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Does Your CNC Machining Facility Offer Detailed Inspection Of Finished Parts?

To ensure that the parts you have machined for your aerospace application are absolutely precise, your CNC machining facility should have inspection options in place. Parts should be sampled on a regular basis during production; your machining facility can help you set up a customized process control system if you need tighter specifications than are standard. As part of the process, one or more of the following testing procedures may be implemented.

Fluorescent Penetrant Inspection

Fluorescent penetrant inspection (FPI) is a process that works well for machined parts because they are smooth and non-porous. In this test, a fluorescent dye is applied to the surface of the part to highlight defects. The high contrast between the metal part and the dye — usually bright yellow — shows any flaws. A trained inspector can see these mistakes and quickly determine if there are any problems with the machining process used to make the part. 

It’s a popular test because it’s not expensive, takes only about 30 minutes for the dye to thoroughly penetrate any defects and can be cleaned up completely so a flaw-free part can still be used. This is a good initial test to use on a few parts out of each batch to check for problems in the machining process.

The key to the FPI process is to do a complete cleaning of each part. Debris or dust can show up as defects with this method, so if the part isn’t perfectly clean, the process won’t work. FPI is also unable to identify interior defects in parts; it only shows surface flaws.

Magnetic Particle Inspection

If the part to be tested is ferromagnetic — iron, nickel, cobalt and some related alloys — magnetic particle inspection (MPI) is a useful non-destructive test. MPI uses electricity to create a magnetic field around a part. A magnetic powder is then applied and will accumulate in larger quantities around defects, which can be seen by a trained inspector. Either alternating current (AC) or direct current (DC) can be used to create the magnetic field, but DC creates a field that allows better identification of flaws below the surface of a part.

Most MPI requires a specialized machine that can both easily magnetize and demagnetize the part being tested. Two types of testing systems exist: wet and dry. Neither is preferable, as either can show minute cracks in the metal that may impact the part’s performance. 

MPI testing is ideal for oddly-shaped and complex parts that are prevalent in aerospace applications. The test can also show cracks under paint and other thin coatings, as long as they are not metallic.

Coordinate Measuring

Use of a coordinate measuring machine (CMM) for very precise checks on the dimensions of a part is another way to locate errors in the machining process. Computers control the measuring process, which uses a probe on a three-dimensional axis to check the exact size of a part, even one that is complex. Different systems may use probes with lasers, optical detectors or white light. 

Once the dimensions are taken, the CMM can display any variations between that specific part and the ideal design. If there is enough of a variance, the part may be deemed flawed.

Other testing that may destroy the parts being examined can also help identify any issues with the machining process, but ideally, one or more of these non-destructive tests can highlight any problems.

For more information on aerospace machining or to learn more about these quality assurance tests, contact Sage Cheshire Aerospace.