Automated Polishing: Commonly Asked Questions about Peening

Peening itself is not a new treatment, but that doesn’t mean it is fully understood. Peening  is the process of working a metal's surface to improve its material properties, using mechanical means, such as hammering, blasting with shots (shot peening) or blasting of light beams (laser peening). Here are some commonly asked questions regarding the process.


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Peening is not a new process. The ancient Egyptians practiced it to improve the strength and durability of their chest armor and spear tips. They understood that a cold-working process produced better results than heat treating their metals. According to the contributors of our January 2018 feature, “Machine Hammer Peening Automates Mold Polishing,” today this process uses automation to hammer and control each point of impact.

An electronically-controlled hammer head produces machine-hammer peening, and a CNC machine or a robot controls the head positioning, executing CAM software that is programmed specifically for peening applications.


Here, Thomas Pickhardt and Brian Guild, share some frequently asked questions about peening:

Does automated peening require the best CNC machine tool? For most applications, peening is a forgiving process and it is not necessary to use the best cutting machines that are available. Projects with required high-grade surface finishes, such as a grade A diamond finish for a lens mold, are an exception.

What are the consumable costs? While a striker might be considered a consumable item, it does not wear out. Strikers last for a very long time, except when peening very hard tungsten particles into surfaces.

Are robots suited for automated peening? For many applications, robots are well-suited for peening. CAM software will support robots with up to seven axes. Customized manipulating devices can also be tailored for simple or dedicated operations.

Is extra machine capacity for peening necessary? Machining methods are often modified prior to peening where significant milling or turning time is reduced. In some cases, machine time can be cut in half or more before peening operations begin.

How does peening affect the machine spindle? The energy transfer from peening hammers clamped onto spindles does not cause any detrimental effects to the machine spindle. Also, moldmakers may be able to adapt customized clamping fixtures where spindles are not engaged.

Can you peen any material? Moldmakers can peen any ferrous or non-ferrous metals.

Does peening change surface dimensions? Machine hammer peening does not affect surface geometries or dimensions. The peening process flattens cutter cusp peaks into the valleys, maintaining dimensional uniformity. On the other hand, hand stoning and polishing removes high-point materials to the low levels, changing geometric and dimensional accuracies.

How much will hardness increase? A mechanical increase in surface hardness is dependent upon the material being peened and any pre-treatments. Hardness increases can range from 3 to 20 HRC points, and almost always to a depth of 1.5 millimeters on a decreasing gradient. Residual compressive stress can be induced to depths up to 20 millimeters.

How fine of a finish can moldmakers achieve with peening? The highest grades of diamond-buff finish are possible.

How does peening affect mold texturing? Peened surfaces respond well to acid texturing processes.

Can peening create a surface texture? It is possible to peen-stamp a random texture on a mold surface, which eliminates any secondary texturing processes.

How does peening affect coatings? Peening produces a superior surface platform for coating adherence.

What is the purpose of the grease on the surface? A lubricant on the workpiece surface reduces friction. Machine-way oil works well.

How loud is peening? Noise levels depend on the material, the workholding method and the machine tool. Rough milling or turning operations are often louder than peening. Ear plugs are recommended while the system is in operation.