Most arguments about a bore quotation are not about the diameter. They are about location, and about which operation is supposed to produce it. The two requirements travel on the same drawing line and are read as if they were the same thing.
They are not. Size is a tool property: a reamer of the right diameter produces the right hole, within its band. Position is a process property: it comes from the operation that put the hole there and the fixture that held the part.
A reamer copies position; a boring tool can correct it; the fixture decides it. Size tolerance is the only one of the two a tool can be designed against.
Three callout mistakes account for most of the mismatch, and the most expensive one is asking a reamer to hold position it cannot influence.
This page does not teach geometric dimensioning and tolerancing. It covers one rule inside the hole-making chain, and the three ways that rule gets broken on drawings we receive.
The one rule
Size is produced by the last tool that touches the bore. Position is produced by the first operation that establishes the hole, and preserved by everything after it.
That is why a reamer cannot fix location: its edges are guided by the pre-hole, so it follows whatever axis the pre-hole had. A boring tool can move the axis because it is a single edge on a stiff bar, adjusted radially and positioned by the machine.
The fixture sits underneath both. A drill jig, a fixture with two pins, or a pallet located against a datum decides where the hole starts, and no finishing tool can recover a position the fixture gave away.
The practical consequence is simple. When a bore is off location, the corrective action is upstream or in a boring pass. When a bore is off size, the corrective action is the tool.
Mixing the two produces the pattern every tool supplier recognises: a customer asking for a reamer "that holds position", and a quotation that either refuses or prices a trial-and-error programme.
Three mixed callouts
The three patterns below cover most of the drawings that create a mismatch between what is asked and what a tool can deliver.
| Callout pattern | What it asks for | Consequence |
|---|---|---|
| Size only, no position | H7 on the bore, nothing about location | Each supplier assumes the position is held elsewhere, and the first article becomes the place where the assumption is discovered |
| Position on the bore, no requirement on the pre-hole | A position tolerance on the finished hole only | The pre-hole is drilled without a positional requirement, so the reamer copies an uncontrolled position and the first article fails |
| Position treated as a tool requirement | A position tolerance attached to the reaming operation | The tool is asked to do something its geometry cannot do; the quotation doubles or the supplier declines |
The first pattern is the most common and the least damaging, because it fails at first article rather than in production. The customer learns that the position was never specified, and the drawing is corrected.
The second pattern is more expensive. The pre-hole is a separate operation with its own tool and its own setup, and if it carries no positional requirement, the operator has no target. The reamer then faithfully copies whatever came out of the drill.
The third pattern is the one that distorts quotations. A supplier who is asked to guarantee position with a reamer has two options: refuse the requirement in writing, or price a development programme. Both look like an uncompetitive quotation to a buyer who does not know the rule.
What each does to the tool quotation
The first pattern usually gets a quotation with an assumption written into it, and the assumption is worth reading. It says, in effect, that the position is the customer's responsibility.
The second pattern gets a quotation for a reamer plus a polite note asking for the pre-hole requirement. If the note is ignored, the first article fails and the tool is blamed.
The third pattern gets the highest price, because the supplier has to cover the risk of a requirement that cannot be met by the tool alone. The price is not inflated; it is the cost of a specification error being transferred to the supplier.
There is a fourth, quieter pattern: a drawing that specifies both size and position correctly, and a purchase order that asks for a cheaper tool.
The quotation then has to explain why the cheaper tool cannot hold the position the drawing requires, which is a conversation that belongs before the order, not after.
In all four cases the quotation is a reflection of the drawing, not of the supplier's capability. That is why the correction belongs on the drawing.
Where the correction belongs
Position can be corrected in three places, and each has a different cost and a different owner.
The first is the pre-hole operation. A drill jig, a bushing plate, or a boring pass before reaming establishes the axis. This is the cheapest place to correct position when the volume justifies the fixture.
The second is the fixture. A two-pin location, a datum change or a pallet with a known relationship to the machine axes moves the whole hole pattern. This is a setup decision, not a tool decision.
The third is a boring pass. A single-point tool can move the axis within the machine's compliance, which is why the hybrid route of semi-finish boring followed by reaming exists.
The reamer enters only after the position is already inside tolerance. Its job is size, roundness and surface, and it does those well when it is not asked to do anything else.
A guide bushing is not a position correction. It supports the tool at the entry and improves roundness, but the bore's axis is still the pre-hole's axis; the guide changes stiffness, not location.
The full route matrix, including the row for position-dominated bores, is on hole route selection by tolerance and volume, and the matrix sits under the wider sequence in hole machining process route.
The division of labour between boring and reaming on the same bore is set out in fine boring versus reaming. In tool terms, position is corrected by the fine boring tool, not by the reamer that follows it.
