Working An Inside Job

Setting up for I.D. grinding jobs is different from setting up external and surface grinding operations. Because of the wheel-within-a-hole configuration of I.D. grinding, shops must pay special attention to the area of contact between the wheel and workpiece and, most importantly, to the relationships between wheel speed, workpiece speed, rigidity of the wheel spindle and other operating factors.

Most shops believe that selecting the proper I.D. grinding wheel for a particular job involves nothing more than knowledge of the material to be ground. However, in most cases they are wrong, says Bill Bednarski, an application engineer at Saint-Gobain Abrasives (www.nortonabrasives.com).

Consider a shop that does a lot of O.D. cylindrical grinding using 32A aluminum oxide wheels (a strong sharp monocrystalline grain) on hardened tool steels, then uses the same wheel specifications for its I.D. grinding operations. The result can be that the shop sees problems with chatter and grinding burns. In addition, to make the wheel perform properly, operators must dress it several times per part. On the other hand, if this shop used a 25A wheel (a more friable grain in J grade — two grades softer) for the I.D. grinding operation, that chatter and burning would be eliminated and dressing cycles would be put back to once per part.

But Bednarski said there are several other factors, besides the material to be ground and its hardness, that shops must consider when choosing the grain and grade for a grinding wheel for "inside" work. These factors include the amount of stock that is to be removed, grinding accuracy and surface finish, area of contact, wheel size, workpiece speed, wheel speed and operating factors and grinding system rigidity.

MATERIAL TO BE GROUND
The workpiece material degree of hardness and its susceptibility to cracking principally influences the choice of an abrasive grain, grain size, and the wheel grade. Hard materials resist the penetration of abrasive grains and cause them to dull quickly. Therefore, the harder the workpiece material, the more operators should use wheels with friable grains in softer grades. These softer grades permit abrasive grains to break away as they dull, exposing new, sharp cutting grains to the workpiece.

Here are several types of abrasives to use for the given materials:

• Aluminum oxide or CBN abrasive wheels for high tensile strength materials such as carbon steels, alloy steels, and high-speed steels.
• Silicon carbide or diamond abrasive wheels for low tensile strength materials such as cast iron, soft bronzes and aluminum, as well as cemented carbides.
• Diamond wheels for most non-metals, such as high-density ceramics, boron carbides, carbides and Pyrex.

Three principal kinds of aluminum oxide abrasives are classified by their fracture qualities. These include:

• Dark, durable aluminum oxide (57A) and monocrystalline aluminum oxide (32A) for general purpose internal (inside the workpiece) grinding of steel parts.
• White, friable aluminum oxide (38A) is well-suited for dry or light grinding extremely hard steels because of its cool and almost self-dressing action. (A friable grain will fracture as the grinding pressure increases, causing a sharp edge to be exposed.)
• Pink aluminum oxide (25A) has grinding properties similar to 38A and also exhibits good form-holding qualities.

In addition, 53A is a blend of dark aluminum oxide and white aluminum oxide and has a grinding action intermediate between 57A and 38A abrasives.

STOCK REMOVAL, GRIND ACCURACY AND SURFACE FINISH
As a rule, coarser grain sizes make for higher stock-removal rates because these wheels ordinarily permit heavier cuts. However, if the workpiece material is hardened steel or some other difficult-to-penetrate alloy, then a slightly finer, rather than coarser, grain wheel may cut faster because of the greater number of cutting points it presents to the workpiece.

To maintain close dimensional accuracy, a free-cutting wheel is essential. Free-cutting wheels cut with considerably less pressure, and this is a particularly important factor in close-tolerance work. For example, if an I.D. form calls for a 0.005-in. to 0.008-in. corner radius, the wheel grain size typically should be 100 to 150.

In finishing operations, the rule is that better finishes require finer grain sizes and cooler cutting wheels. Cool cutting minimizes the deformation of the workpiece from grinding heat and allows the fast and consistent production of parts with close tolerances.

AREA OF CONTACT
The contact area is much larger for internal or I.D. grinding compared with cylindrical or surface grinding. It is especially large in some small bore grinding, where it is necessary to use wheels just slightly smaller in diameter than the hole itself. A large area of contact with the workpiece necessitates using relatively soft-grade wheels with open structures to provide sufficient chip clearance.

In regards to wheel size, a wheel that is too large creates low pressure and dulls it. Oversize wheels also block coolant flow, and that results in poor chip removal from the grinding area. Conversely, a wheel that is too small grinds inefficiently because it does not present enough area of the abrasive to the workpiece, and it operates under excessive pressure and exhibits excessive wear. Generally, wheels should be replaced when they reach 70 percent of their original size.

WORKPIECE AND WHEEL SPEEDS AND OPERATING FACTORS
The faster a workpiece spins in relation to a grinding wheel, the greater the force imposed on the wheel and the "softer" it will "act." In contrast, as wheel speed increases, imposed force on the wheel decreases, and it "acts harder." According to Bednarski, typical operating wheel speeds should run about 8,500 sfpm, but they can range from 4,500 to 12,500 sfpm.

Wheel speed factors

• Maintain proper sfpm and use a constant surface feet speed if possible.
• sfpm = wheel rpm x wheel diameter x 0.2618
• Less wheel speed makes for a "softer acting" wheel, and more speed makes for a "harder acting" one.

Work speed rules of thumb

• Work speed to wheel speed ratio should be between 25:1 and 65:1.
• For material hardnesses of Rc 50 and less: sfpm_work = sfpm_wheel ÷ 30.
• For material hardnesses of Rc 51 or more: sfpm_work = sfpm_wheel ÷ 55.

Operating factors

• Infeed rate starting points for material hardnesses Rc 50 and softer are: Rough rates use 0.00015 in. —0.00010 in. stock removed per part wheel revolution. Finish rates use 0.00005 in. to 0.00003 in.
• Infeed rate starting points for material hardnesses Rc 51 and harder are: Rough rates use 0.00010 in.
• —0.00005 in. stock removed per part wheel revolution. Finish rates use 0.00005 in. — 0.00003 in.
• Oscillation rates are typically 75 ipm to 125 ipm. For a starting point, use 0.2 x wheel width x rpm_work.

GRINDING SYSTEM RIGIDITY
Successful I.D. grinding requires rigidity, and selecting the proper wheel quill or spindle is imperative. To determine proper quill diameter, shops must consider wheel wear. They should use the shortest quill that allows the wheel to grind the workpiece and to be dressed.

Regarding quill length to quill diameter ratios, the shortest quill length and the largest quill diameter possible should be used.

• Ratios less than 3:1 will produce relatively trouble-free performance and the shortest sparkouts.
• Ratios of 3:1 to 6:1 are typically viewed as borderline.
• Ratios greater than 6:1 can result in problems such as deflection, taper, chatter and long sparkouts.