Monday, November 11, 2013

CNC Router Tools and Tool Holders

This post provides information on common CNC tools used in the lab. It also shows examples of the two types of tool holders we employ. Lastly it shows how the tools are measured for length so the CNC machines knows exactly where the tooltip is for accurate cutting.


Tools

This section discusses a few different CNC router bits and their common cutting application.

The most common types are:
  • Straight Bits: These tools cut on both the end of the tool and on the sides. There are many variations of the geometry of these bits. Most of the material below is devoted to explaining the differences and their use.
  • Ball-End Bits: These bits have a circular profile at the tip. This allows them to cut 3D surfaces.
  • Chamfer and Engraver Bits: These cutters come to a point at the tip. They are commonly used to label parts by cutting the numbers as a shallow V shaped groove.
The following illustrations show the difference in cut pattern and the effect of wide versus narrow step-over in toolpath setup.



Tool Material

There are a number of common materials used to make bits. 
  • HSS (High Speed Steel): Typical applications in non-abrasive plastic, solid wood and aluminum where keen edges perform best. High Speed Steel tools a tougher core which helps to prevent tool breakage. These are the least expensive.
  • Carbide Tipped: Used for a variety of applications in composite woods, hardwoods, abrasive plastics and composites plastics to hard aluminum. Limited by geometry in some applications due to the brazed nature of the tool. These last longer than HSS but are more expensive.
  • Solid Carbide: Typically used for widest variety of applications including man-made board, solid wood, abrasive plastics, and some aluminums. Solid Carbide does not deflect allowing the tool to be fed at higher feedrates. Solid tools also have major edge keenness advantage thought only possible in HSS until a few years ago. The Taubman Fab Lab uses most solid carbide tools.

Flute Geometry

The geometry of the flutes in the cutters has a great influence on their application. 

  • Up-shear (up cut) Flute - This is a spiral geometry. It provides the best surface finish and allows for good chip extraction. May cause part lifting if vacuum or fixturing is not sufficient to hold the parts in place.

  • Down-shear (down cut) Flute - This is also a spiral geometry and provides a downward force which helps eliminate part lifting. Chip overheating may occur if there is no space below the part for chip expansion.

  • Compression - Visualize combining an up-shear and a down-shear and you've got a Compression Cutter.  The flutes go one way on the bottom of the flute length and the other way at the top. They are particularly suitable for double-sided melamine or laminated material. They’re used because they pull towards the middle of the cutter, which reduces chipping on both the top and bottom.

  • Straight Flute - Offers a neutral cutting action. There is no spiraling, augering action to move the chips up or down. These bits are the least expensive because they are easy to manufacture. The downside is that they apply the cutting force all at once rather than progressively like the spiral flute geometry.
Here's how you can look at a bit and determine if a bit is up-shear or down-shear: The router bits we use in the lab are designed to spin clockwise. Hold the tool shank up and the tip facing downward. Look down the length of the tool. Spin the tool in a clockwise direction. Note if the spirals are pulling upward, or pushing downward. If they are pulling upward you have an up-shear. 

    Chip-Breaker Grind: Another variant on flute geometry are Chip-Breaker grind bits. These usually take the form of a serrated edge on the flutes. The break in the edge fractures the chips into smaller pieces which can improve chip removal. Some chip-breaker bits can leave a striated finish unless the serrations are staggered between the flutes. 





    Here's a picture of a test cut showing the difference between up-shear and down-shear bits. This example shows the direction the tool is moving with the arrows. This sample is White Oak.
    The bit is turning clockwise. Note the upper edge of the up-shear cut. It's pretty clean. That direction is like petting a dog along its back in the direction the fur flows (head to tail). The other side of the bit is like petting the dog from tail to head - backwards. In the down-shear case it doesn't make much difference.

    Number of Flutes

    The Fab Lab has single, double, and 3 and 4 flute cutters. Note: As the number of cutting edges increases, your feed rate should increase to prevent burning and premature tool dulling. 
    • Single Flute - Allows for larger chip loads in softer materials. Certain materials produce large chips - for example Aluminum. 
    • Double Flute - Allows for better part finish in harder materials. Because of the two flutes the feed rate must be increased over a single flute.
    • Multiple Flutes - Allows for an even better part finish in harder materials. Again, because of the multiple flutes the feed rate must be further increased. 

    Drill Bits

    Drill bits are for making holes in your work. There are three types commonly used in the lab: 
    • Twist - these leave a cone shape at the bottom of the hole. Thus you need to drill deeper to get fully through the material.
    • Brad Point - the edges of the bit are sharpened and extend to nearly the same length as the tip. This allows the end of the hold to be less cone shaped. Thus, you don't need to cut as deep to ensure that the material is fully cut through.
    • Forstner - These are used for cutting flat bottomed larger holes. There is a small detent in the middle of the hole from the point in the center of the bit. Note that if you want truly flat bottom holes you can use a "boring" toolpath and an endmill rather than a drilling operation.

    Tool Length

    A note about tool length. In choosing a bit for any application, always select one with the shortest cutting edges and the shortest overall length that will reach the required cut depth. Excessive length intensifies deflection and vibration, which degrade cut quality and lead to tool breakage.


    Formulas Relating Chip Load, Feed Rate, and Spindle Speed

    Chip Load = Feed Rate / (RPM x Number of cutting edges)
    Feed Rate (IPM) = RPM x Number of cutting edges x Chip Load
    Spindle Speed (RPM) = Feed Rate / (Number of cutting edges x Chip Load)

    Definitions:

    • Feed Rate = The speed of the tool moving through the work.
    • IPM = Inches Per Minute
    • IPR = Inches Per Revolution
    • RPM = Revolutions Per Minute


    Tool Holders

    There are two types of tool holders used in the lab. One uses a conventional collet, the other a shrink fit collet.

    Conventional Collet

    The conventional collet is a holding device that forms a collar around the tool to be held and exerts a strong clamping force on the tool shank as it is tightened by means of a tapered outer collar.A torque wrench is used to apply the correct amount of clamping force.

    Shrink Fit

    A shrink fit collet holds the tool in place by the collet shrinking and tightening around the tool shank.

    Tools are inserted and removed using this machine.

    The collet is heated until the tool barely slips in. Once hot it shrinks enough that the tool can be slid in or out. Compressed air is then blown onto the collet to cool it down. This locks the tool in place.

    You cannot use HSS (high speed steel) tools in the shrink fit machine due to the very high temperature. 

    Tool Measurement

    The tools are measured for length using a digital height gauge. First the gauge is zero'd on the base ring. 

    Next the tool is placed on the right and the height is measured. The height is entered into the tool table of the machine.

    For the 3-axis routers in the Fab Lab we always manually measure the tools. The Roland 4-axis and the Onsrud 5-axis routers have the ability to measure the tools automatically, right on the machine.


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