Guides / Process
How Two Bores a Metre Apart Are Held in Line
September 7, 2026
A single large bore is straightforward work for a boring mill. Two bores that must line up across a housing are a different job, and the difference is where the cost sits.
Why the span is the problem
Boring a 420 mm hole to a few hundredths is a matter of a rigid setup, a good bar and a finish pass. The machine does it every day.
Making the second bore, a metre or four metres away, share an axis with the first brings in everything that varies over distance: machine geometry across the travel, thermal growth in the part, table deflection under the load, and the accuracy of any re-clamping in between.
That is why the same 0.03 mm number is routine in one context and expensive in another.
The four methods
One setup on a boring mill. The part is clamped once and both bores are machined without releasing it. Accuracy comes from the machine’s travel between the two positions. The practical limit is whether the spindle can reach both from one clamping.
Rotary table index. On a table type boring mill, bore one face, index the table 180 degrees, bore the other. The relationship comes from a calibrated rotary axis rather than from re-indicating the part by hand. This is the standard route for housings with bores on opposite faces. More in floor type versus table type boring mill.
Line boring with a supported bar. A single boring bar passes through both bores, supported at both ends or at the outer bore. One rotating axis cuts both features, so they are coaxial because they were made by the same axis. This is the most certain method and it is what portable line boring equipment does in the field, as covered in on-site machining and line boring.
Two setups plus alignment. Bore one face, turn the part over, re-align to the first bore, bore the second. This works and is sometimes the only option, but the accuracy depends on the alignment procedure and it is the slowest of the four.
The cost ranking is roughly the reverse of the certainty ranking, which is why a shop will always ask whether the two bores really do have to line up.
Deep bores
A deep bore brings its own problem: the bar has to reach into the part, and a long bar deflects.
Deflection rises steeply with length, so the usable overhang for a given diameter is limited. Shops manage it with:
- The largest bar diameter the bore allows
- Heavy metal or damped bars, which shift the point at which chatter starts
- Lighter finishing cuts at the far end of the bore
- Support bushes in the part or in the fixture
- Approaching from both ends where the geometry allows
This is why the aspect ratio matters as much as the diameter. A 200 mm bore 200 mm deep is easy. The same bore 900 mm deep is a different conversation.
What to tell the shop
The relationship, not just the sizes. “Two bores 420 mm H7, coaxial within 0.05 mm” is quotable. “Two bores 420 mm H7, each positioned within 0.05 mm of datum A” is harder, more expensive and probably not what the assembly needs.
The depth and the wall. Bore diameter, bore depth, and how thick the material around it is. A thin wall bores differently from a thick one.
What runs in it. A bore that takes a bearing has a different surface finish and roundness requirement from one that takes a bolted bush. Say which.
Whether it is bored before or after anything else. Bronze bushes are usually bored after the housing is finished, so the bore matches its seat rather than a number on a drawing.
The condition it is measured in. Clamped or free, and at what temperature. On a large housing both change the answer.
A quick specification checklist
- Nominal diameter and fit or tolerance
- Depth, and from which face
- Surface finish, only where it matters
- Coaxiality or parallelism to the mating bore, with the tolerance
- Roundness and cylindricity, if the function needs them
- What the bore carries in service
- Measurement condition
Seven lines. They turn a bore from something a shop has to price defensively into something it can plan.
What to do next
Look at your drawing and ask whether the two bores are toleranced to each other or to a distant corner. If it is the corner, change it. It is the single cheapest change you can make to a heavy housing drawing.
Related: tolerance across a large part and inspecting large machined parts.
Frequently asked questions
How coaxial can two bores across a large housing realistically be?
It depends on the span and the method, but a few hundredths of a millimetre is a normal target when both bores are machined in one setup or line bored on a single bar. Across separate setups on a long part it gets much harder.
What is line boring?
A single boring bar passes through both bores and machines them in one pass, supported at both ends. Because one rotating axis cuts both features, they are coaxial by construction rather than by alignment.
Should I call out coaxiality or position?
Coaxiality, when the requirement is that the two bores line up. Position from a distant datum is harder to hold, harder to measure and usually not what the assembly needs.
Need a quote for a part like this?
Send the drawing with the envelope and finished weight. We match you with up to 3 shops that take parts this size and reply within 24 business hours.
Get matched with a large part shop