CNC Technology a Presentation

CNC Practicum 2

Milling Machine Tapers

               This is an attempt to clarify some of the murkiness surrounding Milling Machine Tooling Tapers. Given the number of terms used to describe Milling Machine Tapers, it may be surprising that there are really just a few tapers in widespread use. To make it easy for you, many manual milling machines use "R-8" tooling, but CNC or larger machines usually will have some other toolholder taper. One of the things to recognize is that a toolholder consists of 2 sections separated by the gage line. The taper fits into the spindle allowing the machine spindle to transfer rotary motion to the tool which is held in the toolholder portion. The toolholder section ... holds the tool.

So a toolholder has a 2 part specification, taper and tool. For instance, an NMTB #30 collet chuck and an NMTB 1/2" endmill holder would fit the same machine, but might not hold the same tool. Conversely, the same NMTB 1/2" endmill holder and a Universal Kwik-Switch 200 1/2" endmill holder will hold the same tool, but WILL NOT fit into the same machine. If you look at the drawings below, you'll see only the taper portion of the toolholder, not the toolholder portion, since we are dealing Milling Machine Tapers, not toolholders. But since you were nice enough to read this far, here is a link to a toolholder family.

So why are you here reading this page?

1. You have a machine and don't know what kind of taper it uses.
2. You have a toolholder and don't know what kind of taper it is.
3. You don't know anything about toolholders and tapers.
4. You were surfing the NET and stumbled across this page. 

Identify the Taper:

Tapers, like most things in life, can be separated into 2 groups. Those with ears and those without them. If they have ears, they fall into the Kwik-Switch or Rapid-Switch family. If they have a collar or flange instead of ears, they are either single collar or dual collar tapers. The single collar tapers go by lots of names, most notably NMTB or Erickson Quick Change. There are two common dual flange types: V-flange and BT-flange. V-flanges are often referred to as Caterpillar V-flanges. Somebody said: "Caterpillar V-flanges use inch threads for the retention knob and are used to hold inch-dimensioned cutting tools. BT-flanges have metric threads for the retention knob, but their adaptors can be designed to accommodate a wide range of inch-dimensioned cutting tools. BT-flange holders are widely used in Japanese and European made machining centers."

There are some size definitions from the ANSI standard and the relationship of machine size to NMTB taper used. They say "Larger machines use toolholders that have larger shank taper numbers." (Kinda' makes sense in an engineering sort of way.)

Taper Shank No.	Type of Machine
#60	Very large machines
#50	Medium size machines (20 to 50 HPs)
#40	Small size machines
#30	Very small machines

All of the major tapers, except the Universal Kwik-Switch family, use the 3 1/2" in 12" model of the NMTB. Of course, the Europeans claim that this is really 7 in 24, but why argue. While the taper per se may not help much, the mass might. A 3/8" endmill #50 NMTB toolholder is about 9 pounds of serious metal compared to 3/4 of a pound for 1/4" endmill #30 NMTB toolholder. One might almost say "it is dainty" by comparison.

Identify the Machine:

The above assumes that you are holding a chunk of metal in your hand. What do you do if you have a machine with no tooling and would like to get some? (After all, there is no sense in letting a multi-thousand dollar piece of machinery sit idle just because you aren't sure what you've got.) Ask an expert? "What a great idea!" However you need to keep in mind that machine tools are not commodity items produced on high-volume assembly lines. A machinery dealer notes in the year 2000, that

"With more than 350,000 machines sold since 1939, Bridgeport continues to set the standards of machining industry."

Hardly what one would call high volume production compared to automobiles or washing machines. Milling machines seem to much more like Semi-Truck Tractors. You can go to the factory and get it made "any way your heart desires". Unless your expert is live and in-person, take what you hear with a grain of salt. The odds are that he may never have seen a  fill in the blank  just exactly like yours.

Beg, borrow or steal any documentation you can find. While this will get you on the path to enlightenment, it probably won't answer all your questions. Go out and look in the maw of the machine. The spindle will exactly opposite of the toolholder. What is void in the spindle is solid in the toolholder, and vice versa. Take some measurements, make some simple drawings, get more advice. When you do buy toolholders that don't fit your machine, mention them on Rec.Crafts.Metalworking, someone will take them off your hands. Above all, treat this as an adventure in the pursuit of knowledge, because goodness knows,

"This is a not-for-profit operation."

