Dear Doc: I creep-feed grind slots in hardened steel and am constantly battling burn. Some days I can crank up the feed rate and grind without burn, but other times burn occurs even at a low feed rate. Why?
The Doc Replies: My customers think “grinding faster” is the primary cause of burn. Therefore, I first have them test for burn and chart burn severity vs. specific material-removal rate (SMRR). The SMRR is calculated by the following equations, both of which give results in mm2/sec.
Courtesy of J. Badger
Higher SMRRs are not always the cause of burn.
DOC (mm) × feed (mm/min.) ÷ 60, or 10.75 × DOC (in.) × feed (ipm).
If you keep all other grinding conditions constant, burn should increase as you increase the SMRR. However, other factors play an even more important role than SMRR, and if these aren’t chosen correctly you’ll get results like those shown in the figure above, taken from a company flute-grinding HSS, which shows the severity of burn from acid cooking. Acid cooking is a process whereby the workpiece is boiled in hydrochloric acid to show the severity of residual tensile stresses.
Why are the results all over the place? The biggest cause of burn is not an SMRR that’s too high. Instead, it’s caused by dull dressing, a timid grit-penetration depth (i.e., not grinding aggressively) or bad cooling when creep-feed grinding. If you have one—or all—of these conditions, you’ll burn the workpiece regardless of the SMRR.
The theoretical surface temperature vs. SMRR for creep-feed grinding at a 1mm depth and increasing feed rates can be calculated using “The Grinder’s Toolbox,” a program I use to predict grinding temperatures. For a given set of dressing and grinding conditions, temperatures rise with an increasing SMRR. That’s fine and reasonable, but if you have dull dressing, a timid grit-penetration depth and bad cooling, you’re going to experience burn at a much lower SMRR.
Let’s say your burn threshold is 600° C. If you dress aggressively and choose an aggressive grit-penetration depth, you can grind about as fast as you want and not burn. But if you don’t dress right or if your speeds and feeds are timid or your cooling is poor, be prepared for burn at low SMRRs.
Dear Doc: I ID grind tungsten carbide with 240-mesh electroplated diamond wheels. When I first use a wheel, the first few parts have a poor surface finish and chatter marks before good parts are produced. Is there a way around this?
The Doc Replies: I recently visited a company that had the same issue. To overcome it, the operator mounts the wheel and then uses a dial gage, a piece of wood and a mallet to tap the wheel as close to true as possible—within 0.001 " and preferably less. He then grinds several IDs undersize and allows the wheel to break in. This knocks off the high, rogue grits. Once the wheel is broken in, he reruns those parts to size while producing the others. CTE
About the Author: Dr. Jeffrey Badger is an independent grinding consultant. His Web site is www.TheGrindingDoc.com. He’ll be giving a Rollomatic-hosted grinding course about grinding tungsten carbide Nov. 6-8 in Mundelein, Ill., near Chicago.
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.
- 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.
- 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.
- feed
feed
Rate of change of position of the tool as a whole, relative to the workpiece while cutting.
- 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.
- 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.
- inner diameter ( ID)
inner diameter ( ID)
Dimension that defines the inside diameter of a cavity or hole. See OD, outer diameter.
- tap
tap
Cylindrical tool that cuts internal threads and has flutes to remove chips and carry tapping fluid to the point of cut. Normally used on a drill press or tapping machine but also may be operated manually. See tapping.
- tungsten carbide ( WC)
tungsten carbide ( WC)
Intermetallic compound consisting of equal parts, by atomic weight, of tungsten and carbon. Sometimes tungsten carbide is used in reference to the cemented tungsten carbide material with cobalt added and/or with titanium carbide or tantalum carbide added. Thus, the tungsten carbide may be used to refer to pure tungsten carbide as well as co-bonded tungsten carbide, which may or may not contain added titanium carbide and/or tantalum carbide.
- 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.
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