What Is Hole Finishing?
Hole finishing is any secondary operation that improves a drilled or rough-bored hole beyond the capability of the drilling process itself. Drilling can produce holes to IT10–IT11 tolerance with Ra 3.2–6.3 µm surface finish. Many applications require tighter tolerances or finer finishes, which require a secondary operation.
The three common hole-finishing processes:
Boring: a single-point tool enlarges the hole to the final dimension. Boring is flexible, accurate, and applicable to most materials and tolerances.
Reaming: a multi-tooth tool removes a small amount of material to achieve a precise diameter and good surface finish. Reaming is fast, economical, and limited to standard sizes.
Honing: an abrasive stone removes a small amount of material while rotating and reciprocating in the hole. Honing produces the finest surface finish and the most accurate roundness.
The choice depends on the required tolerance, surface finish, lot size, and the part material. The Yuqing processing of mechanical parts capability covers all three processes.
When Is Boring the Right Process?
Boring is the right process when:
The hole diameter is not a standard size (i.e., not a standard drill or reamer size).
The tolerance is IT6–IT8.
The lot size is small or medium.
The material is anything that can be machined.
The part requires tight concentricity or perpendicularity to a datum.
Boring uses a single-point tool that can be adjusted to any diameter. The tool is fed into the hole, and material is removed on one side. The flexibility of boring makes it the default process for custom parts that do not match standard sizes.
Yuqing's CNC machine tool processing capability includes boring heads that can be adjusted to any diameter within the machine's spindle envelope.
When Is Reaming the Right Process?
Reaming is the right process when:
The hole diameter is a standard size (e.g., 10H7, 12H7, 16H7).
The tolerance is IT6–IT8.
The lot size is medium to high.
The material is machinable.
The part does not require extreme surface finish.
Reaming uses a multi-tooth tool that cuts on multiple edges simultaneously. The process is faster than boring (typically 3–10× faster for the same material removal) and produces a better surface finish (typically Ra 0.8–1.6 µm vs Ra 1.6–3.2 µm for boring).
The limitation is that reaming is limited to standard sizes. A non-standard reamer is expensive (typically 200–500 per reamer) and is amortized over the lot size. For small lots, boring is more economical even at standard sizes.
When Is Honing the Right Process?
Honing is the right process when:
The tolerance is IT5–IT7.
The surface finish requirement is Ra 0.1–0.8 µm.
The hole is a cylinder or cylinder-like shape.
The part requires excellent roundness, straightness, and surface finish.
The material is machinable (typically steel, cast iron, aluminum).
Honing uses an abrasive stone that rotates and reciprocates in the hole. The cross-hatch pattern produced by honing is ideal for retaining oil on cylinder walls (e.g., engine cylinders, hydraulic cylinders).
Honing is slower than boring or reaming (typical material removal is 0.01–0.05 mm per pass) and requires dedicated honing equipment. It is justified when the surface finish and dimensional accuracy are critical.
What Are the Achievable Tolerances and Finishes?
|
Process |
Tolerance (IT) |
Surface Finish (Ra) |
Material Removal |
|
Drilling |
IT10–IT11 |
3.2–6.3 µm |
Bulk |
|
Boring |
IT6–IT8 |
1.6–3.2 µm |
0.5–5 mm |
|
Reaming |
IT6–IT8 |
0.8–1.6 µm |
0.1–0.5 mm |
|
Honing |
IT5–IT7 |
0.1–0.8 µm |
0.01–0.05 mm
|
The process chain for a tight-tolerance hole typically starts with drilling, then boring (if needed) or reaming, then honing (if needed). Each step adds cost and lead time.
How Is Boring Performed?
Boring is performed with a boring head that holds a single-point insert. The boring head is mounted in the machine spindle, and the insert is fed into the hole.
Steps:
Drill a pilot hole: the pilot hole is drilled undersize (typically 0.5–2 mm undersize for finishing).
Mount the boring head: the boring head is mounted in the spindle, and the insert is set to the desired diameter.
Bore the hole: the boring head is fed into the hole at the appropriate speed and feed.
Measure and adjust: if the hole is out of tolerance, the boring head is adjusted and the hole is re-cut.
For CNC boring, the boring head is adjusted automatically by the machine (e.g., digital boring heads). The operator sets the target diameter in the control, and the boring head adjusts to that diameter with high accuracy.
How Is Reaming Performed?
Reaming is performed with a reamer that has multiple cutting edges. The reamer is mounted in the machine spindle or in a floating holder.
Steps:
Drill or bore a pilot hole: the pilot hole is drilled or bored to within 0.1–0.3 mm of the final diameter.
Mount the reamer: the reamer is mounted in the spindle or in a floating holder to allow self-centering.
Ream the hole: the reamer is fed into the hole at the appropriate speed and feed.
Measure: the hole is measured with a plug gauge or bore gauge.
Reaming is faster than boring because the multiple cutting edges remove material on each pass. The process is also more repeatable because the reamer diameter is fixed.
How Is Honing Performed?
Honing is performed with a honing tool that has abrasive stones. The honing tool rotates and reciprocates in the hole, removing material with the abrasive stones.
Steps:
Pre-finish the hole: the hole is bored or reamed to within 0.02–0.05 mm of the final diameter.
Mount the honing tool: the honing tool is mounted in the honing machine.
Hone the hole: the honing tool rotates and reciprocates; the abrasive stones remove material.
Measure: the hole is measured with a bore gauge or air gauge.
