Reducing Chatter In Reaming Operations
The most common problems associated with poor reamer life and hole finish can be attributed to chatter. Chatter is characterized as synchronized vibrations that are set up in the cutting tool, work piece, and machine, or a combination of vibrations in all of these elements. This vibration causes the tool to deflect against the work piece at a continuous, rapid and often irregular pace. The tools attempt to restore balance and resume its natural position against the vibration thus creating chatter. Consequently, chatter leaves poor or torn finishes and possibly tool failure.
Chatter can be caused in one of several ways and can have devastating effects on the quality of the application. Some of the more common reasons include:
1. Excessive speed
Due to the relative small amount of stock being removed from the work piece, reamers perform much better at higher feed rates and lower speeds. A general rule of thumb is to run the tool at feed rates ranging from 200-300% higher than those for drilling. Feed rates will vary depending on the material being reamed. Speed rates should be 2/3s of typical drill requirements. This will allow the tool to cut rather than burnish or rub the material. Increasing the feed rate will promote tool stability within the work piece, reducing deflection between the tool and work piece.
If chatter persists, check the rigidity of the tool in the bushing or machine. Ensure that the holder is securely placed in the spindle. Often the chatter is caused not by the tool but by some facet of the machine setup. Be sure to troubleshoot the holding mechanism for worn or loose bushings or holders. Replace any worn parts that cannot be adjusted and take out all play where adjusting can be done. It is also helpful to check the spindle and other driving parts for adequate strength. Weakened driving parts may cause deflection under the cut to be taken.
Eliminating any unnecessary overhang of the reamer or holder. Remember that using the shortest possible tool significantly increases rigidity within any tool. Shorter shanks create less vibration and reduce the danger of deflection and chatter. It is also important to select the correct style or design of the tool.
Also consider the type of material being reamed. Choose a style that provides sufficient rake or clearance. A straight fluted design is common in applications of a general purpose nature. This style is best used in a horizontal position for through holes due to its inability to lift chips from the hole. However, right hand helical flutes provide a more positive cutting face, which is advantageous in lifting chips that are free cutting and non-ferrous materials such as aluminum alloys and copper. Left handed helical flutes push the material forward requiring more thrust. This action takes up the slack in the machines setup and aids in containing chatter. Using the proper tool type will reduce chatter and also produce better surface finish.
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