Speeds and feeds are the most important considerations to achieve the best results from cutting tools. Improper speeds and feeds often cause low production, poor quality, and damage to the tool. Speeds that are too high or feeds that are too light can lead 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 usually 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 are based on 2-flute drills.

To establish operating conditions, all feeds rates should be calculated from the chipload or feed per tooth. The highest possible feed per tooth will usually provide longer tool life. However, excessive feeds may overload the tool and cause the cutting edges to chip or break.
 
Following are many of the commonly used formulas for calculating operating parameters for cutting tools:

NOTE: for all formulas, D = diameter and T = number of teeth.

SFM: surface feet per minute/cutting speed:  .262 x RPM x D
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
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

Technical Tip #30 - Formulas for Cutting Tool Speeds & Feeds

Kennametal routinely conducts tests of our tools against competitive brands to validate our tool performance.

There are many variables to consider when testing tools against each other in the same application. Every aspect of the application should be considered and measured for each tool in the test.

Many parameters are not changeable, such as desired hole depth, thread size, finished diameter, machine cost per hour, material, etc.

But some performance factors can vary significantly and include:

• Feed Selected: IPR (inches per revolution); IPM (inches per minute); CLPT (chip load per tooth, end-mills) are variable, while tap feeds are set by the pitch.
• Speeds Selected: rotational RPM’s (rotations per minute); SFM (surface feet per minute)
• Tool Geometry/Style: point, web, helix, length, chamfer (taps), etc.
• Tool Materials: HSS, premium HSS, powdered metal, carbide
• Number of regrinds possible can vary by style
• Regrind time can vary by style and type
• Coatings recommended can affect results significantly
• Price per tool has minimal affect in most tests

Lot size may impact how important these variables become. Goals of the comparisons also may vary from tool life, tolerance, scrap reduction, and amount of material removed in a given timeframe, etc. Machine condition and type also may limit or increase options available.

One common mistake is to compare the exact same tool types, styles, coatings or operating parameters to define cost effectiveness.

The goal of testing should be to examine each element of the operation and select the best possible combination to maximize performance.

Technical Tip #46 - Elements of Tool Testing

Work hardening of materials is a condition that should be avoided while machining. It is caused when heat generated by the cutting tool transfers to the workpiece material and causes plastic deformation. The process is similar to a heat treatment of the workpiece but on a lower scale.

When a part work hardens during machining, its surface becomes a shiny glaze and appears slippery. The machined part can even take on the same hardness as the cutting tool.

How to avoid work hardening:

• Make sure the cutting tools are always sharp!
• Run at the recommended feeds and speeds for the material being machined. If these are incorrect, tool rubbing (versus cutting) will increase heat.
• Use coolant-fed tools. Water-based coolant should be used at about 8% to 10% mix.
• Do not dwell the tool in one position.
• When drilling, run with constant feed whenever possible.
• If peck drilling, reduce the number of pecks and withdraw each peck one tool diameter.
• When experiencing tap breakage, the cause may not be the tap, but a work-hardened hole.
• Stainless steels and high-temperature alloys are prone to work hardening.
• Proper tool maintenance will help reduce work-hardening problems.

Technical Tip #60 - Work Hardening During Machining

Coolant can dramatically affect the performance of cutting tools, which can impact the cost of your operation. Consider these guidelines for using coolant:

Cutting fluids perform two basic functions in drilling, milling, and threading:

1. reduce heat generated in cut
2. lubricate the tool

Water-based coolant helps to cool the chip when it is sheared from the workpiece material.

Coolant acts as a lubricant to reduce friction between the chip and the tool. This improves the surface finish and helps force chips out of the flutes. Coolant can improve tool life because excessive heat and friction can dull the tool.

Sometimes users will reduce the coolant/water ratio significantly because they assume this will help cut costs. However, tool life can decrease rapidly if the proper amount of coolant is not used when operating the tool.

If the recommended soluble oil concentration of 8%-10% is reduced to 5%, tool life will be sacrificed. A drill that typically makes thousands of holes may now get only hundreds of holes at the reduced coolant level. Once coolant levels are returned to the recommended amount, drill tool life should return.

For high-speed machining with carbide tools, it is very important to provide adequate water-based fluids to cool the tool. However, if you are high-speed steel tapping, heavier oil-based coolant is recommended.

Always follow the tool manufacturer’s mixing recommendations for coolant.

Technical Tip #76 - Coolant Considerations