Design for Manufacturing: Why Machining Deep Holes Increases Your Project’s Costs

At KT Tech, Inc., we’re passionate about design for manufacturing (DFM). Our aim is to make our customers as successful as possible with our efficient and cost-effective manufacturing services, and that goal requires more than advanced equipment. We have to identify and resolve potential manufacturing challenges early in the design stage, and our team’s extensive experience enables us to quickly pinpoint potential issues and provide solutions.

By sharing our DFM guidance with customers on our blog, we hope to help you proactively avoid any common pitfalls and eliminate the need for back-and-forth discussion in the design stage. In the past, we’ve covered a variety of DFM topics, including internal corners and compound radii. 

Now, we’re focusing on machining deep holes—learn why they add time and expense to your project and how you can avoid them.

What Is a Deep Hole in Machining?

In machining, a deep hole is defined by its depth-to-diameter ratio, not its total depth. For our precision CNC machining services, we typically consider a deep hole to be one with a depth-to-diameter ratio greater than five, and we find that most machinists in our industry agree with this cutoff. In machining terminology, such holes are often referred to as 5D.

5D is not a strict cutoff, of course. For holes with a depth-to-diameter ratio below this threshold, standard drilling is usually simple. Even beyond this limit, a standard drill may still be effective, but only at reduced speeds. It should be noted, however, that this slow pace will add to production time and expense. 

As hole depths reach 7 or 8 times the diameter, machining becomes extremely difficult, and 10X  is essentially impossible without extremely specialized equipment.

Why Machining Deep Holes Adds Time and Money to Your Project

In multi-axis machining, a standard twist drill tends to wander slightly as it drills. This happens for a number of reasons, such as chip interference. If a drill deflects even a small amount, the hole can become slightly crooked, and as the drill bit follows that path further, the effect is amplified.

These effects can be managed below a 5X ratio. However, as holes become deeper, the chances of deflection increase significantly.

End mills are sometimes suggested as an alternative method, but they’re rarely suitable for deep holes. These tools have limited depth capability and tend to create tapered holes.

Our Guidance for Deep Holes

In our California machine shop, we always advise customers to simply avoid deep holes when possible, as this will save on time and cost for your project. However, we realize there are, of course, many parts that require these holes. In such cases, we always try to offer several alternative machining or design strategies to achieve these features:

Gun Drills: An Effective but Costly Option for Machining Deep Holes

Gun drills are specialized drilling tools for creating deep, precise holes with small diameters. They are named for their original purpose of drilling gun barrels, which often have extreme depth-to-diameter requirements.

Gun drills offer several benefits:

• These tools are capable of achieving depth-to-diameter ratios of 20:1 or more.

• They have a self-centering capability, which reduces deflection and maintains straightness over long depths. 

• Gun drills deliver high-pressure coolant throughout the machining process to flush out chips. This helps to maintain consistent cutting conditions and prevent overheating. 

Gun drills do have one significant drawback: they can add notable costs to your parts, even in low volumes. Because they are job-specific tools, we typically have to charge that tooling cost to our customers. As such, it’s best to avoid holes that require a gun drill when possible.

Step-Down Holes for Targeted Precision

We sometimes see applications where precise dimensions are only needed at the bottom portion of a hole, such as for fastener seating or controlled fluid flow. In these cases, we recommend a two-stage approach, as depicted in the image at the top of this blog. 

First, use a larger drill bit to drill the hole up to the section requiring tight tolerances. Then, drill the small, precise hole only at the bottom section where the tight tolerances are actually required. 

This approach significantly reduces machining time and costs while maintaining the functionality you need in the critical area of the hole.

Smarter Shaft Assembly Designs 

Candid communication is one of our values, so we always inform customers that machining deep holes for shafts can be expensive and time-consuming. Instead, we suggest a more cost-efficient alternative: create a larger clearance hole with relaxed tolerances and then add precision counterbores at each end.

These counterbores can be used to install bushings or bearings that will hold and support the shaft. This approach maintains functionality for the shaft while eliminating the need for costly deep-hole machining. 

How Can We Help Your Design Process?

We’re ready to help you make small changes to your design to drastically improve your project’s bottom line. If your next part has deep holes but you’re looking to save money, let’s discuss your options. 

At KT Tech, our focus on quality doesn’t stop at DFM. From 5-axis machining services to manufacturing microparts, we exceed your expectations in every part we deliver. Request a quote and let’s discuss your next project today.