Machine Tool & Automation Group - Chip Control

Wide Ductile Chips Challenge Machine Tool Builder
Question: What do you get when you have a high volume part which, when machined, sees significant stock removal of highly ductile steel? Answer: A mess of chips.

The brake output rod is 1006 cold-formed steel.

A high-volume automotive supplier approached Baltimore-based New Vista Corporation, a specialty machine builder, with a tough assignment: perform a number of operations (including removing .100 on a side of a counterbore) for a brake system output rod that is AISI 1006 cold-formed steel. New Vista, which has often taken other difficult jobs that no one else would quote, took up the challenge.

But first some background. The output rod begins life as Ø.740 rod stock. One end is cold-drawn through a die to a diameter of .354; the other end is cold headed to create a Ø1.519 by .680 long head with Ø1.150 counterbore.

Machining operations are in the tail end (face, drill hole, apply 2 chamfers) and the head end (machine O.D., counterbore, face, apply 2 chamfers). Operator interface is limited to loading the raw parts magazine and unloading finished parts that accumulate O.D. to O.D. Required cycle time is 16 seconds floor-to-floor. The quick cycle time and variety of machining requirements makes a standard CNC machine unacceptably slow and too labor-intensive for this application.

To do this part, New Vista built a five-station transfer machine on its standard PT Frame. The first station is the load-in magazine. The second station, with the part stationary, performs all operations on the tail end, except drilling to full depth, and machines the O.D. of the head. The third station performs as a lathe, gripping the new-machined head, in order to perform final operations on the head end. The fourth station changes the part orientation from horizontal to vertical. The fifth station does full depth hole drilling on the tail end.

Head end O.D. is machined at center left; the counterbore, face, and chamfers are applied at lathe station at center right.

Most stock removal for this part is minimal and with the proper tools and coolant flow, chip control is manageable. This is not true at station number three, the lathe station, where a stationary five-insert boring bar, along with performing a number of other operations, removes .100 on a side from the head end counterbore. This would not be a problem with most steels, but this part’s wide ductile chip and machinability rating of 50% (cold drawn 1212 = 100%) means the chip is practically impossible to break and difficult to control.

Machining of the counterbore creates two chips: a wide chip from the plunge in, and a thin chip from the finish cut (.005 depth of cut back bore). Employing a positive-rake carbide chip-breaking insert (Ø.375 I.C., with .020 nose radius) and constant feed of .016/rev. (surface feed of 719 sfm), the chips would normally birds’ nest around the boring bar. New Vista engineers, anticipating chip management issues in this area, prioritized possible ways to break and/or better control these chips.

With the boring bar mounted to a servo-driven slide, New Vista first developed a series of interrupts in the feed in order to break the chip. Still, the size and volume of chips would be too much to clear from the work area. So the servo was programmed to completely withdraw after each third of the .510 total feed, while still maintaining the series of interrupts to break the chip. (This added a nominal amount of time to the cycle, but not enough to exceed the 16-second cycle requirement). These features allowed for predictable results but by themselves would not guarantee 100% consistency.

High-pressure through-the-tool coolant was the other element required. A 1,000 psi system would improve tool life and surface finish since a lot of the cooling and lubricating potential of flood coolant is lost as it is vaporized prior to entering the cutting zone. Although some applications will see chip breakage with high-pressure coolant, these chips are just too wide & ductile. However, the high-pressure stream is successful in directing the chip as it comes off the tool. The directing of chips combined with shorter chip lengths attained by interrupted cuts led to acceptable uptime in this area.

Raw parts are loaded in magazine at top right. Finish machined parts can be seen accumulating O.D. to O.D. at bottom center of picture.

Still, safeties were added to prevent any damage from the now-remote possibility of chip nesting. New Vista took two measures. First, a safety wire “free space” monitoring system was added to swing tightly around the boring bar after each withdrawal to check for chip nesting. If the wire is not able to complete its full rotation around the bar due to chip presence, then it registers an obstruction and requests operator clearance. Also, spindle motor amperage monitoring was added. The machine control is programmed to shut down if a spike in amps is detected. This protects against damaging the machine if an insert has failed due to chip interference or from normal end-stage wear.

After the chips are clear of the part and tool, they still need to be flushed from the unit. This is relatively easy. Flood coolant washes the chips off the base and into the coolant gutter, which has been specially designed with a 5” x 19” rectangular hole and chip chute. Chips and coolant are channeled onto a reservoir-mounted gooseneck conveyor; coolant flows through the conveyor into the tank while the chips are lifted and dropped into a scrap recycling bin.