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Single Flange Tapers

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NMTB - National Machine Tool Builders' Association, 1927. (Also known as DIN 2080 / IS 2340 SHANK)
This is the granddaddy of tapers, as evidenced by the numerous names it has. You'll see it called Quick Change, NMTB, MM, National Standard, American Standard Machine Taper, ISOXX, & QCXX... The American Standard Steep Machine Taper (ASA B5.10-1963) defines 12 tapers: #5, #10, #15, #20, #25, #30, #35,#40, #45, #50, #55, #60. Those numbers in bold are the "Preferred Series" while the others are in the "Intermediate Series". ANSI mentions six taper shank sizes including #30, #35, #40, #45, #50, and #60. For all the sizes the taper is 3 1/2" per foot.
	Shank Slot A B C 30 16.1 [.634] 31.75 [1.250] 68.4 [2.75] 1/2-13 40 16.1 [.634] 44.45 [1.750] 93.4 [3.75] 5/8-11 50 25.7 [1.012] 69.85 [2.750] 126.8 [5.12] 1-8

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DUAL FLANGE TAPERS

The following dual flange tapers are alleged to be both different and the same. You'll see the CAT flange referred to as DIN69871 as well as ISO 7388-1, and the BT flange referred to as ISO 7388-1. Does that make the DIN69871 the same as ISO 7388-1? Your guess is as good as mine. They do all have the same taper and they are all about the same size for the same taper number.

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CAT or V-flange
These tapers come in the following sizes: 30, 35, 40, 45, 50 AKA: ANSI B5.50 Caterpillar "V-Flange" standard ISO 7388-1 IS 11173 (TC) DIN69871
	SHANK SIZE [in.] A  GAGE LINE  B  C  D  V30 1.250 1.812 1.875 1/2-13 V40 1.750 2.500 2.687 5/8-11 V45 2.250 3.250 3.250 3/4-10 V50 2.750 3.875 4.000 1-8

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BT Flange
AKA: JMTBA MAS-403 "BT" JIS B 6339 - 1986 JIS B6339 - 1992 ISO 7388/1 - 1983
	MODEL D1 D2 t1 t2 L1 L2 L3 L d1 d2 G BT30 46 38 20 2 48.4 34 24 70.4 31.75 12.5 M12 BT40 63 53 25 2 65.4 70 30 92.4 44.45 17 M16 BT50 100 85 35 3 101.8 90 45 139.8 69.85 25 M24

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DIN 69871-A TAPER
AKA: BT? CAT? V-Flange?
	MODEL D1  D2 t1 t2 L1 L2 L3 d1 d2 M max.  G 30 50 44.30 15.9 3.2 47.8 33.5 24 31.75 13 45 M12 40 63.55 56.25 68.4 42.5 32 44.45 17 50 M16 50 97.5 91.25 101.75 61.5 47 69.85 25 80 M24

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Universal Kwik-Switch

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Universal Kwik-Switch
The Kwik-Switch is supplied as alternative tooling in Bridgeport and other machines. The equivalent to the NMTB #30 is the 200 series which has more mass and a more positive lock. This taper style is also made by Collis who call it Rapid-Switch.
	Kwik-Switch Size A B C E F H J K L M 100 .875 1.44 0.16 0.312 0.250 1.12 1.56 0.375 1.06 0.68 200 1.312 2.50 0.22 0.375 0.312 1.75 2.12 0.500 1.68 0.97 300 1.625 2.94 0.28 0.438 0.375 2.00 2.50 0.500 1.94 1.22 400 2.250 3.62 0.34 0.500 0.438 2.81 3.25 0.625 2.75 1.75

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Other Toolholding Information

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Hollow Shaft Taper (HSK) Tooling
Position before clamping Spindle HSK - Tool	(after) Face and taper contact position Spindle HSK - Tool
This is just thrown in as an added attraction. It is another way to hold a tool tightly onto/into the spindle.

Morse Taper
Also seen as MTx, ie MT3 for a #3 Morse Taper. The taper range is from #0 to #7, and while all have different tapers, they are approximately 5/8" per foot. With or without tang, this is a very common lathe and drill bit taper. With a tang it is the same as DIN 228 Form B.

Brown & Sharpe
Range from 1 to 18, taper is approximately 1/2" per foot. B&S 9 and 11 seem to be the most common. But I have no other information on this taper.

R-8 Also refered to as M1TR taper
This is not truly a taper as defined by Machinery's Handbook, since it is held in place by a drawbar. Picture a long 7/16"NF bolt leading down through the spindle, threading onto the R-8 toolholder.

