Dear Doc: I use electroplated CBN wheels to grind cobalt-chrome alloys. I get a lot of wheel wear for the first few parts, creating part-dimension headaches, before the grinding operation settles into a steady-state regime. Also, during this high-wear period, surface finish is atrocious. Is there any way to avoid this period?
The Doc Replies: Yes. Purchase a conditioned electroplated wheel. Unconditioned electroplated wheels, because they are not dressed, have rogue grits sticking up in just a few points around the circumference. These rogue grits wear quickly, but until they do, just a few do the cutting and the space between them is huge. Therefore, the chip thickness is massive and the surface finish is poor. Conditioned wheels are already “broken in,” so you don’t have to endure the break-in period.
Dear Doc: I plunge-roll dress a form onto a wheel with a diamond roll. I like to dress at a 0.4-mm/min. plunge speed, whereas my colleagues say I should dress faster, at 4 mm/min. Who’s right?
The Doc Replies: Avoid thinking about plunge speed—just about any speed will work, depending on the wheel speed. Instead, focus on dressing depth, or the distance the diamond roll plunges in one revolution of the wheel, measured in mm/rev. or ipr.
To calculate dressing depth, divide the plunge speed in mm/min. by the wheel’s rotational speed in rpm. This provides a result in mm/rev. Then, multiply that by 1,000 to convert the result to µm/rev.
Typical values for dressing an Al2O3 wheel are 0.1 to 1.0 µm/rev. or 4 to 40 µin./rev. Small values produce a duller wheel, more grinding power and a finer surface finish. Large values provide a sharper wheel, less grinding power and a rougher surface finish.
In your case, if the wheel is running at 3,600 rpm and you’re plunging at 0.4 mm/min., that’s 0.11 µm/rev. (1,000 × 0.4 ÷ 3,600), a timid dress. Dress more aggressively to sharpen the wheel. If the surface finish goes out of spec, stick with the aggressive dress but switch to a finer-grit wheel.
Dear Doc: I took a high-pressure coolant system from a lathe and installed it on a grinder. It worked wonders for turning, but doesn’t seem effective for grinding. Why?
The Doc Replies: Extremely high-pressure coolant might be fine for turning, but it is not good for grinding. You want a pressure that provides the right coolant exit velocity—one that matches wheel velocity. The basic calculation when applying a water-based fluid is pressure in bar equals (wheel velocity in m/sec.)2 divided by 200, or pressure in psi equals (wheel velocity in sfm)2 divided by 535,000.
For example, for a wheel speed of 40 m/sec. (8,000 sfm), you need a nozzle pressure of about 8 bar (120 psi). Anything more than that is not only wasted, it’s detrimental to cooling performance.
Cleaning nozzles, on the other hand, are a different story. Here, you want a pressure of 50 to 150 bar, depending on the wheel bond.
Related Glossary Terms
- alloys
alloys
Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.
- 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.
- lathe
lathe
Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.
- turning
turning
Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.
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.
