Guide
How do machinability ratings affect CNC cost, and when is a free-machining grade acceptable?
A machinability rating is a relative index of how fast and how cheaply a metal can be cut compared with a reference grade. On the AISI-based charts used for steels the basis is stated as 160 Brinell B1112 steel and 1212 is listed at 100; copper alloys have their own scale with C36000 free-cutting brass at 100. A grade rated around 50 generally has to be cut at roughly half the speed of the reference for the same tool life, so when a part is mostly turning or milling time, the alloy on the drawing can quietly set a large share of the price.
By Saola Manufacturing engineering team · Published · Updated
This guide is for the design engineer or buyer whose quote came back higher than expected and who suspects the material. It explains what the rating does and does not measure, why a slow-cutting grade costs more, how to check whether the alloy is really the cost driver before changing anything, when a free-machining grade such as 12L14, 1215, 303 or C360 is a safe substitute and when it is not, and what to write on the drawing and the RFQ so a supplier can quote the alternative without guessing. The ratings shown are approximate planning numbers from published charts, not cutting data, and the free-machining bar grades they refer to are defined in ASTM A108 for cold-finished carbon steel bar, ASTM A582 for stainless steel and ASTM B16 for brass.
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What does a machinability rating tell you about part cost?
The rating tells you how fast a grade can be cut relative to a free-machining reference at comparable tool life, so it predicts the cost of the time a part spends in the cut. It does not price the bar, the setup, the inspection or the operations that are not cutting.
The steel scale comes from American Iron and Steel Institute (AISI) turning tests, in which a resulfurized free-machining carbon steel, B1112 at 160 Brinell, was set at 100 and other grades were scored mainly by the cutting speed they allowed at comparable tool life. Charts built on that basis list 1212 at 100%, and higher means easier: 12L14 is listed at 170% and 304 at 45% (Enerpac machinability chart; Engineering ToolBox). Copper alloys use their own scale with C36000 at 100, published by the Copper Development Association. Aluminum appears on the steel charts only as generic “cold drawn” or “cast” entries, so alloy-to-alloy comparisons such as 6061 against 2011 come from supplier descriptions rather than one number, and a brass number and a steel number should not be compared directly.
You can usually recognize a material-driven quote by its pattern. The price is high even though the part has no tight tolerances or difficult features; a turned part with heavy stock removal costs far more than a similar milled bracket; two suppliers quote close to each other, which hints that the cost comes from machine time rather than margin; or the supplier’s manufacturability note says something like “quoted in 303, 304 adds cost” or “stringy chips, attended run only.” If any of those apply, the alloy deserves a look before you start widening tolerances.
Why does a low-machinability alloy cost more to machine?
The biggest cost is slower cutting speed, which stretches cycle time on every part. Behind it come faster tool wear, poor chip control that prevents unattended running, extra passes and deburring to reach the finish, and, last, the price and availability of the bar itself.
- Cutting speed. A lower rating means a lower surface speed for the same tool life. On a turned part where most of the cycle is metal removal, cycle time rises almost in proportion, and machine time is usually the largest line in a machined-part price.
- Tool wear and tool changes. Austenitic stainless work-hardens ahead of the tool and alloy steels in a hardened condition are abrasive, so inserts wear faster, offsets drift sooner and more parts are spent on measurement and adjustment.
- Chip control. Free-machining grades break into short chips because of their sulfide or lead inclusions. Plain low-carbon steel and 304 tend to form long, stringy chips that wrap the part and the tool, which is a common reason a lathe cannot run unattended or with bar feeding at full speed.
- Finish and burrs. Gummy grades tear rather than shear, so a finishing pass, a slower feed or a separate deburring operation is often needed to reach the same surface and edge condition.
- Material price. Bar price differs by grade and by size, and an uncommon grade in a specific diameter can carry a premium or a minimum purchase that a common grade does not.
