Short answer: a CNC threaded hole is a machined hole with internal threads so a screw, bolt, or threaded insert can fasten into the part. The hard part is not simply making the thread. The real quality depends on thread depth, hole location, tolerance class, material behavior, burr control, tool choice, and inspection.
For buyers and engineers, threaded holes are easy to underestimate. A part may look perfect, but if a blind thread is too shallow, a tap breaks inside the hole, a soft aluminum thread strips, or a go/no-go gauge fails during inspection, the whole assembly can stop. This guide explains how a CNC factory thinks about tapping, thread milling, inserts, and threaded hole inspection.
Key Takeaways
- Thread size is not enough. Drawings should define thread size, thread depth, drill depth, tolerance class, and whether the hole is blind or through.
- Blind threaded holes need extra planning. Tap lead-in, chip evacuation, and bottom clearance affect whether full thread depth is possible.
- Tapping is fast, but thread milling gives more control. Thread milling is often better for large threads, expensive parts, difficult materials, and critical fit.
- Thread inserts are useful in soft materials. Aluminum and engineering plastics may need inserts when repeated assembly or high pull-out strength is required.
- Inspection must match the function. Go/no-go thread gauges, depth checks, CMM location checks, and FAI help prevent batch problems.
What Is a CNC Threaded Hole?
A threaded hole is an internal screw thread cut into a machined part. It allows a bolt or screw to engage with the part directly. In CNC machining, threaded holes are common in housings, brackets, fixtures, plates, manifolds, robotics parts, medical device components, and automation equipment.
A threaded hole usually starts as a drilled hole. Then the internal thread is created by tapping, thread milling, forming, or installing an insert. The right method depends on material, thread size, depth, tolerance, production volume, and the risk level of the part.
Blind Holes vs Through Holes
A through threaded hole passes completely through the part. Chips can often exit from the other side, and the tool has more room. A blind threaded hole stops inside the material. Blind holes are more difficult because the tool needs room at the bottom, chips need a place to go, and the drawing must separate drill depth from usable thread depth.
For blind holes, a common mistake is specifying a full thread depth without allowing enough extra drill depth. A tap has a chamfered lead at the tip, so it cannot create full thread immediately at the bottom. If full thread depth is critical, the drawing should make that requirement clear.
Tapping: Fast and Economical
Tapping is the most common way to create internal threads. A tap is a cutting tool with the thread form already built into it. It cuts the thread as it rotates into the drilled hole. For standard thread sizes and production parts, tapping is efficient and cost-effective.
However, tapping has risks. In small blind holes, stainless steel, hard materials, or deep threads, the tap can break. A broken tap is difficult to remove and may scrap the part. This is especially serious when the threaded hole is created near the end of machining after many other features are already finished.
Tapping works well when the material is suitable, the thread size is standard, the depth is reasonable, and the production process is stable. For batch production, tool wear and chip control should be monitored to keep thread quality consistent.
Thread Milling: More Control for Critical Threads
Thread milling uses a rotating cutter and CNC toolpath to generate the thread. Instead of pushing a full-size tap into the hole, the tool moves in a helical path. This gives the machinist more control over fit, depth, and chip evacuation.
Thread milling is often preferred for large threads, expensive parts, hard materials, blind holes with risk, or threads that need tighter control. If a thread mill breaks, it is usually easier to remove than a broken tap because the tool is smaller than the final thread diameter.
Thread milling can also adjust thread fit by changing the toolpath. This is helpful when the thread needs to meet a specific gauge condition or when surface treatment after machining may affect the final fit.
Thread Inserts: Stronger Threads in Soft Materials
Thread inserts are installed into a machined hole to create a stronger or more durable internal thread. They are common in aluminum, magnesium, plastics, and other materials where repeated screw assembly can wear or strip the base material.
An insert can improve pull-out strength, wear resistance, and repairability. For parts that will be assembled and disassembled many times, inserts may be more reliable than directly tapped soft material.
Insert design should be planned early. The part needs enough wall thickness, proper hole depth, correct installation clearance, and a defined inspection method. If the insert sits too high, too low, or off-axis, the assembly may fail even if the thread itself is correct.
Thread Depth: Usable Depth vs Drill Depth
Thread depth is one of the most common sources of misunderstanding. A drawing may say M6 x 1.0 depth 12 mm, but the manufacturer needs to know whether 12 mm means full usable thread depth, total drilled depth, or minimum screw engagement. These are not the same thing.
| Term | Meaning | Why It Matters |
|---|---|---|
| Drill depth | Total depth of the drilled hole before threading | Must allow tool lead-in and chip space |
| Full thread depth | Depth where complete thread form is usable | Controls screw engagement strength |
| Screw engagement | Length of bolt thread engaged in the part | Affects pull-out strength and assembly safety |
| Bottom clearance | Extra space below the usable thread | Prevents the screw from bottoming out |
For critical assemblies, it is better to specify minimum full thread depth and required screw engagement rather than relying on a vague hole depth note.
Thread Tolerance and Fit
Thread tolerance controls how tightly the screw and internal thread fit together. A thread that is too tight can make assembly difficult, especially after coating or anodizing. A thread that is too loose can reduce strength, alignment, or vibration resistance.
Metric threads may use tolerance classes such as 6H for internal threads. Inch threads may use classes such as 2B or 3B. The correct class depends on assembly requirement, coating, material, and inspection method. If the part will receive surface treatment after machining, the final thread fit should be reviewed carefully.
For anodized aluminum, plating, or coating, the treatment can change thread fit. If the threaded hole must be functional after finishing, the drawing should state whether the gauge inspection applies before or after treatment. For related dimensional issues, see our article on how anodizing affects CNC part dimensions.
