Cool Shrink Fit System

One of the neat things about metalworking is the wide variety of solutions generated for specific challenges. Shrink fit tooling is one such challenge. Much has been written and demonstrated about the advantages in concentricity, balance and ultimately accuracy that such tool-holders bring to the machining process.

The object of the shrink fit exercise is to get a slightly oversize cutting tool into a slightly undersize cutting toolholder. Several methods have been successfully marketed to accomplish this. Most involve the application of heat to the undersize toolholder to expand it sufficiently to accept the oversize tool. Contraction from cooling the holder then grips the cutting tool. Tool removal is accomplished by reheating the holder and removing the tool.

A new process recently shown at IMTS does away with the application of heat, and consequently, the heating and cooling cycle time associated with it. It's called Tribos power shrinking, and it is being marketed by Schunk Precision Workholding Systems (Morrisville, North Carolina).

The new Tribos system uses controlled deformation to achieve shrink fit on a cutting tool. Viewed on end without a cutting tool, the toolholder ID is shaped in a three-lobed polygon that is slightly smaller than the cutting tool diameter. The OD of the toolholder is round and the wall section between the ID and OD is relatively thin.

To activate the toolholder, force is applied on the circumference at points opposite the ID lobes. This force causes the OD to deform into a three-lobed polygon while the ID changes to a round shape. The round ID is now large enough for the cutting tool to fit inside. Once the cutter is inserted, the force on the OD is released and the ID tries to spring back to its polygon shape thus "shrinking" around the tool and holding it.

Grip forces are spread around the contact area of the lobes and the tool. Runout and repeatability are less than 0.003 mm (0.00012 inch).

A stand-alone unit is required to compress the toolholder. It is hand actuated and mounts on a bench top. A collet-like sleeve that slips over the toolholder provides proper alignment of the lobes. It lines up with marks in the clamping fixture. To activate the fixture a crank is turned until the polygon is sufficiently deformed to accept insertion of the cutting tool. Different diameter tools require different collets and pressure. A dial indicator on the fixture shows the amount of pressure needed to actuate a given size. The process is reversed to remove the cutting tool.

Currently the Tribos system is available in HSK 63-A and HSK 50-E. A 40 taper V-flange is also available. Tool diameters from 6 mm (0.24 inch) to 12 mm (0.47 inch) are available with more sizes in development.

Because of the relatively thin wall between ID and OD on this toolholder, it is effective in applications with geometric features that are hard to reach. Several extension sizes are available to access deep pockets or mill inside bores.