Most large turned parts that go wrong at a subcontractor go wrong before the first cut. The part turns out to be heavier than the machine can carry between centres, the drawing is a revision behind, the forging arrives without enough stock to clean up, or nobody agreed how the finished part would be inspected and shipped. Each of these costs time that is hard to recover on a component that may take days to machine and weeks to replace.
This article sets out the checks worth making before a large shaft, roll, cylinder or sleeve leaves your site, in the order they usually come up. It is written from the subcontractor’s side of the bench: Mollart Cox carries out large CNC turning alongside deep hole drilling, honing and milling at two sites in Chesterfield, and the questions below are the ones that decide whether a job runs smoothly.
1. Will the part physically fit the machine?
Machine capacity is usually quoted as maximum diameter and maximum length, but those two figures alone do not tell you whether a part can be turned. Three other limits matter just as much.
Weight between centres
A lathe that can swing the diameter may not be able to support the mass, particularly on a long part where the load sits away from the chuck. Ask whether the stated weight capacity applies with or without steady rests, because a part that needs steadies needs suitable diameters to run them on. Mollart Cox’s largest turning centre machines parts up to 1,250mm diameter and 6,000mm long, and holds 10 tonnes between centres without steady rests.
The largest diameter, not the typical one
A shaft that is mostly 400mm diameter but carries a 1,100mm flange at one end needs a machine that swings the flange. Check the largest feature on the part, including any lugs or bosses that project beyond the main diameter, against the swing over the bed and over the cross slide.
Internal reach
Bores and internal features are limited by how far a boring bar can reach while staying rigid. For tubular parts, ask about spindle bore size and boring bar length as well as external capacity. Mollart Cox’s second turning centre handles parts up to 600mm in diameter and 4,000mm long, with a hollow spindle and long boring-bar capacity for internal work. Bores beyond the reach of a boring bar are a job for deep hole drilling and boring rather than turning. The plant list gives the full set of capacities.
2. Is the drawing ready for someone outside your business?
An in-house machinist will often know which features matter and which dimension is really the datum. A subcontractor works from what is on the drawing, so it needs to stand on its own.
Before sending, it is worth confirming that the drawing is the current revision and that any marked-up changes have been formally issued. Datums should be clear, with concentricity, run-out and straightness called up against them where they matter. Surface finish should be stated by feature rather than as a single general note if some surfaces need a finer finish than others. Where a dimension is critical to fit or function, saying so helps the subcontractor plan inspection around it.
A 3D model alongside the 2D drawing speeds up programming, but the drawing should remain the controlling document. If the two disagree, the subcontractor will have to stop and ask.
3. What is the starting material, and who is supplying it?
Large turned parts often start as forgings, castings or heavy bar, and the condition of that material shapes the whole job.
Machining allowance
A forging or casting needs enough stock on every surface to clean up fully. Too little and a surface may not clean up; too much and the job takes longer and costs more. Supplying the forging drawing with the finished part drawing lets the subcontractor check this before the material arrives.
Free issue or subcontractor supplied
Free-issue material keeps control of the supply chain with you, but any delay or defect in that material becomes a delay to the job. If the subcontractor sources the material, agree the specification and certification required at the quoting stage.
Certification and traceability
Material certificates should travel with the material and match the specification on the drawing. On regulated work, such as oil and gas, nuclear or defence, check whether heat numbers need to be transferred to the finished part.
Heat treatment
Agree whether the part will be machined before or after heat treatment, and whether stress relieving is needed between roughing and finishing. Large parts can move when internal stresses are released, and allowing for that in the process plan is far easier than correcting it afterwards.
4. What else does the part need besides turning?
Many large turned components also need bores drilled through their length, keyways or flats milled, cross holes drilled, or internal surfaces honed. Each time a heavy part moves between suppliers, it has to be lifted, transported, set up again and re-referenced to its datums, and each of those steps carries cost, time and the risk of handling damage.
Sending the whole job to one subcontractor that can complete it in sequence keeps the part on a consistent datum from start to finish. At Mollart Cox, turned parts can go on to deep hole drilling, gun drilling up to 6,000mm deep, CNC honing, large CNC milling and 5-axis machining under one roof. Where a long bore needs a precision internal finish, our article on why honing after deep hole drilling matters explains how the two processes work together.
5. Are the tolerances the ones the part needs?
Tight tolerances on a large part are achievable, but they cost more to hold and inspect as diameter and length increase. On a long, slender shaft, for example, cutting forces and the part’s own weight cause deflection that needs to be managed through support, tooling, and the number of passes.
Mollart Cox turns to tolerances of 0.0125mm and surface finishes down to 0.2Ra where the part calls for it. The question worth asking before sending a drawing is whether every feature needs that level of control, or whether a general tolerance block has been applied to features that only need a working fit. Opening up non-functional tolerances is one of the simplest ways to reduce both machining time and inspection time on a large part.
6. How will the finished part be inspected and documented?
Agree inspection requirements when the order is placed, not when the part is finished. Points to settle include whether a first article inspection report is needed, which dimensions must appear on the dimensional report, how run-out and straightness will be measured on a long part, and what paperwork must accompany the delivery.
Mollart Cox works to ISO 9001:2015 and ISO 45001, and its quality page has more detail on its quality systems.
7. How will it get there and back?
Transport is easy to overlook until a 6-metre shaft is sitting on a lorry. Check that the part has suitable lifting points or slinging positions that will not damage finished surfaces and agree who is responsible for transport in each direction.
Finished bearing diameters, seal faces, and threads need protecting for the return journey. Agree packaging and corrosion protection in advance, particularly if the part will be stored before fitting. For long, heavy parts, a cradle or crate that supports the part at the correct points prevents it from bending in transit.
Before you send the enquiry
A complete enquiry gets a faster and firmer quote. In summary, it should include:
- the current drawing revision and, where available, a 3D model
- finished weight, largest diameter and overall length
- material specification, condition and certification requirements
- the forging or casting drawing if the material is not bar
- any heat treatment and where it sits in the sequence
- every operation required, not just the turning
- inspection and documentation requirements
- quantities, delivery dates and transport arrangements
To discuss a large turned component, or to check whether a part fits our machines, contact the Mollart Cox team.