Choosing a Hole Route from Tolerance Grade and Annual Volume

A route decision normally arrives as an argument, with each operation defended on its best case. This page turns it into a lookup: two inputs go in, one route comes out, and every cell has a stated fallback.

The two inputs are the tolerance band the drawing asks for and the annual volume the order book supports. Those two variables explain most of the routing disagreements we see, and they are both known before a tool is quoted.

The matrix applies to aluminium and non-ferrous bores with a depth-to-diameter ratio below 5 and no cross holes. Cast iron, interrupted cuts and deep bores fall outside it and are named at the end.

Two inputs, one answer

The first input is the tolerance band, read from the drawing as a class rather than a number. H11 and H9 are clearance fits, H8 is a location fit, H7 is the common precision band, IT6 and tighter is precision work, and the sixth row is for drawings where position, not size, is the hard requirement.

The second input is annual volume, in five bands. The bands are not arbitrary: they are the points where tool amortisation, gauging routine and automation change character.

Below 1,000 pieces a year, a tool's purchase price is the dominant cost and setup time matters more than cycle time. Above 200,000, tool life and in-process gauging dominate, and the route is usually fixed by the line rather than chosen per order.

Two cautions before using the table. The tolerance row is set by the tightest requirement on the bore, not the nominal class; a bore with an H8 size and a 0.03 mm position callout belongs in the position row. And the volume column is annual, not per order, because tool amortisation is annual.

Material changes the tool, not the route, within the stated boundary. A high-silicon aluminium may move a cell from carbide to PCD; it does not move a reaming cell to a honing cell.

The matrix

Each cell gives one route and a short note. The route is the recommendation for that combination; the note is the reason, kept short enough to be read in a quotation meeting.

Read the IT6 row carefully. The route changes across the volume columns because the constraint changes: at low volume the problem is tool cost, at high volume it is holding capability over a long run, and honing is the only route that holds both size and surface at the top end.

The position row is the one that most often surprises a quoting engineer. Position is not a tool property in the way size is; it is produced by the operation that creates the hole, and the reamer only inherits it. That is why the correction is a boring pass or a jig, never a reamer.

No cell says "depends", and that is deliberate. A matrix with a "depends" cell cannot be used in a quotation, and the point of the page is to make the first proposal specific enough to argue with.

Fallback routes per row

A recommendation is only useful if the next step is defined. The fallback is the route to switch to when the recommended one fails a measurable test, not when it feels slow.

The triggers are written as capability indexes rather than as opinions because a capability index is measurable on a sample and reviewable in a meeting. Two consecutive weeks is a deliberately slow trigger; it prevents a single bad shift from changing a route.

The fallback costs more per bore in every row. That is the point: it is a deliberate trade of cost for capability, taken only when the data says the recommendation is not holding.

Failure modes of each choice

Each route in the matrix fails in a characteristic way, and the failure is visible before the parts are scrapped if the right measurement is watched.

Reaming copies the pre-hole position. It appears in every reaming cell, and the symptom is a bore that is on size and off location, consistently, with no trend. The fix is upstream, in the drilling operation or the fixture.

Boring consumes setup time. It appears in the IT6 row and the position row at low volume, where the adjustment that gives boring its capability has to be paid for per order. The symptom is a route that is technically right and commercially slow.

Honing carries a fixed cost. It appears in the IT6 row at high volume and in the surface-driven cells, where the machine and the stones are justified only by the volume behind them. The symptom is a cost per bore that falls steeply as volume rises.

Roller burnishing amplifies form error. It is not a standalone cell in this matrix because it always follows another operation; where a drawing asks for surface below 0.4 micrometres Ra without a form requirement, the burnishing route is in reaming versus roller burnishing.

A combination tool fails on chip evacuation in blind holes. It appears wherever a cell assumes one pass, and the symptom is a tool that works on through holes and fails on the blind variant of the same part. The screening rules are in when to use a combination drill-reamer.

How to use the matrix in a quotation

Three lines of input are enough to use the table: the tightest tolerance on the bore, the annual volume, and the material family. With those, the cell is identified and the route can be written into a quotation before any tool is designed.

Write the route, the fallback and the trigger into the quotation. A customer who sees "H7 at 40,000 a year: drill then PCD ream, fallback bore-then-ream if position capability drops below 1.0" is being given a plan, not a price with an assumption behind it.

Then cost it. The per-bore model for the five finishing routes is in five bore finishing routes compared on cost, and the reaming side of the same arithmetic is in cost per accepted hole.

The order matters: route first, cost second. A cost comparison between two routes that do not both meet the drawing is not a comparison, and the matrix exists to prevent that comparison from being made.

If the drawing's tolerance class and the position requirement disagree with the matrix row, the position requirement wins. That is the rule behind the sixth row and the reason the matrix has one.

Where the matrix does not apply

Four situations sit outside the table, and each one has its own page rather than a cell.

Cast iron and other materials with free graphite or hard phases change the tool and sometimes the route, because edge wear and surface integrity behave differently from aluminium.

Depth-to-diameter ratios above 5 move the problem to chip evacuation and bar stiffness, and the route is set by those constraints rather than by tolerance.

Cross holes and interrupted cuts remove reaming from consideration at most diameters, because the edges chip on the interruption.

Very small bores, below about 1.5 mm, leave the matrix entirely: the tooling exists, but the measurement and the gauging routine dominate the decision.

The matrix is also not a capability promise. It says which route usually holds a band; it does not say that a specific machine will hold it, which is why the fallback column exists.

What published sources say about this

These pages were used for the capacity, capability and tooling points. Each is cited for what that page states, under its own conditions.

• Reaming and fine boring (toolmaker competence page): the two finishing families and their tool designs, the basis for splitting the matrix rows between reaming and boring.
• Tools for fine machining bores (toolmaker interview): the single-bladed reamer and the handling requirements of fine boring tools, the reason boring carries setup cost.
• Reamers, machine reamers (toolmaker technical overview): reamer structure, guide part and flute shapes, which is what makes a reamer follow the pre-hole.
• Dimensioning and Tolerancing (ASME Y14.5): the position and form definitions that decide which matrix row a drawing belongs to.
• What is Process Capability? (NIST/SEMATECH e-Handbook of Statistical Methods): process spread against specification limits, the method behind every capability trigger in the fallback table.
• What are control charts? (NIST/SEMATECH e-Handbook): the time-ordered plot that shows whether a route is drifting before it fails a first article.
• Levene test for equality of variances (NIST/SEMATECH e-Handbook): a test for comparing spread between two processes, useful when two routes are run on the same bore.

Outbound references are listed because the matrix depends on them, not as endorsements. The cell recommendations are HENTRY's engineering convention and are not taken from these sources.

Frequently asked questions

Updated 13 November 2026: first publication. Scope: the two-input route matrix for aluminium and non-ferrous bores under a depth-to-diameter ratio of 5, the fallback route and trigger per row, the failure modes and the quotation workflow. The matrix is an engineering convention, not a standard.