Through holes are one of the most common and practical features in CNC machined parts. A through hole passes completely through the material, creating an open path from one side of a part to the other. This simple definition often hides important design decisions: clearance, position tolerance, edge distance, burr control, surface finish, thread engagement, and inspection method can all affect whether the final part assembles smoothly. For buyers, engineers, and product designers, understanding through holes helps reduce machining cost, avoid avoidable revisions, and communicate drawing requirements more clearly. This guide explains through holes as a CNC machining feature, including their types, functions, processes, difficulties, solutions, and common questions raised during real design and manufacturing discussions.
What Is a Through Hole in CNC Machining?
A through hole is a hole that goes completely through a workpiece. In CNC machining, it is treated as a geometric feature that must be created, measured, and finished to match the part drawing. It may be a simple clearance hole for a screw, a precision hole for a dowel pin, a passage for air or fluid, or a threaded hole used for fastening. The key point is that there is no closed bottom: the tool enters one surface and breaks through the opposite side.
Basic Definition
In engineering drawings, through holes are often called out with terms such as “THRU”, “through”, or a depth condition that clearly indicates full penetration. A through hole may be round, slotted, stepped, counterbored, countersunk, reamed, or tapped. The word describes the depth condition, not the entire manufacturing method. This distinction matters because a through hole can be produced by drilling, milling, boring, reaming, tapping, or a combination of these operations.
Why Through Holes Are Used in Machined Parts
Through holes are used because they solve mechanical, assembly, weight, and routing problems in a direct way. They are also generally easier to machine than blind holes when the design allows full breakthrough. Since the tool does not need to stop at a precise bottom depth, machining can be faster and chip evacuation is less difficult. However, the reason for using a through hole should always come from the part function, not only from manufacturing convenience.
Fastening and Assembly
The most common use is fastening. Clearance through holes allow screws or bolts to pass through one component and engage a nut or a tapped feature in another component. This is common in CNC machined housings, brackets, plates, flanges, mounts, and custom mechanical parts. When the hole is only for clearance, the diameter is usually larger than the fastener major diameter so the parts can assemble without binding.
Alignment and Locating
Through holes can also locate parts when used with dowel pins or precision shafts. In this case, the hole is not just an opening; it becomes a control feature for position and repeatability. A drilled hole may be enough for loose clearance, but a reamed or bored through hole is often preferred for tighter fits. This is why hole-making method and tolerance should match the real assembly role.
Weight Reduction and Flow Paths
Some through holes reduce weight, provide ventilation, route wires, or allow air, oil, coolant, or other media to pass through the part. In waterproof or pressure-related components, the same through condition can become a risk if the hole creates an unintended leak path. Designers should therefore separate functional passages from accidental openings and define sealing surfaces, plugs, O-rings, or thread sealant requirements when needed.
Main Types of Through Holes
Through holes can look simple in a CAD model, but they are not all the same. Their type depends on the functional requirement around the hole: clearance, threading, seating, locating, or weight reduction. A strong CNC design guide should classify through holes by both geometry and purpose, because the same nominal diameter can need very different tools, tolerances, and inspection steps.
Common Through Hole Categories
The table below summarizes the most common categories. It is useful for designers because it connects the feature name with its practical meaning, not only with a drawing symbol. In production, this distinction helps avoid over-tolerancing a simple clearance hole or under-specifying a precision locating hole.
| Type | Typical Purpose | Common CNC Method | Konstruktionshinweis |
| Clearance through hole | Allows a fastener to pass through | Drilling or circular interpolation | Choose clearance by fastener size and assembly tolerance. |
| Tapped through hole | Creates full-depth internal thread | Drill then tap or thread mill | Open bottom helps chip exit and tap clearance. |
| Reamed through hole | Provides accurate fit for pins or shafts | Drill undersize then ream | Use only where fit and location require precision. |
| Counterbored through hole | Seats a socket head or shoulder feature | Drill plus counterbore/end mill | Control depth and flat seating face. |
| Countersunk through hole | Seats a flat-head fastener | Drill plus countersink tool | Control angle, diameter, and surface condition. |
| Patterned lightening holes | Reduce weight or improve ventilation | Drilling, milling, or both | Maintain enough wall distance and stiffness. |
Threaded Through Holes
A tapped through hole is often easier than a blind tapped hole because the tap can pass through the material and chips are less likely to pack at the bottom. Thread milling is another option, especially for harder materials, larger diameters, or cases where tool breakage risk must be reduced. For production parts, the drawing should specify thread size, class or fit, and whether the thread must run full depth.
