Is Your Part CNC-Machinable? Check Before You Send the Enquiry
A designer contacted us recently about a custom stainless steel necklace clasp for a 2027 collection.
We went back and forth for several rounds — the process, sample and production pricing, minimum order, tooling cost, lead time, shipping. We answered every question in detail.
Then he sent the reference image.
One look, and we knew: it was a casting. We cannot make it.
This is not the first time.
The CEO of a Korean golf training equipment company sent us a magnetic connector drawing — one component in it could only be cast. A German craft materials wholesaler wanted findings whose geometry was outside what a lathe can reach. A US stationery founder asked about a connector whose shape needed a mould.
In every one of those cases, the same thing happened: we discussed price, material, lead time and shipping — before anyone had confirmed the single question that actually matters.
Can this part be machined at all?
If the answer is no, every other question is wasted effort — for you and for us.
So this article is ordered differently from most. The 30-second check comes first. The explanation comes after. Use the check before you send your next enquiry, and you will save yourself about a week.
A note on where we stand
We are a CNC machining shop in Dongguan, China. We turn, drill, tap, polish and assemble small precision parts in 304 and 316L stainless steel, titanium and brass.
We do not produce cast parts. No sand casting, no die casting, no investment casting. When a component requires a cast geometry, we tell the customer directly instead of quoting for something we cannot do well.
That is worth stating up front, because it changes how you should read the rest of this article.
When we tell you a part should be cast, we are not trying to sell you anything. We gain nothing from that advice. A foundry writing the same article has a mould to sell you. We don’t. Take that for what it is worth.
Before you send the enquiry: the 30-second check
Look at your part — the drawing, the sample, or the photograph of the product you are trying to match.
Three red flags. If you see any one of these, it is very likely a casting.
- A parting line. A faint seam running across the surface. It means the mould had two halves, and it is the single clearest fingerprint of casting.
- Ejector pin marks. Small circular depressions or raised dots, usually on a non-cosmetic face. These are where pins pushed the part out of the die.
- An enclosed cavity, or a shape that wraps around itself. If you cannot imagine a cutting tool physically reaching a surface — because the material closes over it — the part needs a mould.
Four supporting signs. The more you see, the more likely it is cast.
- Draft angle. Cast parts must release from a mould, so side walls are never truly vertical — expect 0.5° to 3° of taper. A machined part can be a genuine 90°.
- Large internal fillets. Big rounded corners inside a cavity point to casting. A near-sharp internal corner was cut by a tool.
- Raised lettering or logos. Raised text is usually cast, because it is cheap to put into a mould. Engraved or recessed text is machined or laser-marked.
- Orange-peel texture, flow lines, or a grainy feel. Concentric tool marks mean turning. Orange peel means casting.
Now the other direction — what we CAN almost always machine
If your part is primarily a rotational body — cylinders, steps, tapers, bores, threads, grooves — then it can almost certainly be cut from bar stock. No mould, 10-piece minimum, about three weeks to a functional sample. (If you are bracing for a tooling bill, read Do Custom Metal Clasps Require a Mould? — the short answer is no.)
And note this: cross-holes, flats, slots and off-centre features do not disqualify it. A mill-turn machine cuts those with a powered tool. Many machined parts look irregular and are still entirely cut from bar. To see what our machines handle — including mill-turn work at 1 mm scale — see our machining capabilities.
The honest rule
Not “how strange does the shape look” — but “can a cutting tool physically reach every surface?”
If yes → machining works. If the geometry encloses a cavity, wraps around itself, or needs a shape no tool path can sweep → you need a mould.
Run this check before you write to any machine shop. It takes half a minute, and it will tell you whether you should be talking to us at all.
Why one drawing often contains two processes
Here is the part that catches people out even after they understand the basics.
Most products are not made of identical parts. A single assembly usually contains:
- Structural, cosmetic or complex-shaped parts — housings, brackets, handles, decorative bodies, anything with curves, cavities or organic forms
- Functional, precision parts — shafts, pins, collars, threaded bodies, connector ends, anything that must fit, rotate, seal or carry load
Those two groups rarely share an ideal process. The first often wants casting. The second almost always wants machining.
