Machined castings: from drawing to assembly-ready part

A casting is not finished when it comes out of the tool. It is still attached to the runner system, carries overflows, has flash along the parting line, and faces that are too rough or too imprecise to meet the customer part.

Between cast and assembly-ready there are usually seven to nine operations. This article is about who should own them.

The nine steps

1. Casting. The part is formed. The parting line and ejector points land where the tool decided they would land — a decision taken before the tool was built.

2. Fettling. Runners, overflows and flash are removed. This is the step most often underestimated in a costing. Much of the work is manual, and certain geometries put flash in places that are awkward to reach.

3. Heat treatment. Only relevant for gravity cast aluminium parts that need higher tensile strength.

4. Machining. Faces and holes needing tighter tolerance than casting delivers are milled, turned or bored. Threads are cut or formed.

5. Blasting. Gives an even matte surface, removes small casting marks and gives the coating something to key into.

6. Surface finishing. Anodising or powder coating. With us this is outsourced to a sub-supplier — we manage the step, but we do not do it ourselves, and we say so plainly.

7. Inspection. Against the drawing, against the agreed surface reference, against the colour sample.

8. Packing. So the visible face survives transport.

9. Dispatch.

What it costs to own the steps yourself

The usual Norwegian model is that the foundry casts and a machine shop machines. On paper that looks cheaper, because you buy each operation where it is cheapest.

The arithmetic looks different once you include the interfaces:

Lead time. Every handover means packing, transport, receiving and a queue at the next supplier. One handover can add days to the lead time — not because anyone is working slowly, but because the part is standing still waiting its turn.

Tied-up capital. The part is work in progress while it sits in a queue. It has been paid for as a casting and cannot be invoiced as a component.

Incoming inspection. You have to check that the castings are good enough before sending them on, or you machine scrap.

Administration. Two suppliers means two purchase orders, two delivery schedules, two invoices, two complaint routes.

And the most expensive: the grey zone of responsibility. A thread is 0.2 mm out of position. Was the hole cast wrong, or was it the fixturing in the mill? Now you have two suppliers, each with a good argument that it was the other. You own the problem.

What becomes possible when everything happens in one building

This is not only a logistics gain. It is a technical gain, and easier to overlook.

The machining allowance can be optimised. When the person casting and the person milling plan together, the allowance can be placed exactly where it is needed and at exactly the thickness required. Less allowance is less swarf, shorter machining time and a lower price per part.

Faults are caught at the right end. If the threading operation shows a hole consistently a few hundredths out, the message travels ten metres — not to another company that has to find time to discuss it.

Fixturing can be designed into the casting. Datum faces and clamping points for machining can be cast in deliberately, rather than improvised by someone who receives a box of parts.

One point of responsibility. If the part is wrong, it is ours. Regardless of where in the chain the fault arose.

The honest caveat

We do not do everything ourselves, and we do not pretend to.

Anodising and powder coating are outsourced. That means one transport out, one back, and a few extra days of lead time. We order it, follow it up and raise complaints with the coater — you have one supplier and one invoice — but the part leaves the building at that step, and you should know it.

We state the added lead time in the quotation rather than hiding it.

Five questions to ask any supplier

Whoever you are talking to:

1. Which of the nine steps do you do yourselves, and which are outsourced?
2. How many days do the external steps add to the lead time?
3. Who is responsible if a machined dimension is out of tolerance — you or the foundry?
4. Is the machining allowance planned together with the casting, or is it given?
5. What is inspected, by whom, and do I get a measurement report?

The answers say more about what you are actually buying than the unit price does.

What assembly-ready means here

The part arrives cast, fettled, heat treated where relevant, machined, threaded, blasted, finished, inspected and packed. It goes straight into your assembly with no incoming inspection of semi-finished castings and no internal handovers.

One of our product examples is precisely this: a bracket die cast in zinc, threaded and finished here so the customer can use it directly in their own assembly.

Send the drawing and say which faces are to be machined and which are visible. We will then plan the whole chain as one — and you get one phone number for all nine steps.

The documentation that travels with the part

An assembly-ready part is not finished until the paperwork is. In contract manufacturing it is the supplier who has to document what the customer is asked for further up the chain, and four terms recur in every supplier approval.

FAI — first article inspection is the formal check of the first parts from a new tool: every dimension on the drawing is measured and recorded before the series is allowed to start. This is where the tool is adjusted, and it is cheaper to find a deviation on part number three than on part number three thousand.

A coordinate measuring machine (CMM) is what makes that record verifiable. It measures points in three axes against the CAD model and produces figures rather than an opinion.

Material certificate EN 10204 documents what the metal actually is. The standard has several types, from a simple declaration of compliance to a certificate based on analysis of the specific melt. If you need a particular type, say so in the enquiry — it changes how the feedstock has to be bought.

Traceability ties the three together: that a finished part can be traced back to melt, feedstock batch and production date. For load-bearing parts in medical equipment and defence it is usually an absolute requirement.

Two more requirements arrive when the part has a function of its own. Pressure tightness has to be tested, not assumed — a housing that must hold oil or air is checked with pressure or vacuum. Surface is stated the same way: an Ra value is a number, "smooth" is not. And if the part is bolted together, thread cutting in the CNC machine decides whether the thread is a specification or an accident.

One last thing about lead time

Lead time consists of two entirely different figures that are often conflated in an enquiry.

Tooling lead time runs from order to first approved trial part. It is a one-off cost in calendar time and dominates the launch of a new product.

Series lead time runs from call-off to delivery once the tool already exists. That is the figure that matters day to day, and it is where the number of links in the chain shows up.

Always ask for both, and ask specifically whether the external steps are included in the second figure. A promise of two weeks that does not include the coating is not a promise of two weeks.