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Fixture concentrically for precision machining. (Work Holding).

More than ever, precise positioning is required to meet tighter manufacturing tolerances required for machining workpieces. How well the part is loaded and clamped play an important role in the quality of machined castings, forgings and sintered parts.

In theory, to position a part precisely the part can be adjusted according to its surfaces or by imaginary bisecting centerlines. Adjusting to exterior contours (surfaces) is called positioning and adjustment to bisecting lines is called centering. In practice, a combination of both types is possible, with centering requiring somewhat more mechanical means.

Concentric positioning and clamping offer a way to shorten loading and clamping time and provide an important advantage over manual centering in the repeatability that is possible.

For workpieces with cast or machined bores, reliefs or breakouts, as well as molds, Carr Lane Roemheld Mfg. Co., St. Louis, MO, has developed Swiftsure concentric clamping elements. The combination of hydraulically operated two- and three-way elements is designed to reduce tedious manual positioning by clamping with the turn of a valve. By using the "sliding-dog principle," maximum damping and compactness of construction are obtained together with high repetitive accuracy.

"Positioning has a critical influence on the ultimate quality of the machined workpiece," explains Derek Evans, Swiftsure application engineering manager. "With just the quick turn of a valve or the push of a switch, a workpiece is immediately positioned and clamped correctly, ready for machining. Without the concentric elements, it would often take a long time to manually position some workpieces using indicators."

Moreover, the concentric positioning and clamping elements can be used in conjunction with other Swiftsure clamping and supporting elements to make a complete fixture.

A good application can be seen in the fixturing that was developed for two washing machine parts. In the application, a drum bearing is hydraulically positioned, clamped and machined from four sides in one clamping position in a flexible three-spindle machining center with three NC rotary tables and with pallet transfer.

The development of the centering and clamping elements was important to the fixturing of the washing machine workpiece because of their universal use and modular design, as well as compactness.

The demands of the finished machined workpiece and the geometric design require one centering process--which positions the workpiece with reference to X- and Y-axes--and a second centering process, which determines the rotary position of the workpiece around the Z-axis. Positioning in Z direction is made by placing the workpiece onto three fixed rests.

Hydraulic clamping fixtures were chosen due to the high quality demands of the workpiece, the required clearance of operators and the fact that the machine tool would run in three-shift operation.

The hydraulic positioning, clamping and support elements are arranged on an aluminum base plate. Oil supply is made through drilled channels in the base and tubing.

Centering is accomplished with two different clamping and centering elements. X- and Y-axes are determined by a three-point concentric positioning and clamping element in the cast bearing hole of the workpiece. Rotation around the Z-axis is determined by linear centering elements at one of the four brackets.

After centering, the part is clamped against rests by two swing clamps. Since the centering elements can also be used as clamping elements, only one work support has to be used to support the fourth bracket of the workpiece.

Connecting ports for an additional swing clamp are provided in the fixture base plate beside the work support so that, in case of unacceptable vibration at this bracket, clamping can be done against the work support. When doing this, the clamping force should not exceed 50 percent of the support force to maintain a reserve force in the work support to compensate machining forces.

In these elements, two or three centering bolts are equally operated. This uniformity within a range of tolerance of a maximum of 0.02 mm can only be obtained by mechanically linking the centering bolts. The clearances between the mechanical components are completely compensated and do not have any effect on the centering. Return motion of the centering bolts is made by the forward motion of the lower mechanical guide.

In the accompanying drawings, dimension X determines the length of the required intermediate bar for the mechanical linkage of the centering bolts and circular centering is illustrated.

Hydraulically operated elements can be integrated into an automated process.

Swiftsure concentric clamping elements, which are available in diameters from 1" to 10", are also available in larger sizes and with a variety of mounting and connecting options to extend their versatility.

Swiftsure Div., Carr Lane Roemheld, St. Louis, MO, or circle 263

RELATED ARTICLE: Reasons to use concentric clamping elements

* Alignment of functioning surfaces: Functional cast or forged surfaces can be concentrically aligned to reduce or eliminate further machining.

* Centering of interior surfaces: The insides of housing can be aligned in one or two directions to assure clearances for internal parts.

* Reduction of machining allowances: Minimize machining allowances by concentrically aligning workpieces. Savings of material and labor plus machine wear and tear are possible with lower cutting forces.

* Adaptation of exterior surfaces: Cast or forged shapes with two different mating components are concentrically aligned to position parts evenly without unequal projections or steps. Examples include A) housing and cover assembly; B) flush requirements on outside surfaces.

* Centering of large parts: Large castings, forgings or flame-cut parts require generous tolerances. These parts can be concentrically aligned to assure the strength and rigidity of the parts can be maintained. Examples include A) steering axles of heavy-duty trucks; B) bearing eyes for steering knuckles.

* Improving balancing of rotating parts: Round, square and hex workpieces are concentrically aligned to uniformly distribute the workpiece weight. The unbalanced portions of the workpiece are therefore greatly reduced.

* Reduced time for balancing: Strict requirements on the balancing of rotating parts are very time-consuming. Concentric alignment and clamping of the parts reduce the required time to mechanically balance the parts.
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Article Details
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Author:Lorincz, Jim
Publication:Tooling & Production
Geographic Code:1USA
Date:Nov 1, 2001
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