A large auto manufacturer has increased tool life by a factor of five in reaming connecting rods by switching to HFS toolholders from Mapal. Excessive runout in the conventional tapered-flange style toolholders previously used on this operation concentrated the cutting load on just a few teeth, which hurt tool life. Switching to HFS toolholders from Mapal (Piscataway, NJ), which have both taper and face contact, improved both concentricity and changeover accuracy to less than 3 microns. The new toolholders evened out wear over all teeth in the reamer, increasing tool life from 2,000 to 10,000 bores. They also made it possible to reduce machining time by 15% by increasing cutting speeds and feeds.
The connecting rods are made of carbon steel (C 70). Requirements for machining the smaller bore are a roundness of less than 2 microns and a cylindrical form within 5 microns. Since it was impossible to adjust for angular error between tool and holder with the previously used toolholding system, concentricity was a continual concern. Mapal''s HFS toolholding system, however, offers a unique design consisting only of a head, holder, and differential screw. The screw draws the taper in to make face contact, thereby providing exceptionally low runout.
Roundness values and surface quality achieved produce a contact area of up to 97%. Additional tool life improvements are possible because the Mapal system''s high changeover and concentricity accuracy mean the tool diameter can be adjusted closer to the upper tolerance limit. The superior coolant coverage of the new toolholders provides further tool life improvements: whereas the old toolholders had a coolant outlet 1.18 in. behind the blade, making it impossible to provide full coolant coverage at spindle speeds of 1,600 rpm, the coolant outlet on the HFS holders is only 0.27 in. behind the blade, so the blades are now completely flooded.
The new toolholding system is also much easier and faster to change. The tapered-flange holders used in the past were jammed together by the cutting forces, making them very difficult to remove. In many cases, it was necessary to remove the holders from the machine and use a special fixture to change the reamer head. A defined release force in the HFS makes it easy to remove the tool simply by turning the screw used to lock the tool in the opposite direction, so the reamer head can be changed without removing it from the machine. The fact that all functions are activated with a single screw makes the new toolholders very easy to use. The new production feasibility index of Cpk 2.11, which means that quality is significantly higher than six sigma levels, underscores the success of this application.