The table below gives the approximate ratings published for the grades buyers most often ask about, with the trade-off that comes with each. Use it to rank options and to frame a question to the supplier, not to calculate a price: the same grade can rate quite differently in the annealed, cold drawn or pre-hardened condition, and published charts disagree by several points.
| Grade and condition | Family | Approximate rating | Main trade-off |
|---|---|---|---|
| 1212, cold drawn | Resulfurized free-machining carbon steel | 100 (steel basis) | Poor weldability; lower transverse ductility |
| 12L14, cold drawn | Leaded, resulfurized carbon steel | 170 | Contains lead; not for welding or highly stressed parts |
| 1215, cold drawn | Resulfurized, rephosphorized carbon steel | 136 | Lead-free, but still poor for welding |
| 1018, cold drawn | Low-carbon steel | 78 | Weldable and carburizable; long, stringy chips |
| 1045 | Medium-carbon steel | Not listed; 1040 is 64 and 1050 is 54 | Hardenable; resulfurized 1144 is the free-cutting relative |
| 4140, annealed | Chromium-molybdenum alloy steel | 66 | Pre-hardened bar cuts slower than annealed |
| 416, annealed | Free-machining martensitic stainless | 110 | Magnetic; corrosion resistance well below 304 |
| 303, annealed | Free-machining austenitic stainless | 78 | Lower corrosion resistance than 304; not for welding |
| 304, annealed | Austenitic stainless | 45 (some charts 40) | Work-hardens; gummy chips |
| 316, annealed | Molybdenum austenitic stainless | 45 | Best chloride resistance of the three; as slow as 304 |
| C36000 (C360) | Free-cutting leaded brass | 100 (copper-alloy basis) | Contains lead; restricted for drinking-water wetted parts |
| 6061-T6 and 7075-T6 | Wrought aluminum | No alloy-specific value; generic cold-drawn aluminum is listed at 360% | 2011 free-machining aluminum cuts better but has poorer corrosion resistance |
How do you check whether the alloy, not the design, is driving the price?
Ask the supplier to quote the same drawing in one or two alternative grades and to say what share of the cycle is cutting time. If the alternative is much cheaper and the part is mostly in the cut, the alloy is the driver; if the price barely moves, look at tolerances, features and setups instead.
Work through this checklist before changing the material callout:
- How much of the part is in the cut? A turned shaft machined from bar with heavy stock removal is sensitive to machinability. A milled part with many setups, probing and tool changes spends more of its time not cutting, so a better grade helps less.
- Is the stock removal large? Compare the finished volume with the bar or plate it is cut from. Parts that remove most of the stock gain the most from a faster-cutting grade.
- Is the price split known? Ask for material, machine time and secondary operations as separate lines, as described in the guide to CNC machining cost drivers. A material question needs a machine-time answer.
- Why was the grade chosen? Find the reason in the design history: welding, a heat treatment, a corrosion environment, a cold-forming step, a regulation, or simply the grade the last designer always used. A grade chosen by habit is the easiest one to change.
- Is the bar condition the real problem? Ratings fall as hardness rises, so a 4140 called out pre-hardened costs more to cut than the same grade annealed. Where the drawing allows, buying annealed or normalized bar and heat treating after rough machining is a common way to keep the grade and still cut faster (Tech Steel & Materials). Allowing for distortion and a finish pass after heat treatment is part of that trade.
- What else is expensive? Tight tolerances, deep small holes and thin walls raise cost on any grade, and the tolerance cost guide covers those levers. Fix the right one first.
A rough estimate shows whether a switch is worth the paperwork. Multiply the cutting share of the cycle by the ratio of the two ratings and add back the non-cutting share. For a typical turned part where cutting is about 70% of the cycle, moving from 304 (45) to 303 (78) gives 0.3 + 0.7 × 45/78, or about 0.7 of the original machine time. The same arithmetic on a typical milled part where cutting is 30% of the cycle gives about 0.87, which may not justify a new material approval. Treat the result as a reason to ask for a comparative quote, not as a price.
If the quote comparison shows a meaningful difference and the grade was not chosen for a functional reason, move to the substitution rules below. If the grade was chosen for welding, corrosion or regulation, the cheaper grade is usually not an option, and the saving has to come from design, volume or process instead.