CNC Machining Processes for Through Holes
Through holes appear in CNC machining, but they are not limited to a single CNC process. The correct process depends on hole diameter, depth, tolerance, material, surface finish, and whether the hole is perpendicular, angled, or located on a curved surface. Most through holes in milled parts are created on CNC milling centers, while axial or radial holes in round parts may be made on CNC lathes with live tooling.
CNC Drilling
Drilling is the fastest and most common way to create a standard through hole. A center drill or spot drill may be used first to reduce drill wander, especially on flat but critical surfaces. The twist drill then removes material until it exits the opposite side. For deeper holes, peck drilling may be used to break chips and improve coolant access. Drilling is efficient, but it may not deliver the tightest diameter, straightness, or roundness without secondary finishing.
Milling and Circular Interpolation
Circular interpolation uses an end mill to cut a hole by moving in a circular toolpath. It is useful for non-standard diameters, larger holes, slots, and holes that need better control than a simple drill can provide. It can also reduce the need for special drill sizes. However, it takes more cycle time than drilling, and tool deflection can affect size if the setup is weak or the material is difficult.
Reaming, Boring, and Threading
When through hole accuracy is important, drilling is often only the roughing step. Reaming improves size and finish for precision fits. Boring can improve location, straightness, and roundness, especially for larger holes. Tapping creates internal threads, while thread milling cuts the thread with a controlled helical toolpath. A typical precision workflow may be spot drill, drill undersize, bore or ream to final size, chamfer, and inspect.
Design Rules for Better Through Holes
Good through hole design reduces cost without weakening the part or creating inspection problems. Many design issues come from treating every hole the same. A loose fastener clearance hole, a precision pin hole, and a sealed fluid passage should not share the same tolerance strategy. The best design starts by defining what the hole must do in the assembly, then applying only the requirements needed for that function.
Diameter and Depth Ratio
Small deep holes are more difficult than wide shallow holes. As the depth-to-diameter ratio increases, chip evacuation, drill deflection, heat, and tool breakage risk increase. Through holes are easier than blind holes because chips can exit, but deep holes still need correct drilling cycles and coolant. Designers should avoid very small holes through thick sections unless the function requires them.
Tolerance and Position
Hole diameter tolerance controls fit, while position tolerance controls assembly alignment. A common design mistake is focusing only on diameter and ignoring location. For bolt patterns, positional tolerance affects whether all fasteners can pass through at once. For dowel holes, both size and true position matter. For countersunk fasteners, location errors can become more visible because the fastener head self-centers in the cone and may pull parts out of alignment.
Wall Distance and Edge Distance
Through holes placed too close to an edge, pocket wall, slot, or another hole can create weak sections, distortion, burrs, or breakout. The correct distance depends on load, material, thickness, fastener size, and manufacturing method. As a practical DFM habit, leave enough material around each hole for strength and clamping, and clearly identify any holes that must be close to an edge for functional reasons.
Through Holes Compared With Other Hole Features
Designers often compare through holes with blind holes, tapped holes, counterbores, countersinks, and slots. These comparisons are practical because they affect cost, part function, and drawing clarity. The right choice depends on whether the hole must pass completely through the part, hide a fastener head, seal a surface, carry a thread, or locate another component. The table below focuses on discussions that commonly appear during design review and quoting.