This is normal, not a design error. But it means your drawing should be evaluated part by part, not as a single quote request.
With the Korean golf connector, that is exactly what we did: we separated the components, said which we could machine and which needed a foundry, and explained why. No quotation went out until that split was clear.
So even if part of your assembly is a casting, the rest may be entirely machinable — and worth sending us on its own.
Casting vs. CNC turning: the full comparison
| Casting (sand / die / investment) | CNC turning (bar stock) | |
|---|---|---|
| How it forms | Molten metal poured into a mould cavity — near-net shape | Material cut away from solid bar stock — subtractive. Besides solid bar, the blank can also be tube stock — currently stainless steel tube and aluminium tube. For clasp-type parts, solid bar is the more common choice |
| Tooling | A mould is mandatory. High cost, long lead time | No mould. Program, load the bar, run the part |
| Typical geometry | Irregular shapes, curves, internal cavities, hollow sections, raised lettering, multiple features formed in one piece | Rotational forms: cylinders, steps, tapers, bores, threads, grooves |
| Economic quantity | Large volumes — the mould cost must be amortised (usually thousands of pieces) | Small batches are viable. 10 pieces is a real order, not a favour |
| Unit cost behaviour | Very low per piece at high volume | Unit cost falls with volume, but far less dramatically |
| Tolerances | Loose. Die casting around ±0.1 mm; sand casting looser. Critical faces usually need a second operation | Down to ±0.01 mm, held consistently from prototype through production |
| Surface finish | Parting lines, ejector pin marks, flow lines, porosity — usually needs grinding or polishing before it looks right | Mirror, brushed, PVD or passivated straight off the machine |
| Internal structure | Possible gas porosity, shrinkage cavities and coarse grain — affects strength and pressure tightness | Wrought bar stock: dense, fine-grained, no internal voids |
| Mechanical properties | Lower strength, toughness and fatigue life than the same alloy in wrought form | Retains the grain flow of the rolled bar — stronger and more fatigue-resistant |
| Materials | Die casting is limited to low-melting alloys (zinc, aluminium, magnesium). Stainless steel cannot be die cast — it requires investment casting | Almost unrestricted: stainless steel (304, 316 / 316L), titanium, brass, copper, iron, aluminium, and plastics |
| Wall thickness | Handles both thin walls (1–2 mm in die casting) and very heavy sections | Very thin walls (<0.5 mm) tend to deflect and are hard to hold |
| Design changes | New or modified mould — weeks, plus a second tooling bill | Change the program. Parts can be on the machine in a day or two |
| Prototyping | 4–12 weeks before you hold the first part | Around three weeks to a finished functional sample |
| Threads and mating faces | Usually need secondary drilling, boring or tapping | Formed in the same setup, including tapping. Internal threads cut directly on the lathe — a step we call thread turning |
The short version: casting wins on complex geometry at high volume. CNC wins on precision, small batches and fast iteration.
Materials and blank forms: more flexible than you think
Many buyers assume CNC turning means stainless steel bar and nothing else. The actual range is much wider.
On materials, we regularly machine stainless steel, titanium, brass, copper, iron, aluminium and plastics. Stainless is the most common, and within that we run 304 and 316 / 316L — 304 is the general-purpose choice, 316 / 316L offers the best corrosion resistance for saltwater, outdoor and skin-contact use. Which one you should use depends on your operating environment, strength requirement and budget. For a full side-by-side, see Stainless Steel vs Brass vs Titanium; if your part is worn at the beach or against skin, read Why 316L Beats 304.
On blank form, besides solid bar, we also work with tube stock — currently stainless steel tube and aluminium tube. Tube suits parts that are hollow, need a through-channel, or need to save weight, and it uses material more efficiently.
One thing should be said clearly, though: for clasp-type components, solid bar is still the usual choice. Clasps are small, need solid strength and full thread engagement, and cutting them from bar is the most direct route. Tube is more often used on structural parts with a genuine hollow requirement.
