What Is Lapping?

Joshua Jablons Ph.D.

In the right hands, this little-known process can produce parts with highly polished ends, tight length tolerances, and extraordinary flatness.

When we conduct tours of the Metal Cutting facilities to show guests our many capabilities, including our three types of lapping equipment, people often give us puzzled looks and ask, “What is lapping?” The real mystery is why more people don’t know about lapping, since it is a profoundly simple (and beneficial) process. While a lapping machine does have moving parts, they are plates that rotate like the teacups in an amusement park ride — methodically spinning in a single plane (not multiple axes, with the top plate going in one direction and the bottom plate, the other.

So, what is lapping exactly? It is a controlled mechanical sanding or polishing process that involves an abrasive between two surfaces that are rubbed together to create an accurate finish on a part. Depending on the material and application for the part, lapping can be one sided or two sided.

What is lapping when it is one sided versus two sided?

Think of one-sided lapping like polishing an automobile, where an abrasive is applied to a softer material and used to polish the surface of a harder material. With one-side lapping, usually the part is held in place with a ring or other device, weighted down, and then ground as an abrasive slurry is fed onto a rotating lap plate.

However, with two-sided lapping, material from two surfaces of a part (top and bottom) is removed as pressure is applied and the part and abrasive are rolled around between the two lapping plates. For lapping of very small parts — say, from 3 inches (76.2 mm) down to 0.118 inch (3 mm) — where the application of weight might damage delicate materials, a jig is used to hold parts in place, allowing for precise positioning of the part and adjustment of the load during lapping.

What is lapping used for?

Lapping can be used to create a specific surface roughness. For Metal Cutting and our customers, the primary goal of lapping is to achieve a very accurate flat surface; however, lapping can also be used to obtain a domed surface, such as a convex lens or mirror.

The process of lapping can be applied to a wide range of materials, including metal, glass, ceramic, plastic, or silicon substrate. Thanks to the precision with which material can be removed, lapping can produce surface finishes in the nanometer (or 0.001 μm) range.

What is lapping used for when the goal is flatness?

Flat lapping is most often used to process parts that require tight control of thickness and parallelism. It can correct surface irregularities and remove subsurface damage caused by sawing or grinding, producing dimensionally accurate flat parts to high tolerances (generally less than 2.5 μm uniformity). Since the parts being lapped will mirror the shape of the lapping plates, for flat lapping it is vital for the plates to be maintained in extremely flat condition (typically less than 0.004 mm) so that the parts being lapped will themselves be flat.

What is lapping especially good for? Using specialized lapping machines, lensmakers can produce surfaces that are flat to better than 30 nanometers (or 0.030 μm). Lapping is also critical to semiconductor production, providing the precision required for handling very fragile, thin components, using semiconductor-grade lapping machines and specialty mounting carriers designed for wafers including LED, sapphire, silicon, compound substrates, and fiber optics.

What is lapping used for when the parts need to be domed?

Lapping can be used for lenses and other parts that require a slender, uniform dome shape. While the dome cannot be pointy, the heights and radius of the corners can be adjusted to create different degrees of convexity. This convexity is achieved by using a lapping plate conditioned so that it is concave and will thus create a convexly domed part that mirrors the shape of the plate.

Creating the correct convexity is a time-consuming process, often with much trial and error involved in the development stage. What is lapping precision critical for when you have domed parts? Consider the Hubble Space Telescope, a $3 billion project that turned out to have a flaw in its optical system. Specifically, Hubble’s primary mirror was just slightly the wrong shape, causing light that bounced off the center of the mirror to focus in a different place than the light bouncing off the edge. This tiny flaw — about 1/50 the thickness of a sheet of paper — was enough to distort the telescopic images. And while scientists were able to correct for the flaw, it was a costly lesson in dome uniformity.

What is lapping typically used for at Metal Cutting?

In addition to using lapping to create accurately smooth, flat surfaces as discussed above, Metal Cutting utilizes lapping to achieve very precise tolerances and end surface finish requirements. For example, we can move the decimal point on a length tolerance from +/–0.001 inch (0.0254 mm) to +/–0.00010 inch (0.00254 mm). In addition, for applications that require geometry, it is possible for us to (for example) take a part that is a trapezoid and use lapping to turn it into a rectangle or parallelogram.

With our 12 machines and three lapping processes, “What is lapping?” at Metal Cutting means being able to handle both small batches and large volumes, and producing highly polished part ends, tight length and thickness tolerances, and exceptional flatness. Our lapping services can achieve an end finish of Ra 2 microinch (0.050 μm) on both solid parts and tubes, including thin wall tubing and long length components. Surface metrology is verified by multiple in-house linear variable differential transformer (LVDT) systems and computerized profilometers.

Conclusion

As you can see, asking “What is lapping?” reveals a simple yet beneficial process that can produce highly polished part ends, tight length and thickness tolerances, and flatness unavailable by other methods.

You can visit our website to learn more about lapping — or better yet, give us a call! As a precision metal fabricating company, Metal Cutting Corporation is an expert in very tight tolerance lapping as well as cutting, grinding, and polishing of all metals for a wide range of applications. We also provide secondary operations such as bending, angle cutting, and pointing and slotting of small diameter tubes, wires, and rods; and tungsten and molybdenum products, such as wire, ribbon and rod, are available.

 

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