Cast iron quirks (powdery chips, edge chipping, black scale)

?Likely causes

• Gray cast iron produces fragmenting (powdery) chips, and the vibration when they break tends to chip the cutting edgeOne maker^[1]
• Powdery chips tend to advance rubbing wear (cutting force is not that high)One maker^[1]
• Ductile and malleable cast iron produce long chips and show damage close to that of steel (rake face wear, etc.)One maker^[1]
• The casting skin (black scale) is hard and uneven, and sand inclusions greatly shorten tool lifeAgreed by 2+ makers^[1][2]
• Edge chipping tends to occur at the exitOne maker^[3][1]
• Chips do not connect, so a spiral tap cannot push them out upwardOne maker^[4][5]
• Enlargement allowance is small, and internal threads tend to shrinkOne maker^[6][7]

✓Fixes

Cutting conditions

• For edge chipping, reduce the feed and depth of cut and use a sharp cutting edgeAgreed by 2+ makers^[3][8]

Tool

• Straight flute taps (hand taps) are the basic choice. Depth up to about 1.5 times the nominal diameterOne maker^[4][5]
• For better durability, use a powder HSS or carbide tapOne maker^[5]
• For ductile cast iron, tool materials of the same family as for steel can sometimes limit damageOne maker^[1]
• For sand inclusions and hard surfaces, use a strong edge geometry and reduce the feedAgreed by 2+ makers^[2][1]

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