Precision grinder gives hammer mill its ‘impact’ edge

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For hammer mills to reliably function requires extraordinary coordination of all the hammers swinging at extremely high velocity within narrow confines. PHOTO courtesy DCM.

Hammer mills are essential pieces of industrial equipment used to crush or pulverize a wide range of materials such as spices, cosmetics, pharmaceutical powders, rocks, minerals, and recycling waste. Although the design can vary, hammer mills are a steel drum containing a vertical or horizontal rotating shaft or drum on which hammers are mounted. Within the chamber, the hammers swing freely on a rotor that spins at high speeds while material is fed into the hammer mill from a feed hopper. The material is impacted by the hammers and is thereby pulverized and expelled through a screen in the bottom of the mill.

Because the many internal hammers spin at extremely high velocity to pulverize materials, the point of contact of each hammer must be precisely ground to the correct shape and weight with virtually no variation; any excess can not only cause severe abrasion but also throw the carefully calibrated machine dangerously out of balance.

“The tip of the hammer travels about approximately 21,000 feet per minute. That velocity leaves little room for imprecision in the hammers, which must be very precisely ground to certain specifications of size, thickness, and weight,” says Dave Hahn, Lab Manager, Pulva Corporation, a global industry leader in pulverizing machines since 1938.

Hahn points out that there can be dozens or up to a hundred or more hammers in the largest pulverizers. 

To minimize vibration, each of the rotors that the hammers are assembled to are dynamically balanced to an exceptionally tight tolerance. The hammers must also be manufactured to an extremely consistent weight and size. This is critical not only to maintain the balance of all the rapidly swinging hammers but also to provide sufficient clearance within the tight confines of the chamber.

Since the hammers are high wear parts, the tips are hardened to withstand continual impact. However, this requires grinding the “impact” edge to the ideal dimensions for optimal pulverization of the material. 

For hammer mills to reliably function requires extraordinary coordination of all the hammers swinging at extremely high velocity within narrow confines. With great precision similar to a Swiss watch but on an industrial scale, the end result is exceptionally precise pulverization machinery designed to last for many years.

Industrial Scale Precision

With 80 years of experience as a leading supplier of industrial size reduction equipment, Saxonburg, PA-based Pulva provides a wide variety of pulverizers and auxiliary equipment. This includes hammer mills of different sizes and capabilities.

“We offer various hammer mills from a very small lab size with only a few hammers, to our largest, which uses over one hundred hammers. We produce tens of thousands of hammers a year, all of which must be extremely precise,” says Hahn.

The specialized hammers are manufactured by beginning with either forged carbon steel or stainless steel. However, forged parts can have variances in dimensions, surface finish, and material properties. These variations are inherent to the forging process and can result in slight differences between each manufactured piece.

“There is some weight variation in the forged hammers since there are differences in the material used, the heat, and the forging process itself,” says Hahn.

To increase the durability and longevity of the high wear part, Pulva welds a layer of hardened material on the tip, or impact edge. According to Hahn, the thickness of the hardened material welded onto each hammer varies depending on several factors but can generally range between 0.25” to 0.5”.

“Without a hardened surface, the impact would wear the base material, which is not hard enough to withstand what the hammers are put through,” explains Hahn. “So, we weld on an overlay of hardened material at the tip where the impact occurs. However, there is variability in the overlay as well.”

To ensure absolute consistency of size and weight of the hammers installed on new equipment, Pulva has relied on advanced rotary surface grinders from Winona, MN-based DCM Tech, a designer and builder of industrial rotary surface grinders.

“DCM Tech’s rotary surface grinders surface the tip of our hammers down to a specific size, shape, and dimension, which gives us a really good impact edge. The surface grinders take the hardened welded material down to exactly what we need for the hammers,” says Hahn.

Pulva ensures that each hammer possesses uniform weight while adhering to essential internal clearances by grinding the hammers’ base edges, notes Hahn.

“There is a very specific dimension from the tip of that hammer to the inside of our hammer mill. Adequate clearance is required to avoid impact damage to the mill itself, but it also needs to be close enough to provide the fine pulverization needed,” says Hahn.

“We have owned several of DCM Tech’s rotary surface grinders for the last 20 plus years; they work really well and hold exceptionally close tolerances,” adds Hahn.

Still, after 20 years of use, Pulva decided it was time to replace one of them and recently purchased its third rotary surface grinder, a more automated, Robot Ready IG 282 SD with a 24” variable speed table and 20HP variable speed grinding spindle motor. 

The surface grinders are designed with advanced sensors and controls that automatically maintain very tight tolerances, removing material down to within one ten-thousandth of an inch of the final thickness. Digital technology also allows for an interface with easy-to-use touchscreen controls.

The new model also includes advanced features that automate the initial contact between the abrasive wheel and the part, which typically had to be finessed by the operator.  With this updated option, advanced sensor technology detects vibration and can automatically fine-tune not only the pressure of the spindle motor but how quickly it moves the wheel down onto the part.  When the machine senses the abrasive wheel has contacted the part, it automatically begins the grind cycle.

“Automatic part detection eliminates the need for the operator to do time consuming, error prone ‘manual touch offs’ where they would manually feed the grinding machine until it just touches the surface of the part before backing off and restarting it,” says Mike Anderson, a technical specialist at DCM Tech.

The rotary surface grinding process consistently achieves high throughput and eliminates variability, which enables Pulva to achieve high-quality finished hammers with great consistency.

According to Hahn, the advanced rotary surface grinder arrived “robot ready,” allowing Pulva to incorporate robotic elements when needed in the future.

“By adding robotics for the loading and unloading of workpieces, OEMs like Pulva with higher production demands can substantially decrease cycle times while improving precision on unattended machines,” explains Anderson.

Pulva’s Hahn says Anderson travelled to Pennsylvania to oversee the installation and commissioning of the new Robot Ready IG 282 SD rotary surface grinder.

“There’s been no large learning curve and our operator has been very happy with the newest machine, which is performing very well. The equipment does exactly what we need it to do,” concludes Hahn.

Although hammer mills are used by many industries to pulverize materials to the required specifications, the precision required cannot be taken for granted. When the hammers must be meticulously ground to precise weights, dimensions, and surface conditions, advanced rotary surface grinders are a key technology that will be utilized.

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