Dead length collet chucks help turn metal into music

Modern Machine Shop, March, 1996

The UMI Eastlake, Ohio plant, 285 people strong, manufactures brass wind instruments, and if you marched in your high school band, you probably played one of their creations - a trumpet, trombone, sousaphone or maybe even a tuba.

Certainly, close tolerances are important to ensure durability and musical tone. But a flawless finish is an absolute for every UMI brass instrument. It is the brilliant, burnished-brass finish that calls to mind the flashing "Seventy-six Trombones" in The Music Man, and makes UMI's brasses look as good as they sound.

In turning metal into music, one parts family, the valve case caps, proved to be a production challenge. Achieving a high finish on the smooth surfaces of valve case caps, after rough finishing on older machines, would ordinarily be simple. Tumble polishing after machining, then buffing, would do the job.

But the valve caps include a knurled circumference which facilitates opening the cover. Tumbling breaks down the knurl, reducing gripping friction and degrading United Musical Instruments' image of precision manufacture.

For the most part, switching to CNCs retained the finish of parts from pre-polished brass stock, ranging from 3/4[inches] to 1 3/8[inches] diameter. They required only buffing to achieve the final finish. But the steps in the secondary machining of the top and bottom caps for UMI's 5J tuba, as one example, made achieving both high polish and a sharp knurl a dilemma.

To form this part, a 1 3/8[inches] diameter bar is loaded through the main spindle of an EMCO 360 CNC machine, fed out to a bump stop and indexed. A face-and-turn forms two radii in the diameter and cuts a cosmetic groove in the OD of the part. The knurl is then put on the smaller radius, the piece is spot drilled and center drilled, using a Seco mill drill to bore out the ID to the thread size. The program then undercuts for the thread, single points the ID thread, and brings the sub spindle forward, synchronizing it with the main spindle, to clamp the workpiece.

Once a cut-off tool has separated the valve case cap from the bar stock, the sub spindle returns to home position and a facing tool reduces the overall length to size at 0.001[inches] tolerance. A 15-degree cosmetic angle is turned on the face, from the OD to 3/16[inches]. In the final turning operation, a trepan for insertion of the valve stop - a rubber washer - is let into the hole. The sub-spindle moves forward and the part is ejected into a catcher.

The production challenge lies in gripping of the workpiece in the subspindle. UMI's machine shop foreman, Jim Spraggins, recalls that "to avoid scratches on the polished part caused by the pull-back from collet chucks, we used to run the parts with three-jaw chucks. But we didn't want to have to keep reboring them." He also cited the time lost in changing to a three-jaw chuck just for the valve caps and back to a collet to run other workpieces.

"We not only get more machine flexibility with collets than with three-jaw chucks," Mr. Spraggins adds, "but the collets are more widely usable for us, and we don't have to constantly bore them out or true them up. With the three-jaw chucks, we have to rebore the jaws to make certain they're concentric."

Mr. Spraggins wanted a solution that would allow him to keep collets - he favors the Hardinge Brothers' 5C - on his CNCs full-time, yet eliminate scratching from workpiece "pull back" upon closing. About thirty months ago, he asked EMCO/Maier's Tom Jackson for help in the search.

The answer from EMCO: Powerhold's 5C dead length collet chuck. By re-engineering the collet by first attaching a connecting device to the machine tool's existing draw-tube, and joining that to a Powerhold force multiplier, the sleeve is forced forward and closes the collet: no collet movement - no Z-axis "pull back" - no scratches. When Jackson lent Spraggins one Powerhold 5C dead length collet chuck to try, it sold itself. United Musical Instruments has since bought two and they have been in use for 2 1/2 years.

The 5C accepts diameters to 1 1/16[inches], which accommodates a broad family of work for UMI. For the 5J Tuba valve case caps, Spraggins needed to chuck a 1 3/8[inches] workpiece. EMCO recommended using the Hardinge 5C 2[inches] step chuck in the Powerhold assembly, which gave the same non-mar advantage for the larger diameter pieces.

It was creative solution on EMCO's part, because it required deriving a new application for the Powerhold 5C dead length collet chuck (which are also available for the Hardinge 3J). As noted before, though UMI values close tolerances, the absolute Z-positioning of the Powerhold 5C chuck was secondary to its care of the workpiece surface.

Since most ordinary dead length collet chucks work via internal stop mechanisms, many users prefer the Powerhold system because its open thru-hole permits thru-feed of bar stock. One example the manufacturer cites is unloading parts through the tailstock an a spindle liner during high production runs, eliminating the need for parts catchers.