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Don’t let the name fool you, CNC boring is a very interesting topic. In this post, we’ll be discussing everything about precision CNC boring, how it works, and why it matters to you. When it comes to fabricating precision parts, machinists will often use CNC boring to get nearly perfect dimensions for internal holes and features.

What is CNC Boring?

CNC stands for computer numeric controls. It means that the machine in question is using computers and perhaps some automation to run the machine. A non-CNC machine requires the machinist to hand-measure, position, and operate the machine. A CNC version can be controlled with a touchscreen and buttons instead. 

When it comes to boring machines, the important part of the CNC is its ability to digitally determine positions and automatically move the cutting head around to get the perfect position.

Now, let’s talk about what boring is. It’s the process of taking a hole and making it the perfect dimension and size. This is done one of three ways: 

– Spinning material is fed against a stationary bit 

– A spinning bit is fed into a stationary piece of material and moved around 

– A stationary blade is used against a spinning piece of material

This can be done horizontally, vertically, or at compound angles. The speed is typically a little slower and feed rates are lessened to reduce chatter and “noise” from the machine, improving the finish and precision of the hole.

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Comparing Boring to Drilling

Wait a second, that definition of boring sounds exactly like drilling 

— what’s the difference? Repeatability, precision, and the finished state of the hole.

A drilled hole comes with a lot of variability. After all, the bit is rotating so quickly and experiences a big moment arm as it comes in contact with the material. On top of that, a drill bit’s diameter isn’t exact to a thousandth of an inch, and it will wear down over time.

On the other hand, boring tools are specifically designed to be highly accurate, repeatable, and operate with minimal chatter.

Most of the time, a part is drilled to bring it to a “rough state”. The drilled hole will be undersized a little bit before it’s taken to the CNC boring machine. From there, the part will be CNC bored to get a diameter within mils (0.001”). 

Even better, the part can be CNC bore repeatedly and each hole will have nearly identical diameters.

If you take a standard drill press and drill ten holes into a piece of metal, you’ll notice a lot of variability on a microscale. As the tool bit chatters and dances around, holes will become nonidentical and can even become oblong. With CNC boring, this doesn’t happen.

Precision Boring vs. Standard Boring

When it comes to precision machining, the difference usually lies in what the final dimensions are, and how much deviation there is from the dimensions on the 2D designed part. High-precision machining aims to get within a few thousandths of an inch (0.001”) of the designed part. Standard machining might allow for tolerances as loose as 1/32 of an inch (0.0313”) or bigger. 

Precision boring follows the same trend. Standard boring is more akin to drilling — a drill bit will be fed into a part using a lathe.

With precision boring, a specialized boring tool might be used. This tool features a blade positioned at a downward angle. The tool is fed into an internal hole, and then carefully positioned. From there, it will slice off a small, uniform layer from the internal hole.

This process is repeated until the interior diameter reflects the designed diameter, within a few mils.

Different Types of CNC Boring

A CNC boring machine can differ quite a bit, and it all depends on what the shape is of the final hole. The stock piece can either be supported at both ends, oneend, or on the sides. 

In addition, the holes themselves can feature different geometries thanks to a CNC boring machine. For instance, imagine a counterbore through-hole in a plate of metal. The larger counterbore as well as the smaller through-hole can both be bored using a multi-step boring process.

When to Use CNC Boring

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To keep it simple, think of CNC boring as a much slower, more accurate version of drilling a hole. It’s almost exclusively used on pre-drilled, “rough” holes in a piece of material. These rough holes can be made through a casting process, a drill press, or even a mill. 

Since boring is slower and more meticulous, it adds money to the final price of the fabricated part. Since low-volume production can get expensive, it’s important to reserve CNC boring for only the most critical holes that can’t afford to have deviations. 

Specifically, holes should be CNC bore if they’re part of a tight assembly or require precision installation. If a hole has too much devi

ation, then the part can fail a precision inspection and might not fit in the assembly.


CNC boring is a high-precision manufacturing step that comes after a hole has been rough-drilled. It’s used to make parts with very accurate hole diameters that can be repeated hundreds of times. If you want to see if CNC boring is right for your next fabrication part, reach out to our pros at Rapid Axis and get a free quote. We have in-house CNC boring capabilities along with a number of CNC machines that will help us fabricate high-quality parts that meet your needs.