What is Machinability?
Machinability is the ease with which a metal can be machined. It is represented in percentage relative to a reference metal. A smaller value means the metal is harder to machine. Very difficult to machine materials can rate 10-20%, while very easy to machine material can reach 200-400%
- What is Machinability?
- Why is Machinability a vital parameter?
- How is Machinability calculated?
- Determining cutting speed from Machinability
- What are the main factors that affect machinability?
- Machinability Range of material Groups
- Machinability Rate of Popular Materials
- Search for machinability by a material standard
Why is Machinability a vital parameter?
High Machinability metals offer little resistance to being cut (Machined), while Low Machinability metals have high resistance. Higher resistance to machining means faster wear on the cutting tools, resulting in slower cutting speeds that increase the cycle time. Thus, it has a tremendous effect on the price of the product. The engineer who designs a new part has to balance the properties he needs from the metal (Such as strength, wear resistance, heat resistance, and corrosion resistance) and the part’s cost. As it is with many things in life, most of the time, better mechanical and chemical properties go hand in hand with lower machinability rates.
How is Machinability calculated?
Machinability is commonly rated relative to the results achieved on Steel SAE1112 at a hardness of 160 Brinell.
Machinability Rating [%] = (Score of a material / Score of SAE1112) * 100
(So SAE1112 gets a score of 100%)
The score typically factors results of several different tests:
- Cutting Speed Test: The cutting speed in which the cutting tool can “survive” for 60 minutes machining the material.
- Tool-Life Test: In this test, we measure how long it takes to reach specified wear on the cutting tool at certain cutting conditions.
- Force Test: Comparison of the force requirements relative to the normalized material. The theory is that difficult to machine materials require more horsepower.
- Temperature Test: Comparison between cut temperatures developed when machining a material relevant to the reference material SAE1112. The temperature developed during the cut has a direct relationship to wear and tool life.
Machinability is not a definite formal figure, like hardness or density, and there is no official standard on how to measure it. You can find different methods and different machinability rates for materials on various sources!
Determining cutting speed from Machinability
We rarely want to find out the machinability rating just to learn what it is. In most cases, we are interested in the machinability value in order to estimate the cutting speed we can use for a certain material. Cutting speed depends on many factors such as stability, desired surface finish, CNC machine abilities (to name just a few). Therefore, speed and machinability recommendations are very general, and only the machinist can take the final decision depending on all the factors. The most reliable information we can learn from machinability tables is the relation between the rate of different materials on the same table. Therefore we recommend the below procedure:
- Choose a material that you are familiar with and that you are relatively confident about what cutting speed you would use for it on the relevant application. We will call this our reference material.
- Preferably, choose a reference material that is in the same material group. (List of material groups).
- Calculate the relation between the two machinability rates.
- Multiply the relation by the reference cutting speed.
- Pay attention that machinability rating is specified for each material at a specific hardness.
- To increase the accuracy of the estimation you can also normalize the machinability rating according to the factor between the hardness of your specific material and the hardness listed in the machinability table.
Example of Calculation:
- You need to machine Stainless Steel 17-7PH and you are not sure what is the suitable cutting speed. However, you know that for the same application with stainless 304 you would run at 360 SFM (110 m/min).
- In the machinability table you see that 304 has MR=40% and 15-7PH has MR=20%.
- Speed[15-7PH]=360*0.2/0.4=180 SFM (88 m/min).
- In the Machinability table, the MR for 17-5PH was given for hardness of 270 BH. However, our 17-5PH is only 230 BH.
- Therefore the final value will be 180*270/230=211 SFM.
What are the main factors that affect machinability?
- Chemical composition: The amount of particular elements such as Carbon, Nickel, and Lead (And many others) have a significant influence on Machinability. For example, higher Nickel (Ni) content tends to reduce Machinability, while higher Lead (Pb) content tends to increase Machinability. However, the effect is complex, and each element can have a different impact collectively with other elements.
- Microstructure: Non-metallic inclusions can have a tremendous impact on the Machinability.
