4 FACTORS TO CONSIDER FOR GRINDER REBUILDS Memphis TN

With more than 20 years of experience working on grinders, Cliff Divine has seen the technology evolve dramatically, particularly since starting Brooklyn Technical Services, Brooklyn, Conn., 13 years ago. Running the small business specializing in rebuild and repair of grinding systems, he has seen a host of misconceptions crop up—mainly about what a grinder rebuild and retrofit can and cannot do. Fabricating & Metalworking spoke with Divine about several of them.

1. KNOW WHEN TO REBUILD; KNOW WHEN TO BUY

   New machines today often cost less than the rebuild, which is why Divine's business focuses on the extremes of grinding—machines that are extremely large, extremely expensive, extremely specialized, or all of the above.

   Divine says he puts together quotes for some rebuilds that, in reality, shouldn't even be candidates for a rebuild or repair. Consider a common example: A company requests a quote for a retrofit and new dressing system on a surface grinder. Yet "you can buy a new grinder for two-thirds of the cost of the price to retrofit and the new dressing system," he says, adding that many companies, including large multinationals, seem to have money in the budget for equipment "repair," but not new equipment purchases. Translation: The problem often has more to do with accounting and red tape than anything else, he says.

2. TO THE MILLIONTHS

   "A grinding machine has to be worked on absolutely correctly, because we're trying to maintain tolerances of 20 to 50 millionths of an inch, versus two thousandths with a machining center," Divine says. "That's a world of difference."

   To get down to those millionths involves strict attention to the straightness and flatness of the ways. "You need excellent bearing surfaces, whether they be a scraped way or roller-bearing or ball-bearing way systems."

   The tuning of the drive system has to be "absolutely perfect," he says, as should the tuning of the CNC. "A lot of CNCs now do not have the resolution necessary to get down into the very small increments of movement that we need with grinders. One micrometer was the typical resolution that you used to set these machines to; now it's a tenth of a micrometer, or at least a quarter of a micrometer resolution to get into the accuracies that customers are looking for."

   Such accuracies require the use of lasers and electronic levels. "For an indicator, we need something in the 50 or even 20 millionths range," he says.

   Divine uses controls from Siemens (www.siemenscnc.com) that offer a sinusoidal wave form in the feedback system, instead of a square-wave form and similar systems that have typically been used. These square-wave systems, he says, "can only get a resolution of four times better than the encoder resolution times the ball-screw pitch. Whereas with the sinusoidal system, we can see resolutions of several hundred times the calculation of the feedback resolution times the ball screw pitch. We're absolutely assured with a sinusoidal feedback system that we can get the resolution we need."

3. CONSIDER PLAIN-TEXT COMMANDS

   Typical on newer CNCs, operators can define routines as mathematical functions (R1 to R10, etc.) in the NC program to augment the G-code commands. "You can now say G0, X=R1," with "R1" representing a pre-programmed value, he says. Of course, those values must be recorded on look-up tables, and in situations with multiple operators, mistakes can abound. Modern controls now "allow me to write real words into those variable tables," says Divine. "Now I can say, 'G0, X=Grind Start Position' ... I can now use common industry grinding terms [in the program]. For example, 'Y grinding position for the head,' 'Z grinding position for in-and-out on the table.' So now, an operator only goes into a table and changes it just like he changes an offset. He doesn't have to go into the program, or even know how to read a program."

4. A CONTROL CAN'T FIX MECHANICAL PROBLEMS

   A control cannot electronically fix most mechanical problems, says Divine. Most commonly, he says, people have the idea of upgrading to a control with backlash compensation to compensate for backlash problems. The backlash compensation can help, "but if you have forces acting upon your part, that mechanical movement forward and backward, within the looseness of the ball-screw, is not detected by the machine. It is time to replace the ball screw or have it rebuilt."

   Another concern: Be wary of compensating for old ball screws with a linear scale that measures the position of the table. Yes, the scale detects minute table movement, and ball screws adjust as the whole system "hunts" for the correct position, he says, but such actions can lead to premature wear. "We wear our motors, belts and ball screws much faster with a direct feedback such as a linear scale versus having high-accuracy ball screws and an encoder on the screw," he says.

   This, of course, applies to all machine tools, not just grinders. But the problems become greater with grinding systems simply due to the accuracies they require.

Editor's Note: Photos courtesy of Brooklyn Technical Services, 860-774-2125, 294 Tripp Hollow Rd., Brooklyn, CT 06234.