Shrink Fit: The High Accuracy Toolholder of Choice(1)

Requirements in the mold industry are much more precise than general machining, so more attention must be paid toward the selection of the appropriate toolholder in regards to its features and benefits.

Within the last 10 years, the acceptance and integration of shrink fit toolholders in the mold and die industry has continued in aiding North American-based mold shops in remaining globally competitive.

Toolholding for milling machines in general had often been overlooked by most manufacturing facilities throughout the North America. However, it was the mold and die industry that was really the first segment of the manufacturing market to look for more precise toolholder options. Due to numerous inherent benefits, shrink fit toolholders have predominately become the high accuracy toolholder of choice for the mold and die market.

The Challenge
As the mold and die industry in North America has become much more of a globally competitive market, the need to reduce expensive labor intensive practices has become an absolute necessity for shop owners. The goal for most shops is to reduce or completely eliminate polishing or spotting time on their molds. This can be accomplished in numerous ways including the use of EDM sinker machines, hard milling of steel or cutting closer to net shape from the beginning of the machining process.

In order to address this challenge, the mold and die shops have been forced to look at their entire process. For example, choosing the correct machine tool for the job is no longer the only consideration. It is important to investigate and decide on the appropriate machine control, CAD/CAM software package, toolholders and cutting tools in order to complete the job most efficiently.

Anti-vibration shrink fit chuck designed to provide better finishes when roughing. Images courtesy of Haimer.

The Toolholder Solution
When looking for toolholder solutions for any type of machining, it is always good to first focus on three main features that a toolholder must bring you:

1. Rigidity
Rigidity comes from sufficient taper contact and proper clamping of the toolholder in the machine tool spindle.

2. Accuracy
Accuracy comes from minimal run-out at the cutting edge of the cutting tool.

3. Balance
Balance comes from a balanced assembly of the toolholder (including all accessories such as pull-studs) and cutting tool combination.

However, the requirements in the mold and die industry are much more precise than general machining, so more attention must be made toward the selection of the appropriate toolholder for the job.

For example, a mold shop often must think of the following:

  • Geometry of toolholder to avoid collisions with the workpiece. In regards to the EDM process, electrodes must be machined accurately and efficiently. Oftentimes, deep ribs are required in the part process and often present challenges. Also, deep cavities such as large door panel or bumper molds require deep reach with extreme clearances.
  • Cleanliness of the toolholder to avoid excessive run-out, especially when machining graphite.
  • Extending cutting tool life since high-end cutting tools are needed to obtain the best performance in the shortest time possible. These cutting tools often have exotic coatings that lend to an expensive price.
  • Finish. Higher speeds and feeds are used with lower depth-of-cuts, which translates into better surface finishes. This makes balance even more important to minimize vibration at the cutting edge of the cutting tool. Also, proper chip evacuation provides better finishes.

Based on these additional requirements, most mold shops have found that due to some inherent benefits, shrink fit toolholders give them the best opportunity to accomplish the job competitively and accurately. Also, the evolution of inductive shrink fit machines has made the shrinking process easier, quicker, safer and less costly to invest in this technology up front.

Shrink Fit Advantages
There are 10 inherent benefits that a good shrink fit chuck can offer a mold shop:

1. Unsurpassed accuracy
A properly produced shrink fit chuck should be able to guarantee 0.00012" (3 microns) maximum run-out at three times the cutting tool diameter. This accuracy is very repeatable from operator to operator.

2. Availability of slim profiles
Shrink fit chucks are available with three-degree draft angles and very slim profiles. They can also be modified to be straight walled if needed in order to prevent toolholder collision with the workpiece.

Slim 3-degree draft angled shrink fit chucks.

3. Gripping torque
A shrink fit chuck grips the cutting tool 360 degrees around the shank. This leads to a very high gripping torque that prevents the cutting tool from moving during roughing or finishing operations. This greatly aids in the reduction of scrapped parts.

4. Extended reach options
Shrink fit chucks can use shrink fit extensions that provide the user with many options with standard products. When machining deep cavities, one can place shrink fit extensions into standard shrink fit chucks, getting unsurpassed toolholder lengths with very little run-out.

5. Balance repeatability and balanceable options
Shrink fit chucks offer the best balance repeatability of any toolholding system on the market since there are no moving parts. In many cases, if a shop purchases a properly balanced shrink fit chuck with correct accessories (such as pull-studs) and uses good cutting tools without inherent unbalance (such as flats) then they can often have good balance characteristics for running at high speeds without doing an additional fine-tune balancing. Of course, if there is a need for additional balancing of the toolholder on a balancing machine after the assembly of the toolholder setup (toolholder plus cutting tool, plus pull-stud or coolant tube) then many shrink fit chucks on the market come with simple to use balanceable options already built into the chucks.

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