Start From the Job, Not the Grade
Most drawings that reach us without a grade come with a job instead: a bracket that lives outdoors, a stud that carries two tonnes, a bushing that must not seize, a washer that must insulate, four spacers that hold 5 kg under a roof. The table answers those directly. EKINSUN LTD is a custom parts manufacturer that machines every material in it from 1 piece, so the choice is written on the quote, not left to the buyer.
| The job | First choice | When that is not it | Avoid | What decides |
|---|---|---|---|---|
| Outdoors, rain and humidity, no salt — brackets, spacers, fixings on a building or a vehicle | 304 (1.4301) stainless | Zinc-plated steel when hundreds of pieces must be cheap and a scratch rusting is acceptable; anodised 6061 when weight matters | Bare carbon steel; plain brass where the look matters (it dulls) | Years of service vs price: 304 costs about three times plated steel per piece and needs no coating |
| Salt water, coast, chlorides, washdown — boat, pool, food plant, road salt | 316 (1.4404) stainless | Brass CW614N for water fittings and low loads; bronze for sliding parts; duplex or super-austenitic for hot chloride service (ask) | 304: it pits in chloride service; plated steel | Chloride exposure: 316 has 2–3 % molybdenum, 304 none; both are 205 MPa yield as bar |
| Carries a load — stud, pin, adapter, lever, shaft | 42CrMo4 +QT (4140 HT): 750 MPa yield at 16–40 mm section | Class 10.9 or 12.9 bolt steel for threaded fasteners (940 / 1,100 MPa); 17-4 PH when it must also resist corrosion | Annealed 304 or 316 unless the load is small or corrosion rules: 205 MPa yield, less than a third of 42CrMo4 | Yield strength per section size, then the thread engagement: 1 d in steel, 1.5 d in brass, 2 d in aluminium |
| Slides, pivots or wears — bushing, guide, roller, cam follower | Bronze CuSn8 or CuSn7Zn4Pb7 against a steel shaft | POM (acetal) for light loads and no lubrication; hardened 42CrMo4 or case-hardened 16MnCr5 for the shaft; PE-UHMW for abrasive, wet sliding | Stainless on stainless (it galls); aluminium as a bearing surface | Load times speed, lubrication, and whether the part or the shaft should be the one that wears |
| Must insulate electrically or not be magnetic — washers, spacers, sensor mounts | POM for parts, PEEK where temperature or chemicals are high | PA6 / PA66 for toughness; brass or 6061 when it must be metal and non-magnetic | 304: it becomes slightly magnetic when machined or cold worked | Service temperature: POM to about 100 °C, PA66 to about 100–110 °C, PEEK to about 250 °C |
| Touches food, drink, drinking water or medical fluids | 316 (1.4404) stainless | Brass fittings where the local water standard allows the grade; PEEK, PTFE or POM-C for plastics | Leaded free-cutting brass CW614N in contact with drinking water in markets that restrict lead; plated steel | The regulation of the market the product is sold in; we quote the grade and its certificate |
| Hot — above 150 °C, exhaust, oven, process line | 304 or 316 stainless | 42CrMo4 up to its tempering temperature for loaded parts; PEEK (to about 250 °C) or PTFE (to 260 °C) for plastics | 6061 above about 150 °C (T6 strength falls); POM and PA above about 100 °C | Temperature at the part, not in the room, and how long it stays there |
| Must be light — hand-held, moving, on a drone or a robot arm | 6061-T6 aluminium: 2.70 g/cm³, 240 MPa minimum yield | 7075-T6 when strength per gram matters (about 500 MPa yield); Ti-6Al-4V (4.43 g/cm³, about 880 MPa) when it must also be strong and corrosion-proof | Steel of any kind if weight is the brief; 7075 outdoors without anodising | Strength per gram and the price per piece: titanium bar costs several times aluminium and machines at a fifth of the speed |
| Cheapest single piece or small batch — a prototype, one spare, five pieces | Free-cutting brass CW614N or 6061 aluminium | Free-cutting steel 11SMnPb30 / 12L14 when it must be steel; 304 when it must be stainless | Titanium, 17-4 PH, 316 for a part that only needs to exist | For one piece the price is machine time, not bar: brass and 6061 cut at two to three times the speed of 304 |
| Cheapest at 100 to 1,000 pieces — spacers, standoffs, pins for a product | Zinc-plated free-cutting steel or 6061 | 304 when the finish must last unplated; brass when it must not spark or rust at all | Over-specifying: 316 for an indoor part, 42CrMo4 for a 5 kg load | Bar price starts to matter at volume; machinability still decides most of the unit price |
| Will be welded or brazed | 304 / 316 (304L / 316L for thick sections) | Low-carbon steel S235 / 1018; 6061 welds but loses its T6 strength at the weld | Free-cutting brass and 11SMnPb30 (lead); 42CrMo4 without preheat and post-heat | Whether the weld or the base metal sets the strength |
| Replacing a worn part of unknown material | The same family as the original, identified from its colour, weight and a magnet | One grade up when the original wore out too soon (bronze for a brass bushing, 42CrMo4 for a C45 pin) | Guessing stainless for every grey metal | A photo, the weight and the magnet test tell us the family; see the reverse-engineering pages |
Not sure which grade? Send the job. Where the part sits, what it carries, what it touches and how many you need; we propose the grade and price a second option when two are close. From 1 piece; standard parts in common grades can also be supplied from stock; the more pieces, the lower the unit price. Quote in 12 hours.
