Built-up edge (BUE)

Mitsubishi Materials: “A built-up edge is a piece of work hardened material that adheres to the top of the cutting edge.” It protrudes from the edge and “actually machines the workpiece”. The textbook slides of Manufacturing Engineering Technology describe it as “layers of material from the workpiece that are deposited on the tool tip”, and treat it as a chip type of its own, as does the chip types page.

How it forms

Heat, pressure and friction weld work material to the tool. Harvey Performance: “When chips adhere to the cutting tool due to high heat, pressure, and friction.”; a Turkish study of aluminium alloys names the temperature and very high pressure at the tool, which depend especially on the cutting speed, as the cause (Sekmen, Günay and Şeker). Mitsubishi adds “High affinity between a tool grade and the workpiece material.” The lump does not stay: “As it grows larger, the BUE becomes unstable and eventually breaks apart” (Pearson slides), and it “periodically breaks off and reforms” (Stephenson and Agapiou). Fragments can end up on the new surface (a label on Groover’s figure in İTÜ’s course slides; PMPA: “As the BUE sloughs off the tool, it can weld to the workpiece”).

Cutting speed

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When it forms

• Sticky, ductile materials. ISCAR lists “Machining of very sticky materials such as low-carbon steel, stainless steels, and aluminum”; Machining Doctor names low-carbon steel, austenitic stainless steel and aluminium, and Sandvik Coromant’s Turkish brochure low-carbon steels, aluminium and duplex stainless steels. PMPA adds alloys that work-harden, and notes that resulfurized steels usually have a stable BUE, because their manganese sulphides have “anti-weld properties”.
• A cutting zone that is too cold. Mitsubishi’s cause is “Low cutting temperatures under the recrystallization temperature of a workpiece material.”; ISCAR’s “Cutting zone temperature is too low”. No source read for this page puts a number of m/min or °C on it. Not every source points the same way: PMPA advises reducing speed with HSS tools, Sandvik Coromant writes that in duplex stainless both too low and too high temperatures are the root cause, and in the Turkish turning tests AA7075 showed less BUE at 600 m/min than at 400 m/min while AA2011 showed no clear change.
• Negative geometry. ISCAR names “Negative cutting geometry” as a cause; in the Turkish tests rake angles of −5°, 0° and 5° gave more built-up material than 10° to 20°, where the sticking was mostly a thin layer (Sekmen, Günay and Şeker). The angles themselves are on the rake, clearance and wedge angles page.

What it does

• Surface finish. “The built-up edge has the greatest influence on surface finish” (Pearson slides). In the Turkish aluminium tests, Ra fell by about 12 % (AA2011) and 16 % (AA7075) on average when the rake angle went from −5°…5° to 10°…20°; the authors put this down to less built-up material.
• Size. Mitsubishi: it generally causes a greater depth of cut and so inaccurate parts; Harvey: “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”; PMPA speaks of losing statistical capability in dimensional control.
• The tool. When it breaks away it can take tool material with it and leave “a pit on the cutting edge” (Machining Doctor); Mitsubishi names chipping of the edge. A 2004 study of turning medium- and high-strength steels at low cutting speed starts from the point that BUE “increases the wear of the tool”; with uncoated carbide tools it observed more wear and premature fracture.
• Not always harmful. PMPA writes that a thin, stable BUE can mean less wear, lower power and a better finish, and Mitsubishi that chip welding forming a BUE can delay tool wear; most other sources treat it as damage (MIT’s slides ask why it can be a good thing, without an answer).

Harvey’s way to spot it: material on the rake face or flank of the tool, which may be visible without a loupe.

Remedies

What the makers recommend, with the points where they disagree:

ISCAR also has the oil percentage of the coolant checked, and Harvey asks for a higher concentration; how that is measured is on the coolant concentration page, and why the cutting speed falls towards the bar centre is on parting off and grooving.