A spool bore is not a difficult hole in the usual sense.
It is often a straight bore with several lands and a moderate tolerance, and it still causes more scrap than a tighter bore elsewhere in the same valve body, because the failures come from cross holes, shoulders and particles rather than from size.
This page sets out what makes the bore different, the three process combinations that cover most cases, and why cleanliness has to be a tooling criterion rather than a final inspection step.
A spool bore has four properties that change the tooling decision: several lands that must share one axis, shoulders that need a clean root, cross holes that interrupt the cut, and a particle sensitivity that is set by the hydraulic function rather than by the drawing tolerance.
Three combinations cover most work: a combination tool that forms the lands in one pass, a form reamer for multi-land bores at volume, and fine boring where position or size must be adjustable.
The valve body as a component is on the valve body spool bore machining page. This page covers tooling selection and the burr question.
What makes a spool bore different
The lands are the first difference. A spool bore has two or more diameters that must share an axis, separated by shoulders, and each land has its own fit.
Cutting them in separate operations introduces a position error at every transition, and the error shows up as a spool that binds at one end of its travel.
The shoulders are the second. A shoulder that meets a bore at a sharp internal corner cannot be machined by a tool with a radius larger than the corner, so the root is either left with a radius or left with a burr. The drawing decides which, and the tool has to match the drawing.
The cross holes are the third. A cross hole that intersects the bore creates an interrupted cut at a known angular position, and at that position the tool loses support. The result is a step, a burr turned inward, or both.
Particle sensitivity is the fourth and the least visible. A hydraulic valve fails from a particle lodged at a land edge long before it fails from a bore that is a few microns out of size, so the burr and the surface finish matter more than the tolerance band suggests.
Three process combinations
The three combinations are not alternatives of equal rank. Each one is the right answer for a particular bore geometry and volume, and each has a failure mode that tells you when it is the wrong choice.
| Combination | Where it fits | What it needs | Failure mode when misapplied |
|---|---|---|---|
| Combination drill and reamer, one pass | Through bores with few lands, moderate depth, volume high enough to justify a special tool | Stable pre-hole condition, adequate machine thrust, coolant through the tool | Land position error where the pre-hole was off axis, and poor chip evacuation on deep bores |
| Form reamer with the land profile ground in | Multi-land bores at volume, where the profile is fixed and the tolerance is tight | A pre-hole that is round and on axis, and a tool that can be reground without losing the profile | Scrap on the whole bore when the form wears, because the tool cannot be adjusted |
| Fine boring with an adjustable tool | Bores where position or size must be corrected, or where volume is low | A capable machine, a skilled operator, and time per bore | Cycle time and operator dependence; chatter when the bar is slender |
The choice usually follows the land count and the volume. One or two lands at high volume favour the combination tool; three or more lands at volume favour the form reamer; low volume or an uncertain pre-hole favours fine boring.
A mixed route is common: a combination tool for the rough lands and a fine boring pass for the land that carries the spool fit. That split puts the adjustable operation where the tolerance is tightest and the fixed tool where the metal removal is heaviest.
The combination tool option is described on the combination drill reamer page, including what the tool needs from the machine.
Cross holes: burr and deflection
At a cross hole the tool leaves the cut and re-enters it. The exit is where the burr forms, and the re-entry is where the edge takes a load it was not designed for.
The burr has a direction. On a bore, the burr is usually turned inward at the exit side of the cross hole, which puts it exactly where the spool travels. A burr on the outside of the body is an appearance problem; a burr inside the bore is a functional one.
Three measures address it, and they are usually combined. The first is to slow the feed and, in some cases, the speed as the tool crosses the intersection, so the edge does not take the full load at the moment of re-entry.
The second is to design the cross hole so that its exit is on the low-pressure side or outside the spool travel, which moves the burr away from the functional surface. The third is to put a chamfer or a radius on the tool so that the exit edge is deburred in the same pass.
Where the burr cannot be avoided at the tool, it has to be removed in a controlled operation rather than by hand. The options are set out in CNC hole deburring tools, and the intersection behaviour itself is covered in PCD reamers in blind, through and cross holes.
The deflection side is separate. At the intersection the tool has less material to guide it, so a slender reamer can drift. The result is a bore that is on size before and after the cross hole and slightly off across it, which a two-point bore gauge will report as scatter.
Cleanliness as a tooling criterion
Cleanliness specifications are written as particle counts or as maximum particle size by size band, and they are set by the hydraulic function.
Where the customer references a standard such as ISO 16232, the acceptance limits in that standard are the customer's to state; this page does not substitute a limit of its own.
