SPEED, FEED RATE, AND DEPTH OF CUT

Tool life and tool performance in HSM are determined largely by how much load there is on the tool. Because speed increases the effect of small differences resulting from factors such as toolholding, tool path and control, this will vary from process to process. How fast each process can be machined may require that the starting point should be at the lower range and increased until ideal machining conditions are reached. The following are general guidelines when using HSM with a ball-nose tool for finishing operations:

• Speed - At or near maximum spindle r/min.

• Feed rate – Use light cuts at chip load equivalent to that of lower speeds.

• Depth of cut - No more than 10% of cutter diameter, even less for harder materials.

Note: It is important not to exceed a tool’s maximum safe speed. Always follow safety precautions appropriate to higher speeds.

SAFETY AT HIGH SPINDLE SPEEDS

Centrifugal force at high spindle speeds can turn any loose insert, screw, or tool fragment into a dangerous projectile, Fig. 2-1-25. Use the following precautions when machining at high spindle speeds:

• Do not exceed a tool’s maximum spindle speed rating.

• Check tools and toolholder components regularly for fatigue cracks.

Fig. 2-1-25 Any tool fragment from a high-speed operation can be compared to a bullet. (Modern Machine Shop)

MACHINING HINTS

In comparing HSM with conventional machining there are many different factors that must be considered to make the change to HSM cost effective. Some of the changes associated with HSM are cutting forces, cutting tools, speed rate, feed rate, toolpaths, and material removal. The following conditions may occur:

• Cutting tool edge buildup – This can be a problem at high speeds for some combinations of cutting tools and materials.

• Chip removal – Because of the large volume of chips created, they must be removed quickly form the machining area. Horizontal machining centers (HMC), where the chip falls away from the machining area, seem to resolve this issue.

• Rigidity – Higher speeds can produce some unwanted vibration that may require a sturdier machine to overcome.

• Cutting tools – Longer-life tools, such as coated, CBN, or diamond, are required to avoid the time lost due to frequent tool changes.

CNC

In high-speed milling, the control system electronics can make all the difference, Fig. 2-1-26. The right CNC, together with other elements of the control system, can let a slower machine tool mill a given form faster than a machine with a higher top feed rate. The reason is that in any milling routine that is relatively complex, the control system determines how much of the available feed rate can be put to use.

Overall System

A control system is only as fast as its slowest component. Improvements in CNC control systems have made HSM possible and the limiting factor seems to lie within the machine tool itself. Technologies related to HSM touch on every link in the CNC loop.

Fast Processing

Fast CNC processing speed is fundamental to HSM. This is particularly true where the CAM software has defined a complex tool path as a series of numerous short moves. If the CNC cannot process these blocks faster than the machine can move through them, then the machine will stutter as it waits for data. Slow data input to older CNCs can produce a similar effect.

Input Baud Rate

With an older CNC machine, input baud rate is the bottleneck that can severely limit feed rate. When a program must be drip fed through a serial port, the CNC can’t execute the program commands any faster than it can receive them across this connection, Fig. 2-1-27.

Fig. 2-1-27 A slow baud rate can severely limit the feed rate. (Modern Machine Shop)

Newer CNCs overcome this bottleneck in either of two ways. They provide enough memory for a long program to be stored at the control so that drip feeding is no longer necessary. This also allows for network connections (like Ethernet) that permit much faster program transfer than the serial link.

Machining Rate

The need to drip feed across a serial connection can impose a feed rate limit for effective machining. The maximum feed rate is a function of the serial connection’s baud rate.

Assume each character commands 10 bits of data. A serial connection with a baud rate of 38,400 bits per second can therefore transfer 3,840 characters per second.

Factors including the memory required for DNC overhead will limit the maximum effective feed rate to a value somewhat lower than this.

·         On older CNCs, a common serial baud rate is 9,600. At this rate, the maximum feed rate drops to below 60 in/min. Therefore it may not be possible to perform effective high speed milling of complex regions of the part where such a slow drip feed is required.