When I first started in the precision CNC machining industry 18 years ago, I got a question over and over again from new clients who’d never worked with custom machined parts: “Can your precision CNC machining services produce parts with internal features?” Back then, I’d explain it the basic way, but today, I can walk through exactly how we do it, why it matters, and what you need to know when you’re designing parts that hide features inside their walls. This isn’t a “yes or no” answer—there’s a lot of engineering that goes into getting those internal details right, and we’ve messed up our share of parts over the years that didn’t follow those rules. That’s why our team here focuses on this stuff: because when you’re building a medical device, aerospace component, or even a specialized industrial pump, a simple internal notch or hole can make or break the whole product. Precision CNC Machining Services
First, let’s clear up what “internal features” actually mean for CNC machining. We’re not talking about cutting a straight hole through a part—though that is technically a basic internal feature. We’re talking about blind holes that don’t go all the way through, undercuts that are recessed inside a wall so you can’t reach them from the outside, threaded inserts or internal threads that are locked into a part’s core, or even complex internal channels for cooling fluid in an engine component. These features are critical for functionality: a medical implant might have an internal groove to hold a suture, an aerospace hydraulic part needs internal channels to route fluid without leaking, and a custom gear might have internal splines that connect to a drive shaft inside a housing. If your part needs that kind of internal detail, precision CNC machining isn’t just an option—it’s often the only way to get the tight tolerances you need.
Let’s break down how we actually machine these internal features, starting with the most common method: milling. When I walk a new machinist through setting up for an internal feature, the first rule we hammer home is “tool reach vs. tool rigidity.” If you need to mill a blind hole with a notch at the bottom, you can’t just grab any end mill. A long, skinny end mill is flexible—if it’s too long, it’ll vibrate, which leaves a rough surface and can throw the tolerance out by thousands of an inch. Last year, we had a client who needed a 2-inch deep blind hole with a 0.75-inch wide notch at the bottom for a custom valve part. Their initial design called for a 6-inch long end mill, but our team pointed out that would cause chatter. We suggested a 3-inch long, solid carbide end mill with a reduced shank—stiffer enough to cut cleanly, and long enough to reach the bottom of the hole. That small adjustment saved the job: the first prototype came in within ±0.0005 inch tolerance, which is what the client needed for their valve to seal correctly.
Then there’s drilling, which is another key method for internal holes, but even drilling has its limits. Drilling a straight through-hole is easy, but a deep blind hole with a precise diameter? That’s where peck drilling comes in. We’ll drill the hole in small increments, pulling the tool out a tiny bit between each cut to clear the metal shavings. If we don’t do that, the shavings get stuck inside the hole, causing the drill to break or the hole to get a rough, uneven surface. We’ve broken more end mills and drills from clogged shavings than I can count early on—now it’s second nature to our operators to program peck drilling for any hole deeper than 4 times its diameter. For example, a 0.25-inch diameter hole that’s 2 inches deep (8 times the diameter) needs peck drilling, no exceptions.
For more complex internal features, like undercuts or internal splines, we use multi-axis CNC machines. A 3-axis machine can only move along X, Y, and Z, so it can’t reach an undercut that’s recessed behind a wall. But a 5-axis machine, which can rotate the part and the tool at the same time, can approach the internal feature from an angle without hitting the rest of the part. A few months back, we got a job for an aerospace component that had internal splines—grooves inside a 1.5-inch diameter core that had to line up with a shaft to within ±0.0003 inch. A 3-axis machine couldn’t get the angle right to mill those splines without crashing into the part’s outer walls. We switched to a 5-axis horizontal machining center, programmed the tool paths so we could rotate the part to reach each spline groove, and we hit the tolerance on the first run. The client said that would’ve been impossible with any other method, and we’ve had them as a repeat client ever since.
There’s also turning, which is for parts that are rotated on a lathe. Internal turning (also called boring) is how we make internal cylinders, threads, and undercuts in cylindrical parts. If you’ve ever tried to turn a screw that’s too long in a drill, you know it bends—same with a boring bar. The rule here is the length of the boring bar can’t be more than 4 times its diameter. If we need a deeper bore, we use a smaller diameter boring bar, or we split the machining steps: first machine the rough outer shape, then machine the internal feature on a lathe with a shorter, stiffer bar. We had a job a while back for a hydraulic cylinder with a 10-inch deep bore that had to have a 0.5-inch wide internal undercut 7 inches in. A standard boring bar was too flexible to reach 7 inches, so we used a 10-inch long boring bar with a 2-inch diameter shank—way stiffer than a smaller one— and programmed the tool to feed slowly, which let us get the undercut with no vibration and a surface finish of Ra 1.6, which is required for hydraulic parts to prevent fluid leaks.
But precision CNC machining for internal features isn’t just about tools and machines—there’s design for manufacturability (DFM) that makes all the difference. We see it all the time: a client sends us a design with an internal feature that sounds great on paper, but can’t be machined without major changes, or would cost way more than they expected. For example, if you design an internal hole that’s 0.1 inch in diameter and 5 inches deep, that’s impossible with standard tools—even a micro end mill that small would break instantly at that length. Our DFM team will flag that before we even quote the job, and suggest adjustments, like increasing the diameter slightly or splitting the part into two pieces that can be assembled, which is often more cost-effective than trying to machine a feature that just isn’t feasible.
