Screw-machine parts made from free cutting brass cost less than leaded carbon steel.
TRUE! For typical high-production multi-spindle automatic screw-machine jobs, Free Cutting Brass (Alloy C3600) is the most cost-effective material you can specify. Net piece costs will be LOWER than with any other equivalent material, including leaded plain carbon steel.
But doesn’t brass cost more than steel?
Sure it does, off the shelf, up to two-and a half times as much, on the average. But that’s only part of the story. The net cost of Free Cutting Brass as a raw material is almost the same as that of steel.
You mean brass can actually cost less than steel?
YES! For two reasons:
The real cost of raw materials is their delivered cost MINUS their scrap allowance for turnings, rod ends and so forth. Most screw-machine parts generate between 50 percent and 75 percent turnings, so the value of the scrap can be a very significant part of the overall cost picture. You’d be surprised at how many people overlook this factor when selecting a material. Free Cutting Brass turnings have a high value; it’s not unusual for them to command up to 70 percent of the cost of the initial bar stock, and this allowance can usually be negotiated into the purchase agreement. Leaded plain carbon steel turnings are almost worthless screw-machine houses sometimes have to pay to have them hauled away. So when you factor in the value of the scrap, Free Cutting Brass costs are a lot nearer to leaded steel than you might think.
Free Cutting Brass permits much higher production rates,and this more than makes up any remaining difference in cost. Consider this: When you send a job out to a screw-machine house, what you’re really doing is buying machine time by the hour. The more parts a shop can produce per hour, the lower your per piece cost.
But doesn't free cutting brass and leaded steel both claim high machinability?
YES... BUT . . . (and that’s a very important but). . . its true that both Free Cutting Brass and leaded plain carbon steel are designed for enhanced machinability and that each has an index rating of 100; however, this doesn’t mean their machinability ratings are the same . . . not by any means.
Take this simple screw-machine part, for example. It’s the standard test piece called for in ASTM E618,“Tentative Method for Evaluating Machining Performance Using an Automatic Screw/Bar Machine.” To evaluate machinability according to this ASTM test method, tens of thousands of these little parts must be turned out on a standard production automatic screw machine. The “machinability rating” the test gives us is defined as the maximum number of parts that can be produced in an eight-hour shift while still holding dimensional tolerance and surface roughness within pre-selected limits, i.e., the highest production rates for acceptable tool wear.