The Need for Speed
Dear Doc: I cylindrical plunge-grind carbide with diamond wheels and steel with CBN and Al2O3 wheels. There’s disagreement in the shop about what to do with the workpiece rpm when burn occurs. Some say speed it up. Some say slow it down. Which is correct?
The Doc replies: The typical philosophy for many grinders is “When something bad happens, slow things down.” When it comes to burn in cylindrical plunge grinding, this is absolutely wrong. Let’s look at the general trends.
Increasing the workpiece rpm leads to: lower workpiece temperature and lower risk of burning or cracking (almost always); more wheel wear (usually); rougher (higher Ra) surface finish (almost always, but often the change isn’t that large, and spark-out frequently negates it anyway); and increased risk of chatter (usually).
Conversely, decreasing the workpiece rpm leads to: higher workpiece temperature and greater risk of burning or cracking (almost always); less wheel wear (usually); smoother (lower Ra) surface finish (almost always, but often the improvement isn’t that large, and spark-out frequently negates it anyway); and decreased risk of chatter (usually).
Keep in mind that all these trends are for cylindrical plunge grinding. If you’re doing cylindrical traverse, then everything I’ve written here is wrong. To further complicate things, if you’re doing cylindrical plunge with oscillation, all these trends apply if the workpiece always stays on the wheel — provided that you don’t have significant wheel wear, such as in grinding PCD or PCBN with diamond or grinding high-vanadium HSS with Al2O3. If the workpiece leaves the wheel or if there's huge wheel wear, then the trends may apply. If you have a swivel on your wheelhead — for example, the typical 30° swivel — then all these trends apply on both the OD and the shoulder. Finally, they all apply for both cylindrical OD plunge and cylindrical ID plunge.
A common situation with my customers who grind carbide with a diamond wheel is as follows: They want to increase feed rates, but when they do, they get too much wheel wear or form breakdown. This is caused by the increase in aggressiveness — or if you prefer, chip thickness — when they increase the plunge velocity. The figure shows a quick, easy formula to keep aggressiveness constant while increasing the feed rate.
Let’s say you’re cylindrical plunge grinding at 60 rpm for the workpiece, 1,800 rpm for the wheel and a 1 mm/min. (0.04 ipm) feed rate. You want to increase the feed rate to 1.5 mm/min. (0.06 ipm). If you increase only the feed rate, the diamonds will dig in deeper, causing more wheel wear. Do this instead: Keep the wheel speed the same, but decrease your workpiece speed to 40 rpm (60 × 1 ÷ 1.5 × 1,800 ÷ 1,800 = 40). By doing this, your aggressiveness and wheel wear will stay the same.
If you want to get fancy, increase the wheel speed to 2,100 rpm and decrease your workpiece to 54.4 rpm [60 × 1 ÷ 1.5 × (2,100 ÷ 1,800)2 = 54.4]. Why increase the wheel speed? With the faster plunge velocity, you can keep the same aggressiveness by either decreasing the workpiece rpm or increasing the wheel rpm. For a fixed aggressiveness, a slower workpiece rpm means higher grinding temperatures. A faster wheel speed doesn’t. What’s more, the square term means that wheel speed has a bigger effect than workpiece rpm.
Play around with it and see what happens. The beauty of this equation is that you can use any units you want — rpm, sfm, meters per second, furlongs per fortnight — as long as you’re consistent.
Related Glossary Terms
Rotary tool that removes hard or soft materials similar to a rotary file. A bur’s teeth, or flutes, have a negative rake.
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.
- 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.
Rate of change of position of the tool as a whole, relative to the workpiece while cutting.
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.
- outer diameter ( OD)
outer diameter ( OD)
Dimension that defines the exterior diameter of a cylindrical or round part. See ID, inner diameter.
- polycrystalline cubic boron nitride ( PCBN)
polycrystalline cubic boron nitride ( PCBN)
Cutting tool material consisting of polycrystalline cubic boron nitride with a metallic or ceramic binder. PCBN is available either as a tip brazed to a carbide insert carrier or as a solid insert. Primarily used for cutting hardened ferrous alloys.
- polycrystalline diamond ( PCD)
polycrystalline diamond ( PCD)
Cutting tool material consisting of natural or synthetic diamond crystals bonded together under high pressure at elevated temperatures. PCD is available as a tip brazed to a carbide insert carrier. Used for machining nonferrous alloys and nonmetallic materials at high cutting speeds.
- spark-out ( sparking out)
spark-out ( sparking out)
Grinding of a workpiece at the end of a grind cycle without engaging any further down feed. The grinding forces are allowed to subside with time, ensuring a precision surface.