Another option: Use a bake-hardening steel grade with 210-MPa minimum yield strength, which will have added formability, allowing the use of tighter die radii and the ability to hold the panel tighter without splitting. These actions lead to more stretching, increasing the amount of work hardening.

Flank wear

Built-up edge can be caused from running a tool at incorrect cutting parameters. Usually, when BUE is an issue, it’s due to the speed or feed rates being too low. Heat generation is key during any machining application – while too much heat can impact a part material, too little can cause the tool to be less effective at efficiently removing chips.

Serrated chips

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Take this a step further: Transitioning from draw beads to lock beads holds the panel even tighter, possibly allowing the use of mild steel while still achieving the same strength in the formed part. In the example highlighted in the table, stretching mild steel by just a few percentage points results in more than a 60-percent increase in yield strength as compared to the strength of the incoming flat sheet.

Crater wear

Forming forces exceeding a material’s yield strength create the plastic deformation necessary to produce an engineered stamping. As deformation continues throughout the press stroke, metal alloys strengthen from a process known as strain hardening (or work hardening), which reduces the tendency for localized thinning in highly deformed areas. This leads to the characteristic parabolic shape of a stress-strain curve between the yield and tensile strengths.

For example, consider the task of creating a Class A panel that can withstand 260 MPa before permanent plastic deformation. One approach: Purchase a bake-hardening steel grade with 260-MPa minimum yield strength. However, this grade of steel has relatively low formability, which may limit design flexibility.

This condition can create a lot of problems with your machining operations, such as poor tool life, subpar surface finish, size variations, and many other quality issues. The reason for these issues is that the centerline distance and the tool geometry of the cutting edge are being altered by the material that’s been welded to the rake or flank face of the tool. As the BUE condition worsens, you may experience other types of failures or even catastrophic failure.

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Built up edgechips

Work hardening is characterized by the n-value—related to the slope of stress-strain curve. More formable material grades have higher n-values, meaning that for the same amount of strain created by the part design, more formable grades strengthen to a greater extent.

continuous chip with built-upedge

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what causes built-upedge

A built-up edge is perhaps the easiest mode of tool wear to identify, as it may be visible without the need for a microscope or an eye loupe. The term built-up edge means that the material that you’re machining is being pressure welded to the cutting tool. When inspecting your tool, evidence of a BUE problem is material on the rake face or flank face of the cutting tool.

Even when using a turning tool with correct geometry, you may still experience BUE. As the tool starts to wear and its edge starts to degrade, the material will start building up on the surface of the tool. For this reason, it is very important to inspect the cutting edge of a tool after you have machined a few parts, and then randomly throughout the set tool life. This will help you identify the root cause of any of the failure modes by identifying them early on.

Exploiting this phenomenon, it may be possible to create a lighter-weight high-strength steel from mild steel.  While this may sound too good to be true, achieving this may require a change in forming methods from draw forming to stretch forming.

Forming forces exceeding a material’s yield strength create the plastic deformation necessary to produce an engineered stamping. As deformation continues throughout the press stroke, metal alloys strengthen from a process known as strain hardening (or work hardening), which reduces the tendency for localized thinning in highly deformed areas. This leads to the characteristic parabolic shape of a stress-strain curve between the yield and tensile strengths. Work hardening is characterized by the n-value—related to the slope of stress-strain curve. More formable material grades have higher n-values, meaning that for the same amount of strain created by the part design, more formable grades strengthen to a greater extent. Exploiting this phenomenon, it may be possible to create a lighter-weight high-strength steel from mild steel.  While this may sound too good to be true, achieving this may require a change in forming methods from draw forming to stretch forming.  Estimating the yield strength of a formed panel requires knowledge of the forming strain and the as-received (flat sheet metal) mechanical properties determined from tensile testing. Several techniques measure forming strain, such as manual circle-grid strain analysis, camera-based noncontact analysis methods or even commercial simulation programs. These techniques may show individual strains in each direction. The formed panel yield strength (σf) can be estimated as:  where we determine n (work hardening exponent) and K (strength coefficient) from the true stress-true strain data at strains up to εu, the strain at uniform elongation. εeff is the effective strain, a way to combine the biaxial effects of ε1 (major strain) and ε2 (minor strain) into a single term.   Note: The equations shown here are for steel grades. Estimating the strength of aluminum alloys requires different equations, but the same general concept applies.   For more than 50 years, researchers have been defining terms and methodologies to best describe effective strain. The simplest way is to just define effective strain as the sum of the major and minor strains: Hill developed a more complex relationship, in 1948, using r̅ (or r-bar), the plastic anisotropy ratio. There have been refinements over the years, and this equation suffices here:   This equation provides the tools needed to choose between designing the forming process for higher-strength steel (likely also requiring a higher purchase price) or obtaining the needed strength by forming a panel made from lower-strength steel.

Built up edgein metal cutting

Too slow cutting speeds lead to edge build-up and blunting, while too fast results in quicker insert wear, deformation and poor finish. Tool feed calculator F.

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Built-up chips

This equation provides the tools needed to choose between designing the forming process for higher-strength steel (likely also requiring a higher purchase price) or obtaining the needed strength by forming a panel made from lower-strength steel.

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For more than 50 years, researchers have been defining terms and methodologies to best describe effective strain. The simplest way is to just define effective strain as the sum of the major and minor strains:

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Note: The equations shown here are for steel grades. Estimating the strength of aluminum alloys requires different equations, but the same general concept applies.

Hill developed a more complex relationship, in 1948, using r̅ (or r-bar), the plastic anisotropy ratio. There have been refinements over the years, and this equation suffices here:

Putting more strain into the panel results in another major benefit: Major strain, minor strain and thickness strain are related. In the areas of the panel experiencing a 3-percent by 2-percent stretch, the thickness is nearly 5 percent lower than the starting material. A 5-percent weight reduction and 60-percent strength increase using mild steel sounds like a great way to stretch your steel dollars! MF

Built-Up Edge is oftentimes caused by using a turning tool that does not have the correct geometry for the material being machined. Most notably, when machining a gummy material such as aluminum or titanium, your best bet is to use tooling with extremely sharp cutting edges, free cutting geometry, and a polished flank and rake face. This will not only help you to cut the material swiftly but also to keep it from sticking to the cutting tool.

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where we determine n (work hardening exponent) and K (strength coefficient) from the true stress-true strain data at strains up to εu, the strain at uniform elongation. εeff is the effective strain, a way to combine the biaxial effects of ε1 (major strain) and ε2 (minor strain) into a single term.

Reaching these levels of percent-stretch with bake-hardenable steel leads to a substantially stronger panel, approaching 350 MPa before the paint-bake cycle, which may add another 40 MPa. In-vehicle panel strength after forming and baking correlates with dent resistance, leading engineers to specify bake-hardenable grades on many Class A automotive panels.

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In turning operations, the tool is stationary while the workpiece is rotating in a clamped chuck or a collet holder. Many operations are performed in a lathe, such as facing, drilling, grooving, threading, and cut-off applications. It is imperative to use the proper tool geometry and cutting parameters for the material type that is being machined. If these parameters are not applied correctly in your turning operations, built-up edge (BUE), or many other failure modes, may occur. These failure modes adversely affect the performance of the cutting tool and may lead to an overall scrapped part.

Estimating the yield strength of a formed panel requires knowledge of the forming strain and the as-received (flat sheet metal) mechanical properties determined from tensile testing. Several techniques measure forming strain, such as manual circle-grid strain analysis, camera-based noncontact analysis methods or even commercial simulation programs. These techniques may show individual strains in each direction.