Flexible Active and Passive Universal Holding Fixturing Systems for Aircraft and Aerospace Applications

Abstract

Universal Holding Fixtures (UHF) avoid the high costs, single purpose fixtures and long set-up times traditionally required for holding large, contoured and/or variable geometry parts and structures.

This paper describes some unique solutions of passive and active holding fixtures used in the aircraft-, aerospace and defense-industry for cutting, drilling, milling, riveting and assembly purposes.

The Universal Holding Fixtures shown in this technical paper allow a high density of jack posts and withstand very high axial and radial loads without deflection. The active UHF systems concentrate on highly accurate solutions and "smart" vacuum cups incorporating "intelligent" features.

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Background

KOSTYRKA GMBH from Stuttgart, Germany, is involved in research, development and production of sophisticated methods and fixtures to position, secure and clamp many types of workpieces, both large and small. In 1969 the company began manufacturing and distribution of a previously unknown clamping device: The Tension- or Clamping-Sleeve. Within a short time this unit became a versatile element with great benefits to modern workholding equipment.
Very soon the aircraft-, aerospace and defense-industry took the advantage of the KOSTYRKA™ Clamping Sleeve for supporting and workholding applications.
The method of the clamping operation is shown in the Figure: Flexible sleeves made from a compound of metal and plastic are axially held in housings. The sleeves surround the part and clamp it by applying hydraulic pressure to the sleeve jacket. The pressure is converted, without loss of efficiency, into radial acting clamping force. These clamping sleeves work with no part influences: The part to be clamped is neither axially pushed nor twisted as the clamping occurs.

When the oil pressure is completely removed, the sleeve I.D. springs back elastically to it’s starting position, releasing the part once again. In spite of high wall strength, clamping sleeves are very elastic: This is due to the fact that individual sleeve sectors are flexed, rather than a closed cylinder jacket having to be compressed.

The described clamping sleeves are available with internal diameters from 8 to 1200 mm. They have been approved elements since 1969 for over 200 000 applications, including for clamping jack posts in Universal Holding Fixtures..

Figure Clamping Sleeve

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Why UHF Systems ?

Recent polls of UHF system users cited significant advantages of (active and passive) UHF systems over conventional (fixed) fixturing. These large composite structures that hold a part to it‘s designed contour are created for each detail. They are larger than the parts they hold and are extremely heavy. The part changeover time is extremely high, and the use of riggers and a crane crew is traditionally required. Additional storage and handling equipment is also necessary.
UHF systems use up to several hundred computer controlled variable height shafts to rigidly hold curved and shaped panels. When implementing this flexible fixturing, recurring and non-recurring costs become minimized. UHF combine today‘s technology with the flexibility required for most fixturing applica

Passive and Active UHF Sytems
Passive UHF

Passive jack posts, as shown in the Figure, consist of actuators only, capable of extending, retracting and clamping. These jack posts are positioned through external means, such as a robotic gantry or the machining center itself. The gantry system then sets the jack post to the correct heights.

Note: The passive UHF system leaves total position responsibility up to the CNC controller!

The setting process: With clamping sleeves released all jack posts will be pushed to their outward position by use of compressed air. Then a low oil pressure will be built up around all sleeves simultaneously generating a clamping force just great enough to hold all jack posts in place

Now all units will be precisely positioned by the milling head or robotic gantry. Setup accuracy and speed are dependent on the machine tool or robot.

The hydraulic clamping pressure will then be maximized to insure secure clamping. Once clamped, the jack post‘s axial and radial stiffness has become extraordinary, allowing immense loading of the system. KOSTYRKA jack posts can withstand up to several thousand pounds vertical weight with only (nearly immeasurable) elastic compression deflection of the jack post shaft.

UHF / Passive Jack Post

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Modular UHF Sytems

The cross section in Figure shows schematically the Modular Universal Holding Fixture System described on the previous page. Three (3) stations on the left side of the fixture are unused and covered, then 6 jack post units are shown in their bottom and top position.

Active Jack Post

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The "Bed of Nails" configuration

Because of their slim dimensions, clamping sleeves, serving as a „brake“, allow a high density of jack posts in so-called „Bed Of Nails“ fixtures with a maximum of supporting and clamping points. The illustrated set-up consists of 504 jack posts. All can be locked in any position within their 380 mm (15 in.) stroke.

Each post will not deviate by more than 1 micron (.000040“ ) in Z-axis height from the time the machine locates the post till the brake is engaged.

3-way solenoid valves, tightly sealed and located adjacent to each jack post are incorporated into a carrier frequency network. This system actuates any given solenoid using only one looped supply cable. The „brakes“ (clamping sleeves) can now be controlled (on/off) individually at each jack post. Unused units remain at rest in their bottom position.

This fixture consists of three „units“ each 1420 mm (56“) long by 1220 mm (48“) wide. The assembly of these 3 units form a Universal Holding Fixture, in total 1420 mm (56 in.) wide and 3660 mm (144 in.) long. Each of these units is subdivided into 6 heavily ribbed containers with individual cover plates holding 28 jack posts each.
This design has a distinct advantage over conventional solutions by putting together relatively large posts (O.D. = 50 mm (2“) together in as close as 100 mm (4“) center distance pattern in both X and Y axis. All supply lines are out of the debris and coolant area in the totally enclosed interior of the fixture.

Modules may be added at either side without losing the center line distance.

Illustration opposite demonstrates how this fixture is being used for routing processes for honeycomb composites added to wing parts in a national defense program.

"Bed Of Nails"

High density "Bed Of Nails"

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