- Grain size: Small and well-ordered grains are easier to cut. Large disordered grains are harder to cut.
- Hardness: Both very hard and very soft materials have low Machinability. Hard materials cause fast wear on the cutting inserts, while soft material tends to be gummy and sticks to the cutting edge. Intermediate hardness yields the best Machinability.
- Heat Treatment: Heat treatment processes can shape various properties that influence Machinability: toughness, hardness, microstructure, and stress.
- Fabrication method: Different production methods such as Hot rolled, cold rolled, cold drawn, cast or forged affect properties such as grain size, uniformity, hardness, and toughness.
Machinability Range of material Groups
Steel – ISO P
There is a wide variety of steel materials used in machining. The range of Machinability is vast and spans from about 40% for difficult to machine steels, such as bearing steel (SAE 52100 / DIN 100Cr6) up to Ledloy free-cutting steel (SAE 12L14 / DIN 9SMnPb36) that has a machinability rate of about 170%.
The Machinability is mainly affected by:
- Carbon (C): 0.3-0.5% is ideal. Lower content creates soft and gummy material, which is hard to machine. Higher content increases the strength and is also difficult to machine.
- Adding alloying elements such as Chromium (Cr), Molybdenum (Mo), and Nickel (Ni) tweak the steel properties and tend to decrease Machinability.
Learn more about Steel Machinability
Stainless Steel – ISO M
Additional alloying elements such as Nickel (Ni), Chromium (Cr), Molybdenum (Mo), Niobium (Nb), and Titanium (Ti) supply different characteristics, such as resistance towards corrosion and strength. Higher content of Chrome and Nickel tends to decrease Machinability.
One of the most popular materials in this group is 316, which has a machinability rate of only 36%. But there are other common grades such as SAE 303 & the 400 series that have a Machinability of over 60%.
Learn more about Stainless Steel Machinability
Cast Iron – ISO K
Cast iron is a steel-like material with Carbon (C) content of more than 2% and Silicon (Si) content of 1-3%. Cast iron produces tiny chips, and because of that, it is very convenient to machine. On the other hand, it is very abrasive and creates a lot of wear. The main factors affecting the physical properties and Machinability of different types of Cast Iron is the fabrication method. (Nodular, Gray, Malleable) and hardness.
Learn more about Cast Iron Machinability
Nonferrous – ISO N
A group of metals with no Iron (Fe) content. Most of them are very soft, with a hardness under 150 HB. The popular materials in this group are Aluminum and Copper-based alloys. Machinability rates are very high compared with ferrous materials and are typically in the range of 200-400%. Because of this, a component made from these materials has low production costs. They are attractive as long as their inferior mechanical properties are acceptable for a given part.
Learn more about Aluminum Machinability
SupperAlloys – ISO S
Titanium alloys have a very high strength-to-weight ratio. This means that it is possible to design parts that weigh much less (And very strong) when compared with other metals. For this reason, the material is popular in the aerospace industry in components where there is no exposure to heat. Titanium also has excellent corrosion resistance and chemical stability and, therefore, a favorite material for medical implants. The commonly used grade is Ti-6Al-4V, with a machinability rate of 20%.
Learn more about Titanium Machinability
Heat resistance Superalloys (HRA):
A group of Nickel (Ni), Cobalt (Co), and Iron (Fe) based metals. Their main feature is the ability to preserve mechanical properties and corrosion resistance also at very high temperatures. They are mainly used in jet engines, power turbines, and turbochargers. The most commonly used HRSA material is Inconel 718, which has a machinability rate of 10%.
Learn more about Superalloys Machinability
Machinability Rate of Popular Materials
|0||P||Carbon Steel (1010)||66%|
|0||P||Steel 12L14 (LeadLoy)||170%|
|0||P||Steel 52100 (Bearing Steel)||40%|
|0||P||Alloy Steel (4340)||57%|
|1||M||Austenitic Stainless Steel (316)||40%|
|1||M||Stainless 17-4 PH||45%|
|2||K||Gray Cast Iron (GG25)||112%|
|2||K||Nodular Cast Iron (GGG50)||100%|