Reaming finishes a prepared hole with a multi-edge tool; boring enlarges or finishes an existing hole using a boring tool. Reaming often suits repeated holes of one size when the incoming hole is suitable. Boring offers diameter adjustment and can correct hole position when the setup, tool access, and remaining material allow it.
For precision CNC parts, choose from the required diameter, location, shape, and finish. Neither process name alone guarantees that a hole will fit or function correctly.
Reaming vs boring at a glance
Consideration | Reaming | Boring |
|---|---|---|
Main purpose | Finish a prepared hole to size and surface requirements | Enlarge, correct, or finish an existing hole |
Typical finishing tool | Multi-edge reamer | Single-edge fine-boring tool or internal turning tool |
Hole position | Generally follows the prepared hole | Can establish the required axis within the setup's capability |
Diameter changes | A fixed-size reamer targets one diameter; adjustable systems also exist | Adjustable tooling can cover a range of diameters |
Production fit | Often attractive for repeated sizes and suitable quantities | Useful for size flexibility and position-critical features |
Main planning concern | Incoming-hole quality, allowance, and alignment | Tool stiffness, cutting forces, adjustment, and access |
These are selection tendencies. The actual material, geometry, equipment, and inspection requirements decide the route.
How the two processes finish a hole
Reaming follows a prepared hole
A reamer removes a controlled amount of material from a hole that has already been drilled, bored, or otherwise prepared. Its cutting edges bring the hole toward the required diameter and finish.
Reaming allowance is the material intentionally left for that finishing cut. Too little can cause rubbing; too much can overload the tool. Use the tool maker's guidance for the selected diameter, material, and application rather than one allowance for every hole. Sandvik's reaming guidance provides material-dependent recommendations.
Reamers are not all fixed-size tools. MAPAL's reamer range includes fixed, expanding, and finely adjustable versions. That distinction matters when comparing tooling flexibility.
Boring cuts to a controlled axis and diameter
Boring removes material from an existing hole. On a machining center, a boring tool typically rotates; on a lathe, the part typically rotates around an internal turning tool.
Rough boring prepares the hole for later work. Fine boring is a finishing operation for close diameter control and surface quality. Sandvik describes finish boring as a way to achieve close tolerances and correct positioning.
Correction still needs enough material around the intended axis. If a drilled hole has already cut beyond the permitted finished boundary, boring cannot replace the missing metal.
The part's location in the fixture also matters. The machine axis must be related correctly to the drawing datums before boring can produce the required hole position.
Which process is more accurate?
Both can produce accurate holes, but diameter accuracy and position accuracy are different requirements. Separate four questions:
- Size: Is the diameter within its specified limits?
- Position: Is the hole located correctly relative to the drawing's datums—the reference features used for measurement?
- Form: Is the hole sufficiently round, straight, and cylindrical?
- Surface texture: Does the bore surface meet its specified roughness requirement?
A hole can pass its diameter check and still place a locating pin in the wrong position. A visually smooth bore can also be tapered or oversized.
Sandvik states that reaming should not be expected to correct positional or straightness errors in the prepared hole. Boring can address those requirements, but its result depends on the machine, setup, stock, and tool behavior.
For wider context, see our CNC milling accuracy guide. For this decision, ask the supplier how each critical hole requirement will be produced and checked.
Check the incoming hole and tool access
Before selecting a finishing tool, review the hole that it will enter. An uneven allowance or misaligned axis can make an otherwise suitable process unreliable.
For reaming, check the prepared diameter, alignment, straightness, and consistency of the remaining material. The workholding must also keep the part stable without distorting it.
For boring, check whether the tool assembly can enter, reach the controlled depth, and withdraw without collision. A long, slender tool is more sensitive to deflection and vibration. Chip removal becomes especially important in blind holes. Sandvik's boring application guidance discusses these access and stability factors.
A blind hole needs more detail than a diameter and drilling depth. State the depth over which the finished diameter must apply. The reamer's lead, boring insert, shoulder, and chip space can all affect how close the controlled surface reaches to the bottom.
Material matters too. Review the actual alloy and condition with the supplier; a process proven in aluminum is not automatically suitable for stainless steel or a flexible plastic part.
Reaming vs boring cost: compare the complete job
The quicker finishing cut does not always produce the cheaper accepted part. Compare:
- Hole preparation and any additional setup.
