WHITE PAPER - RACKMOUNT COMPUTER SHEETMENT FABRICATION

Transit Case and Rack System Integration

WHITE PAPER - SHEET METAL MANUFACTURING

Table Of Contents

Material Selection
Programming
Shearing
Punching
Folding
Install Hardware
Finish

Introduction
Please note this section is provided for information only. You should keep in mind you don't have to know the intricacies of sheet metal fabrication to have a chassis manufactured. Chassis Plans will handle the prototype through production and integration. All you need to specify is the color you want. If you need minor modifications, our shop will be able to work from marked-up prints and can help you with the details.

The lead-time for having a chassis manufactured is dependent of drawing and modification to prototype and how busy the shop is. Average turn around time is to get low volume production in six weeks or less. The steps to manufacturing a part from sheet metal are:

choose the material
generate the machine programs
shear the metal sheets
punch holes
fold to shape
install the hardware
finish

For low volume quantities of less than 5000 pieces, the parts are punched and folded. For larger quantities, other technologies are available such as stamping or injection molding for plastic parts. However, the tooling charges for these technologies can be in excess of $50,000. This is generally not cost effective for small run industrial applications.

Material Selection

Before a part is manufactured, a material must be selected. Materials for all Chassis Plans parts have been optimized for strength, ease of manufacture, and cost and are clearly specified on all the drawings. Aluminum makes for a lightweight chassis, but is expensive. Steel is strong and inexpensive, but can be heavy. Steel rusts so it must be plated, or a pre-galvanized material can be used (Paint-loc). Different gages of material are available so that weight can be controlled by using thinner material. But there is a limit to how thin a piece can be made without becoming weak and flimsy. Sheet metal thickness is specified as a gage. Typical structural parts such as the chassis body are made from 16ga cold rolled steel (c.r.s.), which would be approximately .055 inches thick. The front panel would be made from 12ga (.104"). Nonstructural parts would be made from 18ga (.047") or 20ga (.035").

Programming

All sheet metal manufacturing equipment is now computer controlled. This includes the shear, punch and break. Our sheet metal house will first convert the parts into flat patterns as though the part was unfolded. We will then determine bend allowances and offsets for their particular machines. With these numbers, they can program the various pieces of equipment to minimize waste and provide the accuracy you require. For small quantities of parts, the majority of the cost is in programming and setup. The actual material cost may be on the order of $3 per part but the programming will cost $200. Thus, 1 part is very expensive but the price curve for any quantity over 25 parts flattens out as the programming is spread out over more and more parts. 1 part may be $250, 2 parts $130 each, 25 parts $12 each, 100 parts $5 each.

Shearing

Large sheets are cut or sheared to their finished size plus a couple of inches on an edge for the punch machine to grab. The backstop in the shear is numerically controlled and provides for highly accurate sheet sizes and square edges.

Punching

Punch Photo

Holes in sheet metal aren't drilled, but are punched in a large computer controlled punch press. These punches provide a multi-station turret where all the different punch sizes and shapes required for your part can be stored. Your sheared-to-size sheet is loaded and indexed to a 0,0 corner and the punch then shifts the sheet rapidly to the different x-y coordinates and punches the appropriate sized hole, including counter sinks and threaded extrusions. In some cases, even the hardware inserts can be inserted automatically by the punch press. This is all computer controlled and accuracy's of .001 inches are easily achievable across the part. The finished part may be `time saved' or lightly sanded on both sides after punching to break any burrs and provide a scratch free surface.

Folding

Break Photo

The punched metal is folded to shape in large press brakes, which, like the shear and punch, are computer controlled. These brakes provide highly repetitive and accurate parts with the ability to make small bends in tight corners.

Install Hardware

Depending on the material, hardware may be installed now or after the part is finished. For steel parts to be painted or plated, the hardware is pressed in now. Generally a PEM insert is used to provide high-strength screw threads, both as internally threaded nuts and as studs. PEM stands for Penn Engineering and Manufacturing Corp. (215-766-0143).

Finish (Paint or Plate)

Chassis Plans is dedicated to serving the special needs of its customers. Our goal is to become a valuable extension of your business by providing high quality and service at very competitive prices. We offer painting, powder coating, silkscreening, pad printing, and mechanical assembly.

Our facility is equipped with several large-capacity spray booths, ovens, powder coating systems, silkscreen and pad printing equipment, and a large vapor degreaser.
There are basically four finishes to consider: anodize, plate, pre-galvanize, and paint.

Aluminum can be painted or anodized. Anodizing converts the aluminum surface to Aluminum Oxide which is very durable. Some clients want anodized front panels to match other equipment or for a particular look. Anodizing front panels can be expensive because the hardware is inserted after the part is finished and can cause cracks in the finish so the reject rate can be high. It is better and less expensive to paint front panels. Anodizing is also nonconductive and can make it difficult to certify the chassis for FCC and CE requirements for electromagnetic interference.

Steel rusts and must be protected. Several types of plating are available with the most common being zinc and nickel. Nickel plating is very attractive and looks like chrome. However it is expensive, tends to build up on threads, and can be porous allowing moisture incursion and subsequent plating degradation. Parts with zinc finish stand up much better to a salt spray test than do parts with nickel plating. Zinc plating puts down a very thin layer of zinc on the part. The zinc is self healing meaning small scratches will not cause rusting.

Zinc can be clear or gold colored. Clear zinc is silver-gray in color but is subject to fingerprints. A part made in clear zinc will show somewhat rusty fingerprints from the assembly operation handling about 2-3 months after manufacture. Gold zinc is clear zinc with a chromate chemical wash. The chromate provides a layer of protection over the zinc and parts will not show fingerprints. <

Pre-galvanized or pre-zinced material is available in flat sheet form. Generally, this is a good material to use for painted parts as no additional finishing is required after punching before the paint is applied. Pre-galvanized material is subject to rusting on the sheared edges if they are filed or sanded after shearing. The shear will smear some of the zinc across the new cut but this layer of zinc is often filed off when the sharp edges are cleaned up or broken. The visual consistency of the finish is not good so there may be stains and other `defects' in the appearance if the part is used in an unpainted form. Some shops do not like to weld or spot weld pre-zinced material because of the fumes.

Any part can be painted. Generally only the parts on the front panel are painted. Paint provides excellent protection against the environment and looks good. However, painted parts will require masking to allow proper electrical conductivity for EMI protection and grounding. Local paint manufacturers such as Sherwin-Williams or Cardinal can manufacture any color in just a couple of days. The sheet metal shop will coordinate this for you. You simply provide a color sample and the shop will order the appropriate paint. Paint is available in a variety of textures from smooth glossy to a rough splatter finish.

A finish similar to paint is powder coating. The part is sprayed with an electrostatically charged paint powder. The part is then baked in an oven, fusing the powder. This provides the most durable finish. Any color can be manufactured, but usually requires longer lead times and larger quantities than for paint.