There are as many opinions of high speed milling with small milling tools and their proper use in the manufacturing industry as there are nuts and bolts. The benefit of high speed milling is that it allows for lower cutting loads and chip rates producing better surface finishes. In our world of high speed, high tolerance, high production machining there is a marrying of many points to create a perfect part. At DATRON we promote a strategy of the proper DATRON Machine configuration, material holding and cutting tools with optimized speed and feeds to define this strategy.
5 Must Haves for Success in High Speed Milling:
- Vibration free high speed/frequency spindle
- Rigid and vibration free X, Y and Z axis
- Rigid, full support, vibration free work holding.
- Vibration free tooling
- Optimized program using proper feeds and speeds
This discussion will concern the fourth item, tooling.
So your shop has made the investment of a high speed milling machine and you have been presented with a program to run. Might be one test piece, or several, or a production run of hundreds. There isn’t a question if the milling machine will perform, that process was confirmed prior to purchasing it. So, work holding and tooling will make or break the success of this piece.
For discussion we will assume the other four points above have been met (vibration free speed/frequency spindle, vibration free X Y and Z axis, rigid full support vibration free work holding, optimized program) so let’s discuss High Speed Milling Tool Strategies.
Composition of High Speed Milling Tools
High speed steel or carbide? We can all agree that vibration is a killer and more so in high speed milling. The operation at higher speeds and feeds to meet production, cost perimeters and to achieve the best finish dictates the most rigid and vibration-free tooling. There is the potential of flex with high speed steel tooling and vibration at high speed rates. So, the need for rigid tooling leans toward carbide … high-quality, solid, micro-grain carbide tooling.
Design of High Speed Milling Tools
The geometric design of high speed carbide tools, and specifically DATRON Tools, allow for the most proficient and clean cut of material at high speed and feed rates. This would allow for heavy roughing passes creating a full-shaped chip without curl putting the full cutting energy of the spindle into material removal.
Holding of High Speed Milling Tools
Next comes the proper holding of the tool. Vibration in tooling is usually a result of tooling not made for high speed milling, improperly balanced tools or tools chucked improperly. With both direct shank spindles or balanced tool holders for HSK spindles vibration free cutting can be achieved. Loading the tool properly (direct shank tools with brass stop ring or HSK tool holders) requires a space of 20% or 4-6mm between the top or fade out of the flute to the stop ring or HSK tool holder. This allows for the chips to move freely away from the tool when milling. However, there should only be approximately 80% of the shaft filling the direct clamp collet or HSK tool holder. The top of the tool should not come into contact with the top of the collet or HSK tool holder. This could cause damage to your spindle.
Feeds and Speeds for High Speed Milling Tools (Plus Chip Removal)
Now, comes the fun part. What is the best speed and feed rate? And for what type of chip removal should you have for your program?
Generally stated the milling strategy with high speed mills will not follow the same path as a conventional, commodity (non-high-speed) milling machines. The ability to use smaller tools faster, cleaner, and at high feed rates, allows for multiple passes with a shorter cycle time than standard commodity machining practices. The high finish removes the need for added second and third step processes normally needed in commodity machine milling.
With this in mind your strategy will include a selection of tools that will meet your most common milling practices. It also enables you to “dial in” the feed and speed rate of the tools selected and to look for the best speed and feed rate for each tool and milling operation. This can be done in a variety of ways: Tap Testing, Vibration Testing, Octave (harmonic pitch) Testing or the old fashioned Trail and Test method. The end goal is to find the best feed and speed rates for the best results. The majority of successful DATRON operators will use the Trial and Test method.
Published Speed (rpm) and Feed (cutting travel) are good starting points but are only starting points at best. You must consider the machine, spindle, material and tool? Remember it is a marrying of several factors to maximize your strategy: high speed/frequency spindle, rigidity and tolerances of X Y and Z axis, rigid work holding. Type of tool (geometric design, diameter, flute, length and tool holding) and depth of pass will all affect the feed and speed rate.
There are also a few anomalies in high speed milling. In high speed milling chatter and vibration are a killer and usually tell the operator to dial down the speed and or feed rate. However in the high speed milling community there are many times where dialing it up, increasing speed rate allowing for a better feed rate, better chip load will actually solve the issue.
Checking the Chip Load.
Chip load is the actual thickness of chip that is produced in the machining process. It is a reflection of the combination of the cutter feed rate (moving in the material) and speed rate (how fast it is turning; RPM). Too high of a chip load will cause an unsatisfactory edge finish and/or part movement from increased cutting pressure. It will place higher stress on the spindle and tool (risking breakage) and certainly cause higher wear on the tool … ultimately shortening is useful life. The need for a good chip load helps reduce the major cause for tool loss: Heat. The heat is created from the friction of the cutting action of the tool, cutting edge to material. With the proper evacuation of the chip comes the release of heat generated from the cutting action reducing possible damage to the tool.
Chip load is measured in thousandths of an inch (i.e. 0.010) and is influenced by the spindle speed and the feed rate of a CNC machine. Number of flutes, length of flute, shank diameter and depth of cut will also have an effect on the chip load. The number of cutting edges on a tool determines how the chip load is divided. A single edge tool provides all the chip load during a revolution, while a double edge tool divides the load over two edges and so on.
Formula for Determining Chip Load:
chip load= feed rate/(rpm x #cutting edges)
Manipulating the Chip Load:
To Increase Chip Load:
- Increase the feed rate
- Decrease the RPM
- Use less flutes
To Decrease Chip Load:
- Decrease the feed rate
- Increase the RPM
- Use more flute
For your reference the chip range for DATRON End Mills will be:
1mm Dia = .01mm (.000394)
20mm dia = .15mm (.7874)
This will depend greatly on shank diameter and flute length. Always use the shortest flute length for the milling depth requirement. Slowing down production to avoid the additional cost of a tool is rarely justified.
Chip load ability is also affected by the manufacturer’s geometrical design. Therefore it is always required to be familiar with the manufacturer’s specifications for the tool.
Lastly, dial in the best speed and feed in real world application. Watch and Listen. The finish of the material and the pitch the spindle makes with the tool during operation will give a good indication if you are going in the right or wrong direction. Having tested the tools prior to specifying them in production will give you the “cutting edge” for your application. Knowing when the vibration of the tool is introduced (at what speed) will allow you to bring the tool operation as close to this without introducing the devastating results. This will give you the ability to maximize the use of the tool, machine, spindle and program to mill the best part and finish.