To understand the concept of feeds and speeds, it is necessary to visualize what is happening at the cutting edge of the tool itself.
When the cutting edge of the tool makes contact with the material being cut, a chip of material is being removed from the base part. The size of the chip is controlled by the rotation speed of the spindle (RPM or speed) and the forward movement of the tool into the material (feed rate) .
Most of the energy expended during this action is released as heat (friction) .Heat is one of the major factors in tool wear.
Always remember that heat is the enemy of the tool.
The best way to get rid of the heat is to have it carried away in the form of the chip. Larger chips will both dissipate heat and will leave you a high quality edge finish due to minimization of re-cut chips.
This is especially true when dealing with plastics which tend to re weld to the router bit and can cause a multitude of complications to the tool and the material.
To achieve proper cuts and finishes you must posses a tool with the proper geometry. Once the proper tool is used you can refer to my feed and speed chart which will guide you to the proper chip load you require to get the finish you desire along with getting the max tool life to your router bit.
The feed and speed chart is broken down by the cutter diameter, number of flutes, RPM's and chip load. The chip load is essentially the thickness of the chip being created by the cutter. If the chip load is too small the cutter will heat up. This can result in burning, melting or premature wear of the cutter.
Remember to make chips not dust.
The folmula to follow is as shown:
[Chipload] x [# of flutes] x [RPM] = [feedrate]
To Increase the chipload: Increase the Feed rate, Decrease the RPM, use less flutes
To Decrease the chipload: Decrease the Feed rate, Increase the RPM, use more flutes
Another way to determine if you are using a proper feed and speed is that of the tool temperature. After running a few parts on your CNC and the spindle comes to a complete stop check the temperature of the router bit. If it is hot or warm to the touch, then the feed is too slow or the spindle RPM is too high. If the proper feed and speed is used the bit should be at or near room temperature. Heat is what breaks down the cutting edge of the tool.
If you are encountering this problem please read on.
The first change to make is to the feed rate. This is the controlling factor in productivity.If the feed rate is at its maximum due to part configuration, hold down capabilities, software limits, or machine limitations, then the RPM's should be lowered. This does two things:
- It increases the chip load (thickness).
- It lowers the number of times the cutting edge is presented to the material.
The second factor will give you increased tool life because the cutting edge will have less contact with the material thus increasing tool life by 15 - 20%. This also reduces the spindle bearing temperatures by reducing the heat transmitted into the spindle.
This may take a few tries, yet once the CNC operator finds his sweet spot he will be on his way to a better quality cut and desired speed and finish to increase his productivity and lengthen his router bit life.
You may view over my Feed and Speed chart in the next article.
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