Undercut
Undercut turns up in 4 fields: thermal spray, welding, gears, geosynthetics. Here is what it means to the people in each one.
Undercut in thermal spray
A recess deliberately machined below finished size so the coating can fill it to full thickness.
Groove or step turned or ground into a part before coating, so the sprayed layer reaches the finished dimension with the required thickness and its edges sit protected inside shoulders. The shoulders are chamfered, usually around 30 to 45 degrees, rather than square, to avoid edge chipping. Undercut depth equals the finished coating thickness plus any allowance for repair of worn metal.
"Undercut the journal 0.015 per side with 45 degree shoulders, we'll put 0.025 on and grind back."
Undercut in welding
A notch the arc melts into the base metal beside the weld toe that the weld metal never fills back in.
Notch along the toe where the arc melted base metal that weld metal did not refill, usually from too much current, too long an arc, a poor angle or too fast travel. It thins the section and creates a sharp stress raiser, so codes limit its depth, commonly to about 1/32 inch on statically loaded structures and less on cyclically loaded work. Repairs usually add a small stringer bead along the toe.
"Inspector flagged undercut on the top toe of every horizontal fillet, drop your amps and slow your travel."
Undercut in gears
A notch cut into the base of small-pinion teeth when the generating tool removes part of the working involute.
On pinions with few teeth, the tip of a generating rack or hob cuts into the lower flank and removes part of the involute it just formed, leaving a weak, waisted root. With a 20 degree standard full-depth rack it starts below about 17 teeth. Undercut shortens the active profile, cuts contact ratio and weakens the tooth, and is avoided by positive profile shift, a higher pressure angle or more teeth.
"Ten-tooth pinion with no shift, of course it's undercut, you can see the waist at the bottom of every tooth."
Undercut in geosynthetics
Digging out soft or unsuitable subgrade below grade and replacing it with better fill.
When a subgrade will not carry equipment or fails a proof roll, the usual fix is to undercut a set depth and backfill with stone or select fill. Geosynthetics are often used to cut the undercut depth or avoid it, by laying a woven or grid on the soft soil and building the platform over it. Undercut quantities are a frequent change-order fight, so they are measured and logged.
"Engineer wants three feet of undercut, but with grid and fabric we can bridge it with eighteen inches of rock."