The worldwide use of grinding wheels for grinding a thread form into a precision workpiece is a common process. However, in this case study, the thread grinding operation is unique in that the threads were ground from solid to finish tolerance, eliminating the need for the rough cutting operation, the cutting machine, and complementary cutting tools. Advancements in wheel technology, coolant, coolant delivery, and machine technology have made this approach economically viable.
Benefits of Grinding Threads from Solid
Grinding threads from solid can provide a quicker response to customer needs, competitive cycle times, and the elimination of the traditional milling and turning tooling lead times. It also reduces tooling cost — to change the form, simply redress the wheel — and it reduces tooling inventory and cutter costs and logistics. This approach offers additional design freedom to allow thread form modifications to improve strength or fit a custom application, and it eliminates capital equipment cost, as there’s no need for a thread milling or turning machine.
Those who could benefit from the operation of grinding threads from solid include job shops, shops that have large varieties of products manufactured in small batches, or worm reducer manufacturers with aging and inefficient thread milling machines.
Economics play a major factor where a job shop supports a large mix of products that have short lead times. With a rush job, they can’t wait for special cutting tools, or the products they manufacture are so varied that the cost to hold tooling to support their jobs is cost prohibitive. Producers of small to medium size lots are the best candidates for grinding from solid. Tracey Gear is one such company that has successfully embraced the new Drake grinding machine technology and the new Norton wheel technology.
Coolant Needed To Grind Threads from Solid
For coolant, oil is the best option — premium ester-based high-performance grinding oil for multiple metals with a high flash point. Coolant velocity matches wheel velocity. For an untested grind cycle, use 25 GPM/inch of form width. If power is known, use 1.5–2.0 GPM/HP.
A coolant nozzle should help provide a solid jet flow of coolant. This means the coolant coming out of the nozzle looks like a solid bar, void of any air entrapment. This is accomplished through correctly sized supply pipes and limit bends in the coolant lines. Lastly, a properly designed coherent jet nozzle will help to reduce turbulence and air entrapment. See Figure 1.
Coolant should be maintained at ambient temperature whenever possible. Tank size should be able to satisfy coolant flow without interruption of coolant during a continuous grind cycle and, ideally, provide for 8-10 minutes of settling time for degassing. Filtration is necessary to eliminate reprocessing grinding swarf.
Grinding Wheel Selection for Deep Cut Grinding
The makeup of a grinding wheel is:
Grain + Bond + Porosity = Wheel
For all but very fine pitch threads (Drake uses the product to grind medium to coarse pitch threads), the wheel makeup is (see Figure 2):
High-Performance Ceramic Grain + Strong Bond + High Porosity
The self-sharpening grain technology, stronger glass bonds, and porosity of the wheel enhanced by the shape of the grain help to remove material in a rapid, efficient process. Wheel specifications are customized to suit the applications. The high unit material removal rates (Q’ to 25 mm3/mm sec.) keep high unit pressure on the ceramic grains to force self-sharpening from micro-fracturing.
High-quality dressing rolls are required to condition the wheel. Quality hand-set diamond rolls with synthetic CVD reinforcing logs are best. The roll and wheels should run in the same direction at the point of contact — unidirectional. For peak sharpness of the wheel, run the diamond roll at 80 percent of the grinding wheel speed measured in SFPM. On this particular operation, traversing the diamond roll across the wheel at 6–12 IPM would provide a sharp aggressive wheel. The dress depth should not exceed 0.0007″ (0.015 mm). See Figure 3.
Thread Grinding Machine
Machine stiffness and accuracy is critical to the success of grinding threads from solid. In this case study, the test machine is a Drake GS:TE 200 external thread grinder equipped with a 12 kW variable speed grinding spindle motor. It is a rigid machine with high static and dynamic stiffness and linear motors on linear ways. It has micron-scale feedback with a Fanuc CNC controller and a diamond dressing roll. The machine is equipped with a high-volume, high-pressure coolant system with engineered coolant nozzles. Furthermore, software and a typical interactive screen make for easy operation. See Figure 4.
At Tracey Gear, the thread was ground using two metal removal rates (MRR). MRR is the term used to quantify the amount of material removed in a given time period. The first 50 percent of the cut had an MRR of 4.3 cubic inches per minute. The last 50 percent of the cut had a reduced MRR of 2.7 cubic inches per minute with one light cleanup pass. The effort produced promising results. In this test, the customer was able to remove up to 4.3 cubic inches of steel per minute, resulting in very short processing times for these particular parts. See Figure 5 and Figure 6.
Grinding from solid, referred to as machining to grinding (MTG) at Norton, is a more efficient process that can provide additional opportunities for manufacturers to be more flexible and competitive. Tracey Gear is one of those companies that is thinking outside the box and achieving its goals by using this approach.