Greenfield Industries (Lyndonville, VT) reduced scrap rate 60% in special tap production by installing CNC thread grinders. Older mechanical grinders often produce scrap parts because of difficulties in achieving special-profile forms, and because machine positioning variations affect size. New CNC thread grinders eliminate these problems by dressing the wheel and grinding relief under full CNC control. The ability to quickly program the CNC grinder to make taps with special forms opened up new markets by substantially reducing the cost of producing tools in smaller lot sizes.
Greenfield Industries is one of the largest producers of high-speed, steel cutting tools in the US. The Lyndonville plant produces special taps that differ from standard ones in their geometries, materials, or applications. The automobile, aerospace, off-road equipment, and piping fabrication industries use most of the tools produced in this facility. They produce the tools to extremely close tolerances- + 0.00025″ (0.0064 mm) pitch diam tolerance is typical.
Greenfield managers examined a range of CNC thread grinders in an effort to increase accuracy and productivity. They selected MX16 universal thread grinders from Normac Inc. (Arden, NC). This machine has a rotary dresser mounted on the same slide as the work, and it dresses the wheel more accurately than mechanical methods because the dressing tool is under CNC control. It can also grind relief to the same high.levels of accuracy. The CNC machine can achieve accuracies of 0.0001″ (0.003 mm) for both dressing and grinding, which is considerably better than mechanical grinders.
The MX16 with a CNC dresser produces any profile in minutes by creating a simple 2-D program. The control includes a routine that programs special thread forms by selecting the basic form from a menu and filling in the dimensions on a profile attribute page. The operator can program most special thread profiles, while a CAD/CAM system or special software usually generates more complicated profiles.
The CNC thread grinder has one rotary and two horizontal linear axes. The grinding wheel mounts on a precision, belt-driven cartridge spindle, which attaches to a cast-iron slide moving on precision linear bearings. The other slide carries the workhead (rotary axis) and the tailstockmounted rotary dresser. A doublenut preloaded ballscrew coupled to a brushless servomotor with encoder feedback drives each linear slide.
The machine control supports G code programming and an RS-232 interface that allows downloading CAD/CAM-generated programs. The machine uses a rotary diamond with a smaller radius than the thread crest for radius profiling. The control has automatic tool radius compensation and calculates the diamond path tangent to the required profile radius.
The new machine has improved control over machining parameters. Mechanical grinders have a dial that lets the operator enter spindle speeds, but actual grinding-wheel surface footage changes as the wheel changes size. Improper speeds or feeds can cause excessive heat that leaves a visible burn on parts. The burn is metallurgical damage, resulting in scrap parts. In other cases, cutting speed activates the natural frequency of the machine or part, causing chatter which also generates scrap tools.
The MX16 controls spindle speed to much finer levels and automatically adjusts for changes in wheel size. This feature provides significant additonal reductions in scrap. The machine reduces possible chatter by measuring wheel balance and adjusting the weights within to bring it back into balance. Improved accuracy eliminates the need to measure every part, and the machines can take faster cuts to reduce the passes needed to finish-form many parts, resulting in a 25% increase in production rates. While higher volumes and simpler forms still run on mechanical grinders, the company uses the CNC grinders for the vast majority of special tool output, including all the most complicated and low-volume work. The new machines quickly paid for themselves by reducing scrap and increasing production rates. They generate considerable additional business by making it more economical to produce low-volume parts.
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