A sentence that fixes the drawing
The change is usually one sentence on the drawing, plus one on the process sheet. The sentence separates the two requirements and assigns each to the operation that produces it.
For the drawing note: "Bore position is established by the drilling operation. Position tolerance applies to the pre-hole step; size, form and finish apply to the reaming step."
For the process sheet: "Pre-hole position to be verified at first article and every 20 parts; reaming to be accepted on size, roundness and surface only."
For a supplier note: "Position capability is the responsibility of the pre-hole operation. The reamer is supplied to the size band and is not a position-correcting tool."
Those three sentences are enough to stop the third callout pattern from reaching a quotation. They also make the first article a test of the right thing: the pre-hole position and the reamed size, measured separately.
If the design cannot accept a positional requirement on the pre-hole, for example because the pre-hole is a casting, then the correction has to be a boring pass, and the drawing should say so rather than leaving the reamer to attempt it.
The callout itself carries the split: the six items a reamed bore should specify, and the two notes that assign position to the pre-hole step, are set out in how to call out a reamed bore on a drawing.
Reading your own drawing before you send it
Five checks take a few minutes and prevent most of the back-and-forth. They are worth doing before an RFQ leaves the building.
First, is there a position or profile requirement on the bore at all? If not, the supplier will assume the position is held upstream, and that assumption should be written down.
Second, is the requirement also stated on the pre-hole step? If the pre-hole is drilled in-house, this is a process sheet change; if it is purchased, it is a purchase specification change.
Third, does the datum scheme match the fixture? A position tolerance referenced to a datum that the fixture does not touch cannot be verified at the machine.
Fourth, is the tolerance band on the bore achievable by the finishing tool without a correction pass? The class alone does not answer this; the pre-hole quality does.
Fifth, is the gauge specified? A position requirement without a stated gauge is a requirement that will be measured differently by each party.
The definitions behind these checks are in ASME Y14.5, listed in the sources below. The rule on this page is narrower than the standard: within a hole-making chain, size belongs to the tool and position belongs to the process.
What published sources say about this
These pages were used for the tool behaviour and the tolerance definitions. Each is cited for what that page states, under its own conditions.
- Dimensioning and Tolerancing (ASME Y14.5): the position, profile and datum framework that separates location from size on a drawing.
- Reamers, machine reamers (toolmaker technical overview): reamer structure, guide part and chamfer lead, the geometry that makes a reamer follow the pre-hole axis.
- Reaming and fine boring (toolmaker competence page): the two finishing families described together, with the fine boring tool as the adjustable member of the pair.
- Tools for fine machining bores (toolmaker interview): the single-bladed reamer and its handling requirements, the background to the adjust-versus-copy distinction.
- What are the issues for characterization? (NIST/SEMATECH e-Handbook of Statistical Methods): why a requirement has to be defined together with the measurement that decides it, the reason a position callout needs a gauge.
- What is Process Capability? (NIST/SEMATECH e-Handbook): process spread against specification limits, the method behind judging whether a position requirement is being held.
- What are control charts? (NIST/SEMATECH e-Handbook): the time-ordered plot that shows position drift before a batch is lost.
Outbound references are listed because the argument depends on them, not as endorsements. The three callout patterns are HENTRY's observation from quotations and are not taken from these sources.
Frequently asked questions
Can a reamer be made to hold position by adding a guide?
A guide bushing supports the tool and improves roundness and surface, but it does not move the bore's axis relative to the part datum.
Position still comes from the pre-hole and the fixture; a guide reduces deflection, not location error.
Why do some suppliers accept a position requirement on a reamer?
They usually price it as a development programme rather than as a tool, or they accept it and rely on the customer not measuring it.
Either way, the requirement is not being met by the reamer's geometry, which is why the quotation looks different from a normal one.
Does a boring pass always fix position?
It can move the axis within the machine's compliance and the bar's stiffness, so it fixes position when the error is small relative to the bar's capability.
On a deep bore the bar deflects and the correction is partial; the fixture or the pre-hole remains the better place to fix it.
What should a position tolerance on a drawing reference?
It should reference the datums the fixture actually locates against, and it should state the gauge or the measurement method.
If the datum scheme and the fixture disagree, the requirement cannot be verified at the machine and the first article becomes an argument.
Where do I send a drawing that mixes the two?
Send it as it is, with a note about which requirement is which. The route can still be quoted, and the mismatch can be flagged in the reply.
Use the custom cutting tool RFQ for the drawing and the process data; the reply will separate what the tool holds from what the process has to hold.
Updated 13 November 2026: first publication. Scope: the rule separating size from position in a hole-making chain, the three mixed callout patterns and their quotation consequences, the three correction points and the drawing sentences. Not a GD&T tutorial.