Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)

by Larry Zitko, ChromeTech, Inc.
January, 2003

Grinding, Polishing or Honing?

Q. What determines whether a part is ground, polished or honed before/after chrome plating?

A. I have designed and installed several turnkey hard chrome plating lines for the hydraulic industry, and my customers have used the following techniques for preplate and postplate finishing:

• Centerless grinding (common for valve spools)
• grinding between centers
• wet-belt polishing
• dry-belt polishing
• Superfinishing (where very smooth profiles are required)
• Honing (especially I.D. work)

The reasons for choosing one method over another are varied, but here are some considerations:

• What surface condition is required or preferred by specifications or service use?
• Size and shape of parts: what type of machines can hold or process > them?
• What kind of finishing equipment is available, affordable at the shop?
• What are the precision requirements for dimensional considerations?

Hydraulic repair shops often used belt-polishing techniques to provide dimensional tolerance and surface profile and finish on rods and cylinders. I am told that many modern hydraulic seal types are quite "forgiving", thus can prevent fluid leakage throughout a significant dimensional range. This often encourages shops to polish, rather than grind, because it is easier and faster. When possible, wet-belt polishing is preferred over dry-belt polishing because it can promote smoother chromium deposits, especially for heavy buildups.

Having said this, there are a lot of applications where the dimensional requirements cannot be met with belt polishing. Valve spools, for instance, are typically centerless ground to keep the parts within tight size tolerances (some manufacturers can measure and hold to ten-thousandths or even millionths of an inch). When hydraulic shops make the effort to O.D. grind rods, they often have to machine the required "centers" so that they can be chucked up in the grinder. This can be time consuming and costly. It has been my experience that grinding will typically produce a part with less dimensional variation - i.e. a straighter or truer part, compared to polishing. Many other industries, like the aerospace industry, use grinding techniques almost exclusively.

Both grinding and belt-polishing techniques have their own set of problems associated with equipment and operator technique. For example, a polisher should have an assortment of fresh, new belts, and should work his way down through finer and finer grits. The belt should be wetted continuously with a coolant, and the speeds should be right for the belt, and rotation and traverse of the rod. All too often, I see polishers using worn-out belts and skipping intermediary grits. Grinders need to have a wheel that is appropropriate for the material (steel, chrome, etc.), the proper coolant, wheel speed, rotational and traverse speeds for the rod, and correct contact pressure. Damage to the preplate steel or plated chromium may result if the grinding parameters are not right.