Honing Services

A special capability of my shop is precision honing. It was a skill I picked up many years ago, sizing production parts to about 15 millionths of an inch. These days CNC machining is so good that honing is a distant memory in many shops. Most don't miss the oily mess or the smell, though it does bring back some pleasant memories for me. The fact is, when you need a hone, nothing else will do. Nothing else will be as fast and nothing else will be as controllable and precise.

Honing can bring an already good bore into almost perfect roundness and straightness, plus size it within close tolerance. The surface finish can range from a medium cross-hatch to a near mirror polish. Almost any material can be honed, given the right tooling. The value of honing can be seen when one begins honing what appears to be a very good reamed bore. The lobes and imperfections produced by almost all reamers immediately show up as the hone starts removing the high spots. The effect is even more dramatic on holes produced by circular interpolation.

How Does it Work?

The machine appears simple, a spinning mandrel that carries an abrasive stone in a slot, but a few subtle details make it capable of extraordinary accuracy. The stone is raised up from the mandrel by a shallow wedge. Because the wedge is shallow, the machine can easily raise the stone, but pressure from the work can't force it back down. That means any high spots in the work receive all the pressure and are quickly ground down. The machine applies pressure to the wedge through an adjustable spring, so the stone is automatically raised up as the diameter of the bore increases. Naturally the geometry of the mandrel and stone has to be perfect. This is achieved by running the mandrel and stone in a sacrificial piece called a truing sleeve, with the honing oil shut off. A lapping paste quickly builds up, wearing the mandrel and stone into perfect cylindricity. When the oil is turned back on, the wear reduces to near zero, with all the grinding action taking place on the part.

Here's a complete Sunnen honing mandrel alongside the wedge and stone it contains. The mating faces of the wedge and stone are shown.

photo of Sunnen honing mandrel, wedge and stone

This photo shows the wedge and stone in their normal relationship. One wedge is hidden but you can see how the stone will slide up parallel to the wedge as the wedge is forced forward.

photo of Sunnen wedge and stone

It took me several years to find the right machine at the right price, but I now have a Sunnen MBB-1660 machine that I've refurbished to near new condition.

photo of Sunnen MBB-1660
      honing machine

I don't run a job shop or offer machining services, other than for support of my development work, but honing is sufficiently specialized that I will entertain small runs of parts or repair work. In particular, I can easily refurbish most turntable bearings to better than new. I can also offer training and advice for others doing honing, though the Sunnen factory and reps can certainly provide this as well. If you're considering using honing as a process, I can run a few parts to give you an idea of what to expect.

If I have the right size mandrel and stones, the cost is minimal. The downside of honing is that each mandrel covers a limited range of sizes, and several different types of stones may be required for best results. Each size also requires a machine adapter and a truing sleeve. If one has to tool up for a new size, everything needed can be $200-$300. Sometimes suitable surplus tooling can be found on eBay or other machine tool auction sites, but it's a matter of timing and luck. Even then, a complete mandrel with stones, usually the wrong stones, might be $50 or more. My regular tooling covers many common fractions from about 1/4" to 1 1/8". Special tooling is needed for very long parts, parts with keyways and certain materials that are only compatible with bronze or other special mandrels and stones. Though new machines have become quite expensive, the honing companies have always followed the razor and blade business model- If they can get you to buy a machine (the razor), they know you'll be buying expensive tooling (the blades) every time a new job comes up.

If you describe your needs in an email I'll get back to you with the feasibility of the job and an approximate cost.