When designing parts for CNC machining, the type of hole you specify affects tooling, cycle time, inspection methods, and cost. Blind holes and through holes may seem like a simple distinction, but each presents different challenges in the machine shop. Understanding these differences helps designers communicate requirements more clearly and helps ensure parts meet specifications the first time.
What Are Blind Holes and Through Holes?
A through hole passes completely through the workpiece from one side to the other. The tool breaks through the material, creating an opening with two ends. Through holes are common for fasteners, pins, shafts, and any application requiring clearance through the entire part thickness.
A blind hole has a closed bottom. The tool machines to a specific depth without breaking through the opposite side. Blind holes are used when a fastener needs to thread into solid material, when a feature must be concealed from one side, or when the design requires a pocket or recess that doesn't penetrate the part.
The distinction matters because blind holes CNC machining requires different tooling strategies, depth control, and inspection methods compared to through holes.
Machining Considerations for Blind Holes
Blind holes introduce several machining challenges that through holes don't present. The most immediate concern is chip evacuation. When drilling or milling a blind hole, chips accumulate at the bottom of the hole with nowhere to go. Without proper peck drilling cycles or chip-breaking strategies, chips can pack into the hole, causing tool breakage, poor surface finish, or dimensional problems.
Depth Control and Tool Geometry
Controlling depth accurately is critical for blind holes. The drawing typically serves as the controlling specification, so the machinist must account for tool geometry when programming depth. Standard twist drills create a conical point at the bottom of the hole. If the print specifies a flat-bottom hole, a flat-end mill or spot drill must be used, which adds operations and time.
The depth callout on the drawing should clarify whether the dimension refers to the full depth including the drill point, or to the depth of the cylindrical portion of the hole. Ambiguity here leads to rejected parts or requests for clarification that delay production.
Tool Length and Rigidity
Deep blind holes require longer tools, which reduces rigidity and increases the risk of deflection, chatter, and poor surface finish. A hole depth-to-diameter ratio above 3:1 generally requires special attention. Ratios above 5:1 may require gun drilling, specialized deep-hole tooling, or multiple operations with progressively longer tools.
Designers should avoid specifying unnecessarily deep blind holes. If the function allows, a shallower hole or a through hole simplifies machining and improves quality.
Machining Considerations for Through Holes
Through holes are generally simpler to machine. Chips fall through the hole or are easily cleared by coolant flow. The tool exits the material, which reduces heat buildup and tool wear compared to blind holes of similar depth.
Exit Burrs and Breakthrough
The main challenge with through holes is managing the exit side. When the tool breaks through, it can create burrs or tear-out, especially in softer materials like aluminum or plastics. The size and severity of exit burrs depend on material, tool sharpness, feed rate, and whether a backing plate or sacrificial material is used.
Deburring adds time and labor. If the application requires burr-free edges, that requirement should be noted on the drawing or in the RFQ so the shop can plan appropriate processes or secondary operations.
Tolerance and Alignment
Through holes are easier to inspect than blind holes because both ends are accessible. A pin gauge or plug gauge can pass completely through the hole, and alignment or perpendicularity can be checked from either side. If positional tolerance or true position is critical, through holes generally offer more options for verification.
Inspection Differences
Inspection methods differ significantly between blind and through holes. Through holes allow direct measurement from both sides and permit light or gauges to pass through, making it easier to verify diameter, straightness, and surface condition.
Blind holes require depth measurement, which typically involves depth micrometers, pin gauges with depth stops, or coordinate measuring machines (CMM). Verifying the bottom condition of a blind hole—whether it's flat, conical, or radiused—requires either specialized gauges or optical inspection.
If a blind hole includes internal threads, inspection becomes more complex. Thread plug gauges can verify thread fit, but confirming full thread depth in a blind hole may require go/no-go gauges with specific lengths or CMM inspection if positional tolerances are tight.
When requesting a quote, specify any inspection requirements beyond standard dimensional verification. If you need a specific thread class, surface finish measurement, or certification of hole depth, include that information up front.
Design Guidelines for Holes in CNC Machined Parts
Designers can make machining easier and more cost-effective by following a few practical guidelines:
- Specify standard drill sizes when possible. Non-standard diameters require end mills or reamers, which increases setup time and tooling cost.
- Avoid unnecessary depth. Blind holes should be only as deep as the function requires. Extra depth adds cycle time and tool wear without adding value.
- Call out flat bottoms only when necessary. A standard drill point is faster and less expensive. If the bottom geometry doesn't affect function, let the shop use standard tooling.
- Consider hole location and accessibility. Holes near edges, in tight pockets, or at odd angles require special tooling or additional setups.
