Large Part Machining Processes

How a part that outgrows a machining centre is actually milled, bored, turned, stress relieved and measured. Typical results for each process, and what to put on the drawing so the quote matches the method.

A large part rarely sees one process. A welded machine base is typically stress relieved, milled on a gantry, then bored on a horizontal boring mill. Knowing the route helps you write an RFQ a shop can price without guessing.

Cutting processes first, then setup and stress relief, then measurement. Machine envelopes and table loads are on the machine class reference, and how each grade behaves is on the materials reference.

Processes at a glance

Typical results across machines from several builders and several decades. They are not a capacity we hold, because we own no machines. The shop that quotes you confirms what it holds on your geometry.

Process	Typical machine	Part features it suits	Typical result
Large part milling	Gantry or bridge mill, travelling column mill	Large flat faces, pads, pockets, slideways, bolt patterns on one face	Flatness 0.02 to 0.05 mm per metre, Ra 1.6 to 3.2 µm
Floor type boring	Floor type horizontal boring mill on long rails	Bores and faces on parts too heavy for a rotary table, frames and beds	Bore H7, position between faces about ±0.05 to ±0.1 mm
Table type boring	Horizontal boring mill with rotary table	Housings with bores on several faces that must line up	Bore H7, position between faces about ±0.02 to ±0.05 mm
Vertical turning	Vertical turning lathe (VTL)	Rings, flanges, drums, casings, tube sheets, anything wider than it is long	Diameters IT7 to IT8, face flatness 0.02 to 0.05 mm, Ra 1.6 to 3.2 µm
Line boring	Boring mill with outboard support, or a portable boring bar on site	Two or more coaxial bores far apart: bearing lines, hinge bores, stern tubes	Coaxiality 0.02 to 0.05 mm in the shop, 0.05 to 0.1 mm on site

The cutting processes

Four routes that cover most large part work. Background on the boring process itself is on Wikipedia.

Large part milling

Also called Gantry milling, bridge milling, travelling column milling

How it works: The part is clamped once on a fixed table and the spindle travels over it, so a whole face is cut inside one reference frame.
Typical parts: Machine bases and beds, press platens and bolsters, mould blocks, welded frames, large plates.
Typical result: Flatness of 0.02 to 0.05 mm per metre and Ra 1.6 to 3.2 µm. Slideways that need better are ground or scraped after milling.
Watch for: Deep horizontal bores need a right angle head, which costs rigidity. A face flat on the table can move once the clamps come off.

Floor type boring

Also called Floor borer, floor type horizontal boring mill

How it works: The column runs on long rails beside a floor plate and comes to the part, so there is no rotary table and no table weight limit.
Typical parts: Press frames, mill stands, large gearbox cases, turbine frames and welded structures too heavy for a rotary table.
Typical result: Bores to H7 with Ra 0.8 to 1.6 µm after fine boring. Position between faces typically ±0.05 to ±0.1 mm, because the machine is re-aligned for each face.
Watch for: Ask how the shop transfers the datum between faces: a rotary table on the floor plate, laser alignment or re-indicating a reference bore.

Vertical turning

Also called VTL, vertical boring mill, VBM

How it works: The part turns on a horizontal table and the tool comes down from a rail, so the weight goes straight into the table bearing.
Typical parts: Rings, flanges, tube sheets, slew bearing races, drums and casings: round parts wider than they are long.
Typical result: Diameters to IT7 or IT8, face flatness and runout of 0.02 to 0.05 mm. A live milling head cuts bolt holes in the same setup.
Watch for: Height under the rail limits tall parts. Thin rings distort when chucked hard, so the finish pass follows a clamp release.

Line boring

Also called In-line boring, align boring

How it works: A bar supported at both ends passes through two or more bores and cuts them on one axis, on a boring mill in the shop or as a portable bar on site.
Typical parts: Bearing lines in gearbox and crusher housings, hinge bores on booms and buckets, stern tubes.
Typical result: Coaxiality of 0.02 to 0.05 mm in the shop. Portable bars on site usually hold 0.05 to 0.1 mm.
Watch for: A long unsupported bar sags and chatters. Ask how the axis is set out: from a wire, a laser or the existing bores.

Floor or table type is compared in detail in this guide. Site work is covered in on-site machining and line boring.

Large part setup

On a large part the setup decides more of the result than the cut does. A 20 t frame clamped hard onto an uneven support will be flat on the machine and out of flat on the floor.

Support and levelThe part is set on jacks or wedges at planned points, levelled, and checked for sag before clamping. Three point support avoids rocking on uneven blanks.
Clamp at the supportsClamps go over a support so they do not bend the part. Before the finish pass, clamps are released and re-tightened lightly.
Datum firstA reference face or bore is cut early and used for every later setup, so position between faces does not depend on a rough surface.
Fewer turnoversEvery turnover with a crane is a new setup and a risk. Angle heads, rotary tables and long travel reduce them.
TemperatureLarge parts are left to settle to shop temperature before finishing and measuring. Sun on a door or a cold morning moves a long part.

