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    May 24, 2022

    How to Inspect Precision Machined Parts


    After machining a precision part, inspection is a critical step. It ensures that each part is dimensionally perfect and that your customer gets a part that works. Since tolerances are so tight, a simple scale or tape measure won’t work in these cases. 

    In this guide, we’ll walk you through some common ways to inspect precision machined parts. We have plenty of examples in this guide to help you get the best results.

    How to Inspect Precision Machined Parts

    When it comes to inspection, there is a lot of flexibility. In most cases, the methods you use will depend on the situation. Take a look at the following situations and see what our inspection experts recommend. 

    For Small-Batch Production

    A combination of a micrometer and caliper will work wonders for small-batch productions. Since there are only a few units to inspect, you can be more detailed and hands-on with your approach.

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    When you have to inspect a lot of parts, the process needs to be expedited. If you go through with a micrometer on every part, the turnaround time will suffer and you’ll be inconveniencing your customer.

    We suggest using a plain restriction gauge. This gets connected to a micrometer and quickly tells the inspector if the part is outside of the dimensional range or not.

    For some high-volume production runs, it might make sense to create a separate “go/no go gauge”. This is a templatized part that is built to specific dimensions. If the fabricated unit fits inside of this gauge, then it is within the tolerance. If it doesn’t fit, then it’s too large or small and does not pass.

    When done correctly, this gauge can save countless manhours during inspection.

    Go/No Go Gauge Example

    As an example, consider a machined marble. It needs to be within 0.295” and 0.305”. You could take a piece of bar stock, and bore out a hole that’s 0.295” and another hole that’s 0.305”. From there, every marble that you machine will be dropped through both of these holes.

    If the marble doesn’t go through the small hole and does go through the larger one, then it passes. If this doesn’t happen (if the marble is 0.280” or 0.315”, for example), then the marble is dimensionally incorrect and cannot be sent to the customer.

    For High-Precision Parts

    When you’re making high-precision parts, a set of calipers might not be sensitive enough. In this case, you should use a machine like an optical comparator, otherwise known as a profile projector.

    This consists of a camera and screen. It magnifies the part and performs a tolerance check on a much smaller scale.

    The inspector simply positions the machined part into this equipment, and then inspects the screen to ensure the part is within tolerance.

    Inspecting Surface Roughness

    Surface roughness can be measured with either a profile method or laser scanner. With a profile method, an arm drags along the surface of the part. The motion is then graphed based on the motion of the needle on the arm. As the arm goes up over a peak, the graph will also go up.

    With a laser scanner, a contactless laser will scroll over the top of the part. It uses optics to digitally capture the roughness of the part and report back.

    As long as there isn’t a strict requirement for roughness, the profile method is the less expensive option that works well.

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    Whenever you inspect a part, you should record the data. Even if your customers don’t care, it will help grow trust and strengthen the relationship. If nothing else, it’s a paper trail proving that the part is within the allowable tolerances.

    Different companies opt for different inspection sheets, but it’s best to come up with a template. Fill out the paperwork as you take your measurements.

    Repeat Your Inspection Steps

    You might have some customers that require you to take and record repeated inspections for each batch.

    When you repeat the inspection steps, you are taking out some of the potential human error. It also gives you a few measurement values that you can then average and check against the part’s specifications.

    It might be best to have two or three inspectors measure and record each part. By doing this, you’ll get a reliable final figure.

    Consider Inspecting On-Tool 

    In any of these cases, it might be better to perform on-tool inspection. This means that you would take measurements before removing the material from the CNC machine.

    Since the part is already oriented and zeroed, you will be able to correct any problems you find. If you remove the part, you might have to scrap it after finding a dimensional deviation. On-tool inspection will save you time and money in these events.


    There you have it, a few ways to inspect precision machined parts. Use this guide to improve your machining process — cutting down on time spent, improving the quality of parts your customers receive, and saving you money.

    For more of your machining questions answered, you can reach out to our experts at Rapid Axis. We’re a full-scale production shop with 3D printing, CNC machining, and molding capabilities. Contact us for a free quote today

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