"I'm sure [Kantus] is right, and I'm also sure that I'm right. Both statements are true."
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Now, if you feel like you just fell down a flight of steps built by M.C. Escher, it's just because what Kinney said is pure paradox. It's simply not possible for forging to be progressing while at the same time not changing.
Kinney says it is. Kantus says it's not. Who should you believe?
Both.
A Heated Debate
To even begin to dissect this paradoxical problem, it would be of great use to first understand the basics.
Here's a quick crash course:
production. The [forged] form is made as close as possible to the finish contour of the part to reduce machining time and to keep the amount of wasted material low. Forging tolerances are [commonly] held within ±0.030" (0.8 mm) and machining stock allowance is in the range of 0.080 to 0.120 per side. The surface finish of forged surfaces [for steel parts] is around 500 micro-inches.
"The advantages of forgings include: [shortened] machining times, as the raw material is shaped close to the final contour of the part and excess material can usually be removed in a single [operation], thus less machining and nonfunctional surfaces do not require machining."
That's per gear blanking expert Robert Endoy — it's accurate, informative and completely up-to-date.
And it was published in an issue of Gear Technology in . Seems impossible that almost 25 years have gone by and key data remains totally untouched, but here we are. Of course, none of this is to say staying the course is a bad thing. Some great things remain steadfast and constant through the years, stubbornly defiant in the face of Father Time himself.
There can be elegance in simplicity, after all. And, by most accounts, creating forgings is just that.
"The process looks very simple," Kantus said of Ajax's method of making rolled rings. "We start out with bar material and we cut it to a weight, and then we forge it into a pre-form so it looks like a donut. That then goes on a rolling mill where you can roll it out to the required dimensions. Then most of them go to heat treat and then machining.
Shawn O'Brien, vice president of sales and marketing at McInnes Rolled Rings (Erie, PA) also peppered the word "simple" into his explanation of McInnes' forging process.
"We keep our process simple by focusing exclusively on seamless rolled ring forgings with rectangular cross sections," O'Brien says. "We have three distinct ring manufacturing areas.
Each area has dedicated furnaces, blanking presses and material handling equipment located and optimally sized to provide the mills with hot metal in the most efficient manner possible." Rolled rings, which both Ajax and McInnes specialize in, are a unique type of forging that offer a distinct set of pros and cons when compared to products made from die forging, the process that Clifford-Jacobs uses.
forging method is best," O'Brien says. "Open die forging covers the broadest size range but typically requires most gross input weight and processing time. Impression die methods are best suited for high volume, near net shape orders that will likely repeat. However, the initial time and expense of creating dies make this a cost prohibitive method for low volume, short lived programs."
Clifford-Jacobs Forging (Champaign, IL), a company owned by IMT that produces steel forgings for the mining, gear, aerospace, energy, and defense industries, normally makes its forgings from special bar-quality steel. As steel bars are rolled, the grain structure within the steel is forced to flow along the centerline of the bar. When a standard or custom forging is produced from the bar, this inherent grain flow bends to follow the contour of the forged shape.
"In some cases, we actually use pre-forms," Kinney says. "We produce more near net products than, in most cases, a rolled ring could produce. We actually could use a rolled ring as a preform for what we do. That's really what, in a sense, differentiates us. [IMT] has a lot of companies, but Clifford-Jacobs is probably our premier company on the gear side."
O'Brien adds:
"For gears or bearings with a basic OD × ID × height geometry,
seamless rolled rings provide the best of all forging options,"
he says. "There are a wide variety of ring mills in terms of size
and control technology. The latest in ring rolling technology
enables quick set-up times, minimal stock allowances and size
consistency from ring to ring."
So, as O'Brien clearly states, the process for making forgings hasn't changed in — wait what?
The latest in ring rolling technology.
Somewhere at Château d'Amboise da Vinci is rolling over in his grave.
Forging Ahead
To truly understand the forgings paradox you must first realize that preservation and change aren't always mutually exclusive.
It is possible, for instance, to make alterations to a car without completely transforming the car into an entirely new entity (though detractors of the "Ship of Theseus" thought experiment would argue differently, but that's a paradox for a different day).
Such is the case with creating forgings — the process is fundamentally the same, there have just been a few recent upgrades.
"While forging as a process remains quite similar to what was done generations ago, the application of engineered innovations such as direct-from-forge-heat-treating makes it considerably more efficient and effective today," Kinney says.
Forgers and ring rollers aren't trying to reinvent the wheel, so to speak. They're simply trying to make the wheel easier to produce and easier to use.
"There have been a lot of innovations in the press and forging market to use modeling and simulation to give you better results," Kinney says. "We're really immersed in that process — giving you a better idea of the grain flow and what the final near net part is going to look like.
"Specific to Clifford-Jacobs, we are also involved in development work with SCRA (South Carolina Research Authority) to produce what we call direct forge intensively quenched parts that we believe will be a game-changing approach to the forging and heat treating process. It's a development project that's underway, but our goal is to better integrate forging and heat treating to produce superior parts. We're really excited about that."
For companies that specialize in rolled rings, like McInnes and Ajax, innovation has come in the form of newer machinery and more advanced, intuitive software. "It's like comparing a car being manufactured a decade ago to now," O'Brien says. "You didn't have XM Radio but you had radio — is it really different if the music is still coming out? "A lot of the difference is user interface and how much more you're able to rely on technology. That's not to say the operator's experience isn't valuable, but if you combine an experienced operator with the newest technology, then the horsepower behind it is that much greater.