When is a free-machining grade an acceptable substitute, and when is it not?
A free-machining grade is usually acceptable when the part is not welded, not cold formed, not highly stressed in fatigue, not exposed to a severe corrosion environment and not governed by a lead restriction. If any one of those applies, keep the standard grade or choose a different improved grade rather than the free-machining one.
Free-machining grades cut well because they carry deliberate inclusions, manganese sulfides in resulfurized steels and stainless, and lead in leaded steels and brass. Those same inclusions are the source of every trade-off:
- Welding. High sulfur promotes hot cracking and porosity in the weld, and lead adds fumes, and 303 is generally not recommended for welded assemblies. Keep 1018 or 304 on any part that will be welded.
- Forming and fatigue. Sulfide stringers run along the bar and lower ductility and toughness across the grain. Parts that are crimped, swaged, flared, staked or loaded in bending fatigue are safer in the standard grade.
- Corrosion. Sulfide inclusions in 303 and 416 are sites where pitting starts, so their corrosion resistance is below 304, and 316 has no widely stocked free-machining equivalent. Passivation of free-machining stainless also needs a procedure suited to sulfur-bearing grades; the passivation guide covers it.
- Lead restrictions. Lead in steel for machining (up to 0.35%, exemption 6(a)-I) and lead in copper alloys (up to 4%, exemption 6(c)) are allowed under Annex III of the EU RoHS Directive, but those exemptions carry expiry dates and renewal reviews, so check the current annex before relying on them (Directive 2011/65/EU). Drinking-water parts sold in the United States must meet the Safe Drinking Water Act lead-free limit of 0.25% weighted-average lead on wetted surfaces (Public Law 111-380), usually shown by certification to NSF/ANSI 372, as covered in the lead-free brass guide.
- Heat treatment. 12L14 and 1215 can be case hardened for wear, but they are not a replacement for a through-hardened medium-carbon or alloy steel. Where strength comes from the grade, look at resulfurized relatives such as 1144 or a leaded or resulfurized version of the alloy steel, if the mill supply allows it.
| Specified grade | Faster-cutting candidate | Acceptable when | Keep the original when |
|---|---|---|---|
| 1018 | 1215 or 12L14 | Turned, not welded, lightly loaded, plated or painted for corrosion | Welded, cold formed, or lead not allowed (12L14) |
| 1045 | 1144 (resulfurized) | Strength needed but no welding; similar hardenability is acceptable | Welded or induction hardened to a specification tied to 1045 |
| 4140 | Resulfurized or leaded 4140 bar, or annealed bar heat treated after machining | Mill supply exists and fatigue is not the limiting case | Fatigue-critical or welded |
| 304 | 303, or a 304 bar melted for improved machinability within the 304 chemistry | Indoor or mild environment, no welding, no cold forming | Welded, chloride exposure, food or pharmaceutical wetted surfaces |
| 316 | A 316 bar melted for improved machinability within the 316 chemistry | Same grade designation and certificates as 316 | Never swap to 303 for chloride or marine service |
| C36000 brass | None; it is the copper-alloy reference | Lead is allowed for the market and application | Drinking-water contact; move to a lead-free brass and expect slower cutting |
| 6061-T6 | 2011-T3 or a lead-free free-machining 6xxx alloy | High-volume turned parts with little corrosion exposure | Welded, anodized for appearance, or outdoor exposure |
What should the drawing and RFQ say about the material and allowed substitutes?
Call out the grade, the standard and the condition on the drawing, and say in the RFQ which alternatives the supplier may quote. A supplier cannot offer a cheaper grade that the drawing does not allow, and should not substitute one silently.
A material block that a supplier cannot misread names three things: the grade, the product standard and the condition. For example: “Material: AISI 1215 cold-drawn bar per ASTM A108, or AISI 12L14 per ASTM A108 as alternate.” Or: “Material: UNS S30300 per ASTM A582, annealed; S30400 per ASTM A276 permitted.” If an alternate is acceptable only with a restriction, write it next to the grade, such as “lead-bearing grades not permitted” or “no welding on this part.”
