A tight-tolerance bore in a tungsten-family material is usually lost in one of two places that have nothing to do with the cutting: the temperature at which it was measured, and the measurement system used to accept it. Both are supplier-selection questions, and both belong in the RFQ.
This guide separates size, form, and location; shows what tolerance is realistically published for tungsten work versus conventional materials; quantifies the thermal error that a steel master introduces on a tungsten part; and gives the questions that tell you whether a shop can hold and prove the number on your print.
- Decide first whether "tungsten" is the workpiece or the boring bar. The query covers both, and the two jobs share nothing but a word.
- A bore is three specifications — size, form, and location — and no single process fixes all three. Honing corrects form but not location; reaming corrects neither.
- Tungsten machining specialists commonly publish ±0.0005″, while jig-bore specialists in conventional materials publish ±0.0002″. "Tight" means different things to the two shops.
- Tungsten-family materials expand at roughly half the rate of steel. A Ø50 mm carbide bore checked against a steel master on a 25 °C shop floor carries about 1.5 µm of pure thermal error — near 30% of a ±0.0001″ band.
- At ±0.0001″ the 10:1 gage rule demands 0.00001″ resolution. Ask for the measurement method and the gage study, not just the tolerance.
What "Tungsten Bore" Means Before You Quote It
The phrase covers two unrelated jobs. In one, tungsten is the workpiece and you are trying to produce an accurate hole in a hard, dense, low-expansion material. In the other, tungsten carbide or tungsten heavy alloy is the boring bar, chosen for stiffness and damping so a deep hole in something else can be held to size. Both are legitimate readings, and a supplier working from the wrong one will quote the wrong job.
When tungsten is the workpiece, three different materials share the name, and the bore process changes with each:
- Unalloyed tungsten — brittle at room temperature. Bores are normally produced by EDM and finished by grinding or lapping rather than cut with a boring bar.
- Tungsten heavy alloy (ASTM B777 Classes 1–4) — 90–97% W in a ductile nickel-iron or nickel-copper matrix, 22–26 HRC. This is the one that genuinely bores, reams, and hones with carbide tooling.
- Cemented tungsten carbide — a metal-bonded ceramic. Bores are ground, honed, lapped, or EDM'd; conventional boring is not a route.
A drawing that says only "tungsten" will be quoted three different ways. Naming the material and its standard is the single change that most improves quote comparability on this type of part.
A Bore Is Three Specifications, Not One
Most bore arguments come from treating diameter as the whole requirement. A bore can measure in size at every position and still refuse a pin, because size, form, and location are produced — and corrected — by different operations.
- Size
- The diameter and its tolerance. This is what a bore gage or air gage reports, and it is the only one of the three that a two-point measurement can confirm.
- Form
- Roundness, cylindricity, straightness, taper, bell-mouth. A lobed bore can pass a two-point check at every angle and still be out of round.
- Location
- Position and orientation relative to datums. Established by the machine and the setup, and largely locked in once the hole exists.
- Surface
- Ra and the surface's structure. On a bearing or sealing bore this is functional, not cosmetic, and it constrains which finishing process is acceptable.
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| Process | What it can correct | Diameter tolerance | Typical Ra |
|---|---|---|---|
| Jig boring | Location and size together; the only route that fixes a misplaced hole | ±0.0002″ or tighter; location within 1–5 µm of datum | Depends on tool and material |
| CNC finish boring | Size, with limited location correction | IT6–IT8 class | Depends on tool and material |
| Reaming | Size and finish only; follows the existing hole and locks in its error | ±0.010–0.050 mm | 0.4–3.2 µm |
| Internal (ID) grinding | Size and form; good geometry correction | ±0.005–0.020 mm | 0.2–1.6 µm |
| Honing | Form and size; corrects roundness and taper to 1–5 µm, but not location | ±0.001–0.010 mm | 0.1–0.8 µm |
Two consequences worth carrying into the RFQ. First, honing removes only 0.002–0.02 mm, so it refines a bore that is already near size — it cannot rescue one that is 0.05 mm small. Second, if position is the critical requirement, it has to be produced correctly, because no finishing operation moves a hole. A supplier who proposes "bore then hone" for a positional callout has not addressed the callout.
What "Tight Tolerance" Actually Means in Tungsten
The word travels badly between shops. Precision machining suppliers that specialize in tungsten and refractory metals commonly publish capability at ±0.0005″ (0.013 mm). Jig-boring specialists working in steel, aluminum, and superalloys publish ±0.0002″ on diameter with 1–5 µm of positional accuracy. The two figures are roughly 2.5× apart, and they describe different equipment and different process routes.
That gap has a practical reading. A tolerance a general precision shop treats as routine can sit at or beyond the published envelope of a general tungsten machining service. If your print calls for a tungsten bore tighter than about ±0.0005″, you are not asking for careful CNC work — you are asking for ground, honed, or lapped finishing, on equipment and masters that a machining-only supplier may not own.
