Get A (Tight) Grip
High toolholder gripping torque reduces the likelihood that a tool will slip within a toolholder during a cutting operation. A mechanical toolholder design provides a very high gripping torque via a worm gear that compresses a shallow-taper collet tightly around a tool shank.
Do you have any idea how much gripping torque your toolholders exert on cutting tools? Feedback received by Tom Sheridan, director of marketing for Albrecht, suggests that most shops understand the importance of high toolholder gripping torque, but they usually don’t know exactly how much torque their toolholders produce.
High tool gripping torque reduces the likelihood that a tool will slip within a toolholder during a cutting operation. This can enable more aggressive cuts and faster speeds and feeds. At the same time it increases overall system rigidity, extending cutting tool life and improving workpiece surface finish. These are reasons why shops have traditionally used hydraulic and shrink-fit toolholding systems. Hydraulic toolholders work by uniformly compressing fluid within a chamber around a tool’s shank. Shrink-fit systems use a heating element to slightly expand a toolholder’s inner diameter to accept a tool shank. Then the toolholder body cools around the installed tool and creates an interference fit that secures the tool.
Mr. Sheridan says Albrecht’s APC toolholding concept provides higher gripping torque than both of those methods. For a 0.5-inch-diameter tool, an APC toolholder delivers 1,700 inch-pounds (200 Nm) of torque, which Mr. Sheridan says is 95 percent higher than some hydraulic toolholder designs.
The APC’s internal components are shown in the cutaway image above. Cutting tools install in a collet that has a proprietary coating to prevent tools from sticking during changeovers. An internal set screw allows accurate presetting of tool height. Operators then use a hex key to turn a worm gear which serves to compress the collet around the tool’s shank. Because the collet has a taper of only 1.5 degrees (3 degrees included), the tool is fixed both axially and radially. This provides the high gripping torque.
The APC system accommodates tool shank diameters ranging from 0.125 to 1.25 inches. It is compatible with tools that have interrupted shanks, which may not be the case when using hydraulic or heat-shrink toolholders, Mr. Sheridan notes.
In addition to standard and extended-length versions, the APC is available in a small-body-diameter model that provides tool clearance to reduce potential workpiece interference. (This is well-suited for moldmaking applications.) The APC’s design does not use a collet nut, which also improves tool clearance.
Mr. Sheridan suggests that shops use quality cutting tools with an H6 or better shank tolerance to avoid variation in tool shaft diameters. An undersized shank, for example, would force the collet to compress further than it would on a tool that has an accurate diameter. That could angle the gripping surface in relation to the tool, resulting in a thin ring of contact instead of full surface contact between the collet and tool shank. Mr. Sheridan also recommends that shops adhere to proper maintenance practices with each tool change. Wipers should be used to clean a toolholder’s inner taper prior to inserting new collets. The external surface of collets should be cleaned during each change as well.
Decisions about the cutting tools used in machining operations are arguably among the most important in modern manufacturing.
The book on hydraulic toolholders is that they are fussy to set, fragile to operate and expensive to buy. So why do many shops choose them over other holders that seem less demanding? This Chicago mold builder has good reasons for its choice.
Companies concerned about strict quality requirements regularly check toolholder tapers for wear or inaccuracy because these conditions can jeopardize the results of a critical operation. However, a shop can check tapers quickly and reliably with air gages. These devices can be used effectively without special operator training. For measuring taper in a production environment, few other methods can match the speed and performance of air, as multiple-circuit air jets can be placed in very small taper gages.