CNC Machining: Understanding Feeds and Speeds

CNC machining is the backbone of many manufacturers’ processes. Unfortunately, they are also a major cost center and can be a bottleneck when it comes to getting the product quickly through the plant. Far too often we find that a company has invested in quality inserts and tooling, but is not running them at the proper feeds and speeds.

Through Martin’s Tool Analysis Program, companies can find ways to improve their processes and make sure they’re getting the most out of their products.

Feeds and speeds refer to two separate velocities for machine tools: feed rate and cutting speed. They are often considered as a pair because of their combined effect on the cutting process.

• Cut speed (or surface speed) is the speed at the outside edge of the part as it is rotating. It is expressed as a unit of distance along the work piece surface per a unit of time.
• Feed rate is the velocity at which the cutter is advanced along the spinning work piece. The units are typically distance per spindle revolution.

Cutting Speed

Cutting speed is the speed that the material moves past the cutting edge of the tool. Cut speed can be defined as revolutions per minute (RPM) or as surface feet per minute (SFM).

Revolutions Per Minute (RPM) relates directly to the speed, or velocity, of the spindle. It represents the number of turns completed in one minute around a fixed axis. RPM maintains the same revolutions per minute throughout the entire operation.

What are Revolutions Per Minute (RPM) modes useful for?

• Center cutting operations (drilling)
• When the diameter at the beginning and end of a cut only differs slightly from the beginning.
• During threading to allow the perfect synchronization between spindle revolution and Z-axis motion to allow precise threads

Surface Feet Per Minute (SFM) is a combination of the cut diameter and RPM. The faster the spindle turns, and/or the larger the part diameter, the higher the SFM.

For example, if two round pieces of different sizes are turning at the same revolutions per minute, the larger piece will have a greater surface speed because it has a larger circumference and has more surface area. As the tool gets further into a workpiece, the same spindle speed will produce a decreasing surface speed. This is because each revolution represents a smaller circumferential distance, but takes the same amount of time.

Most CNC lathes have Constant Surface Speed (CSS) to counteract the natural decrease in surface speed. This speeds up the spindle as the tool moves closer to the turning axis. By utilizing CSS the lathe is adjusting the revolutions per minute to maintain a constant surface speed at every distance from the center.

What are Constant Surface Speeds (CSS) useful for?

• A uniform surface finish.
• When the diameter at the beginning a cut will differ significantly from the diameter at the end of the cut.
• Better tool life and machining time because tools will always cut at the appropriate speed.

Materials will run better at specific SFMs. SFM is a constant, with RPM as a variable based upon cut diameter.  When the SFM constant is known for a specific material, the formulas below can be used to determine spindle speed:

 RPM = SFM x (12/Pi) / Cut Diameter

So, for an SFM of 400 and a cut diameter of 5″, RPM can be found:

RPM = 600 x 3.82 / 5 = 458 RPM

When RPM is known, SFM can be found as follows:

SFM = RPM x Cut Diameter x Pi/12

For example, if you have a cut diameter of 5” and a spindle speed of 458 RPM, then:

SFM = 458 x 5 x .262 = 600 SFM

Feed Rate

Feed rate is the velocity at which the cutter is advanced along the work-piece. Feed rate is expressed as units of distance (inch) per minute or per single revolution.

Feed rate can be defined as an inch per minute (IPM) or inch per revolution (IPR). IPR is more commonly used.

How can the values for Inches Per Revolution (IPR) and Inches Per Minute (IPM) be easily converted?

IPM = IPR x RPM
IPR = IPM / RPM

So, for a spindle speed of 306 RPM and a feed rate of .01 IPR, the IPM can be calculated as follows: 

IPM = .01 x 306 = 3 IPM

 

Selection of Tools, Feeds, and Speeds

Cutting tool selection has a direct impact on the proper programming of feeds and speeds at the machine.

That said, there are many other variables that affect feeds and speeds, such as:

• Work-piece material class and condition
• Work-piece diameter
• Cutter material
• Cutter geometry
• Type of cut
• Depth of cut
• The condition of the machine

Cutting tool manufacturers publish the general feeds and speeds and recommended usage for the application. That is often a good place to start for recommendations on tool selection and feeds/speeds, but best practice is to work directly with a cutting tool expert at your machine.

At Martin, we pride ourselves on understanding feeds and speeds and optimizing them for our customers. Our Metalworking Team is ready to share their expertise and find custom-tailored solutions that fit your needs. To learn more about how we can help you maximize the profitability of your machine shop, contact your Martin Sales Rep or call 800.828.8116.