Milling Operation

Milling

The Product Realization Lab has several different types of milling machines, including manual Bridgeports, 2-axis CNC Anilam Crusaders, 3-axis HAAS vertical machining centers, and the 3-axis Roland.  Each of them has specific guidelines for how it can be used, but this section documents the fundamentals that are common to all of them.

General Mill Information

Standard Mill Tools:

Endmills

·        Both the side and the end of an endmill can cut

·        They are available in different styles

-         2 flute

-         3 flute

-         4 flute

-         6 flute

-         Ball end, for cutting rounds and doing 3D CNC surfacing

-         Corner-rounding, for rounding corners

·        End mills are available in different lengths

-         Stub, a good idea when using a small diameter cutter if the workpiece is not very thick

-         Standard

-         Long, for reaching into deep cavities

-         Extra long, for reaching into extra deep cavities

You should always use the shortest endmill available that will cut the geometry you need.  Long and extra-long end mills may leave a relatively poor surface finish because they are prone to chatter and can break easily.   Ask a TA for advice when a long end mill seems necessary.

·        Available endmill sizes depend on style and length

-         2 flute standard end mills are manufactured in 1/64" increments down to 1/16" diameter

-         Ball end mills available in  1/32" increments down to 1/16" dia

-         Because small end mills break so easily, the machine shop will typically stock down to 5/16" diameter endmills.  We sell 1/4", 3/16”, 1/8”, 3/32”, and 1/16” in standard and long lengths.

·        Endmills are either center cutting or non-center cutting.  Beore plunging into a part with an end mill, check the end teeth to see if it is center cutting.  If the teeth meet at the center, it is OK to plunge.  If they don't meet, don't plunge with it.

Slitting Saws

·        Used for general slotting or cut-off operations

·        Available down to  0.006"  thick (thin)

·        Because slitting saws break easily, the machine shop has a limited stock.  Find out what's available.  Get assistance before cutting.

Single Lip Cutters

·        Custom made cutters for special applications:  in our shop they are typically ground for putting draft angles onto casting patterns

·        If you need something specific, you may need to make your own, so ask the staff for advice and refer to the Single Lip Cutter Grinder document in the TA Handbook.

Fly Cutters

·        Used for taking light face cuts on large surfaces

·        Max depth of cut depends on material being cut

steel : don't flycut

aluminum, brass : 0.025" max

plastic : 0.010" - 0.050"  (be careful with plexiglass, the edges  will chip with too heavy a cut)

·        CAUTION- A spinning flycutter is often difficult to see.  Keep your hands well away from the workpiece and tool when performing flycutting operations.

Drilling, Reaming, Boring

When drilling or reaming holes on the mill, straight-shank tools with diameters less than 1/2" are mounted in a mill drill chuck.  Large straight-shank tools mount directly into collets.  Large tapered drill bits are mounted into tapered sockets. 

NOTE: Do not mount taper shank drills in chucks. Chucks cannot hold tapered shanks securely.

NOTE: Do not mount end mills in drill chucks.  Drill chucks cannot withstand the forces induced on an end mill in use.

Center Drilling

·        A center drill is used for starting holes to assure accurate location.  Center drills are short and stubby to maximize their stiffness and keep them from deflecting under load.

·        Cut to depth illustrated - halfway up the larger taper.  This creates a funnel to guide the drill bit accurately.

Drilling

·        Twist drill sizes less than 1/2"  diameter are called out by

Fraction:  in 1/64" increments

Number:  to fill in the spaces between fractional sizes <1/4"

Letter:  to fill in the spaces from 3/16" - 7/16"

Metric:  ask about availability

·        Our drill chucks will hold twist drills  <1/2".

·        The chuck should grab onto the drill's shank, not its flutes.

·        Larger drill bits are available in fractions at  1/32" increments

·        Most of the larger bits have tapers that fit into tapered sockets.

Measuring Depth

·        Hole depth is usually measured from the surface of the part to the shoulder made by the drill bit, as shown at right.

·      When designing parts with blind holes, keep in mind the length of the tip of the drill bit.  Allow adequate wall thickness so the tip does not poke through the bottom of the part.  In depth critical applications, it makes sense to measure to the tip.

·      Twist drills do not make flat-bottomed holes.

Drilling to Depth

·        Set the quill stop to allow adequate spindle travel to drill hole.                     

·        Lower quill to the stop and lock in place.

·        Raise table until the shoulder of the drill bit just enters the part as illustrated.  This is the zero point.

·        Raise the quill to retract the drill bit.

·        Raise the table the distance equal to the depth of the hole to be drilled.

·        Now, using the quill, drill into the part.  Periodically, back the drill bit out to clear chips.

·        When you hit the quill stop, the shoulder of the drill bit is at the desired depth.

Making Large or More Accurate Holes

·      When drilling large holes, you will need to start with a smaller drill bit and gradually increase to larger drill bits.

·      For more accurately sized holes, you will want to drill the hole undersized and finish with a reamer.  Ask the TA for assistance.

·      Use a boring bar mounted in the boring head to enlarge or true an existing hole or to make an accurately sized hole that is not available with our twist drills or reamers (in-between sizes or >1")

                                             

Using an Edge Finder

An edge finder is used to locate the machined edge of a workpiece.