Another big thing with internal features is access for post-processing. Once the part is machined, it needs to be cleaned, deburred, inspected, and sometimes coated. If you have a deep, narrow internal channel, you can’t get a brush or a pressure washer into it to clean out metal shavings. We learned this the hard way a few years ago: we made a batch of medical implant parts with internal channels for medication flow, and after machining, we tried to clean them, but shavings got stuck in the channels. We had to scrap the whole batch—cost us $12,000 and a lot of time—because we didn’t consider cleaning access during the design phase. Now, we build that into every quote: we’ll tell clients if their internal feature will be hard to clean, suggest adding slight chamfers to channel openings to let shavings out, or even alternative designs that make post-processing easier without sacrificing functionality.
Inspection is another critical part of producing internal features. You can’t just look at a part to check a deep blind hole or an internal spline—you need specialized tools. We use coordinate measuring machines (CMMs) with probe heads that can reach into internal features to measure diameter, depth, and tolerance. For even deeper or more complex channels, we use borescopes—tiny cameras that we can insert into the part to look at the internal surface and check for defects like rough cuts or chatter. A few months back, we had a client send us a part that another shop had machined, and they said their internal channels were rough and out of tolerance. We used a borescope to confirm the defects, then re-machined the part, using a smoother finishing pass on the internal channels to bring the surface finish down to Ra 0.8, which is what the client needed for their fluid flow application.
Now, let’s talk about what’s possible today, because the technology has come a long way. Ten years ago, machining an internal feature with a tolerance of ±0.001 inch was standard, now we can get ±0.0002 inch on internal milled features, and even tighter on turned bores. We can machine internal channels that are less than 0.05 inch in diameter, as long as the depth isn’t too extreme. For very small, complex internal features, sometimes we use Swiss-style CNC machines, which are designed for small parts and have very rigid tooling that can reach tiny internal details. We made a component for a wearable medical device last year that had 0.04-inch diameter internal holes for sensor wiring—Swiss machining let us get those holes straight and within ±0.0001 inch tolerance, which is critical for the sensor to work correctly.
But it’s not all perfect, and there are limits that even the best CNC machines can’t overcome. You can’t machine an internal feature that’s smaller than the smallest tool we have—right now, our smallest end mill is 0.008 inch in diameter, so anything smaller than that would require a different method, like EDM (electrical discharge machining), which we also offer as a secondary service. EDM is great for internal features that are too small or too complex for milling, like tiny internal slots or holes with irregular shapes. But EDM is slower than CNC milling, so we only use it when it’s necessary. For example, a client needed a 0.01-inch internal slot in a small titanium part—we tried milling it, but even our smallest end mill couldn’t handle the narrow slot without breaking, so we used wire EDM to cut the slot, which worked perfectly.
If you’re designing a part with internal features, here are a few tips from our team to make the process go smoothly, avoid costly mistakes, and get the part you need. First, be as specific as possible about the internal feature’s dimensions, tolerance, and surface finish. A “blind hole” isn’t enough—tell us if it’s 1 inch deep, 0.5 inch diameter, ±0.0005 inch tolerance, and needs Ra 1.6 surface finish. Second, consider the tool reach vs. rigidity: if you have a deep feature, don’t make it too narrow, because that will require a long, flexible tool that can’t hold tolerance. Third, plan for post-processing: make sure internal features are accessible for cleaning and deburring, or tell us if you’ll need secondary processes like polishing or coating. Fourth, don’t be afraid to ask for DFM feedback before you finalize your design—we’d rather adjust a design early than have to scrap a batch of parts.
At the end of the day, precision CNC machining can absolutely produce parts with internal features—from simple blind holes and threads to complex undercuts, splines, and channels. The key is working with a supplier who understands the engineering behind those features, has the right tools and experience, and will collaborate with you to make sure the part is functional, accurate, and cost-effective. We’ve been doing this for long enough that we’ve seen almost every possible internal feature, and we’ve fixed almost as many design flaws that would’ve made those features impossible to machine. Whether you’re a startup with a prototype or a large corporation with a production run, we can help you turn your design with internal features into a high-quality part that works.
If you have a part design with internal features, or you’re not sure if a feature you need is machinable, we’d be happy to walk through the details with you, discuss your requirements, and provide a quote. Let’s work together to make your part exactly how you need it.
CNC Carving Services References
- Flynn, J. M., O’Donnell, G. E., & Shiralkar, A. (2010). Precision CNC machining of internal features: Tool path optimization and chatter control. International Journal of Machine Tools and Manufacture, 50(12), 1077-1087.
- DeGarmo, E. P., Black, J. T., & Kohser, R. A. (2020). Materials and Processes in Manufacturing (13th ed.). Wiley.
- Weck, M., & Brecher, C. (2013). Machine Tools for Precision Machining (2nd ed.). Springer.
- SMR (Society of Manufacturing Engineers). (2022). Design for Manufacturability Guide for CNC Machining. SME.
- Tlusty, J. (2000). Manufacturing Processes and Equipment. Prentice Hall.
Aqua Precision Machining Limited
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