Does your production include some CNC machining of metal parts? You might have recurring issues about finishing.

One of the major challenges a CNC programmer faces every day is identifying critical metal machining parameters such as depth of cut, spindle speed, and feed rate. Traditionally, the starting point for this has been either a machining data handbook, or the experience of senior machinists on the floor.

Getting the best possible results from a CNC machine covers a lot of ground. To begin with, there are several goals the operator might be trying to optimize for:

Best material removal rates

Maximizing tool life

Good surface finish

The best practice for establishing the best feeds and speeds for the material to be machined is to use a feeds & speeds calculator. By utilizing a materials database and knowing the very specific specification for each CNC machine, optimal feeds and speeds can be calculated that provide the three goals identified above.

Different formulas are needed for the different types of tool being used within the CNC machine. Here we will go over the formulas for milling.

Cutting Speed

Surface Feet Per Minute (SFPM) for Common Metals:

Mild Steel 100

Tool Steel 70

Cast Iron 60

Aluminum 250

Brass 300+

SFPM is a unit of velocity that describes how fast the cutting edge of the cutting tool travels, or in other words, it’s the speed at which the material moves past the cutting edge (outside diameter) of the tool in feet per minute.

The simple calculation to work out the RPM of the cutting tool is:

(4 x SFPM)/Cutter diameter

Example for a 5/8” (0.625) end mill cutting mild steel:

(4x100)/0.625 = 640 RPM

>>Learn everything you need to know about the metal machining process. Request our FREE white paper here<<

Feed Rates

Feed rate is the velocity at which the cutter is fed or advanced against the work piece. It is expressed in units of distance per revolution for turning and boring (typically inches per revolution [ipr] or mm per revolution).

It can be expressed thus for milling also, but it is often expressed in units of distance per time for milling (typically inches per minute [ipm] or millimeters per minute), with considerations of how many teeth (or flutes) the cutter has then determining what that means for each tooth.

FR = RPM x T x CL

Where:

FR = the calculated feed rate in inches per minute or mm per minute.

RPM = is the calculated speed for the cutter (as per calculation above).

T = Number of teeth on the cutter.

CL = The chip load or feed per tooth. Chip Load is a term used to describe the thickness of a chip removed by one cutting edge of the tool. Chip load is sometimes referred to as ‘feed per tooth’. The chip load is the radial depth of cut of the cutting tool in one revolution. The chip load value is normally provided by the tool supplier similar to the example shown below:

Example which uses the same 5/8” end mill cutter above and assumes it has 4 flutes:

FR = 640 x 4 x 0.004

FR = 10.24 inches per minute (ipm)

Reference Point

The actual values for cutting speeds and feeds are a good reference point to start and should be verified by experiment depending on the job.

A common consideration for establishing cutting speed is the time it takes to dull a tool. It may be more profitable to run faster for higher production and replace the tools more often. The other way might be to run the tools more slowly for endurance and machine unattended. There is a big variation here so, after selecting a starting speed, tune it up for the work to be done and the desired productivity.

Coolant/Cutting Fluid Selection

There is a wide variety of cutting fluids available today. Many new coolants have been developed to meet the needs of new materials, new cutting tools, and new coatings on cutting tools.

The goal of metal machining operations must be to improve productivity and reduce costs. This is accomplished by machining at the highest practical speed while maintaining practical tool life, reducing scrap, and producing parts with the desired surface quality. Proper selection and use of cutting fluids can help achieve all of these goals.

In machining, almost all of the energy expended in cutting is transformed into heat. The deformation of the metal to create chips and the friction of the chip sliding across the cutting tool produce heat.

The primary function of cutting fluids is to cool the tool, work piece, and chip, reduce friction at the sliding contacts, and prevent or reduce the welding or adhesion on the contact edges that causes a built-up edge on the cutting tool or insert. Cutting fluids also help prevent rust and corrosion and flush chips away.

Did you find this blog helpful? Do you have any questions on how to improve finish through speeds and feeds for metal machining? Leave a comment below, we'd be delighted to hear from you!