Feature Comparison
A through hole is usually simpler when breakthrough is acceptable. A blind hole is used when the opposite side must remain closed, clean, sealed, or visually unchanged. A counterbore or countersink is not a replacement for a through hole; it is an added seating feature that may be combined with one. A slot is chosen when adjustment is more important than precise circular location.
| Merkmal | Beste Verwendung | Main Concern | Compared With Through Holes |
| Blind hole | Closed-bottom fastening or hidden feature | Depth control, chip packing, bottom clearance | More difficult when exact depth or full thread depth is required. |
| Tapped through hole | Internal threads across full thickness | Thread quality, entry/exit burrs | Still a through hole, but requires threading operation. |
| Counterbored hole | Flat seating for cap screws or shoulders | Counterbore depth and concentricity | Adds a flat-bottom seat to the opening. |
| Countersunk hole | Flush fastener head | Angle, major diameter, surface damage | More sensitive to alignment and inspection method. |
| Slot | Adjustment or tolerance compensation | End radius, width, and positional control | Less precise for round locating unless specifically designed. |
When a Through Hole Is Not Ideal
A through hole is not ideal when the far side must remain sealed, cosmetic, wear-resistant, or electrically isolated. It may also be unsuitable if breakthrough creates burrs in an inaccessible internal cavity. In those cases, a blind hole, threaded insert, boss, or redesigned fastening strategy may be better. The designer should also consider cleaning: an open hole can trap debris if it connects to a pocket or internal channel.
Key Machining Challenges for Through Holes
Although through holes are often easier than blind holes, they still create real machining challenges. The most common problems are drill wander, oversize holes, poor roundness, burrs at the exit side, misaligned hole patterns, tool deflection, and cosmetic damage around the hole mouth. These issues become more important when the hole is small, deep, close to an edge, placed on a curved surface, or used for a precision fit.
Accuracy Problems
A drill can follow the path of least resistance, especially in long holes or uneven material. If the hole must locate a pin or match another component, this can cause assembly failure even when the diameter looks acceptable. Thin plates may flex under drilling load. Hard materials and work-hardening stainless steels can increase heat and tool wear, which changes size and finish.
Burrs and Breakthrough Damage
Exit burrs are one of the most common through hole defects. As the tool breaks through the far side, the remaining material can deform and leave a raised edge. Burrs may prevent flat assembly, damage seals, scratch mating components, or interfere with threads. Burr control is especially important on sealing faces, sliding surfaces, and holes used near O-rings or gaskets.
Common Defects and Practical Fixes
| Challenge | Typical Cause | Praktische Lösung |
| Drill wander | No spotting, long tool, angled entry surface | Spot drill, use shorter rigid tools, machine a flat pad if needed. |
| Oversize hole | Tool wear, runout, wrong feed, heat | Use correct tool holder, coolant, speed/feed control, and finish reaming. |
| Poor position | Weak setup, stacked tolerance, tool deflection | Use datums, rigid fixturing, probing, and boring for critical holes. |
| Exit burrs | Breakthrough deformation | Chamfer both sides, back deburr, or use controlled deburring tools. |
| Thread damage | Chip packing, wrong tap, poor alignment | Use through-hole taps, thread milling, lubrication, and inspection gauges. |
How Manufacturers Control Through Hole Quality
Reliable through hole machining depends on process planning, not only on machine accuracy. A CNC shop usually reviews the drawing, identifies functional holes, selects the correct tooling sequence, and checks whether the tolerance is realistic for the material and geometry. Quality control should focus on the features that affect assembly, sealing, motion, or customer acceptance.
Tooling Strategy
For standard holes, a drill may be enough. For critical holes, the process may include spot drilling, drilling undersize, boring, reaming, chamfering, and final inspection. Tool selection changes with material. Aluminum may allow faster cutting, while stainless steel, titanium, engineering plastics, and hard alloys require more attention to heat, chip control, and tool sharpness. The manufacturer may also choose thread milling instead of tapping to reduce risk in expensive parts.
Inspection Methods
Inspection depends on the requirement. A simple clearance hole may be checked with calipers or plug gauges. A precision hole may need pin gauges, bore gauges, CMM measurement, or a coordinate inspection report. Threaded through holes may be checked with go/no-go thread gauges. For patterns, checking one hole alone is not enough; the relationship between holes and datums must also be verified.
Deburring and Surface Finish
Finishing is part of hole quality. Chamfers can protect edges, improve assembly, and remove sharp burrs. However, excessive chamfering can reduce bearing area or alter countersink seating. If the part will be anodized, passivated, plated, or polished, the hole edges should be compatible with the finishing process. For sealing parts, the manufacturer should avoid scratches and raised burrs around the sealing face.
