This technical information has been contributed by
Devon Precision Industries, Inc.

Switch to Whirling Process Improves Accuracy of Worm Gear

Worm Gears Thread Rolling
Worm gears produced by Devon Precision Industries using the whirling process show no burrs or distortions.
Photo courtesy of Devon Precision Industries.

WOLCOTT, Conn.--Devon Precision Industries, Inc., a specialist in Swiss screw machine products, manufactures a variety of high-precision turned parts on its Swiss screw, CNC Swiss, multi-spindle Swiss, and Escomatic (coil) machines. Virtually any custom designed turned part is fair game for the company's Swiss and CNC machines, which regularly turn out special screws and custom-designed fasteners requiring tight tolerances and high precision, according to Don St. Hilaire, executive vice president at Devon Precision Industries. Parts manufactured by Devon are used in industries such as automotive, medical, military, firearms, aerospace, and industrial drives and motors.

The Wolcott, Connecticut-based company also provides gear and pinion hobbing for various  mechanical timing devices, gear drive trains, and safety and arming assemblies for military fuses; centerless and form grinding; mechanical assembly; broaching; and secondary operations. Within the last two years, Devon has added two new CNC worming machines. One, a CNC worm milling machine, uses a solid cutter to dive into the part. The other is a CNC whirling machine that uses a ring with replaceable inserts to cut orbitally to the part.

According to Devon Sales Manager Steve Burns, the highest precision worm gears in the industry are produced by whirling and worm milling, processes that allow worm gears to operate more smoothly and quietly in mechanical drive trains. Both processes can improve product life, Burns says, because they provide better surface finish on worm threads and tighter pitch diameter concentricity, which have the effect of lowering the vibration of the worm gear during operation.

Both St. Hilaire and Burns agree that product manufacturers choose Devon Precision for its "dedication to perfection and ability to manufacture very difficult parts with very tight tolerances."  In addition to its technical experience and depth of machining knowledge, the company offers comprehensive design reviews that can lead to cost-saving suggestions for solving customers' problems.

In one case, Devon was able to significantly improve the operation of a worm gear by changing the part manufacturing process from worm rolling to whirling. The customer, a Midwestern manufacturer of high-end hair clippers used by barber shops and beauty salons, wanted to improve the functional operation of a worm gear in a motor drive for shearing cutters. Its goals were to reduce the weight and noise of the shearing instrument by using an alternative method of manufacturing. 

Devon listened carefully to the customer's desire to improve the long-standing design, including its account of the problems associated with the current design specifications. "The problem was that the customer was having an unfavorable scrap rate at final inspection, as well as some complaints from the field concerning excessive vibration," said Burns. "The root cause was eventually determined to be the worm drive shaft that we manufacture. However, it was determined that the parts we supplied were perfectly made to the current design specifications."

To come up with a reliable solution, Devon worked very closely with the customer's QC, engineering, and purchasing departments. When the "total composite" of the worm gear was identified as the contributing factor, the customer requested cutting the tolerance in half for the total composite, which is, essentially, the concentricity of the worm gear's pitch diameter.

"On the clipper motors that worked the smoothest, the total composite was running about half of the allowed spec," Burns said. "We had been making the part since the early 1990's and had always rolled the worm gear onto the shaft, a method also used by their previous supplier. This provided an economical and proven process, but normal variations inherent in rolling prohibited the ability to achieve the goals of the tightened total composite tolerance."

The worm rolling process, he explains, is actually a forming process in which the worm form is produced by being squeezed between a set of cylindrical worming dies. "This puts a tremendous amount of pressure and physical stresses into the worm that make it challenging to maintain a 'near-perfect' total composite," says Burns. Another problem was that hot metal chips would attach themselves to the flank angles. These, along with the residue of the material left on the top of the worm itself, were found to be contributing factors to the inconsistent performance of the shearing instrument. The overall straightness of the shaft was also found to be affected by the worming process.

Devon tried many things to see if they could force the process to work, including trying to constrict the raw material physical properties and running a tighter spec on the pre-worm blank diameter during the screw machining process. "We even sorted the blanks in 0.0001-inch increments and worm rolled these in families to maintain even stress limits during worm rolling," said Burns. "Although some of these efforts helped, they did not solve the problem on a reliable and capable basis."

Devon's desire to help its customer's bottom line never quit, however. Having recently acquired the new CNC worm whirling and worm milling machines, Devon decided to invest in some tooling and run some samples through the milling process. The results, Burns said, were "outstanding."

"The total composite could routinely be held to 0.0005-inch to 0.001-inch, maximum, which is using only 25%-50% of the allowable tolerance," he explained. "The variations in physical properties of the raw material and normal variations in the pre-turned blank diameter were no longer a problem. The customer built the new samples into their clippers and [they] were absolutely thrilled with the performance."

Thread whirling is essentially a threading process in which a hollow cutter rotates around the outside diameter of a part. The part rotates slowly while the cutter rotates around the part at a high spindle speed; the part and cutter rotate in the same spindle direction. Thread whirling is reported to produce clean contours with very little part deformation. Because the cutting forces are low, no burrs are generated and chip production is eliminated because only fine particles are produced.

Also, according to Burns, the orbital cutter path of thread whirling produces an arc contact between the cutter and the part, resulting in a superior finish and fast cycle time. The cutter is advanced in the Z-axis at the thread pitch, and the direction of the Z-axis movement determines either the left- or right-hand direction of the thread.

Burns said that the difference between the tightened total composite produced from the whirling process and the one produced from the older rolling process was like the difference between night and day. The customer immediately wanted to know what could be done to restrict all future production of the worm to whirling only.

"Of course, we had challenges," he said. "One factor was that we now had to present the part on centers, which is not required for rolling. The part has a center hole on each end, used for installing a steel ball on the final shaft. The concentricity spec of the hole is wide open at 0.005-inch. However, we will now be using this hole to hold the part and needed to re-design our secondary drilling process for the back hole to now hold a much tighter TIR tolerance than we had been running. This way, we can avoid any negative effects during the new whirling process. So we had to examine the entire part design and process. We had to tighten internal specs tighter than the customer design requires, as a pre-control for subsequent processing."

In the end, the switch from worm rolling to the whirling process resulted in a major improvement in the customer's product. Whirling is a process that's hard to find in the U.S., according to Burns, and it's one that Devon is proud to offer.

"The whirling process opened up a whole new market in which we can now produce not only the highest-quality worm gears available anywhere in the world, but also complex shapes, such as worming parts with tightly-toleranced holes in them, and multiple starts," he said. "These parts simply cannot be produced by the traditional rolling process because the rolling forces will crush or distort the hole. And, if attempted to be machined out of pre-rolled worm rod, the worm gear will pick up nicks and damage from collet and bushing marks, as well as part handling."

Most of the parts that Devon produces on the whirling machines are for automotive OEM applications, such as door locking and door closing mechanisms. These parts are often press-fit onto precision ground shafts, and the worm form must be perfect and blemish-free for smooth operation over millions of cycles.

"This was a case where good communication and customer service were a key," added Burns. "We really listened to the customer's needs. Of course there were frustrations, but mutual respect, patience, and a spirit of working together all contributed to a major improvement and a resolution that strengthened our customer relations."

This technical information has been contributed by
Devon Precision Industries, Inc.

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