A drilled hole is rarely round enough or accurate enough to serve as a bearing seat, a seal bore, or a precision dowel hole. Boring and reaming are the finishing processes that turn a rough hole into a precision bore with controlled diameter, roundness, and surface finish. This guide explains how each process works, the bore tolerances and roundness you can expect, when to bore versus ream, and what precision bores actually cost.
Boring vs Reaming: What Each Process Does
Both processes refine an existing hole, but they solve different problems.
Boring
Boring uses a single-point tool to enlarge and true a hole. Because the tool is adjustable, boring corrects the position, straightness, and roundness of the hole rather than just following the existing path.
- Corrects location and axis alignment left by drilling
- Holds diameter tolerances down to +/-.0005 in, and +/-.0002 in on critical bores
- Ideal for large, non-standard, or deep bores
- Produces excellent roundness and cylindricity because the tool cuts on a rigid, controlled axis
Reaming
Reaming uses a multi-flute tool to finish a hole to a precise size. A reamer follows the existing hole, so it improves diameter and finish but does not correct a mislocated or crooked hole.
- Fast and repeatable for standard hole sizes
- Holds diameter tolerances of +/-.0002 to +/-.0005 in
- Delivers fine finishes of Ra 16 to 32 uin
- Requires a properly sized pre-drill, typically leaving 0.010 to 0.015 in of stock for the reamer
Roundness, Cylindricity, and Surface Finish
For precision bores the diameter is only part of the story. Engineers usually care most about form and finish.
| Characteristic | Drilled Only | Reamed | Bored |
|---|---|---|---|
| Diameter tolerance | +/-.003 to +/-.005 in | +/-.0002 to +/-.0005 in | +/-.0002 to +/-.0005 in |
| Roundness | .002 to .005 in | .0003 to .0008 in | .0001 to .0005 in |
| Surface finish | Ra 125 to 250 uin | Ra 16 to 32 uin | Ra 16 to 63 uin |
| Location correction | None | None | Yes |
When a print calls out roundness or cylindricity in a feature control frame, or a bore diameter with an H7 fit, that is a signal the hole must be bored or reamed, not just drilled. We verify bore diameter, roundness, and cylindricity on our Brown & Sharpe CMM and document them in the first article inspection report.
Deep Bores and L:D Ratio
Bore depth relative to diameter, the L:D ratio, drives both process choice and cost. Below roughly 4:1, standard boring bars and reamers work well. Between 4:1 and 8:1 we shift to anti-vibration bars and reduced feeds. Beyond 8:1 the job may need a dedicated deep-hole strategy, and roundness and straightness get harder to hold. Calling out the true functional length of the bore, rather than toleranceing the entire depth, keeps deep-bore cost in check.
When to Bore and When to Ream
- Hole location or axis must be corrected? Bore it. A reamer cannot fix a crooked or mislocated hole.
- Standard size and location already good? Ream it. Reaming is faster and cheaper for a hole that is already close.
- Large or non-standard diameter? Bore it, since off-the-shelf reamers stop at common sizes.
- Very tight roundness under .0005 in? Bore it, and finish-bore in a separate light pass.
- High volume, tight diameter, standard size? Ream it for speed, or combine drill, bore, and ream in one setup.
Precision Bore Cost Guidance
Boring and reaming cost is driven by tolerance, depth, and inspection. The ranges below are typical illustrative estimates for finishing a precision bore, not binding quotes. Request an instant quote for firm pricing on your drawing.
| Cost Driver | Typical Range | Notes |
|---|---|---|
| Setup and programming | $150 to $600 per job | Higher for line boring or fixtured multi-bore parts |
| Reaming (per bore) | $0.25 to $2.00 | Standard sizes with a stock reamer |
| Boring (per bore) | $1.00 to $8.00 | Finish boring and deep bores at the top of the range |
| Tight tolerance and form (under .0005 in) | Add 20% to 60% | Extra passes, tool control, and temperature stability |
| CMM inspection and FAI | $75 to $300 per lot | Roundness, cylindricity, and diameter documentation |
How to Lower Precision Bore Cost
- Reserve tight roundness and +/-.0002 in for the functional length of the bore only.
- Use standard reamer sizes where possible to avoid custom tooling.
- Keep L:D ratios reasonable, and split deep bores into functional zones on the print.
- Combine drilling, boring, and reaming in one setup to cut handling and preserve concentricity.
Wexmar Boring and Reaming Capability
Wexmar is a precision CNC machine shop in Canastota, NY. We bore and ream on 6 Mazak horizontal machining centers and 10 Mazak and Doosan turning centers, holding precision bores to +/-.0002 in with documented roundness and cylindricity. Every critical bore is verified on our Brown & Sharpe CMM with full inspection and quality reporting. We are SAM.gov and CAGE registered, machine DFARS-compliant material, and every part is made in the USA and shipped nationwide with ISO 9001-aligned documentation and material certifications on request.