Yield values are minimums from the standards named in the next table; service temperatures are the usual continuous limits, not the melting points. Thread engagement rule: one diameter of female thread in steel, one and a half in brass or cast iron, two in aluminium. Where two grades sit close, the quote prices both.
The Numbers Behind the Choices
Strength is the minimum yield of the bar in the condition we machine it; a stainless adapter turned from annealed bar is 205 MPa, not the 450 MPa of a cold-worked A2-70 bolt. Machining speed is given as a band relative to free-cutting steel, because published machinability indexes disagree with each other more than they disagree with the bands; bar cost is an order-of-magnitude band and moves with the metal markets.
| Material | Yield / strength | g/cm³ | Corrosion | Service temperature | Machining speed | Bar cost |
|---|---|---|---|---|---|---|
| 304 (1.4301) stainless, annealed bar | 205 MPa min | 7.9 | Good; not in chlorides | to about 800 °C (scaling), strength falls above 500 | about half of free-cutting steel | medium |
| 316 (1.4404) stainless, annealed bar | 205 MPa min | 8.0 | Good in chlorides, acids | as 304 | about half | medium-high |
| 17-4 PH (1.4542), H900–H1150 | three to four times 304 (by condition) | 7.8 | Close to 304 | to about 300 °C in service | about half | high |
| 42CrMo4 +QT (4140 HT) | 750 MPa min at 16–40 mm (900 up to 16, 650 at 40–100) | 7.85 | Rusts: plate, black-oxide or oil | to the tempering temperature | about two thirds | low-medium |
| C45 / 1045, normalised | lower than 42CrMo4; hardenable | 7.85 | Rusts: plate or coat | general | about two thirds | low |
| 11SMnPb30 / 12L14 free-cutting steel, zinc-plated | low; not for hardened or welded parts | 7.85 | Plating protects until scratched | general | fastest steel | low |
| 6061-T6 aluminium | 240 MPa min (about 270 typical) | 2.70 | Good; anodise outdoors | to about 150 °C | two to three times 304 | low-medium |
| 7075-T6 aluminium | about 500 MPa | 2.81 | Fair; anodise | to about 120 °C | as 6061 | medium |
| Brass CW614N (CuZn39Pb3) | 230 MPa min proof (230–350) | 8.5 | Good in water; dezincifies in some waters | to about 200 °C | fastest of all | medium-high (copper price) |
| Ti-6Al-4V (grade 5) | about 880 MPa | 4.43 | Excellent, including sea water | to about 400 °C | about a fifth of 304 | highest |
| POM (acetal, Delrin) | about 68 MPa tensile | 1.41 | Immune to water; attacked by strong acids | to about 100–110 °C | fast | low |
| PA66 (nylon) | about 83 MPa tensile, dry | 1.14 | Absorbs water (2–3 %, grows) | to about 100–110 °C | fast | low |
| PEEK | about 100 MPa tensile | 1.31 | Excellent; steam and most chemicals | to about 250 °C | fast, but the bar is costly | highest |
Two grades close? Name both; the quote prices them side by side from one drawing. The difference is usually machine time, not bar.
Minimums: 304 / 316 per ASTM A276 (bar over 12.7 mm); 42CrMo4 per EN 10083-3; 6061-T6 per ASTM B211 / B221; CW614N per EN 12164; 7075-T6 and Ti-6Al-4V are typical annealed or T6 values. Plastics: tensile strength, dry, typical. Each material name links to its own page with composition, equivalents and machining notes; the load a thread of each material carries is on the thread adapter strength chart; bolt classes on the bolt grade chart.
What a Grade Costs in a Machined Part
For one piece to a few dozen, the bar is a small part of the price and the machine time is most of it, so the cheapest material is the one that cuts fastest: brass CW614N and 6061 aluminium, then free-cutting steel, then 304 at about half their speed, then titanium at about a fifth. At hundreds of pieces the bar price starts to show, and a plated free-cutting steel or 6061 spacer undercuts the same part in 304 by a wide margin. Over-specifying is the common mistake: 316 for an indoor bracket, 42CrMo4 for a 5 kg load, titanium for a washer that could be 316L. The quote shows the price of the grade asked for and, when the job allows it, the cheaper grade next to it.
Three Recent Cases
Eight hundred spacers and two hundred drilled aluminium plates for a sheltered outdoor installation, four spacers carrying 5 kg together, temperature and humidity swings, the brief “zinc-plated steel, something that does not rust, or aluminium — whichever is cheapest”: at 800 pieces zinc-plated free-cutting steel is the cheapest spacer and holds 5 kg many times over; 6061 for the plates because they are flat and drilled, where aluminium cuts fastest; 304 only if the roof ever comes off.