In the RFQ, add a short material paragraph:
- The grade on the drawing and any alternates you will accept, by standard.
- A request to quote the drawing grade and at least one faster-cutting alternate as separate lines, so the cost difference is visible.
- The functional reasons that rule out substitutes: welding, corrosion environment, forming, fatigue, or a lead or RoHS requirement for the destination market.
- The material certificate you need with each lot, such as a mill test report or an EN 10204 type 3.1 certificate, as explained in the mill test report guide.
- That any substitution not on the drawing needs your written approval before the bar is cut.
If you approve a substitute, change the drawing revision rather than leaving it in an email. The next buyer, the next supplier and the next incoming inspector will read the drawing, and a mismatch between the drawing and the certificate is a nonconformance on the receiving dock even when the part is fine.
How is a material substitution handled on a Saola order?
Turned and milled parts are made at our lead CNC plant in Vietnam, and when a drawing allows alternates the quote can show the drawing grade and the alternate side by side. A grade is never changed without the customer’s written approval, and each quote names the production site and country of origin.
Machining runs at the lead CNC plant described on the CNC turning and CNC milling pages, so the effect of a grade on cycle time is assessed by the people who would run the part. If the drawing grade looks like a cost driver and the drawing does not allow an alternate, the manufacturability note says so and leaves the decision with you. Name the material certificate you need in the RFQ and it is confirmed in the quote, and a RoHS declaration is available on request, as listed on the quality and inspection page. ISO 9001 certification is in progress.
Where the part also needs plating, anodizing or another finish, that step is done at a partner plant we manage, and the quote states the production site and country of origin for each step. A free-machining grade can change how a finish behaves, especially passivation on 303 and anodizing on free-machining aluminum, so mention the finish when you ask for an alternate grade. To compare grades on your own part, send the drawing for a quote and list the alternates you would accept.
Frequently asked
Does a machinability rating of 50 mean the part costs twice as much?
No. It means the cutting portion of the cycle runs at roughly half the speed. Setup, handling, inspection, finishing and material do not scale with the rating, so the effect on total price is smaller, and largest on turned parts with heavy stock removal.
Is 303 always a safe replacement for 304?
Not always. It is a good choice for machined parts that are not welded, not cold formed and not in chloride or other aggressive service. The 303 vs 304 guide compares them in detail.
Which free-machining steel should I pick, 12L14 or 1215?
12L14 cuts faster but contains lead; 1215 is lead-free and still cuts well. The choice usually comes down to whether your market or customer restricts lead, as covered in the 12L14 vs 1215 guide.
Why do different charts give different ratings for the same grade?
Because the rating depends on the condition of the bar, the test method, the tool material and the reference grade used. Treat published numbers as a ranking and confirm the effect on your part with a comparative quote.
Can aluminum and brass be compared with steel on one machinability scale?
Not reliably. Copper alloys use C36000 as their reference and aluminum alloys are usually rated on separate scales. In practice, common wrought aluminum and free-cutting brass both machine much faster than any steel or stainless grade.
Sources
- ASTM A108 Standard Specification for Steel Bar, Carbon and Alloy, Cold-Finished, ASTM International, 2024
- ASTM A582/A582M Standard Specification for Free-Machining Stainless Steel Bars, ASTM International, 2022
- ASTM B16/B16M Standard Specification for Free-Cutting Brass Rod, Bar and Shapes for Use in Screw Machines, ASTM International, 2024
- ASTM B211/B211M Standard Specification for Aluminum and Aluminum-Alloy Rolled or Cold Finished Bar, Rod, and Wire, ASTM International, 2023
- Directive 2011/65/EU on the restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS), Annex III, European Union, 2011
- Reduction of Lead in Drinking Water Act, Public Law 111-380, United States Congress, 2011
- Machinability of metals, Engineering ToolBox, 2009
- Metal machinability ratings (AISI basis), Enerpac, 2021
- Machinability of metals, Tech Steel & Materials, 2019 (updated 2026)