# @stem_antics on Instagram

- **Type:** Video
- **Original URL:** https://www.instagram.com/p/DcclGdWgUe-
- **Gondola URL:** https://gondola.cc/posts/69727804-stem-antics-instagram
- **Thumbnail:** https://img.gondola.cc/tr:w-,h-,fo-auto/postThumbnails/736746252a.jpg
- **Posted:** 2026-08-25T02:37:32.000+00:00
- **Account Owner:** Stem Antics (@stem_antics) — https://gondola.cc/stem_antics

## Caption

There is nothing subtle about the first cut into a rusty forging.

The skin is the enemy. A forged billet leaves the furnace wearing a layer of oxide scale — wustite, magnetite and haematite stacked in that order from the steel outwards — and it is harder than the metal beneath it. Magnetite sits around 5.5 on the Mohs scale, and embedded sand or refractory debris picked up in the forging operation can be harder still. Run a cutting edge along that surface and you are not machining steel, you are lapping tungsten carbide with abrasive grit.

The fix is depth, not caution:

- Set the first pass deeper than the scale layer so the edge engages clean parent metal and the oxide rides up the rake face
- Drop surface speed and raise feed; abrasive wear scales with sliding distance, not chip thickness
- Choose a tough substrate with a thick CVD alumina coating, because Al2O3 is the layer that survives the temperatures roughing generates
- Hone or T-land the edge, since a sharp edge in a forging is a chipped edge

Then there is the interruption. Forged stock is rarely round or concentric, so the tool enters and exits several times per revolution. Each entry is a mechanical shock and each exit a thermal one, and that repeated heating and quenching opens comb cracks, fine fissures running perpendicular to the edge that eventually take the corner with them.

The forces are not trivial either. Specific cutting force for medium carbon steel sits near 2,000 N/mm2. At 8 mm depth of cut and 0.5 mm/rev feed, that is roughly 8 kN at the tool tip, before accounting for an unbalanced forging trying to walk its way out of the chuck. Blue chips are not a warning here; they are proof the heat is leaving in the swarf rather than soaking into the edge.

Rust is only rust until it meets an edge; after that it is a grinding wheel.

#stemantics #machining #materialsscience #manufacturing #engineering

## Stats

- **Views:** 17,205
- **Likes:** 56
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## Tags

machining, engineering, materialsscience, manufacturing, stemantics

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