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Cutting Speeds: From Theory to Practice, Optimizing Cutting Speeds in Machining
Suppliers provide general cutting speed guidelines, but adjustments are needed for real conditions. Machining Doctor helps optimize speeds based on material hardness, stability, and other factors.
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Every supplier of cutting tools offers guidance on the appropriate cutting speeds for their products. However, these recommendations are often general, designed for optimal scenarios such as stable clamping, annealed materials, and ideal carbide grades. Consequently, these suggested speeds may be too high for many applications, necessitating adjustments for specific conditions. This article will provide the knowledge needed to make these adjustments effectively.
Additionally, the strategies discussed here are helpful when you find a cutting speed that performs well under certain circumstances, but changes occur in one of the variables (for example, material hardness or stability).
While you can utilize Speeds and Feeds calculators for automated calculations, understanding the underlying factors that influence these parameters will empower you to make more informed decisions.
Raw Material Identification
The suggested cutting speeds for a product are typically found in the packaging of inserts. However, taking these speed suggestions with a "grain of salt" is recommended. This is because the information provided there is often restricted to just six primary ISO material categories, while the details in catalogs are much more comprehensive and precise.
Supplier catalogs generally contain numerous subcategories within each main group, but there's no uniformity across different suppliers, as each has developed its own classification of material groups. Usually, a catalog will list between 40 to 100 sub-groups.
It’s essential to dedicate sufficient time to accurately classify your material in accordance with the supplier of the cutting tool you intend to use before progressing further. While this may require some effort, it's not a stage where you should cut corners!
Now that we have a solid foundation, we can proceed to the next steps.
Raw Material Hardness
In certain situations, you might possess experience or recommendations regarding cutting speeds for a material in its annealed state. However, it will require heat treatment before it is machined. A common example of this scenario includes PH stainless steels or high-alloy steels.
To modify the cutting speed, consult the chart provided in the main image. The horizontal axis illustrates the difference in hardness in Brinell units between the material you're handling and the one for which you have the initial information. The vertical axis shows the necessary percentage adjustments to apply to the base cutting speed.
Application Stability
The appropriate machining speed for any application is significantly influenced by the overall stability of the setup, which involves a subjective evaluation. The stability depends on the quality of the clamping for both the workpiece and the cutting tool, along with the tool's overhang. To assess the stability of your setup, use a scale from 0 to 10, where 10 indicates ideal stability with minimal tool overhang, and 0 reflects highly unstable conditions. Assume that the manufacturer recommendations are based on a score of 8, and you should modify the speed based on the factors outlined in the chart from image #2. Remember that this assessment is subjective, but it serves as a helpful starting point for understanding how stability influences the adjustment of cutting speeds.

Image 2
Tool-life Vs Productivity
The range of cutting speeds that can be utilized is wide, and there isn't a definitive number that is right or wrong. Choosing a higher cutting speed will increase productivity but may lead to a shorter tool lifespan, whereas opting for a lower speed can prolong tool life at the cost of productivity. The decision depends on your objectives and individual preferences. Various models exist to describe the relationship between cutting speed and tool life, with the Taylor model being the most widely recognized:
- (V1/V2)=(T2/T1)^n
Where:
- V1=Current cutting speed
- V2=New cutting speed
- T1=Current tool life
- T2=New tool life
- n=Material factor
The typical value of n for carbide grades ranges from 0.2 to 0.4, influenced by the specific grade and the raw materials involved. In image #3, a graph of the Taylor model with n set to 0.3 is shown. The graph indicates that a 50% decrease in cutting speed can extend the tool life by up to 10 times. A 50% increase in cutting speed can lead to a 75% reduction in tool life. While the exact figures may differ among various materials, the key point is that cutting speed significantly affects tool life in an exponential manner.

Image 3
Radial Depth (Milling)
In milling, an important aspect to take into account is the radial depth of cut (Ae). When the radial depth is less than the cutter's radius, it becomes possible to increase the feed rate due to the principles of chip thinning. However, many may not realize that boosting the cutting speed is also feasible. A reduced radial depth provides more time for cooling outside of the material for each flute, allowing this speed increase (refer to image #4). The extent to which we can elevate the speed depends on the cutter's diameter and the ratio of the radial depth to this diameter (Ae/d). You should assume that the catalog recommendation is for Ae/d equals 0.5, meaning that the radial depth is equivalent to the cutter's radius. You can refer to the table in image #5 for specific speed adjustments.

Image 4

Image 5
Please note that the information provided in this article is scientific. However, it can be helpful in fine-tuning cutting speeds according to modifications in your application.
www.machiningdoctor.com

Image 3
Radial Depth (Milling)
In milling, an important aspect to take into account is the radial depth of cut (Ae). When the radial depth is less than the cutter's radius, it becomes possible to increase the feed rate due to the principles of chip thinning. However, many may not realize that boosting the cutting speed is also feasible. A reduced radial depth provides more time for cooling outside of the material for each flute, allowing this speed increase (refer to image #4). The extent to which we can elevate the speed depends on the cutter's diameter and the ratio of the radial depth to this diameter (Ae/d). You should assume that the catalog recommendation is for Ae/d equals 0.5, meaning that the radial depth is equivalent to the cutter's radius. You can refer to the table in image #5 for specific speed adjustments.

Image 4

Image 5
Please note that the information provided in this article is scientific. However, it can be helpful in fine-tuning cutting speeds according to modifications in your application.
www.machiningdoctor.com