A tapped hole contains internal threads for a screw or stud, while a through hole is an unthreaded opening that passes through the entire part for a bolt, pin, shaft, clearance, or fluid path. The practical choice depends on how the assembly is loaded, accessed, inspected, and serviced. Use a tapped hole when the fastener must engage the part directly; use a through hole when a separate nut, bolt head, dowel, or uninterrupted passage is required.
What Is a Tapped Hole?
A tapped hole is an internal threaded hole made with a tap, thread mill, or another forming process. The thread gives a screw or stud a direct load path into the part. A callout such as M6 x 1.0 – 6H specifies nominal diameter, pitch, and internal thread tolerance; an inch callout might use 1/4-20 UNC-2B.
Tapped holes may be blind or pass completely through the part. Blind tapped holes need a specified usable thread depth, drill depth, tap chamfer, and chip pocket below the last full thread. A drawing should distinguish total depth from full thread depth so the mating screw does not bottom out before clamping the assembly.
What Is a Through Hole?
A through hole, often called a thru hole, opens through the entire part. It may be a clearance hole for a bolt, a dowel or alignment hole, a passage for a shaft, or a fluid and cable route. It normally has no internal thread, although a counterbore, countersink, spotface, chamfer, or reamed finish may be added for the mating hardware.
Through-hole size should be selected from the bolt or pin diameter, positional tolerance, coating allowance, and required clearance. A loose clearance hole improves assembly access but can reduce alignment. A close clearance or reamed hole can locate a pin accurately but costs more and needs better burr and finish control.
Tapped Hole vs. Through Hole
| Feature | Tapped hole | Through hole |
|---|---|---|
| Internal thread | Yes, for direct screw engagement | No, normally accepts a bolt, pin, shaft, or passage |
| Depth | Blind or through; full thread depth must be defined | Passes through the complete part thickness |
| Typical tools | Drill plus tap, thread mill, or forming tap | Drill, reamer, boring tool, counterbore, or countersink |
| Assembly function | Clamps directly into the component | Provides clearance, alignment, access, or a continuous path |
| Common risk | Stripped threads, tap breakage, bottoming screw | Wrong clearance, burrs, hole-position error, edge breakout |
The correct feature is determined by the joint, not by whether the part is metal or plastic. A thick aluminum housing may need a tapped hole, while a thin bracket may need a through hole and nut or insert to avoid stripping.
When to Choose a Tapped Hole
- The part must provide a compact, captive fastening point without a loose nut.
- The opposite side is inaccessible during assembly or service.
- The wall has enough thickness and strength for the required thread engagement.
- The joint needs a controlled screw preload, clean exterior, or removable cover.
- The design can protect the thread from coatings, chips, and cross-threading.
For soft metals, plastics, and thin walls, consider a threaded insert, helicoil, clinch nut, or larger boss. Directly tapping a weak wall may satisfy the drawing but fail during repeated service or over-torque.
When to Choose a Through Hole
- A bolt and nut can pass through the assembly and provide the required clamp load.
- A dowel, shaft, pin, or bearing needs a continuous path through the part.
- The feature is used for alignment, drainage, wiring, cooling, or fluid flow.
- Inspection or cleaning must access both sides of the part.
- The material is too thin or soft for durable direct threads.
Through holes also simplify some CNC operations because the tool can break through and evacuate chips. That benefit must be balanced against exit burrs, fixture clearance, and the risk of drilling into a hidden component.
Thread Depth and Wall Thickness
Thread engagement is commonly designed around the fastener diameter, but no single ratio applies to every material, load, and thread form. Steel, aluminum, brass, titanium, and engineering plastics have different shear strength and stripping behavior. Use a joint calculation or supplier review for critical loads rather than copying a generic 1.0D rule.
Maintain enough surrounding wall to prevent cracking and distortion. Threads close to an edge can break out; threads in a thin boss can ovalize under clamp load. Add a relief or chamfer at the entrance, specify minimum remaining wall, and make the tool accessible for inspection and deburring.
Design Details for Through Holes
State whether the hole is general clearance, close clearance, reamed, or a locating feature. Add a counterbore for a socket-head screw, a countersink for a flat-head screw, or a spotface for a washer or bolt head that needs a flat bearing surface. Locate the hole from functional datums and control perpendicularity when the bolt or pin must align with another feature.
For a useful clearance reference, see the metric and imperial clearance hole chart. The table should be treated as a starting point; plating, burrs, thermal expansion, and assembly access may require a different fit.
CNC Machining Processes
CNC drilling creates the pilot and diameter, tapping or thread milling creates the internal thread, and boring or reaming can improve size and straightness. Rigid workholding, correct drill-point geometry, coolant, chip evacuation, and controlled spindle synchronization reduce oversize holes, torn threads, and tap breakage.