Thin precision washers, 6 mm outside, 2 mm bore, 0.3 mm thick, 100 to 500 pieces, “grade 5 titanium or 316L or 304, or whatever you recommend”: 316L unless the washer must save weight or sit in a body; titanium bar costs several times more and cuts at a fifth of the speed, which at 0.3 mm thickness means the same flatness for a much higher piece price.
Six threaded reducers carrying two tonnes between them, “recommend the steel”: 42CrMo4 +QT at 750 MPa yield gives a margin above 20 : 1 on the smaller 1/2"-13 thread; annealed 304 bar gives about 5.7 : 1, which passes at a design factor of 4 but leaves less for shock. Stainless if the environment demands it, 42CrMo4 for the margin — both prices on the quote.
From Stock or Made to the Job
A stockist sells bar by grade and a catalogue sells parts by size; neither sells an answer to “which one”. That is the part we add.
| What you need | On the shelf? | What we do |
|---|---|---|
| The drawing names a grade and a stockist has the bar | Yes — metal stockists | Buy it from stock — the bar from a metal stockist if you machine it yourself; for the finished part, send the drawing and the grade and we quote that grade from 1 piece (standard sizes of many parts can be supplied from stock too; the more pieces, the lower the unit price). |
| No grade, only the job: where the part sits, what it carries, how many | No — a stockist sells grades, not answers | We propose the grade from the table above on the quote, with a second option priced next to it when two grades are close. |
| A grade from another country’s system on the drawing | No | Matched on the material grade equivalents pages; the grade used is written on the quote. |
| A material certificate with the parts | Rarely | EN 10204 3.1 certificate for the bar the parts were machined from, with the shipment. |
| Two candidates priced side by side — zinc-plated steel against aluminium, 316L against titanium | No | Both quoted from one drawing; the difference is usually machine time, not bar. |
| A hardness or a heat treatment you are not sure how to specify | No | Tell us the job; we write the condition (for example 42CrMo4 +QT, 28–34 HRC) on the drawing for your approval before machining. |
What to Send When You Do Not Know the Grade
What the part does and what it replaces; where it sits (indoor, outdoor, salt, washdown, heat); the load or pressure, even roughly; what it touches (food, water, chemicals, another metal); how many now and later; a photo or sketch with the main dimensions. The quote names the grade and its condition, the certificate, and a second grade when two are close. From 1 piece; quote in 12 hours; dimensioned drawing before machining.
Material Selection FAQ
304 (1.4301) stainless for rain and humidity without salt; 316 (1.4404) where there is salt water, road salt, chlorine or washdown, because 304 pits in chlorides. Zinc-plated steel is the cheap choice for hundreds of pieces if a scratch rusting is acceptable, and anodised 6061 aluminium when weight matters. Both stainless grades are 205 MPa yield as annealed bar, so the choice between them is corrosion, not strength.
42CrMo4 +QT (4140 heat treated): 750 MPa minimum yield at a 16–40 mm section, 900 MPa up to 16 mm, 650 MPa at 40–100 mm, against 205 MPa for annealed 304 or 316 bar. For threaded fasteners the bolt classes give the same answer: 8.8 is 640 MPa yield, 10.9 is 940, 12.9 is 1,100. Choose stainless for a loaded part only when corrosion forces it, and then check the smaller thread on the thread adapter strength chart.
For one piece or five, the price is machine time, not bar: free-cutting brass CW614N and 6061-T6 aluminium cut at two to three times the speed of 304 stainless, so they are the cheapest parts to make. Free-cutting steel 11SMnPb30 (12L14) is the cheap steel; 304 the cheap stainless. Titanium machines at about a fifth of the speed of 304 and its bar costs several times more, so it is the most expensive single piece.
POM (acetal) for most parts: stiff, dimensionally stable, immune to water, good to about 100 °C. PA66 (nylon) where toughness matters more than stability (it absorbs 2–3 % water and grows). PEEK where the part sees heat (to about 250 °C), steam or aggressive chemicals, at several times the price. For thin washers below 0.5 mm, glass-fibre laminate or PEEK film is often the practical choice.
No. Tell us what the part does: the load, the mating part, whether it slides or is struck, and the environment. We write the condition on the drawing — for example 42CrMo4 +QT at 28–34 HRC for a pin, or case-hardened 16MnCr5 for a wear surface — and you approve it before machining. A hardness number without a load behind it is the most common way to over-specify a part.
EKINSUN — a custom parts manufacturer that machines in 304 / 316 stainless, 42CrMo4, free-cutting steel, 6061 / 7075 aluminium, brass, bronze, titanium, POM, PA and PEEK from 1 piece. Send the job, not the grade, if you do not know it: the quote names the grade, prices a second option when two are close, and comes with an EN 10204 3.1 certificate; quote in 12 hours.