What a Copper Machinability Rating Actually Tells You
September 21, 2026
Every copper alloy datasheet carries a machinability rating. It is a single number between roughly 20 and 100, and buyers use it to guess at price. It is genuinely useful, and it hides more than most people realise.
What the number is
Machinability ratings for copper alloys are relative, not absolute. Free-cutting brass C36000 is defined as 100, and every other alloy is scored against it under comparable cutting conditions. Pure coppers land near 20. Free-machining coppers land near 85. The full set of published values sits in the alloy data from the Copper Development Association.
The rating is fundamentally about how the chip behaves. Alloys that form short, broken chips rate high. Alloys that form long, continuous, gummy chips rate low. That is why adding half a percent of tellurium to copper moves the rating from about 20 to about 85 while changing almost nothing else.
| Alloy | Rating | Chip behaviour |
|---|---|---|
| C36000 free-cutting brass | 100 | Short, breaks clear |
| C14500 tellurium copper | 85 | Breaks well |
| C14700 sulphur copper | 85 | Breaks well |
| C17200 beryllium copper | 20 to 30 | Depends heavily on temper |
| C11000 ETP copper | 20 | Long and continuous |
| C10100 oxygen free copper | 20 | Long and continuous |
What it predicts well
Relative cycle time on chip-heavy work. Turning, drilling, tapping and anything with deep engagement. If your part is a turned connector body with threads, the difference between 20 and 85 is close to the difference in cycle time.
Tool life. Long chips re-cut, wrap and rub. Ratings track that reasonably well.
How much a shop will dislike the job. A low rating predicts complaints accurately.
What it hides
Hand work after the cut. The rating says nothing about burrs. On thin copper edges the deburring can take longer than the cutting, and that time does not appear anywhere in a machinability number. This is the most common reason a copper quote surprises a buyer.
Surface finish. A gummy alloy cut with the wrong tool smears instead of shearing. The rating does not tell you what finish you can expect, only how easily the chip leaves.
Heat. Copper conducts heat out of the cut into the part and the tool. That affects dimensional stability and tool wear, and it is not in the rating either.
Fixturing. Soft material needs careful workholding. Not in the rating.
Geometry. A simple flat bar in C110 can be cheaper to make than a fiddly part in C145. Machinability is a material property, not a part property.
How to actually use it
Use the rating for one decision only: whether a grade change is worth making.
If your part is feature heavy, the jump from a pure copper to a free-machining grade is the largest single lever you have on price. You give up roughly 6 to 8 points of conductivity and gain roughly four times the machinability rating. For terminals, fittings, connector bodies and anything with threads, that trade is almost always worth taking.
If your part is a plain bar or plate with a few holes, the rating matters much less, because you are paying mostly for metal and setup rather than for chips. Changing grade there buys you little and may cost you conductivity you needed.
And if the part goes into ultra high vacuum or gets brazed in hydrogen, the rating is irrelevant. The grade is decided by the application, and you pay what pure copper costs to cut.
C101, C110 and tellurium copper compared walks through the grade decision in detail, and why copper is difficult to machine explains what is happening at the tool that produces these numbers.
One thing to put on the drawing
If the machinability question is open, write the requirement instead of the grade. A note that says “C110 or equivalent, minimum 90 percent IACS” lets a shop offer C145 and quote a lower number. A note that says “C110” gets you C110 at C110 prices, even when nobody needed it.
Frequently asked questions
Does a machinability rating of 20 mean copper takes five times as long to machine?
No. It means the reference material can be cut about five times faster under the test conditions the rating came from. Real cycle time also depends on geometry, tooling, machine rigidity and how much hand work follows the cut.
Why is brass the reference at 100?
Free-cutting brass C36000 was adopted as the benchmark because it is the easiest common copper alloy to cut. The scale is relative to it, so any number above 100 simply means easier than that brass.
Is a higher machinability rating always better?
Only if the property you need survives the change. Free-machining grades trade a little conductivity for a large gain in machinability, which is usually worth it. Brass trades most of the conductivity, which usually is not.
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