What Is Reaming in CNC Machining?
Reaming is the finishing step used after drilling, boring or interpolation has already created a hole. The reamer removes a smaller, planned stock allowance and finishes the bore to an agreed size, form and surface requirement.
The process is valuable only when the incoming hole is suitable. A reamer cannot compensate for every upstream error, and nominal tool diameter alone does not determine the result. The drill or pre-machining process, reamer design, holder, spindle, workpiece, coolant and inspection method operate as one system.
Reaming is commonly considered when a production drawing requires a more controlled bore than drilling alone can deliver. It may be unnecessary for clearance holes or features with broad acceptance limits.
What Reaming Changes in a Hole
| Bore characteristic | What reaming can change | Practical limit |
|---|---|---|
| Diameter | Bring a controlled pre-hole to the required finished size | The result depends on allowance, runout, wear and measurement method |
| Roundness | May improve minor roundness variation when the pre-hole, alignment and stock allowance are stable | A severely lobed, distorted or interrupted pre-hole may remain unstable |
| Cylindricity | Improve consistency along the bore when guidance and stock are controlled | Severe bow, taper or wall movement may require another process |
| Surface finish | Produce a more uniform finished surface through stable edge engagement | Recut chips, built-up material, chatter or poor coolant delivery can damage the surface |
These improvements must be verified against the drawing. A diameter reading does not by itself prove roundness, cylindricity, position or surface finish.
What Reaming Cannot Correct
A conventional reaming section normally follows the existing hole. It can refine a qualified pre-hole, but it is not a universal repair process.
| Existing condition | Can conventional reaming correct it? | Better action |
|---|---|---|
| Major position error | Usually no | Correct the drilling process, fixture or datum; consider a process capable of positional correction |
| Severe straightness error | Usually no | Review boring, guidance or upstream hole creation |
| Unstable pre-hole diameter | Not reliably | Stabilize drilling or boring before changing reamer geometry |
| Poor workholding | No | Correct part support, clamping and datum control |
| Excessive or uneven allowance | Not consistently | Control the measured incoming hole and define an acceptable stock band |
| Chip packing or recutting | No | Correct flute, coolant and evacuation conditions |
If location or alignment is the primary problem, review Reaming vs Boring for Precision Holes. Boring can offer more control over size and, in suitable setups, location.
Where Reaming Fits After Drilling
| Operation | Primary task | Main limit |
|---|---|---|
| Drilling | Create a hole from solid material efficiently | May not produce the final size, form or surface finish by itself |
| Reaming | Finish an existing, qualified pre-hole | Normally follows the pre-hole and offers limited positional correction |
| Boring | Enlarge or finish a hole with an adjustable or single-point path | May require more cycle time, setting and machine capability |
The drawing and the observed failure mode determine the route. The Reamer vs Drill Bit guide covers the drilling-versus-reaming boundary in detail. For an aluminum housing bore, for example, the drill must first create a stable hole; the reamer then becomes a candidate only after position, allowance, material, volume and inspection requirements are understood.
Reaming Allowance and Pre-Hole Requirements
Reaming allowance is the material intentionally left for the finishing cut. Too little stock can cause rubbing or incomplete cleanup. Too much stock can increase cutting load, chip volume, heat and deflection.
There is no universal allowance for every diameter and material. Record the measured minimum, maximum and typical pre-hole diameters rather than relying only on nominal drill size. The acceptable band must reflect the material, hole depth, interruptions, wall condition, reamer design, runout and coolant delivery.
Use the Reaming Allowance and Pre-Hole Size Guide for the detailed engineering method.
Main Production Reamer Types
| Reamer family | Typical production use | Selection boundary |
|---|---|---|
| Machine or chucking reamer | General CNC and machine reaming | Match construction and geometry to the material and hole |
| Solid carbide reamer | Rigid production setups and suitable workpiece materials | Confirm supplier capability and the complete application |
| PCD reamer | Repeat aluminum and suitable non-ferrous production | Consider when abrasive wear, volume and cost per accepted bore justify a custom tool |
| Step or multi-diameter reamer | Related coaxial finished features | Each stage needs access, guidance and chip space |
| Adjustable or replaceable-head reamer | Applications requiring setting or replaceable cutting units | Define adjustment, changeover and inspection control |
Manual hand reaming is outside the production-CNC scope of this guide. For a broader category comparison and XRZ’s supply boundary, see the Reamer Tool selection page. XRZ’s published product focus includes drawing-based Custom PCD Reamers for suitable applications.
