Sunnen turns honing inside out!

It’s not too often that the metal-cutting industry witnesses the launch of a brand-new machining process. But that’s exactly what the arrival of Sunnen’s HES series of external honing machines signifies. Yes, that’s right: external honing!

This eye-catching new development stems from a customer enquiry for a machine capable of repairing long, large-diameter hydraulic cylinders like those common in cranes and ships – a machine that would overcome traditionally slower and more complex grinding processes. Sunnen stepped up to the task of developing a solution that would offer the precision of honing while approaching the speed of turning in certain applications.

“We knew that external honing would prove much faster at removing material than grinding or belt grinding, especially in harder and exotic materials,” explains Gunter Peersman, Sunnen’s Sales Manager. “In fact, during the first runs with the machine we discovered its high stock removal potential in soft steel applications. Compared with belt grinding, we also learnt that the cost of the abrasive stick consumables used in external homing were much lower than the typical special belts needed to remove such hard or exotic material. In combination with lower labour costs, reduced energy consumption and faster processing time, external honing demonstrates faster ROI [return on investment].”

And there are plenty more benefits besides. Almost no boundaries exist in honing hard, special or exotic materials/coatings, while both geometry and homogeneous surface structure tend to be better. For certain applications, the typical surface structure and cross-hatch of honing represents another advantage as it creates a ‘plateau finish’ not possible with grinding, belt grinding or turning. This helps the oil film or lubricant deliver longer seal life in service, for example.

In good repair
One of the main applications for external honing is the repair and new production of piston rods for hydraulic cylinders, as well as general cylindrical parts for the oil and gas industry and aircraft landing gear. Sunnen’s innovative process typically comprises a simple sequence of stock removal after pre-turning, followed by fine finishing. However, for certain applications, there’s an even more efficient option.

“Pre-turning is quite challenging on long, small diameter shafts,” explains Gunter Peersman. “It requires a lot of operator experience and set-up time. We discovered through testing that the external honing of soft steel with the right stone comes close to the metal removal rate [MRR] of turning. Customers processing this kind of work can therefore place many more jobs on our external honing machine that previously required turning, saving a whole operation and generating more profit.”

Another important application is the de-chroming of used piston rods. Again, external honing is fast. A 350mm diameter shaft measuring 8m long that requires the removal of its electroplated hard chrome layer (around 0.3mm thick) will take a maximum of just 3 hours using a Sunnen HES series machine with its four-stone contact.

“In contrast, grinding will take approximately 9-12 hours because it’s a single-point contact process. Furthermore, the inconsistency of typical chrome layers proves challenging for grinding to achieve complete removal. A dechroming bath takes even longer, around 24 hours, and this processes is getting harder to source due to restrictions on chemicals and their disposal. De-chroming is often linked to a very urgent job in the hydraulic repair business, such as the cylinder of a crane or a ship. Time is of the essence to minimise expensive downtime.”

So are there any drawbacks with external honing? All processes have some shortcomings, right? Well, external honing cannot straighten a part. While the process will correct parallelism and ovality/roundness, if the shaft features a slight bend, the honing abrasives will simply follow. The good news is that this inherent tendency is irrelevant for the main applications of external honing, like repairing a hydraulic piston rod.

For the long haul
The Sunnen HES machine is modular, allowing customers to extend the length at a future date if requirements for longer shafts arrive.

“External honing can deliver MRR of up to 1400cm³ per hour, even more in soft steel with the right abrasive – it really can compete with turning,” says Gunter Peersman. “In practice, 900-1000cm³ per hour is typical among HES machine users to date.”

A single variable hone head reaches the whole honing diameter of 50-450mm. Users also benefit from a quick stone-setting system.

Take control
“The Windows® based control system with its 15-inch screen is icon-based and intuitive,” reveals Gunter Peersman. “In just 2-3 hours, even an operator with no programming skills will know everything about the control. A new set-up is possible within 2 minutes, step-by-step. The system recommends the spindle speed and all necessary cycle parameters to optimise the process and protect the tool. In fact, the system indicates high and low spots, correcting geometry such as waviness and roundness automatically without operator intervention. The control also supports automatic stone wear compensation.”

In terms of shaft support, although the machine has fixed steady rests available, there’s an optional automatic steady rest (synchronised with the hone head) for parts featuring thinner walls or small shafts (<6m long and <150mm diameter). There’s also an integral sludge scraper with continuous evacuation to keep the machine clean, supported by a high-end filter-based centrifugal cleaning system.

Four Sunnen HES series external honing machines are already active in the field. A fifth machine accommodating a maximum part length of 14m (with an option to extend on site to 18m) is shortly on its way to a customer in Turkey, while Sunnen also has an enquiry from Poland involving a landing gear application with trials due to commence soon. Word is clearly starting to spread.