What Is a Machinability?

Machinability is a measure of how easily a material can be cut to shape, judged by tool life, cutting forces and power, surface finish, chip disposal and material-removal rate. It is usually expressed as a rating relative to a reference steel, B1112, rated 1.00 (100%), so higher numbers mean faster, cheaper machining.

In short: machinability tells you how hard a material will be on the cutting tool, the machine and the budget.

What Does Machinability Mean?

Machinability is not a single physical property like density. It describes how a material behaves in a particular machining operation, with particular tooling and cutting conditions. A material machines well when it gives:

• longer tool life;
• lower cutting forces and power;
• a better surface finish;
• easier chip disposal;
• a higher material-removal rate.

It is linked to, but not the same as, a material's mechanical properties. High hardness wears tools by abrasion. High strength raises cutting forces, energy and temperature. High ductility makes the metal tear rather than shear as the chip forms, which causes chip-disposal problems and a poor finish. That is why a hard, strong titanium alloy (rated about 0.2) is slow to machine, while soft aluminum (about 2.00) is quick.

Machinability Ratings of Common Metals

Ratings compare each material with B1112 steel at 1.00. A rating above 1.00 means better machinability than the reference.

Material	Typical hardness (Brinell)	Machinability rating
B1112 steel (reference)	180–220	1.00
Aluminum, soft	—	2.00
Aluminum, hard	—	1.25
Medium-carbon steels	140–210	0.65
High-carbon steels	180–230	0.55
Low-carbon steels	130–170	0.50
Cast iron	60–230	0.40–0.70
Alloy steels	170–230	0.25–1.5
Titanium alloys	220–280	0.2

These are group averages. The exact figure for a grade depends on its composition, temper and hardness, and on the operation being rated.

Plastics are not rated on this scale. Most do not need cutting fluid; choosing the correct speeds and feeds is what matters most.

How Does Machinability Affect Cost and Design?

• Low machinability costs time. Materials with low machinability take longer to cut and cost more per part, so the choice of alloy can move a quote as much as the geometry does.
• Choose the easiest material that meets the requirement. Where strength, weight and corrosion allow it, an aluminum alloy such as 6061 or a brass such as C360 will usually be quicker to produce than stainless steel or titanium.
• Tight tolerances magnify the difference. Closer tolerances and finer finishes already add cost; on a hard-to-machine alloy they add more.
• Ask before you commit. Material, tolerance and finish are easier to change at the design review than after the first parts are cut.

At NICE Rapid, the engineering review at the start of every CNC machining project checks the design for machinability and confirms the material, tolerance and finish before programming begins.

Frequently Asked Questions

What is machinability?

Machinability is how easily a material can be machined. A material with good machinability gives longer tool life, lower cutting forces, a better surface finish, easier chip disposal and a higher material-removal rate, which means shorter cycle times and lower cost.

How is machinability measured?

It is usually given as a rating relative to a reference steel, B1112, which is set at 1.00 (100%). A material rated 2.00 rates twice as well as the reference; one rated 0.2, like many titanium alloys, is much harder to cut.

Which metals have the best machinability?

Among common engineering metals, aluminum alloys rate highest, at about 1.25 to 2.00 relative to B1112 steel. Carbon and alloy steels mostly fall between 0.25 and 1.5, and titanium alloys sit around 0.2.

Related Services at NICE Rapid

• CNC Machining — aluminum, brass, steels, stainless, titanium and engineering plastics machined in-house, with a machinability check at engineering review.
• Request a Quote — send a STEP file and your material options, and our engineers will advise on how each affects lead time and cost.

Sources

1. Michigan State University, ME 477 (P. Kwon), Economics and Product Design Considerations lecture notes — definition of machinability as the relative ease of a machining operation; tool life, force/power, surface finish, chip disposal and MRR criteria; effects of hardness, strength and ductility; machinability rating table relative to B1112 = 1.00; low-machinability materials take longer and cost more; closer tolerances mean higher costs — egr.msu.edu
2. UC Riverside Mechanical Engineering Department Machine Shop, Standard Operating Procedures for Manual Milling Machines (rev. V5), §6 "Cutting Fluids" — "Plastics: Most do not need any cutting fluids. Correct speeds & feeds are more critical." — me.ucr.edu