Jarno
Still gathering information.

Inserted by
Rahul KP

Cutting Tool Considerations For High Speed Machining

Fast CNC processing and high pressure coolant all contribute to removing metal at dramatic rates. But, what should a shop know about cutting tools in high speed applications? By Chris Koepfer, Senior Editor

Metal removal rates are faster today than ever before. What was considered high speed machining just a few years ago is regarded as conventional today. Many factors are driving shops to faster metal cutting rates. These include better and more capable machine tools and CNC processors that allow the machine to accurately cut at increasingly higher speeds and feeds.Commercial considerations are also driving shops toward higher rates of productivity. The need to put more work across machine tools has shops looking constantly to improve metalcutting processes. While much of the discussion about high speed machining tends to focus on the role played by the machine tool, the cutter is its partner in high speed machining. And, that's the focus of this article. We're going to look at what a shop needs to know about specifying cutting tools for their high speed applications. To get perspective on tooling considerations for high speed machining, we contacted Kennametal (Raleigh, North Carolina) to discuss high speed cutting tools. What's High Speed? A generic definition of high speed machining is elusive. High speed is relative. What's very fast for one industry segment seems glacial to another. Machining speed is very application specific. Calling a machining process high speed draws a comparison between its current and previous performance capabilities. For example, high speed might mean changing from an HSS tool to solid carbide, which allows you to bump up machine feeds and speeds. Because carbide cutters, in many applications, can remove metal faster than HSS, a shop using carbide is machining faster compared with HSS rates. But it's relative because another shop, using cermet or ceramic cutters, can cut faster than carbide. So, we're not going to assign a definitive value to high speed. Suffice it to say high speed machining means cutting metal faster that is customary for your operation. Some Basics -- SFM And IPT Elusive as high speed machining is to define, there are measurements used for machining speed. These allow comparison between different rates and help a shop determine its place in the machining speed continuum. To quantify how fast a machine actually cuts metal, spindle rpm needs to be converted into something more useful. According to Dan Spanovich, application specialist for Kennametal, this figure is expressed as surface feet per minute (sfm). Likewise, feed rate for the machine tool is usually measured in inches per minute (ipm). But for cutting tools, it is expressed as inches per tooth (ipt). Working from the optimum sfm and ipt for workpiece material, a machine tool's rpm and ipm can be determined and programmed. These two measurements are dependent on each other to determine the speed at which a workpiece can be maintained. For example, titanium can be cut effectively at about 250 sfm. That's using a chip load of 0.005 ipt. However, some shops report machining titanium at close to 500 sfm, but to do that, lighter chip loads are taken. Depending on the cutter material, chip load and surface speeds can be adjusted to deliver the best combination for a shop's application. If heavy metal removal is the goal, cranking up the chip load and sfm will maximize the cutting efficiency. For better finish, backing off the chip load while keeping sfm up will give good surface finish. There are no specific formulas to determine the best combinations and results. It takes a little experimentation to find optimum feeds and speeds for a specific application. Material Differences A cutting tool material has specific attributes that make it usable in a metalcutting application. Because applications vary so widely, there are many cutting material combinations from which to choose. But in general, only two performance criteria are used to determine the applicability of a cutter. These are toughness or resistance to fracture (ductility) and thermal hardness (resistance to heat). A myriad combination of coatings, substrates and base materials can be created to deliver specific proportions of toughness and thermal hardness to fit various applications. Cutting tool materials can be classified into five general categories. The materials are arranged from best toughness characteristics to best thermal hardness: HSS, Tungsten carbide (uncoated and coated), Cermets, Ceramics, and Diamond and CBN. Starting with HSS and progressing to diamond and CBN coatings, a scale can be built progressively from best toughness characteristics to best thermal hardness. High speed steels take a pounding but can't take much heat. Ceramics and diamond coated cutters can take the heat but fracture easily. Generally, tungsten carbide cutters have a working range of 100 to 1200 sfm, according to Mr. Spanovich (HSS goes up to approximately 100 sfm). Ceramics, including silicon nitride, push the envelope up to 4,000 sfm. Polycrystalline