A successful heat-treatment handling fixture combines maximum quantity of the customer’s components with minimized fixture weight while also supporting the component with the minimal amount of damage and distortion. In a specific case, the productivity throughput was increased with a reduction of fixture weight using the customer’s existing furnace equipment. As a result, a proposed new furnace purchase was no longer required.

The modern fixture is no longer considered solely in the heat-treatment environment as many are used as total handling fixtures throughout the customer’s plant. It is also essential that the fixture incorporates features compatible with good foundry practices. In the modern manufacturing environment, individual components of a heat-treatment fixture can range from 1 to 3,500 pounds. To maintain innovation and provide these modern concepts and solutions, a global team of 12 designers uses the latest tools at their disposal.

Fixture Design

Fixtures are designed using Solidworks 3-D solid-modeling software. This software enables detailed designs to be produced, taking into account all of the customer’s design requirements. The working volume of the customer’s furnace is considered, and the component to be treated is modeled. From this, the aim of the designer is to maximize the quantity of parts within the stated volume. The fixture is then designed around the optimized layout of the components.

Rapid PrototypingAn additional string to the design bow is the recent addition of a Rapid Prototyping function. A 3-D ABS printer gives the designer the ability to produce a prototype part, enabling the validation of the design prior to manufacture of a pattern or casting. A plastic replica of all or part of the element already designed on the screen is produced. The resulting prototype, obtained in a very short time and at a relatively low cost, can be used to validate the functional and technological design of the new fixture.

This process enables the technical links between customers and designers to be strengthened by having a physical piece that can be handled and tested with specific parts at an early stage of the design process.

Rapid prototyping helps avoid any unforeseen design problems that may arise during production, which minimizes the risk of having to make costly changes to the design and, therefore, pattern during the foundry prototype stage. All of this allows the lead time from design to supply of fixtures to be reduced significantly.

Finite Element Analysis (FEA)
FEA techniques are used to validate designs by ensuring that the fixture will perform well during the entire life of the product. The fixture is supported and loaded to simulate the forces exerted during the life cycle. The strength of the part can then be validated, and areas of the design can be refined by removing unnecessary material where it is not needed, thus reducing the weight of the fixture.

A Case in Point

A design case that utilized all of the design and manufacturing tools of AFE Cronite came from a U.K.-based manufacturer of tubular components. The challenge was to design a fixture for multiple families of products ranging from 1/2 inch bore to 9/16 inch with lengths varying from 1-3 inches.

These products were previously being heat treated in cast baskets in Super Allcase 30-inch x 48-inch x 30-inch furnaces (Fig. 1). A program of continual improvement of the heat-treatment product led the customer to request a fixture that would stand the tubes vertically. The aim was to provide a more uniform heat treatment and better drainage of quenchant and wash media. In addition to this, the customer required a more manageable-sized fixture – easier for manual handling – to be loaded at the machine center and transported to the heat-treatment shop.

Using Solidworks, a footprint of 14 inches x 11¼ inches was established to sit eight stacks of fixtures per base tray. A configuration of parts was developed to maximize the part quantity while maintaining enough space between them to allow good flow of gas and oil and ensure that the parts would not contact each other during the heat-treatment cycle. Suitable stacking posts were incorporated in positions that would allow the load of the entire stack of fixtures to be transferred directly onto the ribs of the base tray. Appropriate positions for handles were established and designed accordingly to achieve support of the upper layers and to provide a means of manual handling.

A proven three-point support feature for the component was incorporated, and the casting design was completed with consideration of both the in-service function of the fixture (the ability to resist the forces of heating and quenching) and the castability of the product.

It was at this point that the strength of the fixture in service was considered. Using FEA techniques (Fig. 2), the designer simulated the loading and the support of the fixture in service – in this case at 1750°F. The calculated stress values were compared to known values for AFE Cronite-specific alloys at temperatures from alloy data provided by the Research Center in Brno, Czech Republic. For this application, the alloy HUCb was chosen. This fixture was then optimized by removing material where it was not required and by adding material in areas of high stress, thus giving the optimal strength-to-weight ratio.

Before the design was finalized, a rapid-prototyped model of a small area of the stacking tray was made to validate the function of the part loading (Fig. 3). Parts were loaded onto this prototype model to ensure that the components would not touch together. The customer was also able to get a “feel” for the new concept before committing to the design and approving the start of pattern manufacture (Fig. 4). This prototype also enabled the casting department to evaluate the shape of the casting for castability.

On approval of the design and on receipt of the order from the customer, the production of the pattern could begin. This pattern was manufactured directly from the design office’s solid-model data and transferred digitally, allowing cutter paths for the CNC machinery to be created, saving time and cost and providing a more accurate pattern.

The final design resulted in a stacking tray that could hold 36 components. A total of 56 stacking trays per base tray, therefore, provided a quantity of 2,016 customer parts per batch (Fig. 5).

Since this successful first design, two more stacking trays using the same concept but for different component family sizes have been produced. One of these families of parts had an inside diameter of 3/8 inch, necessitating a location pin of 19/64 inch diameter. As this would be a challenge to a greensand foundry, AFE Cronite used the more appropriate lost wax (investment) process to produce this fixture. The final lost wax product resulted in a stacking tray that could hold 124 components. In this case, 57 stacking trays per base tray gave a total part count of 7,068 per batch.

The experienced AFE Cronite design team, using the most modern tools available, were able to provide the design, manufacture and supply of a family of products that gave considerable improvements in the quality and handling of the customer component. IH

For more information: Contact Marco Moser, North American Cronite, 37162 Sugar Ridge Road, North Ridgeville, OH 44039; tel: 440-353-6594; fax: 440-353-6599; e-mail: marco.moser@afecronite.com; web: www.afegroup.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: refractory alloy, heat-treatment fixture, rapid prototyping, finite element analysis, castability