How Do Machinability Ratings of Materials Affect CNC Cost and Tool Life?
What Do Machinability Ratings of Materials Really Tell You?
When you compare machined metals, machinability ratings of materials give you a quick check on how a material may cut, drill, turn, tap, and finish. On a sourcing page like machining materials, the rating helps connect the material choice with tool life, cycle time, surface finish, and quote risk. It is not a perfect number. In daily shop use, it is more like a quick reference that still needs judgment.
A Relative Score, Not a Material Property
A machinability rating is a relative score. It is not the same as hardness, tensile strength, yield strength, or corrosion resistance. Two materials can have close hardness values, but one may cut cleanly while the other smears, grabs the drill, or wears inserts quickly. This happens because machining also depends on chip behavior, heat, tool wear, and how the material acts under the cutting edge.
The B1112 Reference Point
The common rating system uses free-machining B1112 steel as the comparison point. Public machining references describe the AISI method as turning tests run at 180 surface feet per minute, with 160 Brinell B1112 set at 100 percent. A metal above 100 is usually easier to cut than that reference steel. A metal below 100 usually needs lower speed, sharper control, or more tool checks. Source: Enerpac, 2021. (blog.enerpac.com)
Tool Life, Finish, and Cutting Speed
The number is mainly used to plan cutting speed and tool life. It can also give an early warning on finish risk, burrs, heat, and insert wear. ISO 3685:1993 gives a formal frame for tool-life testing with high-speed steel, carbide, and ceramic single-point turning tools. It covers workpiece, tool, cutting fluid, cutting conditions, equipment, wear assessment, and result reporting. Source: ISO 3685:1993, published in November 1993 and confirmed current in 2017. (iso.org)
Which Material Groups Usually Machine Faster?
Ratings make more sense when you compare material families, not only one grade against another. A fast-cutting aluminum alloy does not behave like a gummy austenitic stainless steel, even when both arrive as clean round bars. The material group gives the first hint. After that, the exact grade, temper, hardness, and part shape decide how the job really runs.
Free Machining Steels and Leaded Brasses
Free-machining steels are often used for turned shafts, spacers, fittings, and screw-machine parts because their chemistry helps chips break. Sulfur in steel can form manganese sulfide inclusions, which lower cutting force and help chip control. ASME machining data summarized in public engineering references lists B1112 at 100, leaded brass at 200, phosphor bronze at 40, and a heat-treated nickel-chrome steel at 24. That gap explains why the same drawing can quote very differently after a stock change. Source: ASME Manual on Cutting Metals, summarized by ScienceDirect. (sciencedirect.com)
Aluminum Alloys in High Speed Work
Aluminum often machines at high speed, but it still needs the right cutter, rake, flute space, and chip evacuation. A NIST-hosted ASM chapter on machining aluminum reports that turning 2017-T4 aluminum needs about 35 percent of the cutting force required for low-carbon steel with similar basic mechanical properties. The same source says metal removal per unit kilowatt can be about three times as high. That is why 6061 parts are usually easy to quote in a CNC shop, although soft or sticky tempers can still pack chips in drills. Source: ASM International chapter hosted by NIST. (materialsdata.nist.gov)
Stainless, Titanium, and Nickel Alloys
Stainless steel, titanium, and nickel alloys are usually on the slower side. They can keep heat near the cutting edge, work harden, or spring away from the tool. Shops still choose them for corrosion resistance, strength, heat resistance, or medical and aerospace work. A low rating does not mean the material is poor. It means the cutting window is narrower, and the quote should allow for slower feeds, lower speeds, and closer tool checks.
Why Can the Same Rating Behave Differently in Your Shop?
Two shops can cut the same grade and get different results. That does not make the rating chart useless. It only means the chart cannot see spindle condition, fixture stiffness, insert grade, coolant pressure, operator habits, or a deep pocket that holds hot chips for too long.
Hardness, Heat Treatment, and Microstructure
Steel is a common example. ASM Handbook material notes state that steel machinability is affected by composition, microstructure, strength level, feed, speed, depth of cut, cutting fluid, and tool material. The same ASM source also notes that lead can improve surface finish, sulfur can change machining behavior in alloy steels, and cold drawing often improves machinability in steels with less than about 0.2 percent carbon. These details matter because two bars with the same grade name may not cut the same after different processing routes. Source: ASM Handbook, Volume 1, 1990. (dl.asminternational.org)
Tool Geometry and Machine Rigidity
A rating assumes the cutting setup is reasonable. If the tool sticks out too far, the vise clamps a thin wall badly, or the spindle has poor support, the material may look worse than its chart value. Sharp positive geometry can help aluminum and stainless. Stronger edge prep can help cast iron or interrupted steel cuts. A rigid setup is not exciting, but it protects tools and keeps size under control.
