CNC grinders speed punch production

Manufacturing Engineering, Jul 1998

Punches used in tablet press machines make prescription and non-prescription products from powder. Finished product comes out of a press at rates as high as 10,000 tablets per minute. Thomas Engineering (Hoffman Estates, IL) makes tablet presses that produce predominantly ovalshaped tables with a maximum length of 7/8 (22 mm).

Prosperity and problems came hand in hand as demand for the punches grew. Thomas began searching for new grinders because its old profile grinders could not consistently produce parts of perfect shape and quality. Even slight shape variations can make a punch wear out prematurely or not mate properly with the dies. Punches made by Thomas all require different orientation angles with respect to the grinder keyway, making it difficult to achieve consistent orientation. The company typically produces its punches from steel grades S-7 and D-2 with a hardness of RC 56-60.

After considering its options, Thomas Engineering replaced its old grinders with four CNC cylindrical grinders from Weldon Machine Tool Inc. (York, PA). One machinist can operate all four of the model 1632 grinders; each of the old profile grinders required an operator.

Two of the grinders are Gold models, which use a shear-damping base design. All of them feature a 16 x 2 X 5" (406 X 51 X127 mm) conventional abrasive, aluminumoxide grinding wheel with a 1/4" (6.4-mm) corner radius, a GE Fanuc 15M CNC control, and Weldon's EZpunch software. In the Weldon 1632s, punches are ground with both the C (workhead) and X (wheel slide) axes operating simultaneously. The CNC control synchronizes movement of the axes to perform precise nonround grinding. Thomas grinds the working ends of the tools to tolerances of +0.0000 to -0.0005' (0.013 mm). Specifications require a surface finish of 15 ginch (0.00004 mm), with concentricity to the barrel of less than 0.001" (0.03 mm) TIR. Some punch designs also call for a back taper of 0.001-0.002" (0.03-0.05 mm).

Total stock removal ranges from 0.50-0.020" (13-0.51 mm) per side. Rough grinding takes place at a rate of 0.0025 ipr (0.064 mm/rev) and fine-grinding at 0.0005 ipr (0.0127 mm/rev). Grinding wheels require dressing after production of each part; Weldon's in-process dressing cycle makes this step easier.

"Our 1632 grinders can match punches to dies exactly," says Jeff Simonson, Thomas Engineering's tooling production manager. Weldon's E-Zpunch software speeds grinder programming, according to Simonson. It presents a menu of 13 basic shapes to the grinder operator, all of which can be modified. For more information on Weldon grinders Circle 380.