- Clarify depth dimensions. Specify whether depth is measured to the drill point or to the flat bottom of the hole.
- Specify thread depth clearly. For tapped blind holes, call out both the tap drill depth and the thread depth, leaving enough clearance at the bottom for the tap to run out.
Material and Hole Type Interactions
Material choice affects how blind and through holes machine. Softer materials like aluminum and brass machine quickly and produce manageable chips, but they're prone to burrs and smearing, especially at hole exits. Harder materials like stainless steel or tool steel generate more heat and wear tools faster, particularly in deep blind holes where heat dissipation is limited.
Plastics and composites present their own challenges. Some plastics melt or gum up tooling in deep blind holes. Composites can delaminate at the exit of through holes if feeds and speeds aren't controlled carefully.
When submitting an RFQ, always specify material grade and condition. "Aluminum" isn't enough—6061-T6 machines differently than 7075-T6 or 2024-T3. The material affects tool selection, cycle time, and cost.
Prototype vs Production Considerations
Prototype and production runs have different priorities when it comes to hole features. Prototype jobs often focus on fit, form, and function testing. Tolerances may be tighter in critical areas and looser elsewhere. The shop may use different tooling strategies or inspection methods depending on the quantity and the customer's priorities.
Production runs benefit from optimized tooling, fixtures, and cycle times. If a design will go into production, it's worth discussing hole features with the machine shop early. Small design changes—switching a blind hole to a through hole, adjusting depth, or relaxing a tolerance—can significantly reduce per-part cost at higher volumes.
When requesting a quote, specify the quantity and whether the job is a one-time prototype or the first of an ongoing production run. This helps the shop recommend the most cost-effective approach.
What to Include in Your RFQ
A complete RFQ helps the machine shop provide an accurate quote quickly and reduces the chance of misunderstandings. Here's a practical checklist of information to include:
- Engineering drawing or CAD file: A detailed drawing with dimensions, tolerances, and notes is essential. STEP files are useful for programming but should be accompanied by a drawing that serves as the controlling specification.
- Material specification: Include the material grade, condition, and any certifications or material test reports the customer may require.
- Quantity: Specify the number of parts needed and whether this is a one-time order or the start of a production run.
- Tolerances: Call out critical dimensions and tolerances. If standard tolerances apply, reference a standard or specify a general tolerance block.
- Surface finish requirements: Specify surface finish where it matters. If certain features need specific finishes, note them on the drawing.
- Required completion date: Provide a realistic delivery date. Rush jobs may be possible but often carry premium pricing.
- Inspection requirements: If you need dimensional reports, material certs, or specific inspection documentation, specify that up front.
- Special notes: Include any other requirements such as deburring, secondary operations, marking, or packaging.
For blind holes specifically, clarify depth measurement methods, bottom geometry, and any thread specifications. For through holes, note any deburring or edge break requirements.
Common Issues and How to Avoid Them
Many issues with holes in CNC machined parts stem from unclear specifications or assumptions that don't match shop practices. Here are a few common problems and how to prevent them:
Ambiguous Depth Callouts
A dimension that reads "0.500 depth" on a blind hole drawing can be interpreted different ways. Does it mean 0.500 to the tip of the drill point, to the shoulder where the hole becomes cylindrical, or to a flat bottom? Adding a note or a clear section view eliminates confusion.
Unrealistic Tolerances
Specifying a ±0.0005" tolerance on a deep blind hole drilled with a standard twist drill isn't practical. Drills wander, especially in deep holes. If tight positional or diameter tolerances are necessary, the hole may need to be drilled undersize and reamed or bored, which adds cost. Discuss critical tolerances with the shop before finalizing the design.
Missing Thread Information
Calling out a tapped hole without specifying thread class, depth, or whether it's a through or blind hole leads to questions and delays. Include complete thread callouts and, for blind holes, specify both tap drill depth and usable thread depth.
Ignoring Tool Access
A blind hole located in a narrow pocket or close to another feature may be difficult or impossible to machine with standard tooling. The shop may need a special tool, an additional setup, or a design revision. Reviewing tool access during design prevents surprises during quoting or production.
Request a Quote from Anco Precision
If you're working on a project that involves blind holes, through holes, or any CNC machined features, Anco Precision can help. We work with engineers, purchasing managers, and product developers to produce accurate parts from prototype through production quantities.
When you're ready to request a quote, send us your drawing or STEP file, material specification, quantity, any critical tolerances or special requirements, and your required delivery date. We'll review your project and provide a clear quote based on your specifications.
Whether you're refining a design or ready to order parts, we're here to answer questions and deliver quality CNC machining. Contact Anco Precision today to discuss your project.