Extending CBN wheel life

Author Jeffrey A. Badger, Ph.D.
Published
February 01,2009 - 11:00am

Dear Doc: I OD plunge grind forms into cylindrical parts. I switched to CBN and am very happy with the results. However, CBN wheel life isn’t as long as I’d like it to be. Any advice?

The Doc Replies: First, are you using a water-based coolant or oil? With CBN, using oil instead of a water-based coolant provides much longer wheel life. And I’m not talking 20 percent longer; I’m talking 10 to 50 times longer wheel life. So if your machine is enclosed and has fire-suppression equipment, switch to oil.

If you’re already using oil or can’t make the switch, you can adjust the plunge speed and workpiece rpm.

Performing a simple calculation shows OD grinding is analogous to surface grinding. Divide plunge speed in ipm by workpiece rpm. That provides the actual DOC in inches per workpiece revolution, which is analogous to DOC in surface grinding.

Next, multiply workpiece rpm by 3.14 times the workpiece diameter. That equals workpiece velocity in ipm, just like in surface grinding.

Also as in surface grinding, take slow, deep cuts to reduce wheel wear. This technique generates a longer arc of cut and smaller forces on individual grits than fast, shallow cuts. For example, plunging at 0.1 ipm on a 1 "-dia. workpiece rotating at 20 rpm equals a DOC of 0.005 " (0.1 ÷ 20) and a workpiece velocity of 63 ipm (3.14 × 1 " × 20). That’s a slow, deep cut.

Now, let’s say you keep the same plunge speed but increase work rpm to 200. That’s a DOC of 0.0005 " (0.1 ÷ 200) and a workpiece speed of 628 ipm (3.14 × 1 " × 200). That’s a fast, shallow cut, which accelerates wheel wear.

Controlling burn and chatter is a different ballgame. In OD grinding, fast, shallow cuts almost always produce less burn than slow, deep grinding.

Chatter, on the other hand, is anybody’s guess. Slow, deep cuts can be conducive to avoiding chatter, but they can also cause chatter because of higher normal forces and wheel blunting. To avoid chatter, the best approach is to try it and see.

Dear Doc: I form-grind different radii on our parts, ranging from 0.004 " to 0.02 ". I use a 150-mesh wheel and have trouble getting a clean radius on the smaller sizes. Why?

The Doc Replies: I spent lots of time beating my head against the wall after visiting companies that insist on a one-size-fits-all mentality for grit sizes, especially those grinding threads and gears.

It’s a fact that to grind a small radius into a part you need a small grit.

Here’s a rough-and-ready calculation to put you in the ballpark in terms of grit size. Divide the number 1.0 by the radius (in inches) you’re trying to achieve. So, if you need a 0.004 " radius, use a 250-mesh size (1 ÷ 0.004 "). For a 0.01 " radius, use a 100-mesh size (1 ÷ 0.01 "). If you’re using a single-point diamond dresser, a little coarser is OK; if you’re crush dressing, you’ll need a finer grit.

Although you’re able to grind a 0.004 " radius with a 150-mesh wheel, you pay a heavy price. Check the dressing on your machine, and I guarantee your operator is dressing that wheel quite timidly. If he’s using a single-point diamond or a diamond disk, he’s traversing very slowly. If he’s using a plunge roll, he’s plunging very slowly and probably counter-directionally. This “splits the grits,” creating a dull, closed wheel. He’s able to produce the radius—just barely—but the dull wheel is generating gobs of heat, burn and chatter.

Stick on a fine-grit wheel, dress like hell and open the wheel. You’ll get a nice, clean radius, no burn and no chatter. And I won’t have to beat my head against the wall. CTE

About the Author: Dr. Jeffrey Badger is an independent grinding consultant. His Web site is www.TheGrindingDoc.com. E-mail grinding questions to him at alanr@jwr.com. The Doc’s International Hard Facts HSS Grinding Course takes place March 25 to 27 in Paris. Details are at www.HSSGrinding.com. 

Related Glossary Terms

  • chatter

    chatter

    Condition of vibration involving the machine, workpiece and cutting tool. Once this condition arises, it is often self-sustaining until the problem is corrected. Chatter can be identified when lines or grooves appear at regular intervals in the workpiece. These lines or grooves are caused by the teeth of the cutter as they vibrate in and out of the workpiece and their spacing depends on the frequency of vibration.

  • 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.

  • cubic boron nitride ( CBN)

    cubic boron nitride ( CBN)

    Crystal manufactured from boron nitride under high pressure and temperature. Used to cut hard-to-machine ferrous and nickel-base materials up to 70 HRC. Second hardest material after diamond. See superabrasive tools.

  • dressing

    dressing

    Removal of undesirable materials from “loaded” grinding wheels using a single- or multi-point diamond or other tool. The process also exposes unused, sharp abrasive points. See loading; truing.

  • 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.

  • grit size

    grit size

    Specified size of the abrasive particles in grinding wheels and other abrasive tools. Determines metal-removal capability and quality of finish.

  • high-speed steels ( HSS)

    high-speed steels ( HSS)

    Available in two major types: tungsten high-speed steels (designated by letter T having tungsten as the principal alloying element) and molybdenum high-speed steels (designated by letter M having molybdenum as the principal alloying element). The type T high-speed steels containing cobalt have higher wear resistance and greater red (hot) hardness, withstanding cutting temperature up to 1,100º F (590º C). The type T steels are used to fabricate metalcutting tools (milling cutters, drills, reamers and taps), woodworking tools, various types of punches and dies, ball and roller bearings. The type M steels are used for cutting tools and various types of dies.

  • inches per minute ( ipm)

    inches per minute ( ipm)

    Value that refers to how far the workpiece or cutter advances linearly in 1 minute, defined as: ipm = ipt 5 number of effective teeth 5 rpm. Also known as the table feed or machine feed.

  • outer diameter ( OD)

    outer diameter ( OD)

    Dimension that defines the exterior diameter of a cylindrical or round part. See ID, inner diameter.

  • surface grinding

    surface grinding

    Machining of a flat, angled or contoured surface by passing a workpiece beneath a grinding wheel in a plane parallel to the grinding wheel spindle. See grinding.

  • 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.