diamond and CBN coated tools push sfm above 4,000. These rates are at chip loads of 0.003 to 0.030 ipt. These rates represent optimum cutting potential for the right combination of workpiece material and cutter material. However, there are other factors that must be considered before a shop can hope to approach these kinds of cutting speeds. Hold It The importance of rigid fixturing cannot be overemphasized in high speed machining applications. While the goal of any fixturing or clamping setup is to hold a workpiece securely and allow for repeatable location of subsequent parts, high speed requirements magnify any imperfections in a workholding setup. In high speed machining applications, the fixture should support the workpiece on a solid base and have enough mass to help damp cutter-induced vibrations. Fixtures for high speed need not be overly complex but should follow good shop practice. For example, a good vise is adequate if it supports the workpiece securely. It is recommended that positive stops be used to prevent torquing or movement of the workpiece in response to cutter motion. The Force Is With You Proper selection of a cutting tool, especially an indexable cutter that is rated to spin at elevated speeds, is important. Not to put too fine a point on it, but we're talking potentially serious or even fatal accidents if a shop tries to exceed tooling speed ratings. The reason is simplecentrifugal force. For the same reason tire manufacturers have speed ratings for radials, tooling manufacturers put a "not to exceed" rpm on cutters. The force created by rotating a body is exponential to the speed of rotation. That force is trying to rip the inserts away from their seats. Any part of a cutter flying off at 10,000, 15,000 or 20,000 rpm poses a risk to life and limb. Indexable insert tools for high rpm are different than tools for conventional rotating speeds. Inserts are secured differently to the cutter body for high speed indexable tools. According to Mr. Spanovich, a simple setscrew clamp is not adequate for high rotation. Inserts are secured to the cutter body with a pin that fits into a detent molded in the insert. It is anchored on the cutter body in a direction perpendicular to the centrifugal forces generated by rotation. Cutting Dry At elevated cutting speeds and feeds, coolant may be less necessary than at conventional speeds. Heat is the by-product of machining. Generally it's the enemy of metalworkers. Increasingly however, heat is being used to help the cutting process. In an ideal cut, workpiece material, machine feeds, spindle speeds and cutter are all making their respective contributions in optimum fashion. As the cutter creates a chip, the heat generated by that action is transferred to the chip. When the chip breaks and leaves the cutting zone, the heat is carried away with it. A big advantage of high speed machining is that at elevated rates of speed and feed, the chip is cut and evacuated so fast it tends to transfer little or no heat to the green workpiece. At conventional machining speeds, there is time for heat to move from chip to uncut metal and create a work-hardening condition. This increases the force needed to create a chip, which creates more heat, and on it goes. Coolant mitigates the cycle by reducing the temperature in the cut zone and flushing away the chips. But at very high rpm, the tool rotation throws coolant away from the cut zone so without very high pressure or through-the-tool piping, it never reaches the cutting zone. "In some cases," says Mr. Spanovich, "trapped chips can remain in the cut, allowing them to be recut by the tool. We've found an air blast is very efficient for evacuating chips in high speed applications." Thermal shock is another consideration for users of high speed tools--especially ceramic and harder cutting edges. Irregular distribution of coolant in the cut can create an unstable heat zone for these cutters. Designed to operate at elevated temperatures, the cutter material can undergo successive heat and chill cycles in the cutting zone that can create premature failure from thermal shock. The Right Angle Cutter speed is the major influence in creating heat at the cutting edge of the tool. Maintaining a high chip load or feed is how heat is dissipated. Correct ipt, combined with the right cutter rake angle for the material being machined, produces a chip of sufficient density to carry heat from the cutting zone so work hardening can be avoided. Chip load is feed rate for each cutting edge of the tool. For indexable insert tools, it's the load against each insert. On solid body cutters, chip load is rated against each tooth. According to Mr. Spanovich, a good working range of chip load is generally between a minimum of 0.003 ipt to a maximum of 0.012 ipt. The angle of attack for the cutter edge, its rake angle, influences the chip load for a cutter. Rake angles vary from positive through neutral to negative. Positive rake angles present a sharper edge to the workpiece. It's also a weaker edge. Positive rake tools tend to pull the workpiece toward them during the cut. They also tend to push chips up and away from the cutting