Honing is the slowest of the three processes but produces the best surface finish and dimensional accuracy. The cross-hatch pattern produced by honing is ideal for retaining oil.
What Are the Common Defects in Hole Finishing?
Common defects in hole finishing:
|
Defect |
Cause |
Prevention |
|
Out-of-round hole |
Tool wear, machine spindle runout, fixture error |
Use sharp tools, inspect spindle runout, improve fixture |
|
Tapered hole |
Tool wear, deflection |
Use shorter tools, reduce tool wear |
|
Poor surface finish |
Wrong cutting parameters, worn tool |
Adjust cutting parameters, replace tool |
|
Chatter marks |
Insufficient rigidity, wrong cutting parameters |
Increase rigidity, adjust parameters |
|
Oversized hole |
Tool wear, thermal expansion |
Compensate for tool wear, allow machine to warm up |
|
Undersized hole |
Tool wear, incorrect setup |
Compensate for tool wear, verify setup
|
Each defect is preventable with discipline. For high-value parts, the first-article inspection is critical to identify issues before they are repeated across the lot.
How Are Hole-Finishing Operations Quoted?
Hole-finishing operations are quoted based on:
Cycle time: the time to perform the operation per part.
Tooling cost: the cost of the boring head, reamer, or honing stones.
Setup cost: the cost of setting up the operation, including tool setup and part fixturing.
Inspection cost: the cost of measuring the holes, including plug gauges, bore gauges, or CMM.
For a custom part, the hole-finishing cost is typically 5–20% of the total machining cost. The exact percentage depends on the number of holes, the tolerance, and the surface finish.
How Does Hole Finishing Affect Part Cost?
Hole finishing affects part cost in several ways:
Process selection: boring is more flexible but slower; reaming is faster but limited to standard sizes; honing is the most expensive but produces the best results.
Tooling cost: reamers and honing stones are more expensive than boring inserts, but they last longer.
Cycle time: reaming and honing are typically faster than boring for the same material removal.
Inspection cost: tighter tolerances require more precise inspection, which adds cost.
Scrap rate: tight-tolerance holes have higher scrap rates if the process is not stable.
For a custom part, the hole-finishing cost is a function of the tolerance and surface finish requirements. The engineering response is to specify only the tolerance and finish that the function requires.
What Is the Future of Hole Finishing?
Trends in hole finishing:
Automation: robotic loading and unloading of parts, in-process gauging, and closed-loop control of boring heads.
Improved tooling: longer-lasting reamers and honing stones, with advanced coatings and geometries.
Hybrid processes: combination boring-reaming tools that rough and finish in one pass.
In-process measurement: bore gauges integrated into the machine for real-time measurement and compensation.
Additive manufacturing: near-net-shape holes from additive manufacturing, requiring less material removal.
For a custom machine shop, the trend toward more automation and in-process measurement means that hole-finishing operations will become more consistent and more economical.
Conclusion
Hole finishing is the secondary operation that brings a drilled or rough-bored hole to its final dimension, tolerance, and surface finish. The three common processes — boring, reaming, and honing — each have a different range of capability, cost, and applicability. The choice depends on the tolerance, surface finish, lot size, and material. Yuqing's CNC machine tool processing and processing of mechanical parts capabilities include all three processes. The engineering response is to specify only the tolerance and finish that the function requires, and to select the process that meets the spec at the lowest cost.
Frequently Asked Questions
What is the difference between boring and reaming?
Boring uses a single-point tool that can be adjusted to any diameter; reaming uses a multi-tooth tool with a fixed diameter. Boring is more flexible; reaming is faster and more repeatable for standard sizes.
When should I use honing instead of boring or reaming?
Honing is used when the surface finish requirement is finer than Ra 0.8 µm, when the tolerance is tighter than IT6, or when the part requires excellent roundness and a cross-hatch pattern (e.g., cylinder walls).
What tolerance can boring achieve?
Boring can achieve IT6–IT8 tolerance, depending on the machine, the tool, and the material. For tighter tolerances, grinding or honing is required.
What tolerance can reaming achieve?
Reaming can achieve IT6–IT8 tolerance, similar to boring. Reaming produces a more consistent diameter but is limited to standard sizes.
What tolerance can honing achieve?
Honing can achieve IT5–IT7 tolerance with Ra 0.1–0.8 µm surface finish. Honing is the most accurate of the three processes for cylindrical holes.
How is a hole measured after finishing?
Holes are measured with plug gauges, bore gauges, air gauges, or CMMs. Plug gauges are fast for go/no-go checks; bore gauges are used for precise measurements; CMMs are used for full-dimensional inspection.
What is the typical material removal for boring?
Boring typically removes 0.5–5 mm of material per side, depending on the starting condition and the tool. Rough boring removes more material; finish boring removes less.
What is the typical material removal for reaming?
Reaming typically removes 0.1–0.5 mm of material per side. Reaming is a finishing process, not a roughing process.
What is the typical material removal for honing?
Honing typically removes 0.01–0.05 mm of material per side. Honing is the finest of the three processes.
How much does a reamer cost?
A standard reamer costs 50–200; a non-standard reamer costs 200–500. The cost is amortized over the lot size.
How much does honing cost?
Honing is typically priced at 5–50 per hole, depending on the size, tolerance, and surface finish. The cost includes the honing machine time and the abrasive stones.
Can boring and reaming be combined in one operation?
Yes, combination boring-reaming tools are available. The tool roughs with the boring portion and finishes with the reaming portion in one pass.