Which route fits which tungsten material
- Unalloyed tungsten: EDM to shape, then ID grinding or lapping for size and form
- Tungsten heavy alloy: bore with C-2 carbide, then ream, hone, or ID grind depending on the tolerance
- Cemented carbide: ID grind with diamond, then hone or lap; EDM only for shape, with stock left for recast removal
- Any of the three, deep or small bores: EDM makes holes that are smaller and deeper than conventional tooling reaches
Ask the supplier which of these routes they are quoting before comparing prices. Two quotes for the same drawing can differ by a factor of several simply because one assumed a bored-and-reamed hole and the other assumed grinding and honing.
Temperature: The Error Nobody Budgets
Dimensional measurements are defined at a reference temperature of 20 °C (68 °F); ISO 3650 states plainly that gauge block lengths apply at that temperature, and the NIST gauge block handbook puts it more bluntly still: a dimension without a defined temperature is meaningless. On most materials, shops absorb this by measuring a steel part with a steel master, where the expansion largely cancels. On tungsten, it does not cancel.
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| Material | Linear CTE (×10−6/K) | Relative to a steel master |
|---|---|---|
| Unalloyed tungsten | 4.2–4.6 | About 40% of steel |
| Tungsten heavy alloy (Classes 1–4) | 4.8–5.4 | Under half of steel |
| Cemented tungsten carbide | Approx. 5.4 | Roughly half that of ferritic and martensitic steels |
| Steel gauge block or setting ring | 11.5 | The reference the shop is holding |
Work the arithmetic on a real bore. A Ø50 mm cemented carbide bore, measured on a shop floor at 25 °C against a steel setting ring: the part has grown about 1.35 µm from its 20 °C size, the ring about 2.88 µm. The bore reads roughly 1.5 µm (about 60 µin) larger than it is — close to 30% of a ±0.0001″ tolerance band, before any process variation, operator effect, or gage error is counted. Unalloyed tungsten is worse, at about 1.8 µm under the same conditions.
The counterintuitive part is where the error lives. Engineers used to aluminum and steel expect the part to be the unstable element. With tungsten it is the opposite: the part is the most dimensionally stable object in the room, and the steel master, the steel fixture, and the machine's own structure are what move. Gage software that back-corrects to 20 °C using a default steel coefficient will over-correct a tungsten part and make the reading worse.
How capable shops close this out
- State the reference temperature on the drawing, not just the tolerance
- Use masters in a material matched to the part — carbide setting rings and gauge blocks are standard products
- Soak the part and the master together in the metrology environment before final measurement
- Control the inspection room, commonly 68 °F ±1 °F for this class of work
- Record the actual measurement temperature on the inspection report
- If back-correcting, use the part's real CTE, not the gage default
The Measurement System Eats the Tolerance
The customary rule is that a gage should resolve about one tenth of the tolerance being verified. At ±0.0005″ that is a 0.0001″ instrument — ordinary. At ±0.0001″ it means resolving 0.00001″, which puts you in electronic bore gage or air gage territory. Where 10:1 is unreachable, 4:1 is used as a practical floor, but only with a measurement system analysis or gage R&R study behind it.
This is why tolerance and inspection method have to be quoted together. Air gaging is fast and well suited to diameters in the IT2 to IT7 range and measures with almost no contact force. A bore micrometer is a production check that varies with the operator. A CMM characterizes position, cylindricity, and perpendicularity that no two-point gage can see, but its probing force and point density set what it can honestly resolve on a small bore.
Combine this with the previous section and the picture is uncomfortable: on a ±0.0001″ tungsten bore, an uncontrolled 5 °C temperature offset can consume roughly a third of the band and the measurement system another significant share, leaving the machining process a fraction of the tolerance it appeared to have. A supplier who quotes the tolerance without naming the gage and the conditions has not quoted the requirement.
Bore Depth, Bar Stiffness, and the Tungsten Irony
Where a bore has to be cut rather than ground, feasibility is set by overhang. A steel boring bar is generally held to about 4× its diameter in reach; a solid carbide bar extends that to roughly 6×; beyond that the work moves to damped or tuned bars. Deflection rises with the cube of overhang, so a bar one ratio step too long does not simply lose a little accuracy — it changes what tolerance is achievable at all.
Hence the irony in this query. The standard answer to a deep, tight bore is a bar made of cemented carbide or tungsten heavy alloy, chosen for high elastic modulus and, in the heavy-alloy case, density that damps chatter. Tungsten heavy alloy runs 280–345 kN/mm² in elastic modulus at 17–18.5 g/cm³. So on many jobs tungsten sits on both sides of the cut: the material being bored and the bar doing the boring.
For sourcing, the question is concrete: what length-to-diameter ratio can the supplier hold at your tolerance, with what bar, and have they done it in your material? Reach capability is equipment and tooling inventory, not skill, and it is quick to verify at RFQ.
How to Qualify a Supplier
For tight-tolerance bore work, capability splits along inspection as much as machining. These questions separate suppliers faster than a capability list does.
Questions that separate suppliers
- Which tungsten material do you run in production — unalloyed, heavy alloy, or cemented carbide?
