HOW TO GET RID OF MILLING REJECTS?

Production defect - products selected at the production stage that do not meet the established requirements. Transfer of such products to the consumer is not allowed due to the presence of defects.

The best way to get rid of defects is to prevent their occurrence. To do this, let's look at what types of defects generally exist on parts machined with cylindrical end mills and end mills:

1. Faults in the shape of the final part (non-planarity, non-straightness) appear when machining workpieces with large milling depths, especially with uneven allowances with insufficient rigidity of the system (machine-tool-tool-tool-part).

2. Non-compliance with the dimensions of the workpiece due to errors of reference of the machine table movement in the required directions when setting the milling depth or due to measurement errors.

3. Location errors of machined planes (non-parallelism, non-perpendicularity) or deviations from the specified angle of inclination (for inclined planes and bevels), etc. The cause of this defect may be the following.

The cause of this defect may be incorrect installation of the workpiece both on the rotary vise and on the rotary tables of the machine. This type of defect can be observed even if the angles of rotation of the workpiece of the milling axis are correct, but if the surfaces of the table and the supporting surfaces of the vise are poorly cleaned from chips, as well as if there are burrs on the previously machined plane.

To eliminate the possibility of scrap due to errors in the location of mating planes of the part, it is necessary to pay attention to the accuracy of installation of workpieces, the accuracy of angular values, as well as the cleaning of the surfaces of the machine table and the supporting surfaces of fixtures from chips. Also be sure to check for burrs already machined surfaces of the workpiece.

4. When working with a set of milling cutters, defects may be caused by incorrect positioning of milling cutters by length on the mandrel or incorrect selection of the size of milling cutters

5. Reduced surface roughness class may also occur due to improper grinding, runout of the cutter, heavy wear or pitting of the cutting edges of the teeth, as well as the wrong choice of cutting modes and poor choice of coolant.

In this case, it is necessary to identify the cause of the defect, i.e. to check whether the type and size of the cutter, geometric parameters of the cutting part, cutting modes and, above all, feed rate per tooth, conditions of workpiece clamping, runout of the cutter, etc. are correctly selected. Many causes can be prevented by careful observation of machine and tool operation during the milling process.

6. Failure of the machined surface due to undercutting during milling.

Sometimes it is necessary to turn off the feed when the pass is not yet complete and the cutter is still rotating. Since the mandrel was loaded with cutting force and bent a little, and the cutting process is stopped, it will return to its original position (bend) under the action of elastic deformations. At the same time, the cutter will plunge into the metal a little deeper in the place that will be under the cutter at the moment when the feed is switched off

Undercutting phenomenon when milling planes

This phenomenon is called "undercutting" and results in a defective machined surface.

The phenomenon of undercutting can occur in the case of crosscut milling and a large gap in the screw-nut connection. In this case, the cutting process proceeds with uneven feed (jolts), the mandrel deflects and the cutter picks up the workpiece

the cutter tends to rotate around the tooth A with the greatest cutting thickness. If the cutting process is not stopped in time, further work can lead to rapid wear of the screw pair, damage to the machined surface of the workpiece and even to the breakage of the mandrel or milling cutter.

In such cases it is necessary to eliminate play in the screw-nut connection or to perform machining by counter milling.

And in counter milling, catching can occur if milling is done with excessive feed rate per tooth when the cutter is mounted on an unstiff mandrel, especially in non-uniform milling applications.If the "A" tooth is loaded too heavily, the cutter will tend to rotate around it and the subsequent tooth will undercut the surface to be milled.

7. Failure due to vibration during milling.

We should now look particularly carefully at the vibrations that may occur during the milling process. The presence of vibration has a negative effect not only on the roughness of the machined surface, but also significantly reduces the durability of the cutter and reduces the service life of the machine. Vibrations during milling are caused primarily by the unevenness of the milling process itself. To eliminate or reduce vibrations, the number of simultaneously operating milling teeth should be as high as possible. When milling cylindrical milling 

cutters should be observed conditions of uniform milling, when milling end milling cutters in some cases can be applied to the scheme of asymmetric milling, which reduces the intensity of vibrations.