Application

To optimize chip removal, the rotating speed to the cutting tool and the linear axis speed that the tool moves must be balanced to maximize metal removal, machine performance, and cutting-tool performance. Following is background information to help maximize machining performance.

Spindle Speed

Incorrect spindle speed is a common error in CNC machining. Each material type and type of cut has an ideal tool profile and cutting speed. Larger diameter tools require slower cutting speeds. The spindle speed and feed rate for a given cut must be balanced for best work quality, tool life and spindle life. A frequency inverter controls the speed of the spindle. All spindles are 3-phase asynchronous motors with infinitely variable speed from 0 rpm to the maximum rated rpm. The speed can be achieved by correctly programming your frequency inverter drive to match the spindle.

Feed Rate

The feed rate of the cutting tool must be balanced proportionally with the spindle rpm. Changing one influences the other. Feed rates set too slow decrease tool life by causing overheating, a condition that may leave burn marks on the work. A tool moving too slowly causes the tool to heat up because not enough material is removed to cool the cut interface. The best feed rate is determined with experience and trial-and-error to achieve best results. The feed rate chart shown here provides a range for starting point values. For first-time setups, begin at the mid-range. Your cutting tool supplier can recommend cutting data for your specific application.

Chip Load

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 —  radial depth of cut of the cutting tool in one revolution — is calculated as follows:

Chip Load = Feed Rate ( Ipm ) ÷ ( Cutting Rpm X Number Of Cutting Edges )

Chip load is one factor used to determine a starting point to ascertain cutting speed (rpm) and feed rate values for the setup. For wood, a chip load that is too low generates excessive heat; the chips produced are ‘dust-like’ and may leave burn marks on the part. A chip load that is too high is pushing the cutter through the material. This generates high radial loads on the spindle bearings, and over time can cause spindle failure, excessive tool wear, and tool failure. There are two styles of milling: climb milling and conventional milling. For woodworking, climb milling produces a better finish with less tear out. Some say it provides longer tool life.

Chip load is one of many factors used to size the spindle and determine machine requirements such as:

• Speed range and power requirements for the spindle
• Tool load to determine type and grade of cutting tools
• Amount of torque generated to make the cut

We strongly encourage you to consult PDS on new spindle applications to assist in establishing your baseline cutting values. Our staff is here to discuss your technical questions by phone or email. The chart below provides chip-load values for common router tools. These are “not to exceed” starting point values for first-time setups. Actual values will vary as a result of many machine factors such as: rigidity, horsepower, collet condition, spindle integrity, part clamping, hold down, and others.

Cutter Entry Angle (Plunge Angle)

The approach angle that the cutting tool feeds to depth is referred to as Cutter Entry Angle. This angle creates radial and axial stress on the machine spindle and the cutting tool during the material removal process. The following information provides background to help determine base-line starting values for your setup.

High-speed electric spindles designed for routing applications are equipped with bearings capable of achieving high rotating speeds and enduring high radial loads (side cutting). Keep axial cutting load low for best performance and increased spindle-bearing life. One way to do this is by keeping the entry angle of the cutting tool low, 0º to 20º off the table (for vertical-mounted spindles). Using a low entry angle will allow the use of higher feed rates — up to 100% calculated values for most applications. When the work piece features require a steep entry angle, the feed rate must be decreased to reduce axial load, preventing damage to the spindle bearings. Refer to the feed rate chart plunge diagram for various plunge angles. By using low entry angle, axial forces can be minimized to provide longer bearing life. Above all, avoid “crashes” of the spindle into the table, fixtures and work-piece. PDS offers special spindles for applications involving steep plunge cuts producing high axial loads. Contact the PDS Application group for details and recommendations.

Tool Clamping Types

There are two basic styles of spindles: manual tool change spindles and automatic tool change spindles. The type used depends on the application and size of cutting tools for the application.

Manual Tool Change

Electric spindles with manual tool change clamp the tool/cutter using a spring collet in the taper of the spindle shaft. This method offers excellent rigidity at a low cost. The disadvantage is that tool setup is done online, with the machine usually out of cycle for about two or three minutes each change.

Automatic Tool Change

Electric spindles equipped with automatic tool change (ATC) can be integrated with a CNC for quick tool changes without manual intervention. Typical tool change times are less than 5 seconds. On applications requiring multiple cutters, an ATC spindle offers higher productivity. With an ATC spindle tools/cutters are pre-set in tool holders. Tool holders are typically clamped in the electric spindle by a spring loaded clamping system. Tool holders are released by compressing retention springs and opening the clamping system (typically with a pneumatic actuator). Pre-setting tools occurs off-line using jigs for accuracy and without interrupting production.

ISO and HSK are the two most common tool holders used on CNC routers, robots and milling machines. ISO 30 is more popular than the smaller taper sizes ISO 25, 20 and 15. HSK 40, HSK 50 and HSK 63 tool holders provide better cutting accuracy and reliability. Their popularity is growing rapidly due to performance.