Abrasive solution to long-bore sizing.
The answer was a dedicated bore-sizing operation based on unique abrasive tooling--expandable sleeves plated with natural diamond abrasive. By bringing the process in-house and automating it, the user boosted quality and cut part-finishing costs considerably. Caught in the revolving door
Rixson-Firemark Div, Conrac Corp, Franklin Park, IL, makes a variety of architectural-quality automatic door-closing equipment. To maintain close fits for proper closer control, tight tolerances are required between piston and bore over long lengths--particularly in diameter, length, and roundness.
The initial manufacturing plan was to bore the cylinders in the cast-iron housings close to finish size, and leave a few thousandth of metal for removal by finish honing. Unfortunately, it was nearly impossible to find operators with the skill to handle a honing operation of this type, reports plant superintendent, Hank Mlekush. "I'd need a person with at least 15 years experience, and we didn't have that kind of expertise in our plant, nor could we hire it. We'd also have to buy a honing machine. The only answer was to subcontract the job."
That proved difficult, too--the per-unit cost to hone the bores was $3. "They had us over a barrel," adds Leonard Lisak, manufacturing engineering manager. "They could dictate price, production runs, and when the parts would be made. We were at the mercy of a single vendor. The answer was obvious. To stay in the surface-closer business, we had to bring the bore-finishing job in-house." Problem areas
One key technical problem was that too many of the honed cylinders were banana shaped. A straight piston (finished by cylindrical grinding) operating in a banana-shaped bore means a clear sacrifice in bore tolerance, inferior hydraulic action, and unacceptable performance.
Another problem was that the boring operation (before honing) sometimes left excessive stock on the cylinder walls, and the honing subcontractor insisted on higher per-unit pricing, rising to as much as $3.50.
Another very basic problem was that labor-intensive operations should never be allowed to occur late in the manufacturing sequence. Human error at this point can cause costs to skyrocket when parts must be scrapped. Choosing a finishing machine
To resolve these problems, a procurement team sought a machine that would do the job on an automatic or semiautomatic basis. After a false start, they discovered a relatively new diamond bore-finishing process--developed by Engis Corp, Diamond Tool Div, Morton Grove, IL--that takes the part through a series of precise stock-removal operations, using a fixtured rotary indexing table. Finish, machining of the work, bored to within 0.010", is by means of diamond tooling developed by Engis. Skill requirements are minimal; the operator loads the machine and pushes a button. How it works
The process is just 3 years old, a new concept in high-tech finishing. To understand it, you must start with the tooling. The finishing sleeves are expandable sleeves plated with natural diamond particles and mounted on mandrels. The mandrels are mounted on and driven by six spindles, tied together by a timing belt driven by a 5-hp motor. The spindle configuration and motor size can be changed.
The machine operates in the vertical mode with workholding fixtures mounted on a horizontal indexing table beneath the spindles. The work is loaded into the fixtures manually.
The table rotates to bring the fixture under the spindles. The machine head descends, and the diamond-plated sleeves enter the rough-machined bores. Indexing through three stations produces a rough machining (0.0045"), semirough machining (0.0025"), and finish machining (0.0005" to 0.0007") that brings the bores to within 0.0003" of tolerance, a 0.0002" improvement on the 0.0005" tolerance specified by engineering.
This greatly simplifies the producing of "perfect-surface" closers. The surface finish ranges between 15 and 18 rms. The machine can produce up to 204 parts/hr at 100 percent efficiency.
Of special importance is horsepower monitoring, provided by Valentine Corp, Detroit, MI. A digital readout monitors the collective horsepower drawn by the spindles. Variations from the predetermined value indicate the machining operation is leaving either too much or too little stock in the bore. If horsepower exceeds a programmed limit, the entire operation is shut down. Explains Don Bouchard of Engis, "This horsepower monitor is an extremely sophisticated ammeter with control capabilities." Tooling details
Two forms of bore-finishing tools are used: one for roughing, one for finishing. The fixturing on the index table carries two parts per fixture, and microswitches ensure that parts are seated correctly.
The diamond-coated tools are barrel-shaped and adjusted in size by a tapered mandrel to remove a specific amount of material in a single pass. The OD of the sleeve is straight for only a short distance, and surfaces on both sides of the straight portion--that do the final sizing--are tapered.
Once sized, the mandrels run for long periods without adjustment. At Rixson-Firemark, they have been adjusted once every 5400 finished bores, and it is predicted that these tools could finish 100,000 bores.
An adequate flow of honing oil to the working surface of the sleeve is mandatory, and this is assisted by the vertical stance of the machine. A left-hand helical groove in the sleeve imparts an important oil-pumping action in the cutting zone.
although natural diamond was chosen here, CBN (cubic boron nitride) has been used as the abrasive in other applications, and this process has been used to machine both soft and hardened steels.
also, the tolerance here of 0.0003" is relatively loose. Another Engis machine holds size to within 0.000 010". The secret of tight size control is the high degree of accuracy in the sleeve, and the fact that the sleeve/arbor assembly is permitted to float. Although tooling of this type can be used in machining centers, the best results are in dedicated machines. Success is dependent on the use of a high-quality honing oil, not the ideal coolant for a machining center. Also, even operators trained for highly sophisticated machining centers do not generally understand the nuances of diamond bore finishing. Economics
Base price for a VBFM 3030 is $86,000, although it is possible to get into bore finishing for as little as $15,000. The unit purchased by Rixson-Firemark was $137,000 with tooling, gaging, and related equipment. "It's worth every penny," says Hank Mlekush. "It solved a tolerance problem, it gave us top-quality parts, and it also relieved us from an impossible one-source subcontracting situation."
Leonard Lisak sees it as a step forward in dollars and cents. "On a per-piece basis, we've cut our bore-finishing costs from over $3 to something around 50^. From a return-on-investment standpoint, that's over 100 percent/yr. It's enabled us to raise quality while slashing costs, and that's what it's all about in today's manufacturing scene."
For more information on diamond bore finishing from Engis, circle E1.
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|Publication:||Tooling & Production|
|Date:||May 1, 1984|
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