Undercut, Deep Hole, and Overhangs: How to Handle Them

Some features look perfect in CAD but make machinists wince in real life.
Undercuts, deep holes, and overhangs are exactly those features—they push the limits of tool access, stability, and surface quality in CNC machining.

If you don’t design them with process in mind, you risk longer lead times, higher cost, or worse—“we can’t machine this” emails.

We’ve dealt with all of them—and here’s what we’ve learned.

What Exactly Makes These Features Difficult?

Let’s take each one apart:

Undercuts

These are recessed features that standard end mills can’t reach from above. Think of grooves on the underside of a pocket or T-slots inside walls.

Problem: Most CNC tools cut vertically. If the feature isn’t accessible from above or the sides, you’ll need custom tools.

Common issue: Designers forget that machinists need tool clearance in Z or side angle. No clearance = no cut.

Deep Holes

Defined as holes with depth > 6× diameter, often used for shafts, cooling channels, or structural dowels.

Problem: Chips get trapped, tools deflect, coolant doesn’t reach bottom, and heat builds up.

Result: Tapered holes, poor surface finish, broken tools.

Overhangs

Any feature that “sticks out” unsupported—like long tabs, lips, or extended flanges.

Problem: These vibrate, flex, or even snap during machining.

Also: They tend to warp after release from the fixture if not stress-relieved properly.

How We Handle These in Real CNC Jobs

Here’s how we address them—using real examples from jobs at Ekinsun.

Solving Undercuts: Go Beyond End Mills

Case: Internal cable slot inside a sensor housing (aluminum)

Issue: A 2.5 mm-wide slot hidden inside a 5 mm-thick wall. Normal end mill can’t reach it.

Solution: Switched to lollipop cutter with custom toolpath from side-entry. Repositioned part on 5-axis rotary to allow undercut from the side.

Alternative for flat parts: We’ve also used slotting tools, Dovetail cutters, or simply added EDM step when tool geometry fails.

Solving Deep Holes: Short Tools + Long Patience

Case: 6 mm diameter × 60 mm deep hole in stainless steel

Issue: Standard drill broke at 38 mm due to chip jam + deflection.

Solution:

Reduced RPM and feed;

Used peck drilling with through-coolant drills;

Reamed final hole after rough pass to restore straightness.

Rule of thumb:

For blind holes > 6×D, use peck cycle + coolant;

For anything > 10×D, consider predrill + bore from both sides if possible.

Horizontal technical infographic illustrating three challenging CNC machining features—undercuts, deep holes, and overhangs—with side-by-side diagrams showing incorrect and optimized approaches using tools like lollipop cutters, peck drilling cycles, and support fillets with tabs.

Technical diagram illustrating deep hole CNC drilling using peck cycle and coolant flow, featuring a twist drill bit penetrating a gray workpiece in vertical steps, with blue arrows indicating chip evacuation and coolant flow through the bore.

Handling Overhangs: Minimize, Fillet, Support

Case: ABS front panel with 80 mm unsupported top lip, 3 mm thick

Result: During machining, lip vibrated and left chatter marks. Post-machining, it warped upward after release.

Fix:

Added fillets at base corners to reduce stress;

Introduced temporary support tab during machining;

Suggested molded redesign for mass production.

Technical side-by-side illustration comparing unsupported and supported overhangs in CNC machining: the left diagram shows a long thin overhang with vibration marks and deflection, while the right shows the same feature with base fillets and temporary support ribs added for stability.

Quick Design Adjustments That Save Real Money

Replace undercuts with angled walls or open side slots

Add draft angles or clearance slots to allow access

Keep deep holes below 6×D or plan from both sides

Reinforce overhangs with ribs, fillets, or machining supports

Talk to your machinist early—before committing to a design that can’t spin

FAQ: We Get These Questions All the Time

Q: Can CNC machines do undercuts?
A: Yes, but only with special tools (e.g., lollipop or dovetail cutters), or 5-axis repositioning. They're slower and cost more.

Q: What’s the maximum depth I can drill in CNC?
A: As a rule, holes deeper than 6× diameter need special cycles. Beyond 10×, accuracy drops fast unless you bore from both ends.

Q: Why does my overhang warp after machining?
A: Residual stress + unsupported geometry = deformation. Adding base fillets or temporary tabs helps stabilize it.

Q: Can I avoid undercuts completely?
A: In many cases, yes—redesign the feature to be side-accessible, or use multiple parts that assemble later.

Tools & Further Reading

Minimum Wall Thickness Guidelines

Hole Pattern Layout Calculator

Fillets & Chamfers Explained

Google Sites

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