What matters for tooling is that the specification translates into two tool decisions: the surface finish the bore must have, and the burr the bore may retain.
| Cleanliness requirement | What it implies for the bore | Tooling consequence |
|---|---|---|
| Particle count limit with a small maximum size | No loose burr and no torn surface that can shed material | Edge condition and finish become acceptance criteria, not just size |
| Cleanliness verified after flushing | Retained particles matter, not only loose ones | Burr must be removed at the intersection, not just loosened |
| Cleanliness verified on the assembled valve | The bore and the cross holes both contribute | Deburring has to cover the cross hole exits as well as the lands |
The practical consequence is that a tool which produces an acceptable bore dimension and an unacceptable burr is the wrong tool, however good its size capability. That is why the burr question belongs at the drawing stage, when the cross hole position and the shoulder form can still be changed.
Surface finish follows the same logic. A torn bore surface holds particles in a way that a cut surface does not, and a dull edge produces a torn surface before it produces an out-of-size bore. Monitoring edge life by size alone will therefore pass a tool that fails the cleanliness requirement.
Tolerances that decide the tool
Three tolerances matter more than the rest, and each one points to a different combination.
Land-to-land position decides whether a multi-land tool can be used at all. If the position tolerance is tighter than the tool can hold, the answer is a single-land operation or fine boring, not a form reamer.
Roundness decides whether a reamer or a boring tool suits the bore. A reamer follows the pre-hole, so pre-hole roundness becomes bore roundness; a boring tool can produce roundness from an imperfect pre-hole but is slower.
Surface finish and burr allowance decide the edge preparation. A tool ground for size will not necessarily produce the finish that the cleanliness requirement needs, and the two requirements have to be specified together.
What to send with an enquiry
Four items let the tooling question be answered without a round of clarification.
The bore section drawing comes first, with the lands, their diameters and lengths, the shoulder forms and the cross hole positions relative to the lands. The shoulder form is often the item that decides the tool and is the item most often missing.
The cleanliness requirement comes second, quoted from the customer specification with its verification method. A cleanliness number without a verification method cannot be turned into a tool decision.
The pre-hole condition comes third: how the bore is roughed, on what machine, and whether the pre-hole is drilled, cored or cast. Each leaves a different condition for the finishing tool.
The volume and the cycle time target come fourth, because they decide whether a special form tool is justified and how much of the deburring can be moved into the cutting operation.
Send the package through the custom cutting tool RFQ, with the section drawing marked up at the cross holes.
The other two components in this group are covered in bearing bore sequence for EV motor housings and cylinder head guide and seat hole tooling.
The valve body family in the routing view is described in the automotive aluminium bore tool map, together with the bore classes that separate a spool bore from a locating bore.
What published sources say about this
These pages support the tool behaviour, the tolerance framework and the process capability logic used above. Each is cited for what it states, under its own conditions.
- Reamers, machine reamers (toolmaker technical overview): reamer types, guidance and flute forms, the basis for the combination and form reamer options.
- Reaming and fine boring (toolmaker competence page): the finishing families, including adjustable tools for bores that must be corrected.
- Tools for fine machining bores (toolmaker interview): single-blade tools and their behaviour in interrupted cuts, relevant at cross holes.
- Dimensioning and Tolerancing (ASME Y14.5): position, profile and surface texture callouts, which decide whether the land relationship is a position or a profile requirement.
- What is Process Capability? (NIST/SEMATECH e-Handbook): process spread against specification, the basis for judging a land position result rather than a single bore size.
- What is a measurement system? (NIST/SEMATECH e-Handbook): measurement variation as part of observed variation, relevant when a bore is judged across a cross hole with a two-point gauge.
- Levene test for equality of variances (NIST/SEMATECH e-Handbook): a test for equal spread between two runs, useful when two tooling options are compared on the same bore.
Outbound references are listed because the reasoning depends on them, not as endorsements. Cleanliness acceptance limits are always the customer's specification; no particle count or number of lands is promised.
Frequently asked questions
How many lands can one form reamer cut?
The limit is set by the tool's stiffness and by how the profile can be reground, not by a fixed count.
Each additional land adds a cutting edge that must be dressed together, so the practical limit is where regrinding the profile becomes unreliable.
Can a cross hole be drilled before the bore is finished?
Yes, and it is often better: the cross hole is then deburred by the finishing pass, which removes the inward burr at the intersection.
The cost is that the finishing tool meets an interrupted cut, which asks more of the edge and can drift across the intersection.
Why does the spool bind at one end of its travel?
Usually because the lands are not on one axis, so the spool is straight and the bore is not.
Check land-to-land position before checking the spool; a two-point bore measurement will not show this error.
Is a honed finish needed for cleanliness?
Only if the customer specification requires it. A fine bored or reamed surface can meet a particle requirement if the burr is controlled.
The decision follows the specification's maximum particle size, not a general preference for smoother surfaces.
What if the shoulder needs a sharp corner?
A sharp internal corner cannot be machined by a rotating tool with a radius; the drawing has to allow either a root radius or an undercut.
Settling that at the drawing stage is cheaper than deburring a corner that the tool cannot reach cleanly.
Updated 14 November 2026: first publication. Scope: the four properties of a spool bore, the three process combinations and their failure modes, cross hole burr and deflection, cleanliness as a tooling criterion, and the enquiry package. Cleanliness limits are always the customer's specification.