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Thread milling eliminates secondary operations.

Thread milling eliminates secondary operations Burring is a major problem that companies experience when tapping intersecting cross holes in steel. When a tap internally threads a drilled hole that intersects a smaller cross-hole, it pushes a burr into the smaller intersecting hole, usually requiring a secondary operation to remove the burr.

One company that recently experienced this problem, and overcame it, is Yale Materials Handling Corp, Lenoir, NC. Yale, with approximately 300 employees, has considerable experience in the manufacturing of parts used in hydraulic forklift trucks. These parts include hydraulic cylinders and flow-regulator housings.

When the company first manufactured O-ring ports for hydraulic cylinders, using 1117 grade steel, it used a CNC vertical machining center for drilling and tapping the required holes. When tapping one of the drilled holes that intersected with a smaller cross-hole, operators find that steel shavings were pushed into the smaller hole, producing a burr.

After the drilled holes were tapped, a secondary operation was required to remove the burrs. Each part was taken to a manually operated die grinder to grind off the burrs where the threaded hole intersected with the unthreaded cross-hole.

To improve its cost effectiveness Yale purchased a Matsuura Model 400H-11P CNC machining center from Methods Machine Tools Inc, Sudbury, MA. This machining center has 11 pallets (tool-holding fixtures), 42 tool positions, and can be programmed to machine any or all of the 11 pallets in any sequence.

A major recommendation that was given to Yale by Methods Machine Tools Inc was the use of a thread-milling cutter supplied by Detroit Tool Industries, Warren, MI, to thread the drilled holes required for the O-ring ports.

The thread-milling process works in the following manner. Each tooth of a thread-milling cutter is shaped like the thread form it produces. The teeth are arranged in parallel rows without any spiral lead. The cutter rotates at a high speed while its axis slowly moves around the part in a planetary arc of just over 360 degrees. As the spindle axis moves around the part, it advances one pitch in the axial direction.

"By using the thread-milling cutter, the secondary deburring operation has been eliminated and we can now thread to the bottom of a drilled hole," explained Rick Grasty senior manufacturing engineer at Yale. "When we were using the tapping method to thread drilled holes, we could not tap to the full depth."

Grasty indicated that Yale is producing approximately 1300 O-ring ports each month. The previous secondary deburring operation required with tapping took about one-half minute to remove the burr from each part. The use of the new thread-milling cutter has resulted in an approximate time savings of 11 hr per month, or 130 hr per year, not including setup time and handling time.

With the previous tapping method, approximately 200 drilled holes could be threaded with a tap before it had to be replaced. The thread milling cutter can thread nearly 1500 drilled holes per regrind. With a typical cutter capable of five regrinds, up to 7500 holes per tool can be achieved. This represents a considerable savings in tool costs and an additional time savings.

Yale had been receiving complaints about the finish quality on the O-ring ports when the tapping method was used. "Since we began using thread milling, we are no longer receiving complaints about the quality; the finish produced by the thread-milling cutter is far superior," Grasty explained.

Thread milling generally produces a better quality thread because of five basic factors:

1) In tapping, feed rate is determined by the pitch of the thread, and feed rate largely determines the surface finish. So if the pitch happens to be too high, the threads may have a rough finish or gouges may be found on the thread finish. Feed rate is discretionary when thread milling; therefore, much better surface finish can be achieved.

2) The cutting speed of a thread mill is generally two to five times higher than that used when tapping or die threading. This means less tearing and galling.

3) All CNC machine centers have offset routines to allow instant compensation for any variation in tool size. Only thread milling permits taking advantages of these routines. When tapping, an off-size tool results in an off-size part.

4) Of all important threading processes, only thread milling can remove the razor sharp thread produced at the beginning and end of the cut.

5) Reversing the spindle to remove a tap sometimes produces a second root on the machined thread. By eliminating this step, thread milling improves thread quality and saves time.

"Thread milling offers many important advantages on those applications where it can be successfully applied," explained Rick Grasty. "The savings and better quality achieved at Yale by converting to thread milling were surprising. And, the Detroit Tool people are very knowledgeable and good to work with."
COPYRIGHT 1989 Nelson Publishing
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Publication:Tooling & Production
Date:Aug 1, 1989
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