Stress relief before finish machining

Welding, casting and heavy roughing all leave residual stress. Machining removes material that was holding it, and the part moves. Typical methods below; the right temperature and hold time come from the material specification and the governing code.

Method	Typical use	Typical cycle	Notes
Thermal stress relief, furnace	Carbon and carbon manganese steel weldments, S355, A36, A572	Typically 550 to 650 °C, about 1 h per 25 mm of section, controlled heating and cooling	Removes most residual stress. Needs a furnace big enough for the part and leaves scale to clean off.
Thermal stress relief, castings	Gray and ductile iron castings	Typically 500 to 600 °C, slow furnace cooling	Done on the rough casting so the bores and faces stay where they are cut.
Local heat treatment	Parts that do not fit a furnace, site welds	Heating pads or induction coils on a band around the weld	Treats the weld zone only. Temperature gradients need to be planned so the part does not distort.
Vibratory stress relief	Weldments where a furnace is not practical	Part is vibrated near resonance for a set time	Reduces peak stress partly. Not a substitute where a code or drawing calls for post weld heat treatment.
Rough, rest, finish	Plates, forgings, heavily machined parts	Rough with allowance, release clamps, rest the part, re-clamp lightly, finish	Lets machining stress come out before the final cut. Often used together with one of the methods above.

When stress relief is needed and where it goes in the sequence

CMM and laser tracker inspection

Large parts are measured where they fit. The method should be agreed at the RFQ stage, because it changes both the cost and what the report can show. Background on coordinate measurement is on Wikipedia.

Method	Typical range	Typical uncertainty	Best for
Bridge or gantry CMM	Parts that fit the measuring volume, often up to 3 to 6 m	A few microns plus a length term, per ISO 10360	Bore position, form, profile, full first article reports
Laser tracker	Parts from 2 m to tens of metres, in place on the machine or floor	Typically a few hundredths of a mm over several metres	Flatness of large faces, alignment of bore lines, position between faces
Portable measuring arm	Within reach of the arm, typically up to about 4 m	Typically 0.03 to 0.1 mm	Features on parts too heavy to move to a CMM
On machine probing	Anything on the machine	Limited by the machine itself	Setting up and checking before unclamping. Not an independent check.

A measuring method should be several times more accurate than the tolerance it checks. A ±0.02 mm position over 3 m is a CMM job or a careful tracker job in a stable room, not a portable arm job.

Design notes that change the quote

Small things on the drawing that let a shop pick the right process and price it without padding for risk.

Give a machining allowanceCastings and weldments typically need 3 to 10 mm per face on large parts, more on long faces and forgings. Too little and the scale or a distorted face does not clean up.
Put the datums on stable featuresA datum on a machined pad or a bore that is cut early holds better than one on a cast or flame cut edge. It also lets the shop transfer the datum between setups.
Call out the sequenceIf stress relief has to happen between roughing and finishing, say so on the drawing. Otherwise the quote may assume one setup with no heat treatment.
State the measuring temperatureSteel grows about 0.0115 mm per metre for each °C. On a 4 m part, 5 °C is already more than 0.2 mm. Tolerances are normally at 20 °C unless the drawing says otherwise.
Tighten only where it mattersEach step from IT8 to IT7 on a large bore adds passes and inspection time. Keep tight tolerances on functional features.

Questions about large part processes

Anything else, email and a person answers.

Which process does my large part need?

It follows from the shape and the weight. Flat parts with large faces go to a gantry or travelling column mill. Housings with bores on several faces go to a table type boring mill, or a floor type mill if they are too heavy for a rotary table. Round parts wider than they are long go to a vertical turning lathe. Coaxial bores far apart are line bored. Many parts use two or three of these in sequence.

Does my weldment need stress relief before finish machining?

If it carries bores or faces with tight position or flatness tolerances, usually yes. A weldment that is finish machined without stress relief can move by tenths of a millimetre once residual stress redistributes. The usual sequence is weld, stress relieve, rough machine, then finish machine.

How is a part inspected if it does not fit a CMM?

With a laser tracker or a portable arm, often with the part still on the machine or on the floor plate. Trackers cover tens of metres with an uncertainty of a few hundredths of a millimetre, which suits flatness, alignment and position over large distances.

Can line boring be done on site?

Yes. A portable boring bar is mounted on the structure and cuts the bores in place, which is common for hinge bores and bearing lines on heavy equipment. Site work typically holds a little less than shop work and needs access, power and a safe setup.

Do you run these processes yourselves?

No. We are a matching service, not a machine shop, and we own no machines. We read your RFQ and send it to up to 3 shops that run the processes your part needs. The shop that quotes you confirms what its own machines and methods hold on your geometry.

Reading further

Find the part type closest to yours

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Send the envelope, the finished weight, the material and any stress relief or inspection the drawing calls for. We match it with shops that run those processes. Reply within 24 business hours.

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