Choosing A Toolholder For CNC Milling Precision

Using the right toolholder for you milling operations will make a dramatic difference in surface finish, speed and precision.

What are the main points to consider?

Often, toolholders are only an afterthought in the big picture of CNC milling precision .  This is unfortunate because the machine and cutter will not perform any better than the toolholder will allow. Like the weak link in a chain, the entire process can suffer in finish, speed and cutter wear.

What are your needs?

• The obvious place to begin is to analyze your needs. It is easy to get "caught up" in the new technology or sales pitch. If you are cutting cores and cavities from soft tool steel you needs will be quite different than if you are cutting hard steel, for example.
• Doing mold base work will require a completey different type of toolholder than the ones used for cores and cavities.
• High speed machining of electrodes will demand more specific toolholders, since the high speed changes everything.
• There is no need to overkill and overspend. The right toolholder may not be the most expensive or high-end on the market.

What are the main types of toolholders?

1. Hydraulic Toolholders
2. Collet Toolholders
3. Universal Toolholders
4. Heat Shrinking Toolholders

Hydraulic toolholders work for most applications. The cutter is held in place by the force of a screw to distort an expansion sleeve, which has the benefit of even clamping force along the entire cutter diameter. A side effect of the hydraulic oil is its damping effect, which provided for a better surface finish and longer tool life. These holders are also maintenance free and resistant to dirt, as well as being easy to use.

The cutter runout has a best accuracy of .00012 in. (0.003 mm).

Collet toolholders are very popular because, with a collet set, you are able to clamp a wide range of tools at a reasonable cost. Collets come in different degrees of precision-standard or ultr-precise. Many shops begin with the standard set and later upgrade as they need to. Collet sets also come in a wide range of quality, so you cannot expect a poorly finished, cheap set to perform as well as a good set. Cleanliness, proper feeds and speeds will increase the longevity of your collets and help you achieve the finishes and sizes you expect.

Universal toolholders are a cost effective way to achieve high accruracy and good surface finishes. They are available in light duty and medium to heavy duty styles. These are similar to hydraulic toolholders in that they use an expansion technology to tighten the cutter. Instead of hydraulic force, the expansion is done mechanically and, unlike collets, can be tightened against a hard stop without the use of torque wrenches. They also have a very accurate length adjusting screw mechanism for adjusting the length of the cutter.

Heat shrinking technology is based on heating up and cooling down the toolholder through the use of induction heating. Once the toolholder is heated and the cutter inserted, it is cooled so that the holder shrinks around the cutter. This makes the cutter and holder almost as one piece, thus the runout is minimal, less that .00012 (0.003 mm). It also enables a slim toolholder design, which is great for small cutter applications.

Conclusion

Which toolholder is best for you? There is no easy answer because there are so many variables to consider. Many suppliers will allow you to run test cuts, which is the definitive way to go. Who hasn't seen a fantastic display of machining prowess at a tool show, only to find that it doesn't work at home? A careful consideration of your own unique needs will narrow the choices quickly and cost will narrow it further in most cases.