Example 1: an aluminium tube part with internal threads
A connector machined from aluminium tube — the outside diameter is turned, and internal threads are cut in the bore. Because the tube is hollow to begin with, the threaded bore can screw directly onto a mating male thread.
Parts like this one are machined from aluminium tube: the outside diameter is turned to shape, and threads are cut into the bore — a step we call thread turning. Because the tube is already hollow, the threaded bore screws straight onto a mating male-threaded component. That removes the cost of fitting a separate insert nut, and removes the consistency problems that insert fitting tends to introduce.
Example 2: a four-piece mouthpiece set
A mouthpiece component set of four parts (dimensions 8×21, 8×43, 8×3×1.2 and 8×6×5×1.2). Part 2, the long tube, is turned from 304 stainless steel.
In this mouthpiece set, part 2 is turned from 304 stainless steel. Its geometry is regular, with steps and a bore — a textbook rotational part. This is exactly the kind of component that says “lathe” at a glance: no mould needed, bar stock onto the machine, and the outer form, the steps and the bore all finished in a single setup.
What is interesting is that within one product set, the correct process is not the same for every part. Some parts are hollow and need a through-channel, so tube stock suits them. Others need solid strength and precise fit, so they have to be turned from bar. A supplier who understands process does not start with a quotation. They start by assigning every single part to the right process and the right blank form.
The decision tree we actually use
When a drawing arrives, we work through six questions in this order:
- Can a cutting tool reach every surface? If no → it needs a mould. Stop here; the rest does not matter yet.
- Is the main body rotational? Cylinders, steps, tapers, bores, threads → CNC.
- Does it need enclosed cavities, organic curves, hollow sections, or many features merged into one piece? → lean toward casting.
- What quantity? Under a few hundred → CNC. Stable volumes in the thousands → work out whether the mould cost ever pays for itself.
- What tolerance? Anything at ±0.05 mm or tighter, or parts that must be interchangeable across batches → CNC, or casting followed by CNC finishing.
- Is the design still moving? If you might change it, do not open a mould. Machine it now, cast it later once the design is frozen.
Question one is the one that saves the wasted week. It takes ten seconds, and if the answer is “no”, no amount of discussion about price or lead time will change the outcome.
“Irregular shape means casting” — and why that rule breaks
This is the rule most buyers are given, and it is a reasonable starting point. But it is a rule of thumb, not a law, and it fails in both directions.
Where it fails one way: modern CNC turning centres, mill-turn machines and Swiss-type lathes cut flats, cross-holes, slots, off-centre features and contoured surfaces. A machined part can look surprisingly irregular and still be entirely cut from bar stock.
Where it fails the other way: investment casting produces near-net shapes with fine detail and smooth surfaces — including in stainless steel. A cast part can look precise and still require a mould.
So the honest rule is not about how strange the shape looks. It is about whether a cutting tool can physically reach the material.
If your part turns out to be a casting: three ways forward
Finding out that your part needs a mould is not the end of the project. Here is what to do next.
1. Take it to a foundry — and know what you are buying into
We are not one, and we will say so rather than quote for it. Be ready for tooling cost and a 4–12 week tooling lead time before the first part exists. If your volume is in the hundreds, do the arithmetic first: at that volume machining is almost always cheaper and faster.
2. Send us the casting afterwards for finishing
This is the option most buyers miss. A casting is rarely a finished part. The surfaces that actually matter — sealing faces, bearing bores, threaded holes, mating interfaces — are almost always machined afterwards, because casting tolerances are too loose for a reliable fit.
We do not perform the casting operation, but precision finishing of supplied blanks — turning, drilling, tapping, polishing — is work we do every day, at 1 mm scale and ±0.01 mm when the geometry allows.
So “we can’t cast it” does not have to end the conversation. Ask instead: “can you do the post-machining on our castings?”
3. Ask whether the design can be changed
This is often the best option for a small brand, and almost nobody considers it.