Speed, Feed, Coolant and Runout
Reaming parameters must be validated for the actual material, tool, machine and acceptance method. Generic cutting data can provide a starting window, but it is not a production guarantee.
Review cutting speed, feed, entry condition, assembled runout, working projection and chip direction together. Through holes and blind holes may require different evacuation strategies. For coolant, confirm that adequate flow reaches the cutting edges; pressure alone does not prove effective delivery.
Increasing speed or reducing feed is not a universal surface-finish correction. A parameter change may alter heat, chip shape, edge loading and built-up material. Diagnose the symptom before changing the cutting window.
Common Reaming Problems
| Symptom | Possible contributors | First checks |
|---|---|---|
| Oversized bore | Runout, unequal edge loading, excessive allowance, thermal effects or measurement variation | Holder, spindle, allowance band and gauge method |
| Undersized or incomplete cleanup | Insufficient allowance, edge wear, material springback or incorrect setting | Incoming hole and tool condition |
| Taper or bell-mouth | Pre-hole taper, weak guidance, misalignment or uneven wear | Measure the pre-hole along its depth |
| Chatter marks | Excessive projection, unstable holding, interrupted engagement or unsuitable parameters | Setup rigidity, stock, entry and cutting window |
| Scratched surface | Chip recutting, poor evacuation, damaged edge or contaminated coolant | Chip path, flute condition and filtration |
| Built-up material | Adhesive workpiece behavior, edge condition, heat or coolant mismatch | Material grade, edge, speed/feed and coolant delivery |
| Short tool life | Abrasive material, excessive load, runout, interrupted cut or unsuitable cutting material | Wear pattern and the complete process baseline |
Troubleshooting should compare the rejected bore, cutting edge, chips and process history. Check the incoming hole and setup before changing the reamer grade.
How to Verify the Finished Bore
Inspection must match the drawing and functional requirement. Agree on the characteristic, tolerance, measurement method, measurement depth, temperature condition, sampling frequency and acceptance quantity before the trial.
Inspection records should identify the component drawing revision, tool revision and measurement method. Project acceptance may require a bore gauge or air gauge for size, a CMM for geometry or position, a roundness instrument, and a surface-finish instrument. The correct combination depends on the print (see measuring a reamed bore: air gauge vs CMM).
When to Consider a Combination Drill-Reamer
A combination drill-reamer can reduce tool changes and non-cutting time when the holemaking stages form a stable, compatible process.
Consider a combination tool when:
- The drill and ream features are coaxial, accessible and can engage in the designed sequence.
- Chip evacuation remains reliable through the complete cycle.
- Simultaneously engaged sections can use compatible parameters, or staged engagement permits programmed adjustment.
Use separate tools when:
- The pre-hole needs independent correction or inspection before finishing.
- Chips from drilling can damage the finishing edges.
- The process changes frequently or the cutting stages require incompatible conditions.
See the Combination Drill-Reamer page for the product and manufacturing route.
Information to Send a Reamer Supplier
Define the accepted bore before selecting a reamer. Send the supplier:
- The component drawing and revision, including finished bore requirements.
- Workpiece material and condition, including silicon content when relevant.
- Measured minimum, maximum and typical pre-hole diameters.
- Hole depth, blind or through condition, interruptions and coolant route.
- Machine, holder, working projection, runout, current process and inspection method.
This information allows the supplier to decide whether the application fits a standard machine-reaming route, a custom PCD reamer, a combination tool, boring or another finishing process.
Define the Final Bore Requirement
Start with the component drawing and the functional bore. XRZ can review the material, pre-hole, setup and validation requirements before recommending a tool route.
Compare Reamer Types · View Custom PCD Reamers · Submit the Bore Drawing
Written by: Kevin Zeng, CEO
Engineering review: Jiack Liu, Engineering Director
Last reviewed: September 22, 2026
Review scope: CNC reaming process, application boundaries and production-hole selection.