New Balance: Inspection Efficiency

Courtesy of L.S. Starrett

Starrett Master precision level, 199 series 15 “/380mm.

An out-of-level grinding machine can cause major operating and part quality problems. The good news is the solution is simple.

Editor’s Note: This article is adapted from a longer version that includes additional details about the three case studies discussed at the end of the article. To access the longer version in PDF format, click here.

Manufacturers continue to make process improvements in everything from surface finish to grinding wheel life. Many of the improvements require advanced engineering skills, but one of the simplest, most effective—and often overlooked—improvements is leveling an out-of-tolerance machine. When unlevel is corrected, results are immediate and measurable. While this article focuses on grinding, the information applies to any machining process.

An out-of-level machine causes problems such as poor surface finish, the inability to hold size and form, chatter marks, wheel breakdown and excessive scrap. By controlling machine level, manufacturers can help solve these problems while improving wheel life, cycle time and material-removal rates.

Machine Level Definition

All OEMs specify a level tolerance for each machine axis. This information is usually found in installation manuals, but if it cannot be found, 0.0002 in./ft. should be used. However, for some large-footprint surface and creep-feed grinding machines with many leveling pads, this specification may be unrealistic. Multiple leveling points can cause one section of the machine base to be level while another is out of tolerance.

Some grinder maintenance crews feel that carpenters’, masons’ or plumbers’ levels can be used to level a machine, but these tools are not nearly precise enough. Some also believe that newer, more rigid CNC machines overcome any error in level. In fact, the opposite is true for high-speed machines because being level is more important for achieving maximum process efficiency and machine life.

A level’s accuracy depends on the quality of its working surface, straightness, rigidity and level-vial sensitivity. Accuracies are often specified in fractions of a degree, such as 10-second or 43-minute accuracy.

However, because this measurement means little to most people, level manufacturers use inches per foot of elevation. For instance, a 10-second vial accuracy means that if the level is on an incline of 0.0005 in./ft., then the bubble on the vial will move 0.100 “.

There are three general types of level vials: ground vials are typically found in precision levels, and bent glass and plastic vials are in most others.

Most vials have two lines spanning the length of the bubble because most users just want to know if something is level or not. More precise levels, which should be used to level grinders, have vials with numerous reading lines on each side of the bubble. All inch-reading vial graduations are 0.100 “. Metric levels have 2mm vial graduations and accuracies are usually described as millimeters per meter of elevation.

It is important to level the machine—not the fixture or fixture base plates bolted to the machine table. One common mistake when leveling a grinder with a table-mounted component fixture is using the fixture as the surface reference point. A level fixture does not ensure a level machine. All fixtures must be removed prior to leveling.

During leveling, the machine’s axis must be positioned in the center of its respective length of travel. Also, remember that temperature affects bubble size and shape. As temperature rises, the liquid expands, thereby reducing bubble size, so gaps exist at both ends between the bubble and the reading lines at true level. Conversely, the bubble can expand and overlap the reading lines when temperature drops significantly.

To get a correct reading with a level, ensure the level is in calibration according to the manufacturer’s instructions. Next, assess both ends of the bubble. Place the level on the machine and determine how much the bubble overlaps or underlaps the graduations on both ends of the bubble. From there, spin the level 180° to verify the same readings on the other end of the level. If the bubble offers different readings than the original, temperature is affecting the bubble. This should be factored in to the level reading by splitting the difference of the readings in half and applying this to the final specification tolerance.

Note that hand heat on the center of a level for an extended period can expand the center, causing the working surface to become convex. A convex level tends to spin on flat surfaces. This is more noticeable on precision levels.

Inspection Frequency

In general, level should be checked whenever a grinder is moved, when nearby machines are moved or when major service is performed on the machine. Otherwise, a machine should be checked at least once every 6 to 12 months.

Unusual conditions that call for more frequent machine leveling include:

n A machine straddling floor expansion joints. This requires a monthly inspection.

n A coolant spill near a grinder on wooden block floors. Wooden block floors expand and contract during and after coolant spills. Grinding machines must be isolated from this problem. In addition, some companies raise their equipment a few inches upon installation to match shop features, such as catwalks. It is best to avoid risers made of wood or other materials that expand and contract based on temperature or humidity. Cast iron blocks are suitable for elevating a machine a few inches. Beyond that, a raised concrete pad should be poured.

n Shops in a geographic area with seismic activity. Machinery located west of the Rocky Mountains, for example, where earthquakes are more frequent, requires monthly leveling.

Case One: Optical Chatter Marks

A customer making a component with a concave ground surface reported a long-standing problem with chatter marks. The marks occurred only when the wheel OD wore to 15½ ” and 14 “. Thus, extra finishing passes were required.

Courtesy of United Grinding Technologies

A typical grinding machine.

While operating under standard parameters, the majority of the chatter marks occurred as the Y-axis was reversing. The marks were more of an optical phenomenon than a measurable surface irregularity. Most occurred parallel to the grind line; however, during some cycles marks ran at an angle to the grind line.