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For years, users have tried to automate the removal of burrs on internal edges, using tube brushes with spiral wire construction, but the results were disappointing: tube brushes are not sufficiently aggressive and they suffer from very short operating life.
These shortcomings of tube brushes have been overcome by a new generation of nylon abrasive filament (NAF) brushes specifically designed for in-machine deburring in CNC machines. These new brushes are composed of sections, with high-filament density, stacked on top of each other, and securely mounted to a solid shaft.
The high-filament density and robust construction of these new brushes provide very aggressive deburring action for even difficult-to-deburr hard metals, as well as long life and easy integration into the machining process. The effectiveness of the brushes is further enhanced by their ability to rotate in both directions.
Eliminate waste, defects
Now, with these new high-density sectional brushes, the deburring operation can utilize the same CNC equipment that produces the burrs in the first place. Immediately after completion of drilling and boring operations, a brush can be automatically extracted from the tool changer. There is no need to refixture the part in a secondary deburring process, and therefore no WIP inefficiencies.
This arrangement requires no incremental floor space and no capital expenditure on new equipment. For the cost of a brush and some programming time, a user can institute an automated deburring process in a few hours. The reduction of labor costs can be dramatic. The extra time spent moving parts to a secondary deburring process and the operation, maintenance, and management of the secondary process are textbook ex-amples of the type of non-value-added labor waste, called muda in Japanese, that Lean Manufacturing philosophy seeks to avoid.
By eliminating deburring as a secondary process, the in-machine method has the additional benefit of creating single-point accountability for part dimensioning. This ends the bickering and finger-pointing that often occur when machining and deburring are handled by different groups in a manufacturing facility.
Even more significant for many manufacturers is the reduction of scrap, rework, and part variation. Even though the consistency of parts coming from machining processes can be very high, anything can subsequently happen to a part when it is deburred manually. Since manual deburring is at the end of the manufacturing process, defects can be extremely expensive. Scrapping a raw casting due to dimensional problems is not nearly as costly as destroying a machined part by subjecting it to manual deburring.
Another reason that manual deburring is prone to defects and inconsistent results is the difficulty of attracting and retaining competent manufacturing personnel for manual deburring positions which can be perceived as being dirty and demeaning. The repetitive motion involved in manual deburring can also cause Muscular-Skeletal-Disorders (MSD) such as Carpal Tunnel Syndrome—one more factor driving many manufacturers to automate this process.
Sectional brush design
Unfortunately, traditional tube bushes can only be rotated in a single direction, due to the spiral wire construction and its tendency to untwist. The high-density brush design has solved this problem with a design that securely grips the filaments regardless of the direction of spindle rotation. The solid stem in the new design (typically 3/8") provides a more reliable and stable connection to the collet of the machine. Alternatively, the new brushes can be built with an end-mill holder.
A second advantage of the new bushes is the high-filament density achieved by the sectional design, resulting in density ten times higher than that of the tube brush. The increased fill density allows more filament tips to contact the targeted burr. The greater fill density increases the aggression of the brush while extending brush life.
The third advantage of the new brushes is the ability to operate at significantly higher speeds. The new high-density sectional brushes can operate at speeds of 1,500rpm to 8,000rpm, depending on the filament material and the brush diameter. This is more than four times the maximum speed of tube brushes of the same diameter.
The high-density sectional brushes can be equipped with steel filaments or nylon abrasive filaments. NAF brushes are ideally suited for most in-machine automated deburring applications.
NAF brushes are highly compliant, due to their filamentary composition. With NAF brushes, abrasive grains are presented to the work via a flexible carrier, rather than being held rigidly in place (like bonded and coated abrasives). This significantly reduces the aggression of NAF brushes relative to other abrasive products with the same grit size.
When a filament contacts a burred corner, the cutting forces are high because the contact area is small. However, as soon as the filing action of the brush removes the burr and begins to erode the corner, the cutting pressure drops due to the increased contact area.
This means that a nylon abrasive brush acts aggressively when applied to an edge, and yet its aggression diminishes on flat surfaces as soon as the deburring function is performed. As a result, it refines surface finishes without measurably changing part dimensions, making them preferable to many aggressive abrasive products.
NAF brushes are much easier to automate than other metalworking tools, due to their very forgiving nature. Unlike machining and grinding, a high level of precision is not needed in most NAF brush deburring applications. This eliminates the complexities of planning for part-to-part variation arising from casting variances and fixturing tolerances. Brushes normally operate very effectively in a window of +/-0.020".
Brushes at work
Until recently, manufacturers were unable to take advantage of the powerful advantages of nylon abrasive technology for in-machine deburring of internal edges, because tube brushes were not sufficiently aggressive and long lasting.
Now with the advent of high-density sectional brushes, deburring can be performed in the most efficient manner—inside of CNC machine centers, immediately after the machining process.
In-machine deburring is not the optimal solution for every situation. It cannot be implemented when machining is the overall process bottleneck and cycle time increases equate to lost production. However, many plants have excess machining capacity and using it for deburring is more economical than purchasing dedicated deburring equipment or setting up manual deburring operations, and an easy way to improve productivity and product consistency.