zone. Negative rake tools have a much stronger leading edge and tend to push against the workpiece in the direction of the cutter feed. This geometry is less free cutting than positive rakes and so consumes more horsepower to cut. High speed tooling geometry, in general, mirrors the geometry of conventional machining. "What you know about tool geometry for conventional machining transfers to higher speed applications," says Mr. Spanovich. "If there is a trend in high speed, it is toward a positive lead angle tooling. This lead angle effect allows greater ipt, by lifting the chip, while maintaining the same chip thickness. This greater feed rate results in higher speed machining. "The formation of a sufficiently thick chip is the goal," says Mr. Spanovich. "The idea is to use chips as a heat sink. Faster speeds make more heat, so directing that heat into chips becomes critical in high speed machining applications." Watch For Wobble Successful high speed machining is dependent on static and dynamic rigidity among the many components that bring together the tool and the workpiece. Critical to this is a highly rigid connection between the tool, toolholder and the machine tool spindle. Tool balance becomes a big issue at high spindle speeds. "We recommend smooth shank tools, for end mills and drills, held by a hydraulic or roll-lock collet chuck for high speeds," says Mr. Spanovich. "Balance becomes an issue at 5,000 rpm and up. At those speeds, a notch shank with setscrew can move the tool enough off-center to induce vibration--hence chatter." For speeds of 20,000 rpm and up, a custom balance of tools and toolholder combination is recommended. The V-flange taper connection is a potential source for high speed vibration. Until recently, the V-flange taper and measurement gages used by cutting tool manufacturers were made to ANSI/ASME B5.10 standards. "Until high speed applications came along, the ANSI/ASME standard worked well," says David Lewis, staff engineer for Kennametal and vice chairman of the ANSI/ASME B5 Standard committee. Taper fit between a tool body and the machine spindle can be in tolerance (per ANSI/ASME B5.10) and still cause runout and eccentricity problems for a high speed cutter. Mr. Lewis and others representing U.S. tooling manufacturers recommend application of the European ISO 1947 AT3 standard in place of ANSI/ASME B5.10. The ISO standard has twice the accuracy requirement of ANSI/ASME and results in a better connection between the spindle taper and the V-flange tool. To make sure the tooling you purchase for high speed applications is made to the new standards, specify ISO 1947 AT3 or equivalent from your tooling manufacturer for toolholders and collet chucks. For machine tool spindles, specify ISO 1947 AT2 (a lower AT number means a better fit). Mr. Lewis recommends gaging be acquired to check spindle and tool tapers in the shop. A Word About HSK Much has been written about HSK or equivalent tooling as a possible replacement for the V-flange connection in machining operations. "While there are some advantages to the design concept," says Mr. Lewis, "its widespread application is being held up in part by a lack of manufacturing standards." The primary difference between HSK or other hollow shank, short taper toolholders is the way the tool fits into the machine tool spindle. HSK uses a simultaneous fit between the short taper and the face of the spindle. The connection is very rigid. "The problem with HSK," says Mr. Lewis, "is no governing body has established a standard for tooling companies to manufacture to. There is a German DIN standard that's being considered by ISO but so far it has not been approved. There are also some challenges to HSK from Japan, other European countries and the United States. The question of what HSK will look like is not yet decided. "In the mean time, shops looking to do high speed machining on their machining centers may be better off specifying AT3 or better V-flange tooling than waiting for an HSK standard tooling configuration," says Mr. Lewis. It could be a while before HSK or an equivalent standard for tool, spindle and gages comes along. Why Higher Speed? Implementing higher speed machining in a shop has many benefits some obvious but others less so. Obviously, making parts faster helps satisfy customers' demands for quicker deliveries despite shorter lead times. There are also benefits derived from increased tool life. It may seem paradoxical, but machining at high speed with the right tooling matched to the application can reduce tool wear because of the diminished cutting forces at high speed. High speed machining can help a shop manufacture more accurate parts with better surface finishes. Often, because a machine tool and workpiece setup must be very rigid for high speed machining, the results are more consistent workpieces. A less obvious benefit of high speed machining for shops moving in that direction is derived from the exercise of implementing it. Learning to do the things necessary for successful high speed machining can simultaneously elevate other facets of an enterprise to equivalent levels of productivity. MMS Compiled by Rahul KP