Chips, Coolant, and Part Shape
Chip control can change a job more than the rating shown in a table. A simple turned bushing in 303 stainless may run well, while a deep blind pocket in the same family may hold chips and heat. Through-coolant drills, peck cycles, chipbreaker inserts, and the right flute count all matter. One small detail can decide the result, because a chip nest around a small tap can ruin parts faster than a wrong speed chart.
How Should You Use Ratings When Choosing Materials?
Use machinability ratings as a first filter, then check what the part must do in service. The best material is not always the fastest one to machine. Sometimes slower machining is worth it because the part needs strength, wear resistance, food-grade corrosion resistance, or a hard anodized finish after cutting.
Start With a Safe Speed Band
A practical method is to adjust a known cutting speed from a material you already understand. If a chart lists a material at 70 percent of the reference material, you would start near 70 percent of the reference cutting speed for a similar tool and operation. After that, watch the chips, sound, spindle load, burrs, and tool wear. The rating gives the first setting. The machine and the part decide the final setting. See also: Machines.
Match the Rating to the Drawing
For a cosmetic aluminum enclosure, machinability, surface finish, and anodizing response may matter more than ultimate strength. For a threaded stainless fluid fitting, corrosion resistance and thread quality may matter more than speed. For a large batch of small turned pins, a free-machining steel like 12L14 can reduce cycle time, but it may not fit welding, plating, or high-load service. The drawing should lead the material choice, not the rating alone.
Ask for Stock Condition Early
Before placing a production order, check stock form, temper, hardness range, mill certificate needs, and any substitution rules. Bar, plate, forging, and casting do not always machine the same way. Heat-treated stock may remove a secondary process, but it can also increase insert wear. If the tolerance is tight, ask early. A late material change can turn a clean quote into a long shop discussion.
What Rating Mistakes Raise CNC Machining Cost?
Most cost trouble starts when a rating is treated as a promise. It is safer to treat it as a planning note. If the number says easy, the part can still be hard to make. If the number says hard, the part can still run well with the right machine, cutter, and setup.
Treating One Chart as Law
Different charts may use different test conditions, reference materials, tool types, or rating scales. Some focus on turning, while others come from shop experience. This is why a universal cross-family table should not be the only quoting tool. When good public data is not available for a special alloy, the better answer is to quote from test cuts, supplier data, or cautious shop experience instead of making up a clean number.
Ignoring Work Hardening
Work-hardening alloys punish light rubbing cuts. If the tool skims instead of cutting, the next pass may hit a harder surface. Austenitic stainless is the usual case, but nickel alloys and some manganese steels can behave the same way. Keep the tool engaged, use enough feed to make a chip, and avoid dwelling at shoulders or hole bottoms.
Chasing the Cheapest Stock
Cheap raw material can become expensive after machining. A lower-cost grade may need more cycle time, more deburring, more tool changes, or slower inspection because the finish is not steady. A free-machining grade can reduce cycle time, but it may hurt weldability or limit service conditions. In real purchasing work, price per pound is only one line. Machine minutes often carry more weight.
FAQ
Q1: What Are Machinability Ratings of Materials? A: They are relative scores that show how easily a material can usually be machined compared with a reference material. They help estimate cutting speed, tool life, finish risk, and general CNC cost.
Q2: Is a Higher Machinability Rating Always Better? A: Not always. A higher rating usually means easier cutting, but the material must still meet strength, corrosion, heat, wear, welding, and finishing needs. A fast-cutting material that fails in service is not a good choice.
Q3: Why Does 304 Stainless Cost More to Machine Than 6061 Aluminum? A: 304 stainless tends to work harden, hold heat, and make tougher chips. 6061 aluminum usually cuts with lower force and higher speed, so cycle time and tool wear are often lower.
Q4: Can Machinability Ratings Predict Exact CNC Cycle Time? A: No. They give a starting point. Exact cycle time depends on part geometry, toolpath, machine power, fixture rigidity, tool type, coolant, tolerance, and batch size.
Q5: How Should You Choose Between Two Similar Materials? A: Compare rating, required properties, stock availability, finishing needs, and total machined cost. If the job is high volume or the alloy is unusual, a small test run is safer than relying only on a chart.