- Tool purchase, availability, setting, and replacement.
- Cutting time across all holes and parts.
- Inspection effort and required records.
- Expected rework or scrap risk.
For a prototype with several unusual diameters, suitable adjustable boring tooling may avoid buying a separate reamer for each size. That advantage depends on what tooling the supplier already has.
For a repeat order with many identical holes, a suitable reaming process may reduce machining time enough to justify dedicated tooling. Ask for quantity-tier quotes against the same drawing and inspection scope.
Compare total cost per accepted part, including preparation and verification. Request the proposed route and its cost drivers rather than assuming reaming or boring always wins.
A locating-bracket example: prototype to repeat order
Consider a hypothetical aluminum bracket with two nominal 12 mm locating holes. The pins must fit, and the hole pattern must align the bracket with a mating assembly. This is a planning example, not a customer project or capability claim.
For 12 prototypes, the supplier might propose boring the holes after machining the reference faces. That can provide diameter adjustment without committing to dedicated reamers.
For a later order of 300 brackets, the supplier could evaluate reaming to reduce repeated finishing time. The prepared holes would still need to support the position requirement. Depending on the geometry and capability, that could require boring before reaming—or retaining fine boring as the final operation.
The buyer should approve the route against measured sample parts, the inspection plan, and the complete quote. If the bracket will be anodized, agree whether the holes are masked or controlled to size after finishing.
What to specify on the drawing and RFQ
Specify the result clearly before prescribing a process:
- Finished diameter limits or the applicable fit designation.
- Position and orientation requirements relative to identified datums.
- Form and surface-texture requirements where function needs them.
- Through or blind condition, controlled bore depth, and shoulder geometry.
- Material grade, condition, coating, and the stage at which dimensions apply.
- Entry chamfers, edge limits, and restrictions on functional surfaces.
- Order quantities, sample approval, and required inspection records.
A note such as “ream to size” does not define all these requirements. If a process is mandatory for a validated design or customer specification, state that explicitly. Otherwise, allow the supplier to propose a suitable route.
Our technical-drawing checklist covers the wider RFQ package. For hole entrances and internal intersections, also review the CNC deburring guide.
Match inspection to the requirement
Agree how the supplier will verify the hole before comparing quotes.
Requirement | Inspection approach to discuss |
|---|---|
Diameter | Suitable bore gauge, internal measuring tool, or agreed limit-gauging method |
Variation through the bore | Measurements at appropriate depths and orientations |
Position and orientation | CMM or another qualified method referenced to the drawing datums |
Roundness or cylindricity | A suitable form-measurement method and enough measurements to assess the specified surface |
Surface roughness | A surface-texture measurement method with access to the specified region |
Mitutoyo's bore-measurement fundamentals explain that contact arrangements detect bore geometry differently. Instrument choice and the measurement procedure both matter.
A diameter result alone does not establish hole position. Likewise, checking size at one depth does not establish the complete bore's form. The measurement method must be accurate enough to judge the tolerance.
Specify whether acceptance applies before or after coating and in the clamped or released condition.
Frequently asked questions
Can reaming correct an off-position hole?
Generally, no. A reamer follows the prepared hole. If position needs correction, review boring or another suitable preparation step while enough material remains.
Does an H7 hole have to be reamed?
No. H7 identifies a diameter tolerance zone, not a mandatory machining process. It also does not independently define hole position or surface roughness. Select the route from the complete drawing requirements.
Is reaming always cheaper than boring?
No. Reaming can be economical for repeated holes, while suitable boring tooling can offer flexibility across sizes. Tooling, preparation, quantity, and inspection determine the comparison.
Does every precision hole need drilling, boring, and reaming?
No. Some holes can be drilled and reamed; others can be finished by boring. Combining all three operations is useful only when the incoming hole and final requirements justify it.
Choose the process around the finished hole
Start with what the hole must do, define how it will be accepted, and then compare machining routes. Reaming is worth evaluating for repeatable finishing of suitable prepared holes. Boring is worth evaluating when diameter flexibility or axis control is central to the job.
PiPrecision is a Shenzhen-based CNC machining manufacturer supporting milling, turning, and finishing projects from prototype to production. Upload your drawing and CAD files for review, including the critical hole requirements and order quantities. The proposed route and inspection scope can then be considered together.