Case Hardening Steel and Metal

Improvement of Tribological Properties Through Nitrocarburizing 

Structure, Hardness and Depth of the Nitrocarburized Layer.-case hardening-

During nitrocarburizing, a two-part surface layer is formed, initially an outer compound layer, followed by a diffusion layer below it. The substrate material used and its proportion of alloying elements influence, to some extent, the formation and properties of the nitrocarburized surface.

Case Hardening Compound Layer

The nitrogen-rich inter-metallic compound layer mainly contains iron-carbonitrides and, depending on the type and proportion of alloying elements in the base material, special nitrides.

Case Hardening : A unique feature of salt bath nitrocarburized layers is the monophase _-Fe_N compound layer, with a nitrogen content of 6-9% and a carbon content of around 1%. Compared with double phase nitride layers which have lower nitrogen concentrations, the monophase _-Fe_N layer is more ductile and gives better wear and corrosion resistance by improvement with case hardening. In metallographic analysis the compound layer is clearly definable fron the diffusion layer as a lightly etched layer. A porous area develops in the outer zone of the compound layer. The case hardness of the compound layer measured on a cross-section is around 700 HV for unalloyed steels and up to about 1600 HV on high chromium steels. Treatment durations of 1-2 hours usually yield compound layers about 10-20 _m thick (0.0004 - 0.0008"). The higher the alloy content, the thinner the layer for the same treatment cycle. Fig. 2 shows the relationship of layer thickness to treatment time with nitrocarburizing temperature of 580�C (1057�F).

Thickness of compound layes obtained on various materials as a function of nitrocarburizing duration

Case Hardening : Diffusion Layer

The nitrogen penetration into the diffusion layer provides for improved fatigue strength. Depending on the initial structure and composition of the core material, the nitrogen in the diffusion layer is dissolved in the iron lattice and/or precipitated as very fine nitrides.

Influence of chromium on diffusion layer hardness and total nitration depth in various 0.40-0.45% carbon steels

Case Hardening With unalloyed steels, the nitrogen is dissolved in the iron lattice. Due to the diminishing solubility of nitrogen in iron during slow cooling, _'-Fe4N nitrides are precipitated in the outer region of the diffusion layer, some in form of needles, which are visible in the structure under the microscope. If cooling is done quickly, the nitrogen remains in super-saturated solution. With alloyed steels which contain nitride-forming elements, the formation of stable nitrides or carbonitrides takes place in the diffusion layer independent of the cooling speed. With increasing alloy content of the steel, the diffusion layer is thinner for identical nitrocarburizing parameters. However, with their higher level of nitride-forming alloying elements these steels have a greater case hardness. Fig. 3 illustrates the influence of chromium on the hardness and depth of the diffusion layer in steels with a carbon content of 0.40 - 0.45% after 90 minutes treatment at 580�C (1075�F). Total nitrocarburizing depth shown in Fig. 4 is the distance to the point where the hardness of the nitride layer is equal to the core hardness. After a 90 minute treatment the total nitrided depth is about 1.0 mm (0.040") on unalloyed steel, but barely 0.2 mm (0.008") on a 12% Cr steel. (See Fig. 4.)

Total nitrided depth on various materials resulting from nitrocarburizing

Fig. 7 shows the coefficient of friction both under dry conditions and after lubrication with SAE 30 oil, measured by an Amsler machine. All samples were lapped to a roughness of R_ = 1_m after their respective surface treatments and before testing. Without lubrication the nitrocarburized QP had the lowest coefficient of friction, being less than half of that of the hard chrome or case hardened surfaces. The lowest friction level occurred when nitrocarburized QPQ is lubricated. It is 3-4 times lower than that achieved with the chrome or martensitic surfaces.

Coefficient of friction values for various surface layers, with and without lubrication. 

Case Hardening SNC = salt bath nitrocarburized

These results show the direct effect of increased oxidation as it relates to friction on the surface of the nitrocarburized samples. The QPQ sample, with its extra post-oxidation step, has a much higher friction value than the QP specimen, which had part of its original oxidation in the compound layer removed by lapping. However, with this variant, due to the fine microporosity in the QPQ sample which causes the lubrication to adhere better to the surface, this option gives the lowest friction value.

If a uniform running behavior is required the QP process is appropriate. Lubrication has only a slight influence on the coefficient of friction because the oxide layer of the outer surface was removed during the polishing operation.

It has been determined that, unlike with chrome surfaces, the coefficient of friction of nitrocarburized QP and QPQ treated surfaces remains constant, even at varying sliding speeds.

The intermetallic stricture of the compound layer, which contains epsilon iron nitride formed during nitrocarburizing, is extremely resistant to adhesive wear and scuffing. Fig. 8 shows the scuffing loads of gears made from various materials (6). It was established by applying increasing pressure to the flank tooth until galling occurred. Austenitic steel containing 18% chromium and 8% nickel had the lowest resistance to galling, however, after nitrocarburizing its resistance was raised almost five-fold. The performance with SAE 5134 was about tripled. Even SAE 5116, which had already been carburized, more than doubled the scuffing load it could withstand through the compound layer built by the nitrocarburizing treatment.