If your volume is modest, a small design change can remove the mould entirely. Split a complex cast body into two machined parts that assemble, replace an organic curve with a turned profile, or move a cast feature to a machined insert. You lose a little design freedom and avoid a tooling bill and a three-month wait.
We are happy to look at a casting design and tell you honestly whether a machined alternative is realistic. Sometimes the answer is no — and we will say that too.
Four mistakes that cost buyers real money
Discussing price before confirming the process. This is the one that wasted the rounds of email at the top of this article. Confirm “can it be machined” first.
Opening a mould before the design is frozen. The tooling is built for one geometry. Every revision means cutting it again, and the second bill is rarely smaller than the first. Machine your prototypes. Cast your production.
Choosing casting for a 300-piece run. The mould cost dominates, and the per-piece saving never repays it.
Asking a die caster for stainless steel. Die casting is limited to low-melting alloys. Stainless requires investment casting — a different process, a different supplier and a different price structure.
Send us the drawing. We will tell you what is what.
You do not need to know which process your part needs before you contact a factory. That is our job.
But if you run the 30-second check first, you will know whether your enquiry is worth sending us at all — and you will get a useful answer on the first reply instead of the fourth.
Send us your drawing. We will tell you which components we can machine, which ones need a different process, and what each choice means for your cost and your timeline. If you would like to see the range of custom parts we have produced for other customers — from titanium ear picks to gearbox input shafts — see our custom non-standard parts.
Even the parts we cannot make, we will tell you honestly — and we will tell you on the first reply.
Frequently asked questions
Q: How can I tell whether my part is CNC-machinable before I send an enquiry?
Look for three red flags: a parting line, ejector pin marks, or an enclosed cavity / self-wrapping shape. Any one of them usually means casting. If the part is a rotational body — cylinders, steps, bores, threads — it can almost always be machined. Cross-holes, flats and slots do not disqualify it; a mill-turn machine handles those.
Q: Can you produce cast parts?
No. We are a CNC machining specialist. Sand casting, die casting and investment casting fall outside our manufacturing scope. We will tell you directly if your component requires a cast geometry rather than quoting for something we cannot do well.
Q: My part has an irregular shape. Does that mean it must be cast?
Not always. Modern mill-turn and Swiss-type machines produce flats, cross-holes, slots and contoured surfaces that look irregular but are entirely machined. The real test is whether a cutting tool can reach every surface — not how unusual the shape looks.
Q: My part turned out to be a casting. Is that the end of it?
No. Three options: take it to a foundry; send us the cast blanks afterwards for precision finishing (most castings need post-machining anyway); or ask whether a small design change could make it machinable and avoid the mould entirely. That last option often suits smaller brands best.
Q: What materials can you machine?
Stainless steel, titanium, brass, copper, iron, aluminium and plastics. In stainless, our common grades are 304 and 316 / 316L — 304 is the all-round choice, 316 / 316L the most corrosion-resistant for saltwater, outdoor and skin-contact use. On blank form, besides solid bar we also run tube stock, currently stainless steel tube and aluminium tube — though for clasp-type components, solid bar is still the usual choice.
Q: Does CNC production require a mould or tooling?
No mould is required. CNC machining is subtractive: our engineers program the toolpath and the raw bar stock goes straight onto the machine. This is why a 10-piece minimum order works economically, and why a functional sample can be delivered in about three weeks.
Q: Can you machine a part that was cast somewhere else?
Yes. Castings almost always require secondary machining on sealing faces, bores and threads. Send us the cast blanks and we can handle the precision finishing, in stainless steel or titanium.
Q: What is your minimum order for custom machined parts?
Ten pieces — and that applies to custom parts, not just stock items. Because we machine in-house rather than through tooling, low-volume runs are economical rather than exceptional.
Kaiming (kmintjewelry.com) / Dongguan Kaiming Stainless Steel Jewelry Co., Ltd. — CNC machining of precision components in stainless steel, titanium and brass. 50+ CNC machines including 6 Japanese Star Swiss-type lathes, 1,000㎡ workshop, tolerances to ±0.01 mm, 10-piece minimum order.
