RAPID PROTOTYPING IN SHEET METAL

By Gerald L. Yates, Vice President, BalTec Corporation, Canonsburg, PA

Small-quantity production of precision stamped and formed parts has long been a problem for the metalworking industry. The trend today is a demand for smaller quantities, coupled with the highest product quality. In addition, stamping suppliers are expected to deliver a product more quickly than ever. This applies particularly to the production of prototypes and preproduction parts.


Standard geometric-shaped tooling (top photo) is used to punch, notch, semi-pierce and contour punch flat patterns from both ferrous and nonferrous materials. Blanks are formed on the PWS by programming a three axis, or 3D gauging system and segmented press brake style tooling (bottom photo).

 


Laser cutting machine processes a large sheet metal prototype.

 Many manufacturers often believe it is too expensive and time consuming to produce punched and formed prototype and preproduction parts using conventional metalworking methods. Successful production of these low-volume stampings demands creative thinking in relation to tool costs, tool design and production cost.

Prototype Methods

Today's technology is moving at a torrid pace in regard to manufacturing techniques and practice. Only several years ago, with the help of a powerful new generation of computers and innovative software designs, did we begin to investigate an application that has long been a problem to our manufacturing society, namely rapid prototyping. This technology can be defined as "manufacturing small quantities of parts to be used in the development of a new product."

Today we have at our disposal numerous production methods of prototyping, which are well known by their acronyms. The more common are SLA, SLS, LOM, FDM, FEM and CGS or computer generated simulation, where a part can be produced as a 3D model and tested for such things as stress and design integration. Or the latest in rapid prototyping, VR--virtual reality, where the customer can actually interact with computer-generated parts in a cyber world.

How are prototype sheet metal parts produced in today's job shop and R&D facilities? The most conventional method is to use standard machine shop equipment to complete these tasks.

With this method, toolmakers will layout a flat blank and scribe the outline of the part on the material, then remove material by using one or more progressive operations on manual machines. Forming often will be performed in a similarly crude fashion in a vise or arbor press. This method will produce a prototype part, but is very time consuming and offers little chance for repeatability in subsequent parts.

Some job shops may produce small inexpensive soft tooling. Usually each small tool will produce an individual feature of the part. Then the tools are set up in a small punch press, one at a time, and the stock is "stepped" through each tool until the finished part is produced.

These small tool sets usually are designed and built by experienced toolmakers, who will produce the finished prototype parts. Although much more accurate than the machine shop method, this approach requires a very experienced toolmaker and is overly time consuming. Subsequent part changes also require extensive tool modification.

 

Photo-etched parts "tabbed" in a strip.

 

 

 


The precision work station from BalTec Corp., Canonsburg, PA, was developed in Switzerland to produce high-precision stamped and formed  prototypes.

The Laser, EDM Niche

Most of today's modern tool and die shops have additional capacity for producing prototype representatives of a stamped and formed part. Many of these shops will use equipment normally used for the production of permanent tooling. Often a wire EDM machine will be used to cut a stack of sheet metal blanks very accurately for the prototype part. These blanks are formed using either a standard press brake or inexpensive modular tooling.

With the exception of the part forming, this method is very flexible in that the flat blanks can be altered with a simple program change on the wire EDM machine. Thickness of the material stack, and the number of individual openings or cutouts on a part, each of which requires a start hole, affects the speed at which the parts can be produced.

Laser cutting machines are another common method of producing very flexible flat sheet metal blanks used by shops that specialize in fabricating sheet metal parts. These shops usually produce larger parts, such as heavy gauge brackets and sheet metal enclosures of all types, and regularly use laser cutting machines to produce the flat blanks for their products and for prototyping a new part.

The flat blanks produced on the laser can be programmed easily and modified via the CNC programming systems, which drive most of the machines in use today. Again, as with the wire EDM method, forming the parts will be accomplished in a secondary operation, by using a press brake or specially built, modular tooling.

One drawback to prototyping parts on the laser that later will be produced on conventional progressive tooling is the "heat-effected zone" or a hardened edge condition caused by the extreme heat of the laser beam disintegrating the metal. This hardened edge often will crack during forming operations, thus not truly representing the results that will be achieved in a conventionally stamped part.

Some fabrication shops will use an abrasive waterjet cutting system for cutting very thick sheet metal blanks. These CNC-controlled machines use a very high-pressure stream of water to cut the sheet stock. The waterjet systems can easily cut materials 1 in. and thicker. This system is rarely used for formed parts due to the thickness.

For very thin material prototype parts, materials 0.020 in. or thinner, a chemical milling or photo-etch process often is used. This process consists of "tooling" that is simply a photographic sheet of material that transfers the image of several of the flat blanks to be produced onto a sheet of the selected base material. This material then is disintegrated by a chemical process, leaving the finished flat parts "tabbed in" or loosely attached to a skeleton strip of the base material. Several parts to thousands of parts can be effectively produced by this method. Part thickness and secondary forming operations are the only major restrictions.

 

Each method of sheet metal prototyping has its own advantages and disadvantages. It is the responsibility of the engineer in charge of procuring prototype and short run parts to determine which method is most suitable.

METAL PROTOTYPING METHODS
Method Advantages Disadvantages
1) Machine shop equipment Available in most shops, inexpensive Lack of part accuracy and repeatability
2) Soft tooling Represents permanent tooling, cost effective Requires an experienced toolmaker
3) Wirecut blanks Very accurate, easily modified Time consuming, no forming capability
4) Laser cut blanks Fast print-to-part time, large part capacity Heat affected zone, no forming
5) Waterjet cutting Can work thick materials Not applicable to all material types
6) Chemical milling, photo-etching Ability to cut thin (<0.001 in.) material Not applicable in thicker materials
7) Precision work station (PWS) No dedicated tooling, able to punch and form Part size limitations
(10 in. x 24-in. max.)

Stacks of parts can be cut on a wire EDM machine.

Precision Work Station

All of the previously mentioned sheet metal prototyping techniques are using technology derived from some other manufacturing method and adapting these to rapid prototyping. One process has been developed exclusively for rapid prototyping of sheet metal parts. This method uses a machine called a precision work station, or PWS. The PWS was developed in Switzerland to produce high precision stamped and formed prototypes.

This equipment is a CNC, three-axis, X, Y and Z-controlled and hydraulically operated machine that uses standard geometric-shaped punching tooling to punch, notch, semi-pierce and contour punch flat patterns from both ferrous and non-ferrous materials ranging in thickness from 0.004 in. to 0.250 in.

Programming the on-board computer to pierce individual portions of the part generates flat blanks, or parts nested on a sheet of material, while rapidly moving under a punching head. These flat blanks then are formed, on the same machine, by programming a three axis, or 3D gauging system and segmented press brake style tooling. Multiple bends can be easily programmed without the use of dedicated tooling.

Programs for punching and forming then are stored, much like dedicated tooling would be, except on diskettes for fixture use or modification due to design changes.

This machine offers a flexible system upon which one operator, on a single machine, can produce from one single prototype part to several hundred parts with near perfect repeatability and representation of permanent tooling. MF