Material Effects on Threaded Holes
Material has a major influence on threaded hole quality. Aluminum is easy to machine, but threads can strip under repeated assembly or high load. Stainless steel is strong and corrosion-resistant, but tapping can be more difficult because of work hardening and tool wear. Brass machines cleanly and is often stable for small threads. Engineering plastics require care because threads can deform, creep, or lose strength under load and temperature.
| Material | Threading Concern | Practical Factory Advice |
|---|---|---|
| Aluminum | Soft threads, coating buildup | Consider inserts for repeated assembly; check fit after anodizing |
| Stainless steel | Tap wear, work hardening, broken tool risk | Use correct cutting fluid, speed, and tool strategy |
| Brass | Usually clean cutting | Good for small precise threads and fittings |
| Engineering plastics | Deformation, creep, weak thread strength | Use inserts or larger engagement where needed |
Hole Location and Perpendicularity
A threaded hole is not only a thread. Its location and angle matter. If a threaded hole is off-position, the screw may not align with the mating part. If the hole is not perpendicular to the surface, the screw head may not seat correctly, and the assembly can be stressed.
For precision parts, CMM inspection may be used to verify threaded hole location, pattern accuracy, and datum relationships. This is common for fixtures, housings, medical parts, aerospace components, and multi-hole mounting plates.
Burr Control and Chamfers
Threaded holes often create burrs at the entry, exit, or thread crest. Burrs can block assembly, damage screws, create contamination, or affect sealing surfaces. Through holes may create exit burrs on the opposite side of the part. Blind holes may trap chips at the bottom.
A small chamfer at the hole entrance helps screw starting and removes sharp edges. But the chamfer should be controlled. Too large a chamfer can reduce thread engagement, especially on shallow threaded holes.
Inspection: How Threaded Holes Are Verified
Threaded hole inspection should match the risk of the part. A simple non-critical thread may only need a go/no-go gauge check. A critical threaded pattern may need thread gauge inspection, depth verification, CMM location inspection, and FAI.
- Go/no-go thread gauge: confirms whether the thread accepts the correct gauge and rejects the oversize or undersize condition.
- Depth gauge: checks thread depth or drilled hole depth when engagement matters.
- CMM inspection: verifies hole position, pattern location, and datum relationships.
- Visual inspection: checks burrs, damage, coating buildup, and incomplete threads.
- FAI: confirms the first production part before batch machining continues.
- Batch checks: monitor tool wear and thread fit over repeated production.
For high-risk parts, inspection planning should happen before machining, not after parts are finished.
Common Threaded Hole Problems
| Problem | Likely Cause | Prevention |
|---|---|---|
| Screw bottoms out | Not enough drill depth or bottom clearance | Specify full thread depth and drill depth separately |
| Thread gauge fails | Wrong tool, coating buildup, tool wear, poor chip control | Check tolerance class and inspect during production |
| Broken tap | Deep blind hole, hard material, poor chip evacuation | Use proper tap, cutting fluid, or thread milling |
| Thread strips | Soft material or insufficient engagement | Increase engagement or use thread inserts |
| Burr blocks assembly | Exit burr or incomplete deburring | Add chamfer/deburring requirement |
| Hole pattern does not align | Location error or datum issue | Use CMM inspection and clear datums |
Design Tips for Better CNC Threaded Holes
- Specify thread size, pitch, tolerance class, full thread depth, and hole type clearly.
- Allow enough drill depth below full thread depth for blind holes.
- Use thread milling for large, critical, expensive, or high-risk threaded holes.
- Consider inserts for aluminum and engineering plastics when repeated assembly is expected.
- Clarify whether thread inspection happens before or after surface treatment.
- Add chamfer and deburring requirements where assembly or sealing matters.
- For hole patterns, define datums and use CMM inspection when alignment is critical.
- Use FAI for new production parts before releasing a full batch.
How Threaded Holes Affect Cost and Lead Time
A threaded hole may look like a small detail, but it can affect cost. Many small threaded holes increase cycle time. Deep blind holes increase tool risk. Tight thread tolerance may require special gauges or thread milling. Inserts add purchasing, installation, and inspection steps. Surface treatment after machining can require extra planning to maintain final fit.
The best cost-control method is to specify what matters clearly. Use standard thread sizes when possible, avoid unnecessary deep threads, and identify only the critical threaded holes that need special inspection.
Related CNC Machining Considerations
Threaded holes are closely tied to general tolerance planning. Hole position, perpendicularity, bore size, and thread fit all depend on clear datums and inspection methods. For a broader view, read our guide to CNC machining tolerances.
If threaded holes are part of a custom gear, housing, or precision assembly, it may also help to review CNC machined gears and CNC machining services.
FAQ
Is tapping or thread milling better?
Tapping is faster and economical for many standard threads. Thread milling gives more control and is often better for large threads, hard materials, blind holes, and expensive parts where broken tap risk is unacceptable.
How deep should a threaded hole be?
It depends on screw size, material, load, and assembly requirement. The drawing should define full thread depth and drill depth separately, especially for blind holes.
When should I use thread inserts?
Use inserts when the base material is soft, when screws will be installed repeatedly, or when pull-out strength and repairability matter. Aluminum and engineering plastics often benefit from inserts.
How are threaded holes inspected?
Common methods include go/no-go thread gauges, depth gauges, CMM inspection for location, visual burr inspection, and FAI for first production parts.
Can anodizing affect threaded holes?
Yes. Anodizing and other coatings can change thread fit. Critical threads should specify whether inspection applies before or after finishing.
Final Thoughts
CNC threaded holes are small features with large assembly consequences. Good threaded hole design defines not only the thread size, but also depth, tolerance class, material behavior, surface treatment, burr control, and inspection. When these details are clear, the CNC factory can choose the right process and prevent problems before production begins.