How Much Difference in Strength Is There Between CNC Machined Parts And Ordinary Castings? Will They Break?

Machinists are afraid to hear, "This part is cast, and as long as it works, it's fine. "Don't be too picky."
But picky people like to ask: how different is the strength of a cast from a CNC parts? Will they break down?
This question cannot be answered with a simple "almost." In this paper, the strength difference and fracture risk between CNC machined parts machine parts and ordinary castings are analyzed and explained by the data of 2026 frontier material and engineering test. Read this article to find out when casting can save money and when CNC can ensure survival.
First, the conclusion: it's not "almost," but an order of magnitude difference.
Many people think that casting and CNC are just different processing methods, and their strength should be similar.
Wrong. Totally wrong.
According to mainstream material data for 2026, CNC machined parts generally have a tensile strength of 30% to 80% higher than conventional castings, and fatigue life is 2-3 times that of conventional castings at the same volume. It's not just a question of "almost," it's a chasm between "usable" and "good," a life-and-death line between "unbreakable" and "easily broken."
First round: Tensile Strength-CNC-machined Castings
What is the strength ceiling of a regular casting?
Normal gray cast iron The tensile strength of less than a meager 10 kg/mm2 and high strength cast iron has tensile strength of less than 40 kg/mm2, which is approximately 98 -392 MPa in international terms. What does this mean? A common piece of 45# steel can achieve a tensile strength of more than 600 MPa. In other words, even the best ordinary cast iron can hardly meet the threshold of ordinary steel.
This is under ideal conditions. In actual production, castings are prone to internal defects such as porosity, shrinkage and inclusion, resulting in actual strength of only 60% to 70% of nominal value. This means that a piece of high-strength cast iron with a nominal tensile strength of 392 MPa may actually be only 235 to 274 MPa.
More crucially, the performance of castings depends to a large extent on manufacturing processes. Too much carbon content, too excessive alloying elements, too much iron or heat treatment-any one of these stages can lead to a dramatic drop in strength. Actual test data of the Zhengzhou foundry show that both too much carbon and too little silicon lead to an increase in increased hot brittleness, which causes castings to shatter on touch. Excessive use of boric acid in furnace lining can also lead to "boron brittleness" -when iron reacts with boron to form carbon-boron compounds, rendering the entire casting furnace unusable.
What level of strength can CNC machine parts achieve?
The strength of CNC machined parts depends on the machining process.
Take 7075-T6 aerospace aluminum, the most mainstream CNC brake rocker arm material of 2026, which has a tensile strength 570 MPa, close to 45# steel. By contrast, the tensile strength of ordinary cast aluminum alloys is generally only 200 to 300 MPa, nearly double the difference.
Consider a high-end example: C86300 aluminum bronze,also known as SAE430B. It is a copper-based centrifugal cast alloy with a tensile strength of 725 to 830 MPa, yield strength of 380 to 620 MPa and a Brinell hardness 170 to 240 HB. What does this mean? It is tougher than most ordinary steel and close to the level of some low-carbon steel. In addition, the alloy possesses high casting strength, no heat treatment is required to strengthen, and its elongation can be maintained between 10% and 18%, which achieves a balance of strength and toughness.
But here's the kicker: While C86300 is a cast alloy, its performance comes from a precise centrifugal casting process combined with strict composition control. Normal sand castings simply don't measure up.
The conclusion is therefore clear: CNC-machined 7075-T6 (570 MPa) is nearly twice as strong as conventional cast aluminum alloys (200-300 MPa). Both are copper alloys, but precision-cast C86300 (725-830MPa) is at least twice as strong as regular gray cast iron (98-392MPa).
Round 2: Fatigue Life-CNC Parts are 2 to 3 times longer than cast parts and high strength doesn't mean they won't break. It is not static tensile strength but fatigue life that really determines whether a part breaks.
What is fatigue life? It is the number of cycles a part can withstand before breaking under multiple stresses. Your brake rocker arm is pressed hundreds of times a day, and your engine links rotate thousands of times a minute- all fatigue loads.
According to engineering test data, the fatigue life of 7075-T6 aluminum alloy is 2-3 times that of gray cast iron. Simply put, under the same repetitive braking conditions, the CNC aluminum rocker arm can withstand 100,000 laps without cracking, while the cast iron rocker arm takes only 30,000 to 40,000 laps to develop fatigue cracks.
Why is it so different?
First, there were defects in the casting. Porosity, shrinkage cavities, inclusion and coarseness-these are all inherent defects in casting. According to industry analysis, high levels of impurities in casting alloys reduce its plasticity. High sulfur or low silicon content can lead to hot brittleness, which directly leads to a decrease in toughness and ductility, making them vulnerable to breakage during subsequent use.
Secondly, CNC machining involves milling from a single blank. No welding, no seams, no casting pinholes. Integral molding means that there are no weak points, the internal structure is uniform and compact, not easy to produce fatigue cracks.
Third, the surface quality of CNC-machined parts is much higher than that of castings. The surface of the castings is rough and requires secondary machining to improve accuracy and surface quality. However, CNC machining can directly achieve mirror accuracy, making the surface smooth and stress concentration low, making it difficult for cracks to expand from the surface.
Experimental data from Shanghai Jiao Tong University further show that the thermal decay rate of aluminum alloy brake components is 56% lower than that of cast iron. The braking maneuvers power of the CNC-machined part barely decreased after 10 consecutive large brakes, while the cast iron part may start to soften after five or six presses. It's not just a question of strength, it's a question of security.