Design Communication for Through Holes
Many through hole problems start before machining, during drawing or CAD communication. A model may show a hole passing through the part, but the drawing still needs enough information for manufacturing and inspection. Ambiguous notes, missing tolerances, unclear thread depth, or mixed callouts can cause quoting delays and part revisions. Clear communication is especially important when the part includes several hole types in one area.
Drawing Callouts
A good callout identifies diameter, through condition, thread specification if applicable, counterbore or countersink geometry, quantity, and tolerance. For hole patterns, use datums and positional tolerance when alignment matters. Avoid placing unnecessary tight tolerances on every hole. A clearance hole for a cover screw does not need the same tolerance as a dowel pin hole. Over-tolerancing increases cost and can limit supplier options without improving function.
Separate Functional Requirements
If a hole has both a clearance portion and a threaded portion, or a counterbore and a through portion, define each part of the feature separately. This prevents confusion about what should be threaded, what should remain smooth, and what surface should locate the mating component. When the far-side burr matters, add a deburr or edge-break note rather than assuming it will be interpreted the same way by every shop.
CAD and Quoting Files
For online CNC machining quotes, provide both 3D CAD and 2D drawings when holes are functional. The CAD model communicates geometry, while the drawing communicates tolerance, finish, thread class, and inspection intent. If the supplier only sees a model, they may treat many holes as standard features. If the supplier sees a drawing with every requirement clearly marked, they can quote the correct process and avoid unnecessary assumptions.
Applications of CNC Machined Through Holes
Through holes appear in almost every category of CNC machined component. Their importance varies by application: in a simple plate, they may only provide clearance; in a precision fixture, they may define alignment; in a housing, they may affect sealing or assembly sequence. Understanding the application helps decide whether the hole can remain standard or needs tighter process control.
Mechanical Housings and Brackets
CNC machined housings often use through holes for cover screws, mounting bolts, dowel pins, and cable routing. Brackets and mounts use them for fastening and adjustment. In these parts, edge distance and flatness around the hole are important because the fastener must clamp properly. If the surface has a coating, the design should consider whether the coating changes hole size or affects grounding, sliding, or sealing.
Flanges and Fluid Components
Flanges frequently use through holes in bolt patterns. The holes must align with mating parts, and the sealing face must remain flat and free from burrs. Fluid components may also contain functional passages, but these require more careful review because an open path can affect pressure, leakage, cleaning, and contamination control. A through hole should never be treated as harmless simply because it is easy to drill.
Precision Plates and Production Fixtures
Precision plates, tooling plates, and production fixtures use through holes for locating pins, fasteners, clamping points, and modular mounting. These parts often need repeatable hole patterns rather than just individual hole sizes. For this reason, positional inspection, datum control, and consistent deburring are important. A small error repeated across many holes can create assembly problems that are not obvious when inspecting a single feature.
Fazit
Through holes are simple in shape but important in CNC machined part design. They support fastening, alignment, routing, weight reduction, and flow paths, while also affecting tolerance, burr control, sealing, and inspection. A well-designed through hole starts with function, uses the right CNC process, and communicates requirements clearly. When designers match the hole type, tolerance, and finishing method to the real application, parts are easier to machine, inspect, assemble, and repeat in production.
FAQ
Are through holes cheaper than blind holes?
Often, yes. A through hole is usually easier to drill because the tool does not need to stop at a precise bottom depth and chips can exit more easily. However, cost still depends on diameter, depth, tolerance, material, deburring, and inspection. A precision reamed through hole can cost more than a loose blind clearance hole.
Can a through hole be threaded?
Yes. A tapped through hole has internal threads running through the part thickness or through a specified length. Because the hole is open, chip evacuation and tap clearance are usually easier than in a blind tapped hole. Thread milling may be used for larger, harder, or higher-risk parts.
Should every fastener hole be a through hole?
No. Through holes are good for bolts, screws, alignment, and easy assembly, but they are not always suitable. If the far side must remain sealed, smooth, cosmetic, or free from burrs, a blind hole or another fastening method may be better. The choice should follow the function of the part.
How tight can a CNC through hole tolerance be?
It depends on the machining method. Drilling is efficient but not the best option for tight fits. Reaming, boring, or circular interpolation can improve size, roundness, and finish. The material, depth-to-diameter ratio, setup rigidity, and inspection method all affect the realistic tolerance range.