Cutting Tool Engineering
June 2011 / Volume 63 / Issue 6

Material elongation and thickness

By Tom Lipton

Workpiece materials have a variety of characteristics that influence how they are machined or otherwise worked to create parts. Presented here is information about elongation and thickness characteristics.

Elongation numbers provide a clue about material formability and ductility, and modulus of elasticity is a measurement that tells a material’s stiffness. The higher the number, the stiffer the material. Stiffness and strength are two very different qualities, so don’t get them mixed up.

The yield strength measurement provides a value used to determine the upper end of a material’s basic strength. Ultimate tensile strength is not as useful as a design parameter because most materials have already permanently deformed or yielded long before they see the ultimate strength. These numbers just provide a relative strength comparison. Yield strength shows at what point the material will be dimensionally altered.

Toughness indicates a material’s ability to absorb energy. Impact and notch test numbers provide a way to compare different materials. Higher numbers mean tougher materials. They also mean more sweat if you have to apply human energy.

Regarding material thickness, sheet is anything under 0.188" thick and anything thicker is called plate.

Sheets and plates have bow (parallel to the rolling) and camber (perpendicular to the rolling) tolerances. Specifications are available from material suppliers. If they don’t want to provide the information, take your business elsewhere. Use this valuable information for real-world design efforts and tolerance analysis.

Cast aluminum tooling plate sold under various brand names is not ground flat. It is cast against a ground surface, which gives it a ground appearance, which sells more plate. This material is not particularly flat by machinist flatness standards.

First, get the specification sheet from your aluminum supplier. Then get ready for a surprise when you see how much flatness deviation they graciously allow. Keep in mind that many of the cast varieties of tooling plate offered cannot be repaired with welding. There are only a couple I know of where welding is even recommended, so screwups can be difficult or impossible to repair. The major redeeming factor is that large amounts of metal removal have little impact on workpiece geometry.

Figure 4-27.tif
Figure 4-28.tif
Courtesy of T. Lipton

PVC covering (top) and clear shelf paper helps to protect surface finishes on sheet material.

Aluminum sheet is specified by decimal thickness, not gauge number. The salespeople will generally politely overlook this common oversight because they want to sell material and not make their customers look ignorant.

Order sheet materials with PVC covering to protect the finishes. The small incremental cost is repaid in less finishing work. For finishes produced in the shop, I sometimes protect the surface with clear shelf paper. It is readily available at any hardware store and will protect a workpiece’s finish as it moves through the shop. I also use a similar protective sheet material called Surface Armor. It is available in many different levels of tack depending on the material you want to protect and how long you need to protect it.

Carbon steel and stainless steel sheet of the same gauge number have different thicknesses. To avoid confusion, it’s easier to call out the sheet-metal thickness as a decimal dimension with a tolerance on a drawing. Be sure to include an appropriate tolerance for the thickness to encompass the manufacturer’s gauge tolerance.

Be aware that plastics typically have thickness and diameter tolerances of ±10 percent. This has caught many people by surprise because of the large potential variance.

All sheet materials have a manufacturer-specified thickness tolerance. Note this tolerance to avoid mistakes and use it to your advantage. This tolerance shows itself when doing precision forming where the thickness has an impact on the forming operation.

Sheet materials are usually thicker in the center of the sheet parallel to the rolling direction than near the edges. Sheets are also slightly oversize in length and width. You can never count on the accuracy of the sheet width or length. There is a reason they sell materials by the pound—all the extra weight goes on your bill. If you really need an exact width, plan on resquaring the material.

Galvanized sheet is hot-rolled sheet steel that has been pickled in acid to remove the mill scale before plating. It is softer than cold-rolled sheet of equivalent thickness. It’s handy to keep around for durable templates and items you don’t want to paint. CTE

TomLipton.tif About the Author: Tom Lipton is a career metalworker who has worked at various job shops that produce parts for the consumer product development, laboratory equipment, medical services and custom machinery design industries. He has received six U.S. patents and lives in Alamo, Calif. Lipton’s column is adapted from information in his book “Metalworking Sink or Swim: Tips and Tricks for Machinists, Welders, and Fabricators,” published by Industrial Press Inc., New York. The publisher can be reached by calling (888) 528-7852 or visiting www.industrialpress.com. By indicating the code CTE-2011 when ordering, CTE readers will receive a 20 percent discount off the book’s list price of $44.95.



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