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"The hydraulics, the frame upon which they sit, the method of forging, that all remains the same. It's kind of the ‘sheet music' that's different — the software, the computerized rolling methods, et cetera. The newer the machine the latest you have in terms of that.
"Newer is better in terms of control, user interface and maintenance diagnostics."
For gear manufacturers, the main cause for concern isn't necessarily how easy it is to make the forgings — nor should it be — but in the quality of the forgings once they're made.
And the good news for buyers of forgings and rolled rings is that besides being more quickly and efficiently made than ever before from a supplier's perspective, they're also produced at a higher quality, and more consistently so.
"The percentage of rings that are within your nominal size range are going to be better over time," O'Brien says. "If it's a manual controlled mill, you're going to have inconsistencies from piece to piece. Within a single job, a computer controlled mill is going to be putting out the same piece consistently.
"I think [the difference is] repeatability, consistency, and you're going have tighter tolerances and less allowance."
And with that, the paradox that began tangentially with da Vinci hundreds of years ago becomes less of a riddle and more of minor problem solved by good old fashioned persistence and ingenuity.
Now if we can just get da Vinci to tell us where that darn Holy Grail is.
Rolled-ring forging is a specialized forging process used to create seamless rings of different sizes and profiles. These forgings are widely used in aerospace, automotive, wind, bearing, and other industries, where a strong seamless ring is needed.
Rolled rings can vary tremendously in size, from small rings of a few inches in diameter, to those with diameters more than 25 feet in diameter. Weights can vary from a pound to many tons (Figure 1).
Ring rolling is applicable to a large variety of materials, depending upon application. Commonly used materials for ring rolling include:
The initial material billet is heated in a furnace between 1,000-1,200°C depending upon the material. Aluminum alloys are forged at much lower temperatures (375-500°C). Superalloys are heated to 1,100-1,350°C, depending upon alloy grade, generally above the carbide and γ’ solvi. Proper temperature control is important to achieve the desired grain size and minimize grain growth.
Before rolling can begin, the heated billet undergoes an initial shaping process. The preform is positioned between the radial roll and the mandrel. A controlled force is applied to the preform, reducing thickness and initiating radial expansion. The billet is upset (compressed) to the desired thickness and achieves a uniform shape.
In this process, a hole is created in the center of the billet using a punch press or a mandrel press. This transforms the billet into a doughnut-shaped preform, which serves as the starting material for the rolling process. Preform shaping is critical as it determines the initial distribution of material and ensures optimal deformation during the rolling phase.
The pierced preform is often placed back into the furnace to again heat the preform to the proper forging temperature for rolling.
The heart of the rolled ring forging process is the ring rolling operation, where the preformed doughnut is expanded into a seamless ring. This process is carried out using a ring rolling mill, which consists of:
As the preform rotates, the idler roll continuously applies force to expand the ring. The mandrel prevents inward collapse, ensuring uniform thickness distribution. The material undergoes grain refinement, improving mechanical properties such as tensile strength and fatigue resistance.
The axial rolls apply controlled force along the height of the ring to maintain specified dimensions. This stage ensures the final ring does not exhibit excessive width variations or defects.
Grain flow is carefully directed to align circumferentially, which enhances resistance to stress and fatigue. The forging process eliminates porosity and refines the microstructure, significantly improving durability.
As the ring grows in diameter, the idler roll increases pressure on the inside of the ring. In many ways, this is like a potter working in clay, to produce a round vase on a potter’s wheel. Instead of the potter’s finger, the idler roll moves the material outward while the ring is spinning. Like the potter’s hand on the outside of the vase, the drive roll spins the ring and provides back pressure to ensure the proper outer diameter.
This entire process is very quick, taking only a few seconds or minutes. Generally, once the pierced preform is removed from the furnace, the ring rolling process can be accomplished quickly, with internal friction maintaining the temperature throughout the ring rolling process.
A schematic of the ring rolling process is shown in Figure 2. Depending on the specific ring, the rolled ring is heat treated and machined to the final dimensions.
Versatility in Available Materials
Rolled rings can be made from any material that can be forged. This includes carbon steels, alloy steels, titanium, aluminum, and superalloys. Another advantage is the flexibility in size. Rolled rings can be made with a diameter of one inch to diameters of many feet. Wall thickness can also be widely varied depending on material and ring diameter.
Enhanced Metallurgical Properties
During the ring rolling process, the deformation of the preform is radial compression and axial extension. The grain is arranged continuously around the circumference, with no seam. This contributes to enhanced fatigue and creep strength, uniform grain size, and excellent internal quality.
Energy and Material Efficiency
With ring rolling, there is complete use of the material. There are no flash or draft angles to contend with. The outer and inner diameters are close to the desired dimensions, so minimal machining is required. Raw material is conserved.
Ring rolling is cost-effective. It can be highly automated and is a rapid process. The energy consumption is low compared to die forging.
The contact areas between the tool and workpiece are small, but the amount of deformation is large. This means that the required energy or force required to achieve the desired shape is much less than a die press of similar capability. Much lower pressures are required to achieve the final product.
Rolled-ring forging is an interesting forging process that combines precision, strength, and material efficiency.
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