Round 3: Will it break? - Casting the four "breaking points."
After talking about intensity, let's talk about fracture. Could the cast actually break?
Yes. ' Easier than you think.
According to industry figures, there are four main reasons why the cast broke:
Breakpoint 1: Excessive impurity content. Too much sulfur and not enough silicon will cause hot brittleness of steel, which will lead to a sharp decline in toughness and ductility. Even under mild stress, it can snap during subsequent use. This is not alarmism; data from Zhengzhou Foundry clearly indicates that excess carbon content and other alloying elements exceed permitted limits, significantly increasing the brittleness of iron.
Breakpoint 2: Improper heat treatment. Insufficient holding time can lead to rapid cooling after opening the packaging, leading to immediate cracking. Holding it for too long can lead to a decrease in coarseness and strength. In heat treatment steps, even the slightest mistake can lead to disastrous results.
Failure point three: External Impact. During the process of disassembly, cleaning and grinding, strong external shock will break the continuity of the microstructure of the casting and lead to fracture directly. This is particularly true for large castings, which are most prone to failure during removal; a single impact can shatter the castings and render the entire furnace unusable.
Failure point four: Inadequate Structural Design. The thickness of the wall is not uniform, the lack of rounded angle leads to the stress concentration, which leads to linear cracks.
On the other hand, the parts of CNC are made from a single rough processing. Its internal structure is uniform, without porosity and shrinkage, with smooth surface and no stress concentration. The risk of fracture is significantly lower than for castings.
Of course, CNC machining is not immune to fracture. According to a case study of Daxing Machine Tools in May 2026, the main causes of abnormal tool breakage in CNC machining are: uneven stress due to inaccurate machine tool precision, uneven material hardness, incorrect tool selection, excessive cutting force, excessive cutting temperature, insecure clamping and improper use of cutting fluid. However, these are processing problems, not the material itself. In other words, if a CNC part breaks, it's mainly because you're not adjusting the machine well; if a casting breaks, it's mainly because the material itself is defective.
Round 4: Hardness ≠ Strength-Don't Be Fooled Again
Many people can't tell the difference between hardness and strength, saying "hard material doesn't break."
This is completely wrong.
Hardness is the ability of a material to resist surface indentation, scratchings and abrasions, and is the "armor value" of resistance to scratches and abrasions. Strength is a material's ability to withstand compression and stretching, a the "health bar value" of how much force it can withstand.
High-hardness materials may be brittle, while high-strength materials may not be abrasive. tool steel, for example, is extremely hard and abrasive, but it is fragile and vulnerable to shock damage. Titanium alloys, on the other hand, have unparalleled strength and weight, but are extremely difficult to process and extremely expensive.
Back to our comparison:
Ordinary grey cast iron: Although not low in hardness (HB in Britt), its tensile strength is only 98-392 MPa, brittle and prone to cracking on impact.
7075-T6 aluminum alloy (CNC machining): Although less rigid than cast iron, it has a its tensile strength of 570 MPa, good toughness and is not susceptible to brittle fracture.
C86300 aluminum bronze (precision casting + CNC machining): 225HB hardness, 225-830MPa a tensile strength, rugged, truly "indestructible."
So when it comes to choosing materials, it's not as hard as it gets, it's not as strong as it gets. The key is the balance of hardness and strength, and toughness and rigidity. The advantage of CNC machining is that it can select materials and control machining parameters precisely, ensuring that the hardness and strength are in the optimal optimal range. Casting, on the other hand, depends entirely on chance, on pouring temperature, cooling speed and the absence of air bubbles.
Round 5: When Use Castings and when to do NC processing?
This is not to say that casting is completely useless. Casting and CNC machining are just different processing methods, each with its own application.
Castings use scene: large production, complex shape, low precision requirements, cost sensitivity. For example, engine blocks and cylinder head are so complex that CNC machining is impossible and requires casting. Another example is the broken wall and chamber walls of mining crushers, which are made of high-manganese steel and have excellent impact resistance and low cost.
CNC machining indispensable scene: production volume is small, precision is high, strength is high, safety key components. For example, brake rocker arms, aerospace components, medical devices and precision transmission components. For these items, breakage can lead to accidents, and it's not worth saving any money.
According to industry data for 2026, CNC machining costs less than castings initially, but castings cost less in mass production. Casting molds cost more, but cost less per unit, making them suitable for small-scale, high-precision production. Therefore, its core logic is: for small batch, key parts, adopt CNC; for large batch, key parts, adopt casting technology.
Bottom line: how big is the power gap? Will it break? Brief description:
Back to the original question: how different is strength of NC machining from that of ordinary castings? Will it break?
There is a 30% to 80% difference in intensity and a 2-3 times difference in fatigue life difference. Casting can break more easily than you think; CNC parts are not immune to breakaging, but the probability of cracking is an order of magnitude lower.
A 7,075 MPa T6 and 830 MPa C86300 MPa of regular cast iron are "very different," compared to 98-392 MPa of gray cast iron.
But do not forget: casting and CNC machining are just different processing methods, there is no inherent superiority or inferiority. The key is to understand how much force your part can withstand, how it works, and the consequences of breakage. Once you understand these three questions, you will naturally know whether to choose casting or CNC machining.
Every penny saved could be the reason for your next failure. Instead, every extra penny could be the basis for an extra decade.
Choose the correct processing method to prevent breakage. Choosing the wrong approach can damage not only parts, but your life.