- What process route are you quoting: bore and ream, ID grind, hone, lap, or EDM plus finishing?
- Do you own ID grinding and honing at this bore diameter, or does finishing go outside?
- What are your setting masters made of, and when were they last calibrated?
- What temperature is your inspection area held to, and is it recorded on the report?
- How will you measure this diameter — air gage, electronic bore gage, or CMM?
- Can you provide a gage R&R or MSA for that method at this tolerance?
- What length-to-diameter ratio can you hold at this tolerance, and with what bar?
- Are roundness and cylindricity measured, or only two-point diameter?
- If EDM is used, how much stock is left for recast removal?
Treat a published tolerance as a capability, not a commitment. Figures such as ±0.0005″ describe what a shop has achieved on some part, in some material, under favorable conditions. Ask for a feasibility review against your actual material, bore diameter, depth, form requirement, lot size, and inspection method.
What to Send With the RFQ
A bore RFQ that omits form, location, and inspection conditions will come back either padded or wrong. These ten items make the quote comparable.
- Material by name and standard — for example ASTM B777 Class 1 tungsten heavy alloy, a specified WC-Co grade, or unalloyed tungsten — never just "tungsten."
- Bore diameter and tolerance, stated separately from any fit callout.
- Bore depth and whether it is through or blind, plus the length-to-diameter ratio it implies.
- Roundness, cylindricity, and straightness requirements, called out separately from size.
- Positional or orientation tolerance with the datum scheme, and whether position or size is the critical requirement.
- Surface finish requirement and whether an EDM recast layer is acceptable.
- The reference temperature the tolerance applies at, and the temperature range at which the part will function if it differs.
- The inspection method and acceptance criteria you will use on receipt, including master material.
- Mating part material and fit condition if the bore is an interference or transition fit.
- Quantity, prototype versus production intent, and any program requirements such as defense end use or material origin documentation.
Narrow the field by finishing process before you narrow it by price. Once you know whether the bore needs reaming, ID grinding, honing, or lapping, the list of suppliers that can both produce and prove the tolerance gets much shorter.
FAQ About Tight Tolerance Tungsten Bores
- How tight can a tungsten bore realistically be held?
- Tungsten and refractory metal machining specialists commonly publish ±0.0005″ (0.013 mm). Tighter is achievable, but through finishing processes rather than machining: honing reaches ±0.001–0.010 mm and ID grinding ±0.005–0.020 mm on diameter. Below about ±0.0002″, the measurement system and the inspection environment become as important as the process.
- Should the bore be bored, ground, or honed?
- It depends on the material and on which specification is critical. Tungsten heavy alloy bores with carbide tooling and can be reamed or honed to size. Cemented carbide is ground and then honed or lapped. Unalloyed tungsten is normally EDM'd and finished by grinding or lapping. If location is the critical requirement, it must be produced correctly at the boring stage, because finishing operations do not move a hole.
- Why does my bore measure differently at the supplier and at receiving inspection?
- Most often temperature and masters. Tungsten-family materials expand at roughly half the rate of steel, so a tungsten part checked against a steel setting ring away from 20 °C reads large, and the error scales with diameter and with the temperature offset. A Ø50 mm carbide bore at 25 °C carries about 1.5 µm of differential on this basis alone. Agree the reference temperature, the master material, and the gage before production.
- What gage should be used on a tight tungsten bore?
- The customary target is a gage resolving about one tenth of the tolerance, which at ±0.0001″ means 0.00001″ — an air gage or electronic bore gage rather than a bore micrometer. Where 10:1 is not attainable, 4:1 is sometimes accepted with a gage R&R or MSA study to validate it. Add a CMM or roundness measurement if form and position are specified, since a two-point gage cannot see lobing.
- Is "tungsten boring" about the workpiece or the tool?
- Both meanings are in common use. Tungsten carbide and tungsten heavy alloy are standard boring bar materials, chosen for stiffness and vibration damping when overhang is long. If your requirement is a hole in tungsten, say so explicitly in the RFQ, because suppliers indexing on the tooling meaning will quote the wrong work.
- How deep a bore can be held to tight tolerance?
- Reach governs it. Steel boring bars are generally limited to about 4× diameter in overhang and solid carbide bars to roughly 6×, beyond which damped bars are required. Deflection grows with the cube of overhang, so depth affects achievable tolerance sharply rather than gradually. For deep or very small bores, EDM produces holes that conventional tooling cannot reach.
- Does a tungsten bore hold its fit at operating temperature?
- Not necessarily, and this is a design question worth settling before the RFQ. Because tungsten-family materials expand at roughly half the rate of steel, an interference or transition fit set at 20 °C will loosen or tighten as the assembly heats. State the mating material and the service temperature range so the supplier can flag a fit that only works at the inspection bench.
- Why did two suppliers quote the same bore so differently?
- Usually because they assumed different process routes. One may have priced a bored and reamed hole while the other priced grinding and honing with a matched master and a temperature-controlled inspection. Ask each to state the route, the finishing operation, and the inspection method; the quotes become comparable immediately.
