechanical engineering a fitting system is used.

Fitting system

A fitting system helps to optimise production, determines tolerances and thus ensures less rejects, among other things. The system is used when two individually manufactured parts must ultimately be joined together. Such parts must be compatible and interchangeable. An example of this are gear wheels. The basic idea of the system is to shift the tolerances to the other component. In regards of the fitting system, there are two different sub-systems:

• Standard shaft, where the bore is matched to the shaft or
• Standard bore, where the shaft is matched to the bore

So for example, if you have two gear wheels, a big and a small one, you either match the small ones to one standard big one. Or you match the big ones to one standard small one.

Three types of fits

Depending on how the tolerances were selected, one of three fits results:

• Clearance fit: When the tolerances are set, so that the minimum size of the bore is always greater than the maximum size of the shaft, a clearance fit will result. This also applies if the minimum size of the bore is the same as the maximum size of the shaft. Examples of a clearance fit are: centering flanges, change gears, clutches or pump bearings.
• Press fit: When the tolerances are set, so that the maximum size of the bore is always smaller than the minimum size of the shaft, a press fit occurs. This also applies if the maximum size of the bore is the same as the minimum size of the shaft. Examples of a press fit are: bronze rims on gear bodies, handwheels or bearing bushes in housing.
• Transition fit: In the case of a transition fit, depending on the actual size of the bore and shaft, either a clearance or a press fit occurs during joining.

Fitting system and AMPCO METAL

As well as any other material, AMPCO® and AMPCOLOY® have suitable degree of tolerance. This also depends highly on the machining process as for example milling requires higher tolerances than lapping and honing.

For further information about machining, download now our «Guidelines for machining with copper based alloys».