Cleaning up cross-holes

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Smoothing the edges of holes deep inside a workpiece can make for a challenging day. Here’s how to tame those pesky burrs.

The machining world is full of parts with intersecting holes. Hydraulic cylinders; pneumatic manifolds; engine blocks; machinery spindles and valve bodies. These and countless other components contain drilled, milled, reamed, or bored holes that cross the paths of others like them, often leaving the finished part looking like a block of Swiss cheese. 

Whatever the part and its configuration, it’s a fair bet that a machinist somewhere has scratched his or her head over the best way to remove burrs from its interior. Poor accessibility sits near the top of the “why this is difficult” list, especially on very small holes, but there’s also the need for precision and consistency to avoid damaging the area around the holes and quite possibly scrapping an expensive workpiece. Add to this the time needed to scrape or abrade an area that’s not only difficult to reach but might not even be visible and the question arises: isn’t there a better way?

Manual means
Fortunately, there’s no shortage of clever deburring solutions out there. A number of equipment manufacturers and cutting tool providers have tackled this common problem, developing tools and techniques for cleaning up ragged hole intersections and doing so quickly and efficiently. 

Before delving into automated and CNC-driven approaches to cross-hole deburring, it’s important to note that manual deburring techniques remain alive and well, thank you. Providers like Noga Engineering Ltd. and Vargus Ltd. continue to sell their fair share of hand scrapers and swivel blades, some of which can reach deeply enough into a workpiece that you might be tempted to stop reading and get back to work. 

There’s some merit to this temptation. For simpler parts and low-volume work, manual methods can certainly be more cost-effective and flexible than automated deburring. Also, many in the industry might agree with these companies when they say that expensive CNC machinery should be kept busy making parts, not deburring them. This is why most machinists’ toolboxes have long had a drawer devoted to these hand-held deburring devices and their various blades. 

Mounted abrasive points like these Quantum-series points from Norton | Saint Gobain are often an effective solution for internal hole deburring. IMAGE: Norton | Saint Gobain

Companies like 3M and Norton | Saint Gobain will point out the role that advanced abrasives play in machined part finishing. Both offer a wide range of sheets, pads, and wheels—3M’s Cubitron and Norton’s Quantum brands are good examples—that can be used in CNC machinery as an integral part of the manufacturing process, or as a key component in robotic and manual deburring afterwards. When these abrasives are combined with the blade-style tools just mentioned, there’s little that a skilled worker can’t deburr.  

Keep it in the machine
The problem, as many shop owners and managers will attest, is that skilled workers are hard to find, and the ones who are available should spend their time on more advanced tasks like programming, machine setup, and process optimization. This means that parts will ideally emerge from the CNC free of burrs both inside and out or sent to one of the secondary deburring processes we’ll discuss in a moment. 

The COFA from Heule Precision Tools is said to be specifically designed for front and back deburring on even and uneven bore edges using CNC machinery, as well as a wide range of cross hole deburring applications including sloped bores. IMAGE: Heule Precision Tools

Here again, machine shops have plenty of options. One of these comes from Whitney Tool Co. Inc., maker of the Burr-Zit. Often referred to as clothespin tools, these cutters round the top and bottom edges of holes in a single in-and-out pass and can be used on manual or CNC machinery. Heule Precision Tools offers several similar alternatives, among them the COFA, COFA-X, and SNAP-X deburring and chamfering tools, each designed for specific cross-hole applications. 

Spring-loaded and clothespin-style deburring tools can be used manually in a drill press or as part of the CNC machining process.  
IMAGE: Cogsdill Tool Products

Both companies have plenty more to say on the topic, and both suggest that deburring in the machine is more efficient and consistent than the manual alternatives. So does Cogsdill Tool Products Inc., which might recommend one of the company’s mechanical devices for cross-hole deburring. These include the spring-loaded Burraway and Micro Burraway, the single-piece clothespin-style Burr-Off, and the Ellipti-Bur for curved or angled surfaces. 

Before and after (at right) images of holes deburred with J.W. Done’s ORBITOOL. Image: J.W. Done

J.W. Done is another provider of deburring solutions, one that specializes in cross-holes. The company’s ORBITOOL is described as a hemispherical cutter with a polished disk on one side and a flexible shaft on the other. It works by forcing the rotating disk against the hole wall and then driving it along the surface until it reaches the intersecting hole, at which point the cutter springs outwards, thus removing the burr. 

The ORBITOOL is a hemispherical cutter with a polished disk on one side and a flexible shaft on the other. As with many deburring solutions, it is suitable for hand or machine use. IMAGE: J.W. Done

Many of the tools just described can be used in a CNC machine, a robot, or manually on a drill press or other power tool. The same is true for abrasive stones and wheels, as well as lollipop-style ball cutters, all of which are viable cross-hole deburring options. The merits and relative efficacy of each approach depend on a number of factors, starting with the workpiece configuration—clothespin-style cutters will be most effective where small holes break into large ones, while the ORBITOOL is said to deburr any hole combination, regardless of size. 

Other considerations include programming difficulty (cross-hole intersections have a complex potato chip shape), workpiece material (tough or hard metals are more difficult to deburr than aluminum and plastic), hole size, accuracy requirements, part quantity, and much more. This is why machine shops would do well to educate themselves on the available options and set aside sufficient time to master each one’s use.  

Let’s take this offline
Sometimes it makes more sense to use a secondary operation to deburr parts. Doing so eliminates the variation that comes with manual methods, and generally takes less time or reduces costs compared to deburring in the CNC. One of the most common of these offline methods, and one that most shops are at least somewhat familiar with, is vibratory deburring. 

Numerous technologies exist, but all rely on small bits of shaped abrasive media—tumbling stones—that are placed with the workpieces(s) into a drum, bowl, or barrel along with a small amount of detergent and water. The container is then vibrated, tumbled, or spun, forcing the media against the offending burr and removing it while smoothing any sharp edges. It sounds simple, and in many cases it is, but specialists like Rosler Metal Finishing strongly recommend that shops speak with a finishing expert during the project’s process development phase, rather than treating deburring as an afterthought.

The same can be said for abrasive flow machining, or AFM. Here, a pressurized, abrasive-filled, viscoelastic (putty-like) media is made to flow through holes and internal part channels, removing burrs and polishing surfaces as it passes. Other surprisingly effective technologies include electro-chemical machining (ECM), which uses an electrolytic fluid and shaped cathode to dissolve burrs deep inside workpieces, and thermal deburring (TEM), an improbable process that relies on an explosive charge to burn burrs away in an instant. 

This aerospace hydraulic manifold represents the complexity of many cross-drilled workpieces and the precision needed to deburr them. Fortunately, companies like Extrude Hone provide solutions to these and other demanding applications. IMAGE: Extrude Hone
The ROSLER- R150 DL-2 Centerless Vibratory Bowl is used in both the medical and aerospace markets for deburring high-value, high-precision parts. IMAGE: Rosler Metal Finishing

Each of these (and a few others) fall under the purview of Extrude Hone, the company that developed AFM more than half a century ago. Like vibratory deburring, each requires a fair amount of technical know-how to achieve the best results, and in most cases calls for some level of process development and collaboration with the provider, followed by investment in capital equipment. For high-volume parts, extremely complex or difficult to deburr geometries, or where no other option exists, however, they might be the only game in town. SMT

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