The edge finder is made of a shank with a floating tip retained by an internal spring.

·        Mount edge finder in collet or drill chuck

·        Set spindle speed to  >1000 RPM

·        Slide tip off-center

·        Turn spindle on.  Raise the knee or lower the quill to bring the edge finder tip adjacent to the workpiece.

·        SLOWLY move along the x-axis or y-axis to make contact between the edge finder tip and the edge of the workpiece.

·        Continue to slowly advance the workpiece toward the edge finder tip.  As soon as the tip jumps sideways, STOP feed or you will break the edge finder.

·        The spindle is now positioned 1/2 tip diameter off of the edge of the workpiece.  (diameter = 0.200", so offset = 0.100”)

·        Lower the workpiece or raise the quill until the tip of the edge finder is above the workpiece.

·        Now advance the table 1/2 tip diameter  (0.100") farther to bring the spindle directly over the edge of the part.  Set the DRO to zero.

  

Tapping

Taps break very easily and are expensive, so read on ...

Tapping is used to thread a drilled hole.  The tap drill size is found on the chart near the drill index.

·      To accurately locate a hole to be tapped, start with a center drill.

·      Next, drill the pilot hole with the correct tap drill.

·      Tapping is done by hand with the power OFF.

·      Put tap in tap handle.

·      To align tap... Do not move workpiece in  X or Y axes after drilling.  Use a dead center in the milling machine spindle to align tap with pilot hole as illustrated.  Taps break easily if bending is applied.  Proper alignment eliminates bending.

·      Use cutting oil or tapping fluid on metals, and kerosene on plexiglas

·      Turn tap handle by hand 3-4 turns while applying light pressure into the hole.

·      Now, after each 1/2 turn, back the tap out of the hole to break the chip.  Continue advancing the tap until you feel the bottom of the part, or until the through hole has been tapped.  CAREFUL... stop when you feel the bottom of the hole or the tap will break.

IMPORTANT - Use a light touch.  It is VERY difficult  (impossible or expensive)  to remove a  broken tap.  Often a part must be re-started due to a broken tap.

Chatter

Chatter is the rattle or vibration between a workpiece and a tool due to the lack of rigid support for the workpiece or tool.  You may hear screeching or notice a wavy surface finish on the workpiece.

To Prevent Chatter

·        Check spindle speed.  When there is a lot of line contact between the tool and workpiece, keep the spindle speed low.        

·        Provide more support of the workpiece near the cut to reduce vibration of the workpiece.

If these suggestions don't help, ask a TA for assistance.

Other Workholding Devices

You can often hold parts in the vise on the mill, but some projects require other fixturing methods.

Strap Clamps

·        Use at least two strap clamps to hold a workpiece.

·        The parts of a strapclamp are pictured at right

·        To prevent the strap from marring the surface of the workpiece, use a small piece of scrap material, such as plexiglas, as a pressure pad at the point of contact.

·        To get maximum clamping force, the flanged nut should be tightened on the strap as near to the workpiece as possible.

·        Parts should be completely disassembled when you are finished.  If a nut gets stuck, ask for help with removing it.

Backing Sheets 

·        Use a backing sheet whenever there is the slightest chance of cutting through the workpiece and into a fixture or (heaven forbid) the milling machine table.

·        Use a piece of plexiglass or other scrap material.

·        Do not use wood.  Wood chips gum up the slides on the machines.  Wood will also continue to compress when clamped allowing a "loose" set up.

·        When drilling, make sure the backing sheet is thicker than the length of the pointed tip of the drill bit.

·        Set the quill stop to insure that you will not drill through the backing sheet and into the table.

·        DO NOT MACHINE INTO THE TABLE !!!!

Double stick tape

·        Double stick tape works only on thin, plastic sheet stock with lots of surface area to tape to.

·        Use double stick tape when other work holding devices get in the way of machining operations.

·        Do not use cutting oil or kerosene when fixturing with double stick tape as it may weaken the adhesive bond.

·        If you are using a backing sheet, tape your workpiece solid  (no gaps between the strips of tape), and "set" the tape by squishing the pieces together in a vise.  Whenever possible, use an oversized backing sheet and clamp the backing sheet to the table.

·        If you must tape to the table ...

-         Clean the table or fixture with solvent and clean paper towels to remove oil.

-         Apply tape to the workpiece

-         Mount workpiece and backing sheet onto the table or fixture.

-         To set the tape, clamp workpiece to table using a bar or thick plate and strap clamps.

-         Remove clamps and start machining.

-         When you are finished machining, removing your part from the backing sheet will be difficult.  Release it with ethanol, heat it with the heat gun, or get assistance.

Collet Block

·        Used to hold round parts in a collet (<1" diameter) during milling operations

·        4-sided and 6-sided blocks are available

·        Clean outside of collet and inside of collet block before assembly

·        Hold collet block in milling machine vise while tightening collar

How to change a milling machine vise

·        Do not change a vise or other workholding device without first getting approval AND assistance from the TA.

·        The table and vise can be damaged if not treated properly.

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.