Scuffing load limit of gears.
SNC = salt bath nitrocarburized

New Fastener Doubles as Crack Sensor

Alcoa focuses on proprietary aircraft fasteners for composite metal and carbon structures.

Doug Smock, Contributing Editor, Materials & Assembly -- Design News, March 30, 2011

Working with Stanford researchers, Alcoa is developing aircraft fasteners that also function as sensors capable of detecting crack propagation in multilayer composite structures.

The technology could reduce inspection frequencies for wing stringers by one-half. Fatigue cracks forming at fastener holes are a common form of airframe damage.

In the invention, a fastener couples layers of a multi-layer structure together via an opening that traverses the structure. A sensor circuit is inserted into the opening with the fastener, inducing an electrical response in a portion of the multi-layer structure adjacent to the opening. If the structure surrounding the fastener hole is damaged, the electrical response is slowed, indicating a failure.

A new fastener can sense crack propagation in composite aircraft structures. Source: Alcoa

In one example of the technology, a sensor film is embedded on the shank of an aircraft fastener, such as a 1.5 inch shank fastener from Alcoa Fastening Systems. An eddy current is applied to the sensor. The sensor's circuit is established by a coating applied to the fastener and the conformable film.

Alcoa told Design News that the specific materials' technology is proprietary.

The sensor circuit includes an active conductor to induce the electrical response, and a passive conductor to sense the induced electrical response. The active and passive conductors are wound around an outer diameter of the mechanical coupler to form an alternating winding pattern of active and passive conductor lines.

"When you plug this in, you can see if there is a crack and if it has propagated," says Bill Christopher, executive vice president of Alcoa.

Stanford University developed the structural health monitoring (SHM) technology under a research grant sponsored by Alcoa.

Alcoa's SHM system can be used for aluminum aircraft structures as well as hybrid structures that combine carbon fiber-reinforced composite and aluminum. For example, the SHM system can be applied to the joint between aluminum ribs and carbon fiber reinforced wing skins.

Pre-production prototypes of Alcoa's SHM system are currently being tested with select customers for commercial applications.  Alcoa plans to complete comprehensive testing with select customers before SHM reaches full production.

The new fastener is an example of a focus on aircraft assembly technology for Alcoa since it acquired fastener specialist Huck in 2000. In 2002, Alcoa acquired Fairchild's fastener business and formed Alcoa Fastening Systems. Other acquisitions followed, and Alcoa is now the world's largest producer of aircraft fasteners. Alcoa is ramping up fastener production capability in China and other rapidly developing countries.

"Our fasteners aren't the nuts and bolts you buy at Lowes or Home Depot," Christopher told analysts in New York last month. "To give you one example, we have developed a one-inch diameter titanium fastener used on the 787 and A350 that can support the weight of 50 Toyota Camrys."

Alcoa's competitive strategy focuses on design engineering.

"When composites were starting to emerge, we made the decision to be the industry leader in joining dissimilar materials," says Christopher. "One issue that we knew would come up was lightning strike. When you drill through metal, you get a nice hole. With composites, it's serrated."

Voids created by uncut fibers or resin are referred to as machining-induced micro texture. They can trap excess sealant, inhibiting close electrical contact between the fastener and the composite structure. Machining-induced micro texture is associated with arcing between the fastener and the composite structure during lightning strike tests.

Lightning protection of composite structure is more complex because of the high resistance of carbon fibers and epoxy, the multi-layer construction and the anisotropic nature of the structure.

Inherent conductivity of metallic fasteners coupled with the large number of fasteners used in planes creates a high probability of lightning damage on fasteners.

"So we had to develop a sleeved fastener that allows you to have a perfectly close hole," says Christopher.

Conforming fasteners decrease the voltage drop across the interface and reduce the dielectric effect caused by the sealant, minimizing the possibility of arcing between the sleeve and the composite panel. 

Alcoa also developed the Ergo-Tech next-generation fastening system that can be installed by a single person or robotic system instead of two people. The key feature is advanced low-torque installation tooling that reduces strain on installers, making it more compatible with robotic systems, and reducing installation time and cost.

More than ninety percent of Alcoa's assembly systems are specialty structural fasteners and 55 percent of them are either patented or proprietary.