Why Does a Rigid Tap Sometimes Gall in 316 Stainless at a Speed and Feed That Worked Fine in 304?

Because 316 is gummier than 304, not just a slightly different flavor of the same alloy. The extra molybdenum that gives 316 its corrosion resistance also makes it stickier against a cutting edge. It work-hardens readily and has a stronger tendency to weld itself to the tool under pressure and heat. A speed and feed dialed in for 304 can sit right at the edge of what 316 tolerates.

What galling actually is

Galling isn't wear in the normal sense. It's adhesion: material from the workpiece welds onto the tap's flanks under pressure and heat, then tears as the tap keeps turning.

Once a little material has stuck, the tap's effective geometry changes, friction goes up, more material sticks, and the failure runs away fast. It often looks sudden because it is. The tap is fine for most of the hole and then seizes.

304 and 316 have similar hardness on paper, which is why the same numbers "should" work. What differs is the alloy's tendency to work-harden at the shear zone and its lower thermal conductivity relative to something like mild steel. Heat builds up right at the cutting edge instead of dissipating into the chip, and that's exactly the condition galling needs.

What actually helps

Run slower surface speed than you'd use in 304. Keep axial feed consistent to get clean chip formation. Use a tap coating suited to stainless, TiCN or a stainless-specific coating rather than general-purpose TiN.

Cutting fluid choice matters more than people expect. A sulfur-based or extreme-pressure tapping fluid applied at the point of cut does more for 316 than flood coolant washing over the general area. Peck tapping to clear chips and break the heat buildup helps in a blind hole where evacuation is the limiting factor.

A tap that's slightly worn is a bigger liability in 316 than in 304. An edge that's marginal in 304 might still cut. The same edge in 316 is often the difference between a clean thread and a galled one.

The practical takeaway

Don't treat 316 as 304 with better corrosion resistance and everything else the same. Back off surface speed, confirm the coating and fluid suit stainless specifically, and don't push a tap that's due for replacement into a 316 job just because it still cuts fine in easier material.

DigiForge machines both 316 and 304 stainless, and the process difference between them shows up exactly here, in tapping, threading, anything with sustained tool-to-material contact under pressure.