Making large holes in ferrous metals can bepretty slow going.  Big twist drills take a lot of power, and can be expensive and hard-to-find in odd sizes.  Flame cutting will work only if the material is flat plate, plus it is a cumbersome process and leaves a lot of finishing work to be done.  Trepanning, involving expensive, dedicated tools, is limited to repetitive, high-volume work.  Orbital milling is faster and more versatile, but there is always the risk of snapping off the brittle solid-carbide end mills.  

Recently, Baldor Electric Co.’s Gainesville, Ga. plant improved the large hole-making process for one part so successfully that it adapted the approach for 10 other jobs at there — and its now being considering for jobs at several other Baldor Electric plants. 

Baldor manufactures electric motors and adjustable speed drives, and the Gainesville operation has 400 employees producing AC and DC motors (1-1,500 HP).

By switching from orbital milling to corkscrew milling with an indexable, Ingersoll Cutting ToolsHi-Feed Deka face mill for holes over 1.500 inches, Baldor has cut the process cycle time by an average of three to one, and improved tool life by 10 to one. Then, a boring sequence finishes the holes, as before.  The annualized saving from the process change at Baldor Gainesville is estimated at more than $40,000 annually.   

The switch got its start early in 2010 when Darimus Glasper, manufacturing engineering technician at Gainesville, started looking around for a faster way to open three-inch holes in a ductile iron casting with two-inch thick walls.  The part is an explosion-proof cover that encloses the electrical leads in large motors.  Baldor Gainesville’s annual production volume is 100 of these pieces, performed intermittently in 10-piece lots. 

“Because of the workpiece geometry, the holes must be drilled from solid in a long-reach setup,” Glasper said.  “Not only was it slow going with the orbiting method and the solid-carbide too: we also went through a lot of tools.”  Moreover, he recalled that operators were concerned because the spindle load meter read 90% even at that slow rate. 

At the time, Baldor’s standard approach to roughing out the holes was orbital milling with a 3/4-inch, four-flute solid carbide end mill.  In orbital milling, the technique is to plunge about 0.100 inch and then orbit the tool at a very careful feedrate (so it will not snap off, as brittle solid carbide end mills are likely to do.)  Then, the process is repeated until there is a break through, with the hope that the resulting slug doesn’t pose a safety hazard or damage the tool.  It’s basically start-and-stop.  For each hole that Baldor machinists produced in the two-inch thick walls, they had to follow about 20 steps.

Glasper chose the explosion proof cover as a starting point in the search for productivity improvements because, as one of higher-volume items requiring large holes, Baldor would readily recognize the savings because of it cost.  “Solve it on the highest-volume part, and we get the biggest immediate payback,” he explained.