Abrasive brushes are useful tools in all sorts of applications. They’ve been around for many years and are used every day in all types of chamfering, deburring, and finishing processes but when properly applied, they can be used as a precision machining process holding incredible tolerances. Their application is often misunderstood and applied in ways that set the brush up to fail.
What is an abrasive brush?
These are brushes made from a wide variety of filaments fabricated into a wider variety of power brushes. They can be radial (wheel) brushes, disc (cup) brushes, end (face) brushes, bottle brushes, as well as other more specialized configurations.
The filament is made by mixing nylon and abrasives grain and then co-extruding the filaments. This allows the abrasive to be present throughout the filament as the brush wears, constantly presenting fresh grains as the nylon breaks down.
Common abrasives are silicon carbide (most common), but more recent arrivals include ceramic grain as well as CBN and diamond grains.
Brushes can be made to perform as you need them to. The variables include diameter, density, general construction (disc-type brushes), and trim length. Bristle filament types can be round or rectangular, straight or crimped. Each configuration has a very specific use and behavior when applied to the workpiece. Additionally, grit size, filament diameter, and concentration (ratio and size of abrasive grains relative to nylon carrier) all play a role in the end results. There are endless combinations that can be made, and minor adjustments can make significant changes in performance.
How does a nylon abrasive brush work?
Placement and control of engagement are critical for proper results. The brush works best when it passes the edge perpendicularly. It does not work as effectively when contacting the edge parallel. The filament cuts with two elements:
• Impact: This is determined by the size, shape, and trim length of the bristle itself (the nylon carrier).
• Cut: This is determined by the grit size and type of grit, as well as the concentration.
Other important aspects include placement of the brush and, most importantly, maintaining proper engagement of the brush once all process parameters are set. If the bristle can access the area requiring the work, it can succeed. If there are obstructions, the bristle may not reach the intended area. The typical response to this is more pressure or engagement, which usually leads to more problems, including improper chamfer, premature brush wear, or wearing the brush irregularly. Nylon abrasive brushes typically work only on the edges of a workpiece with the exception of the ceramic and diamond media. The radius will generate, and the base material will not be disturbed or changed. The edge will generate quickly at a small level and as it increases, the rate of cut will slow down as the edge becomes larger. Other media types are designed to be more aggressive and can remove base material when brushed heavily or not properly controlled.
Variables, Variables, Variables
Precision machining with abrasive brushes requires control over variables. This is critical. Tolerances of 0.0001” can be held with abrasive brushes. The conditions under which these tolerances are held are highly controlled. Specialized machines are available to perform these tasks and achieve these results.
Variables range from consistent and qualified brushes to controlled machine parameters that monitor and adjust the engagement, dwell time, and position of the workpiece.
Brush tolerances you may not know about. It’s all about control.
As with any cutting tool, consistency and control are key to controlling tight tolerance requirements. Brushes are often overlooked, and their tolerances are misunderstood until you are controlling a radius to a micron level.
Brushes are made with a filament that has a tolerance of ±10 percent. This can be huge in both impact results as well as access to roots and corners. A more controlled tolerance filament is available at a cost and should be employed as a part of the overall process development.
Media concentration is also allowed a ±10-percent tolerance as a standard product. This is also huge and can affect results dramatically. We have proven this in many applications using off-the-shelf brushes. To be fair, in most cases, these tolerances will not be noticed. But they will be in precision-radius applications. We source media from Europe that is verified and provided within stringent tolerances in diameter, nylon type, and concentration of media.
Brush construction varies from manufacturer to manufacturer. The methods used to construct a brush are typically proprietary, and tolerances are not generally provided. Working closely with trusted sources, brush construction can be controlled to an acceptable level. Consider a brush is made in an automated fashion. Bundles of filament are weighed and assembled and inserted into the brush body. These bundles can vary to the point of creating a very different result in the cut. Controlling the bundles and construction are also key to success.
Case in Point
Our application team was approached by a customer that had a challenging requirement. Very specialized materials used in products that fly required some unique results to be obtained. These results required the application of a radius on a square edge of a spline. The tolerances were 0.005” full radius +0.005” -0.000”. Applying a radius to these edges included several angled surfaces as well as long lengths of spline teeth. Applying this level of tolerance was not something that a manual operator can accomplish consistently by applying the part to a pedestal grinder, and this proved to be true. The variables between operators, skill level, fatigue, and the overall difficulty of the repetitive motions make this unpredictable. Although not an ultra-tight tolerance, consistency was an absolute requirement.
The part had a set of spines each with lead and trailing angles. Each of the edges needed to be chamfered equally to meet the print requirements. This is a total of 12 surfaces on each part to be chamfered.
We chose a ceramic brush to do the job. A Pferd 120 grit x 0.028” diameter crimped filament was capable of reaching the roots due to the small filament diameter. The ceramic grit cut predictably, and the process was developed. The results were that all of the edges were predictably capable within the allowable tolerances using a 1.67 Cpk.
Choosing a Suitable Machine platform to apply this level of chamfer
This is a high-demand, high-accuracy application. Developing the process is one challenge but applying the chamfers is entirely another.
We chose the CDMC zero set robot deburring machine. Using the dependable Fanuc LR 200 unit proves to be a reliable and trouble-free approach with minimal maintenance required.
Applying the brush to the workpiece required wear control as the brush wears as well as the correct placement of the brush relative to the part. The zero set is designed for such applications. The brush is applied to the work with a precision pneumatic cutting head that uses differential pressure to maintain exact levels of contact allowing the bristles to do their job. As they wear, the pressure compensates for the reduced diameter and continues to apply the exact pressure assuring results. A reversing air motor drives the brush and provides consistent RPM along with a compact size.
Control is King
This type of application requires collaborative teamwork between the customer and the machine supplier. Having capable and committed people on both sides will make success much easier to attain. All points of the process must be maintained and cannot be deviated from to maintain capable results. If brushes are not as defined, there will be problems. If the machine is not functioning as it should, there will be problems.
Defining and maintaining a level of control is the recipe for precision machining with nylon abrasive brushes.
“A brush is a brush is a brush” can be a true statement depending on the application. The end results have much to do with the understanding of how exactly the brush is, its placement to the part, and the control used to apply it to the workpiece.
To achieve the results of high tolerance control, rules must be followed. In doing so, the repeatability of high tolerances is quite within the capability of most shops.
Abrasive brush machining is a cost-effective, time-reducing method that can complement many workpiece types and solve the problems of many edge finishing challenges.