It all started with a customer's request. They wanted to reduce the number of insert corner changes on a mass production machining line. They also wanted to make complete use of the insert's peripheral edge. There was a definite need for imagination in order to meet this seemingly impossible request. That's when we came up with the idea to rotate the insert itself: and invented the rotary holder. The insert had to be rotated, therefore in the early stages of development we experimented with a sliding and other types of bearings (oil retaining, solid lubricant, carbide + DLC coating). However, these types could not successfully overcome the problem of the insert rotation stopping under certain cutting conditions. After finding that mechanism using sliding type bearings made it difficult to reliably rotate the insert, we replaced the slide bearing with a needle roller type. This solved the rotation issue, but new problems arose.
There were side effects from the cutting temperature, it was difficult to improve lubrication and also to prevent shavings from penetrating the bearing housing. Downsizing was also challenge. We cleared each problem step by step, by using different seals and so on until the tool could finally withstand practical use. When put into actual use, it was found that it was possible to use the entire periphery of the insert, but it was also evident that the effects of lowered relative velocity with the workpiece also contributed to improved wear resistance.
Mitsubishi Materials developed the rotary holder, a turning tool designed so that the cutting forces caused the round carbide insert to rotate automatically; it achieved the following benefits:
1. Unified wear eliminated the need for insert positional changes until the insert was spent.
2. The constantly moving cutting point meant no boundary wear (see column below) to the cutting edge.
3. Non-concentrated cutting heat reduced insert wear.
As shown in the graph below, these three advantages made it possible to create a stable, long-life when compared to a fixed insert tool. If the workpiece is manufactured from a tough material, abnormal damage can occur due to high temperatures when cutting, or the workpiece can be easily work-hardened.
For a standard tool, lowering cutting conditions helps to prevent abnormal damage, however, this also reduces efficiency. The rotary tool negates the need to reduce cutting conditions by rotating the edge of the tool during cutting, thereby improving machining efficiency and extending tool life.
The rotary holder went on sale roughly 20 years ago and was well received by customers because of its novel mechanism and cutting performance.
Unfortunately however, it is now no longer a stocked standard tool because of later improvements to cost and performance of conventional tool holders. However, it remains very effective in suppressing abnormal damage and its value is once again being reviewed because more components are being manufactured from difficult-to-cut materials. By passing down the know-how regarding rotary tools from the developing team 20 years ago to today's young tool development engineers, a next-generation rotary tool, in tune with the workpieces and machinery of today, is currently under development at Mitsubishi Materials.
Standard tools often suffer from damage called boundary wear or notching when the cutting edge is affected by the work-hardened layer of the workpiece, casting or forged surface (see diagram below). Work-hardening of the material occurs where cutting has caused plastic deformation. Extra boundary wear or notching occurs at the point where the insert contacts the work hardened layer. Likewise, cast and forged surfaces have tough surfaces that contribute to notch damage. Compared with other workpieces, INCONEL®718 and stainless steels are particularly vulnerable to work-hardening, which means notch boundary damage is likely to occur.
INCONEL® is a registered trademark of Huntington Alloys Canada, Ltd.