Removing Microburrs from Small-Diameter, Internal and External Threads

If you read online forums you know there is much debate about identifying the optimum technique for removing the inevitable burrs created during machining of threaded parts. Internal threads – whether cut, rolled or cold-formed – often have burrs at hole entrances and exits, on thread crests, and on most slot edges. External threads on bolts, screws, and spindles have similar issues – particularly at the start of the thread.

For larger threaded parts, burrs can be removed by re-tracing the cutting path, but this increases the cycle time for each part. Secondary operations, such as heavy nylon deburring tools or butterfly brushes also can be used.

However, the challenges increase substantially when the diameter of the threaded part or tapped holes measures less than 0.125 in. In these instances, microburrs still are created but they are small enough that removal is more a matter of polishing than aggressive deburring.

At this point, in the miniature range, the choice of deburring solutions narrows considerably. Mass finishing techniques may be used, such as tumbling, electrochemical polishing and thermal deburring, but these require the parts to be sent out at additional cost and loss of time.

For many machine shops it’s preferable to keep secondary operations in-house, including deburring, either by adopting automation using CNC machines, or by using hand drills, or even manual techniques.

For these cases there are miniature brushes that — despite a tiny stem, filaments and overall dimensions – can be rotated using hand drills and even using adaptors on CNC equipment. Now available with abrasive nylon, carbon steel, stainless steel and diamond-abrasive filaments, these tools are available as small as 0.014 in., depending on the filament type.

Given the potential for burrs to affect the form, fit or function of a product, the stakes are high for products that have micro threads, including parts for watches, eyeglasses, cell phones, digital cameras, printed circuit boards, precision medical devices, and aerospace parts. The risks include misalignment of joined parts, difficulties in assembly, burrs that can become loose and contaminate hygienic systems, and even fastener failure in the field.

In short, burr removal remains a critical finishing operation.

Mass finishing techniques — Mass finishing techniques such as tumbling, thermal deburring and electrochemical polishing can be effective for removing some light burrs on small parts. Tumbling, for example, can be used to remove some burrs but it is not generally effective on the ends of threads. Furthermore, care is required to prevent mashing burrs into thread valleys, which can interfere with assembly.

When burrs are on internal threads, mass finishing techniques must be able to reach deep into internal structures.

Thermal deburring, for example, uses heat energy that approaches several thousand degrees Fahrenheit to attack burrs from all sides. Because the heat cannot transfer from the burr to the parent material, the burr is only burned down to the parent material. As such, thermal deburring does not affect any dimensions, surface finish, or material properties of the parent part.

Electrochemical polishing is also used for deburring and works by levelling out any micro-peaks or burrs. Although the technique is effective, there is still some concern that it could affect the threads. Still, generally speaking, material removal conforms to the shape of the part.

Despite the potential issues, the low cost of mass finishing still makes it an appealing process for some machine shops. However, as already noted, machine shops prefer to keep secondary operations in-house if possible.

Miniature deburring brushes — For threaded parts and machined holes less than 0.125 in., miniature metalworking brushes are an affordable tool to remove small burrs and perform internal polishing. Miniature brushes come in various small sizes (including kits), contours and materials. These tools are best suited to address tight tolerances, edge blending, deburring and other finishing requirements.  

“Machine shops come to us for miniature brushes because they do not want to outsource the parts anymore and want to do that work in-house,” said Jonathan Borden, national sales manager for Brush Research Manufacturing. “With a miniature brush, they no longer have to worry about the lead times and extra coordination to send parts out and bring them right back in.”

As a full line supplier of surface finishing solutions, BRM offers miniature deburring brushes in a variety of filament types and tip styles. The company’s smallest diameter brush measures only 0.014 in.

The miniature deburring brushes can be used by hand. However, because the brush stem wires are very fine and may bend, the developer recommends using a pin-vise. BRM offers a double-end pin vise in kits with up to 12 brushes in both decimal (0.032 to 0.189 in.) and metric hole sizes (1 mm to 6.5 mm).

The pin vises also can be used to grip the small diameter brushes to allow them to be rotated under power on a handheld drill and even on CNC machine.

Miniature brushes also can be used on external threads, to remove small burrs that may form at the start of the thread. These burrs may cause problems and should be removed, because any displaced metal may cause critical and potentially hazardous situations in industrial applications that require exceptional precision and cleanliness.

To prevent deflection of the twisted wire stem of the brush, CNC equipment can be programmed to apply the precise pressure and rotational speed.

“These kinds of deburring operations – even with very small diameter miniature brushes – can be automated,” said Borden. “You can use the tools on CNC machines using the pin vise or by making an adapter.”

There are several types of miniature brushes available today that vary not just in size, but also filament type. Carbon steel, stainless steel, brass, nylon and abrasive filled nylon are commonly used. Abrasive-filled nylon may contain silicon carbide, aluminum oxide, or diamond abrasive.

According to Borden, abrasive nylon is particularly effective for removing burrs and polishing thread peaks and flank angles in tapped aluminum holes. “If you cut a single-point thread in aluminum or the part was threaded using diamond tooling, there will be a lot of ‘fuzz’ and rough thread flank angles that need to be polished,” he explained.

Miniature stainless-steel brushes are popular for more aggressive deburring of materials like cast iron or steel, to remove chips or clear break-through burrs. Although abrasive nylon miniature brushes are available as small as 0.032 in., due to the nature of the stainless steel BRM now offers three smaller brush sizes: 0.014, 0.018, and 0.020 in.

It also supplies miniature deburring brushes with diamond-abrasive filaments for harder materials, such as hardened steel, ceramic, glass, and aerospace alloys.  

“The choice of filament depends on the surface finish specifications, or if there is a need for a little more aggressive deburring power,” said Borden.

He added that other factors applying to miniature brushes used in automated applications include RPM of the machine tool, feed rates, and optima; wear-life.

Although deburring of internal and external micro threads can be challenging, using the most suitable tools for a given application can simplify the task and assure all burrs are consistently removed on every part. In addition, by avoiding outsourcing of secondary deburring operations, machine shops can reduce turnaround time and price per part.
Jeff Elliott is a Torrance, Calif.-based technical writer. His recent contributions to AmericanMachinist.com include CBN Hones Improve Surface Finishing for Superalloy Parts and Planar Honing Offers a New Angle for Surface Finishing.