GRINDING RESOURCES

End Mill and Cutting Efficiency - When and How Much To Regrind

A s we remove the material with end mills in the form of chips, a wear landforms on the cutting edge. When this wear land develops to the point of generating excess heat, vibration, roughness, or produces a part that is out of tolerance, the end mill should be pulled.

On high-volume operations, a predetermined number of parts minutes in the cut, power draw on the machine, or cubic inches of material removed can be established as the point to pull the end mill.

When an end mill becomes dull, we choose whether to regrind or discard the end mill. This choice is basically an economic one: will it cost more to regrind than it will to replace it? End mills 1/4" in diameter and above can generally be economically reground if there are sufficient quantities to absorb setup costs.

As the end mill's diameter is reduced through regrinding, there is a significant reduction in the hook or radial rake angle. The original diameter can generally be reduced by the following and still retain up to 80% of the original cutting efficiency.

.250 thru .500 15%
.562 thru .625 12%
.750 thru 1.00 10%
Over 1" 8%

End Mills that are badly chipped or severely worn to the point that the diameter will need to be reduced more than the above guidelines, will need to be refluted to restore the original flute form, and radial rake angle.

Coatings For Cutting Tools

Our group receives many questions regarding coating recommendations for different tool types in an infinite variety of materials. Many of you have also suggested that we cover this topic in your suggestions of things for us to address. We are preparing some future-specific tips to cover this exact topic. As a preface to those tips, we would like you to understand a couple of important fundamental things regarding high-performance coatings.

All seemingly “alike” coatings are not alike: The performance of a TiN/TiCN/TiAlN coated tool from one coatings vendor may perform differently from the TiN/TiCN/TiAlN provided by another. Our independent tests of tools from the same lot and coated by different vendors, including ourselves, revealed a performance spread of 10X when tested in identical test conditions. In other words, in extreme cases, a drill coating from one vendor may yield 300 holes while another vendor's coating will yield 3,000 holes! Consistency day to day and month-to-month performance is also critical for predictable results.
Coatings development is continuous: The process itself is constantly evaluated and improved yielding higher productivity, changes in which coatings we recommend for which tools and workpiece material, and consistency. About every quarter, we receive updates from our R&D department for updated recommendations as well as changes in both SFM and IPR.
Surface feet recommendations are vital: To achieve ANY benefit from HP coatings, they must be run at significantly higher speeds. If coated tools are run with bright tool parameters they can even be counter-productive. Follow the manufacturers recommended starting points (which are typically conservative) and work upward from there.

End Mills; Overhang and Deflection Ratios

End mills, being supported on only one end, are subject to deflection. The amount of deflection is dependent on the material the end mill is made from, the diameter, the overhang, and the amount of radial cutting forces acting on the tool. Steel has a lower co-efficiency of elasticity than cemented carbide. Therefore, HSS end mills will experience more deflection than carbide end mills.

As a general rule:

A 20% overhang reduction will reduce tool deflection by 50%.
A 20% increase in diameter will reduce tool deflection by 50%

Based on these rules we should:

Use the largest end mill the workpiece will allow
Minimize the amount of overhang from the spindle nose
Reduce feed per tooth
Reduce radial or axial depth of cut
Reduce the number of flutes

Formulas For Cutting Tool Speeds and Feeds

Speeds and feeds are the most important factors to consider for the best results from cutting tools. Improper speeds and feeds often cause low production, poor quality, and damage to the tool. Too high a speed or too light a feed leads to rapid wear and dulling of the cutter, reducing tool life.

Speed is measured in peripheral feet per minute. It is often referred to as cutting speed or surface speed. Feed is normally measured and stated in inches per minute (IPM). It takes into consideration the number of cutting teeth (or flutes), the feed per tooth (or cutting edges), and the revolutions per minute. Feed recommendation tables for drills generally are based upon two flute drills.

In establishing operating conditions, all feeds rates should be calculated from the chip load or feed per tooth. The highest possible feed per tooth will usually give longer tool life. However, excessive feeds may overload the tool causing chipping of the cutting edges or breakage.

Following are many of the commonly used formulas for calculating operating parameters for cutting tools:

SFM: Surface Feet per Minute/Cutting Speed: .262 X RPM X D (D=Diameter)
RPM: Revolutions Per Minute/Rotational Speed: (3.82 X SFM)/D or SFM/(.262 X D)
IPM: Inches Per Minute: Machine Feed Rate: RPM X IPR or T X IPT X RPM (T= # of teeth)
IPT: Inches Per Tooth/Feed Per Tooth: IPM/(RPM X T)
IPR: Inches Per Revolution-Feed Per Revolution: IPT X T or IPM/RPM
Inches to MM: Inches X 25.4 or Inches/ 0.03937
MM to Inches: mm/25.4 or mm X 0.03937

End Mills; Reducing Vibration and Chatter

When chatter arises it tends to be self-sustaining until the problem is corrected. This condition causes poor finish on the part and will damage and significantly reduce the life of end mills. Carbide end mills are peculiarly susceptible to damage.

When experiencing chatter problems, the basic reflex action is the reduction of cutting forces. This can be done by:

Reducing the number of flutes
Decreasing the chip load per tooth by reducing the feed or increasing the speed or RPM
Reducing the axial or radial depth of cut

Even though these steps can and will reduce chatter, slowing down the cutting process is not always the best course of action, and reducing the chip load can be detrimental to the cutter.

Better first steps are to improve rigidity and stability:

Use a larger end mill with a larger core diameter
Use end mills with reduced clearance or a small circular margin
Use the shortest overhang from the spindle nose to the tip of the tool
Use stub-length end mills where possible
Use balanced tool holders
Rework the fixture to hold the workpiece more securely
Reprogram the cutter path shifting cutting forces into stiffer portions of the workpiece
Look for optimum spots in spindle speeds then adjust the feed accordingly

A common source of chatter is the machining of corners. A s the end mills enter the corner the percentage of engagement increases the number of teeth in the cut. This drastically increases the cutting forces, causing chatter. Using circular interpolation and producing a bigger corner radius than the part print calls for then going back and removing the remaining stock with a smaller end mill using circular interpolation will reduce the tendency to chatter.

Note: Technical Tip References from Kennametal Greenfield Technical Tip Manual