Thread milling is valuable for large diameters, hard materials, interrupted holes, and flexible pitch requirements. A forming tap can produce strong threads in suitable ductile materials without cutting chips, but it needs the correct forming drill size and lubrication. Through holes can be drilled from one side or opened from both sides when breakout or position error must be controlled.
Review precision machining tolerances and inspection controls when hole position, datum relationships, or thread fit affects assembly performance.
Inspection and Quality Control
Inspect tapped holes with calibrated GO/NO-GO plug gauges, thread depth gauges, and a representative screw or stud. Inspect through holes with pin gauges, bore gauges, CMMs, or functional bolts depending on the tolerance. Check counterbore diameter and depth, countersink angle, chamfer, perpendicularity, and exit burrs where they affect seating.
First article inspection should record tool revision, material condition, hole and thread measurements, and the actual mating test. For batches, monitor tool wear and sample critical holes before drift causes assembly failures.
Materials, Coatings, and Service
Material strength, hardness, coating thickness, and corrosion exposure affect both feature types. Anodizing, plating, powder coating, or paint can reduce clearance and partially fill threads. Mask or chase precision threads after finishing only when the process is approved; indiscriminate rework can damage the profile or remove corrosion protection.
Stainless steel threads may gall, aluminum threads may strip, and dissimilar metals may create galvanic corrosion. Use compatible fasteners, anti-seize where appropriate, inserts, sealants, or isolation washers. Keep chips and blasting media out of blind holes and define cleaning requirements on the drawing.
Assembly and Sourcing Decisions
Choose the hole type with the complete assembly in view. A tapped hole can reduce part count and simplify service, but it concentrates load in the parent material. A through hole with a bolt and nut can provide stronger clamping and easier replacement, but it needs access to both sides and additional hardware.
When requesting a quote, send the model, drawing, thread standard, hole size, depth, coating, quantity, inspection requirement, and assembly torque. Suppliers should confirm tool access, minimum wall, tap or reamer capability, and how the feature will be protected during finishing.
For screw selection in thin sheet, see the self-tapping versus self-drilling screw guide before choosing a direct thread or a prepared through hole.
Applications
Tapped holes are common in machine bases, electronics housings, brackets, fixtures, manifolds, covers, and service panels. Through holes are common in bolted frames, dowel-located plates, shafts, bearing housings, fluid passages, cable routes, and sheet-metal assemblies. Many parts use both: tapped holes for a cover and through holes for dowels or structural bolts.
Supplier and Buyer Checklist
- State tapped or through feature, standard, size, tolerance, depth, and finish.
- Define full thread depth separately from drill depth on blind holes.
- Specify clearance class, reaming, counterbore, countersink, chamfer, and burr limits.
- Identify datums, positional tolerance, perpendicularity, and mating hardware.
- Request material certificates, gauge calibration, FAI, and batch sampling records.
Clear callouts make the feature manufacturable and keep inspection focused on what the assembly actually needs.
FAQs
1. What is a tapped hole?
A tapped hole is an internal threaded hole made to accept a screw, bolt, or stud directly.
2. What is a through hole?
A through hole is an opening that passes through the complete part and is normally unthreaded.
3. Is a tapped hole always blind?
No. A tapped hole can be blind or pass through, but blind holes require separate drill depth and usable thread-depth callouts.
4. When should I use a through hole instead of a tapped hole?
Use a through hole when a bolt, nut, pin, shaft, passage, or two-sided inspection path is required, or when the wall cannot hold a durable thread.
5. How deep should a tapped hole be?
Depth depends on fastener load, material shear strength, diameter, pitch, and engagement efficiency; critical joints need a calculation rather than a universal ratio.
6. What is the difference between drill depth and thread depth?
Drill depth includes the drill point and relief below the usable thread; thread depth is the length of full, functional internal thread.
7. Can aluminum hold a tapped hole?
Yes, when wall thickness, engagement, alloy, torque, and service cycles are suitable; inserts may improve repeated service life.
8. How are tapped holes inspected?
Use calibrated plug gauges, thread-depth checks, visual inspection, and a representative mating screw or stud.
9. Should a through hole be reamed?
Ream when the hole needs tighter size, straightness, surface finish, or locating performance than drilling can provide.
10. What should a hole callout include?
Include feature type, size, standard, tolerance, depth, thread class or clearance fit, chamfer, counterbore, finish, and inspection method.
Related DAXIN Services
DAXIN provides CNC drilling, tapping, thread milling, reaming, counterboring, countersinking, deburring, CMM inspection, first article reports, and finishing coordination for machined parts and assemblies.
Final Thoughts
A tapped hole creates direct threaded engagement; a through hole provides clearance, alignment, access, or a continuous passage. Select the feature from the real load path and assembly process, then control depth, wall thickness, position, finish, and inspection so the part performs reliably in production.
Request an Engineering Review
Send your drawing, mating hardware, material, load case, and hole requirements to DAXIN CNC for a manufacturability review.