Dear Doc: I grind fine threads into hardened stainless steel with a fine-mesh aluminum-oxide wheel. The threads are 0.8mm deep in a 5mm-dia. ID. There are three threads per part. This operation just chews through the wheel, and I get only one part before I have to dress a 0.2mm depth in my 4mm-dia. wheel. What am I doing wrong?
The Doc Replies: Before jumping to conclusions about doing something wrong, let’s calculate the G-ratio, which is the volume of material ground divided by the volume of wheel worn away. Because the maximum wear probably occurs when grinding the bottom of the thread, let’s simplify and calculate the area of the material ground divided by the area of wheel lost—at the thread bottom.
The area of material ground equals depth removed times length removed. The thread depth is 0.8mm. The length of material removed is the circumference of one thread times the number of threads (p × 5mm × 3 threads = 47.1mm). That gives an area of 37.7mm2 (0.8mm × 47.1mm).
Now let’s look at the wheel lost from being worn away. You’re dressing a 0.2mm depth. Let’s assume 80 percent, or 0.16mm, of that is wear. The circumference of the wheel is 12.6 mm (p × 4mm). That gives an area lost on the wheel at the point on the thread bottom of 2.01mm2(0.16 mm × 12.6mm).
Inputting those two numbers gives a G-ratio of 18.8 (37.7 ÷ 2.01). That’s pretty good when grinding hardened stainless steel with a fine-mesh Al2O3 wheel. So you’re really not doing anything wrong. You’re just trying to cope with one challenging aspect of your grinding operation—the small wheel diameter.
A 4mm-dia. wheel just doesn’t have much abrasive around its circumference. Compared to a 400mm-dia. wheel, it has only 1 percent of the abrasive. It’s not going to mean more wheel wear, but it’s going to mean a much larger depth of wheel wear. And you’re just going to have to live with it.
There are ways to reduce that wheel wear, which is not considerable but the depth of wheel wear will be high because the diameter is so small. A harder-grade wheel, such as I to K; a larger grit size, as long as you can hold the corner; an oil-based coolant instead of a water-based one; and a higher wheel speed will all help.
You could switch to a pricier CBN wheel, which will wear less than an Al2O3 one, but wear will still be high. If the goal is to reduce dressing frequency, try it.
Dear Doc: I cylindrical grind IDs with small, bonded diamond wheels. Is it possible to true them in the machine?
The Doc Replies: Absolutely. Mount a vitrified-bond Al2O3 or silicon-carbide wheel that has a hardness grade of at least L onto an adapter. Then, mount that adapter in the chuck of your cylindrical grinder and true away. Take a DOC of around 0.001 " and quickly traverse across the wheel. Use an Al2O3 or SiC wheel with a grit size a few mesh sizes larger than the diamond grits. If it’s a resin- or metal-bond diamond wheel, be sure to stick it after truing.
Dear Doc: I grind tungsten-carbide IDs with 240-mesh electroplated diamond wheels. Initially, a few parts have poor finish and chatter marks before good parts are produced. How can I avoid 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, which 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.
Related Glossary Terms
- G-ratio
G-ratio
Measure of the grinding performance defined as the volume of metal removed divided by the volume of grinding wheel worn away in the operation.
- 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.
- 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.
- chuck
chuck
Workholding device that affixes to a mill, lathe or drill-press spindle. It holds a tool or workpiece by one end, allowing it to be rotated. May also be fitted to the machine table to hold a workpiece. Two or more adjustable jaws actually hold the tool or part. May be actuated manually, pneumatically, hydraulically or electrically. See collet.
- 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.
- hardness
hardness
Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.
- 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.
- truing
truing
Using a diamond or other dressing tool to ensure that a grinding wheel is round and concentric and will not vibrate at required speeds. Weights also are used to balance the wheel. Also performed to impart a contour to the wheel’s face. See dressing.
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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