When is a burr really a burr?
By: David Eliston
Senior Consultant, Product Design
9th August 2017
Burrs on machined metal parts are something nobody wants, but we don’t want to pay more than we have to for their removal. So when is a burr a burr? In many ways, it is only a burr when it becomes a problem.
Burrs are created when a cutting tool interfaces with the material being cut and instead of removing it as we intended, it just moves it and the material doesn’t detach. Instead, it flows plastically and generally ends up hanging off the edges of the part. Good machinists will be able to minimise burrs by using sharp tools with the correct geometry for the material being cut. This is achieved by using optimum speeds and feeds and by carefully planning the sequence of machining operations.
On a recent project, I had to procure some 1.5 metre square installation brackets made from 10mm thick Aluminium. These were laser cut and unfinished with very ragged edges. Being straight off the machine, after a little hand-fettling they were fit for purpose.
However, on a previous project, my 150mm x 100mm x 20mm PCB housing needed to have an auto deburr chamfer operation on a computer numerical control (CNC) machine, followed by an abrasive nylon brush deburr and then a final deburr by hand under a magnifier. This was then followed up by a very detailed inspection process. This is significantly more costly but, in certain situations, is unavoidable.
With the ever denser packaging of PCBs and the narrower track and gap widths that this entails, the deburring of the PCB housings becomes critical. A burr the size of a whisker can cause a short circuit and hence product failure, or, worse still, product recalls and ongoing liability.
This photo (pictured) shows a typical burr which in this case was eliminated by adding an additional abrasive nylon brushing operation to the machining cycle. You can see the deburr chamfer on the majority of the top surface edges; however, in the vicinity of the burr it was not possible to machine the chamfer.
Burrs that are likely to detach cause different problems from those which are firmly attached. Attached burrs can prevent assembly, cut operators or damage o-rings for example. Detached burrs can cause printed circuit board short circuits, block ports or mechanisms.
So, as a designer, what can you do to minimise burrs? Here are 6 suggestions:
1. Design features which can be auto-deburred on the CNC machine with a 0.1mm chamfer on external corners.
2. Minimise the use of intersecting holes or slots and, if necessary, ensure there is access to deburr and inspect the intersection.
3. Use a free machining material that chips well when being cut.
4. Design with generous corner radii on internal features. This will allow the use of a large diameter cutter. Furthermore, it will give you the most rapid rate of material removal with minimum amount of cutter chatter.
5. Where possible, use thread forming taps rather than cutting taps. This eliminates the risk of swarf trapped in tapped holes. But be aware that the pilot hole diameter and the material type/ cutting speed/ coolant flow and type will all affect the form of the thread. Micro section the threads to check thread form and regularly gauge the core diameter to ensure that the process is consistent.
6. Work closely with your machinist to define what level of deburr you’ll need for that application. Agree inspection criteria. Review the design with them and see where improvements can be made.
And of course, come speak to us. We pride ourselves on our close relationship with our suppliers and work with them to understand their processes so we can design our parts to suit them.