Dear Doc: At a trade show, a company displayed a grinding wheel with slots, where coolant is poured into the center of the wheel and then ejected into the grinding zone. What’s your take on this design?
The Doc Replies: Here’s how this gizmo works: Coolant is poured at low velocity into a recess around the rim of the wheel, centrifugally accelerated by the wheel and ejected everywhere through slots around the wheel perimeter. And everywhere includes a small portion—around 2 percent on a good day—that’s the grinding zone. But this technology is nothing new, having been developed in the 1970s for creep-feed operations.
I’ve visited hundreds of grinding shops on five continents and have never seen one of these gizmos in actual production use. Maybe I’m just going to the wrong places. Or maybe they look pretty in a display case, but they’re not doing the hard work on the shop floor.
Why is that? There are several reasons. First, the spindle motor has to accelerate the coolant to the wheel speed, which requires a lot of power. Second, as previously noted, the coolant is ejected not just at the grinding zone, but along the entire wheel circumference. Third, the coolant is ejected from the slots, but not necessarily where it’s needed most, which is the unslotted regions where the abrasive meets the workpiece. Fourth, all that coolant flying out means that if there is any coolant coming from a main nozzle, it gets deflected. Fifth, the wheel is expensive. Sixth … well, let’s stop there.
Whenever a wheel salesman pitches me about some new grinding technology, my first question is always: Can you give me some hard science behind the technology and some verified data from case studies? The answer is usually vague, including references to high quality and state-of-the-art technology, with anecdotes about some company in some distant land that saw cycle times drop drastically. But I seldom get real, hard science.
Keep in mind we’re talking about a coolant gizmo, not just slots in the wheel. Slots alone, when used with a high-velocity main nozzle, can reduce grinding temperatures when creep-feed grinding.
Dear Doc: I grind camshafts and am always battling burn. To assess the burn, I nital-etch some workpieces but it is messy. Is the Barkhausen Noise method an option?
The Doc Replies: Nital etching, a process where the specimen is dipped in a bath of nitric acid and the color on the workpiece surface changes from gray to white in the regions that have suffered rehardening burn, is indeed very messy.
Barkhausen Noise has come a long way in the past 15 years and is now a robust, albeit tricky, method for assessing grinding burn. If you’re looking for a plug-and-play measurement device, where you buy the device, remove it from the box, plug it in, stick the Barkhausen probe on the workpiece and get an immediate answer, then don’t bother. It won’t work for you.
But if you’re willing to take the time—months in some cases—to understand the signal, how it increases with residual tensile stresses and softening, how it can suddenly drop in the regions of the workpiece that suffer from rehardening burn, how different materials impact it and how to establish a baseline signal and then make comparisons from that, then the method could work well for you. Lots of cam producers are generating good results with it every day. CTE
About the Author: Dr. Jeffrey Badger is an independent grinding consultant. His Web site is www.TheGrindingDoc.com. He’ll be giving his High Intensity Grinding Course Jan. 14-16 in Gävle, Sweden, and March 5-7, hosted by Diamond Innovations Inc., in Columbus, Ohio.Related Glossary Terms
- abrasive
abrasive
Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.
- computer-aided manufacturing ( CAM)
computer-aided manufacturing ( CAM)
Use of computers to control machining and manufacturing processes.
- coolant
coolant
Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.
- creep-feed grinding
creep-feed grinding
Grinding operation in which the grinding wheel is slowly fed into the workpiece at sufficient depth of cut to accomplish in one pass what otherwise would require repeated passes. See grinding.
- grinding
grinding
Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.
- grinding wheel
grinding wheel
Wheel formed from abrasive material mixed in a suitable matrix. Takes a variety of shapes but falls into two basic categories: one that cuts on its periphery, as in reciprocating grinding, and one that cuts on its side or face, as in tool and cutter grinding.
- land
land
Part of the tool body that remains after the flutes are cut.
- reaction injection molding ( RIM)
reaction injection molding ( RIM)
Molding process that allows the rapid molding of liquid materials. The injection-molding process consists of heating and homogenizing plastic granules in a cylinder until they are sufficiently fluid to allow for pressure injection into a relatively cold mold, where they solidify and take the shape of the mold cavity. For thermoplastics, no chemical changes occur within the plastic, and, consequently, the process is repeatable. The major advantages of the injection-molding process are the speed of production; minimal requirements for postmolding operations; and simultaneous, multipart molding.
- sawing machine ( saw)
sawing machine ( saw)
Machine designed to use a serrated-tooth blade to cut metal or other material. Comes in a wide variety of styles but takes one of four basic forms: hacksaw (a simple, rugged machine that uses a reciprocating motion to part metal or other material); cold or circular saw (powers a circular blade that cuts structural materials); bandsaw (runs an endless band; the two basic types are cutoff and contour band machines, which cut intricate contours and shapes); and abrasive cutoff saw (similar in appearance to the cold saw, but uses an abrasive disc that rotates at high speeds rather than a blade with serrated teeth).
- web
web
On a rotating tool, the portion of the tool body that joins the lands. Web is thicker at the shank end, relative to the point end, providing maximum torsional strength.
Author
Jeffrey A. Badger, Ph.D., is an independent grinding expert. He will be giving his 3-day High Intensity Grinding Course from Sept. 26-28 in Columbia, S.C., hosted by S.L. Munson. Visit www.thegrindingdoc.com for details. He can be reached by phone at 512-934-1857 or via email at JB@TheGrindingDoc.com.
