1. Cutting speed — the effect you cannot feel
Taylor's relation is vc · Tn = C, with n around 0.20–0.30 for carbide. Take n = 0.25 and the sensitivity is uncomfortable:
| Change in cutting speed | Change in tool life |
|---|---|
| +10% | −32% |
| +20% | −59% |
| −20% | +144% |
Why it stays invisible: the edge does not fail at the moment speed changes. It fails hours later, on a different part, and gets attributed to the insert batch. The only reliable check is to compute the actual cutting speed from spindle rpm and diameter rather than trusting the program, because the two disagree whenever the tool has been substituted.
2. Feed — too low is worse than too high
Low feed per revolution means the edge rubs for the first part of each revolution instead of cutting, and the material work-hardens ahead of the next pass. In austenitic stainless and in titanium this is the dominant wear mechanism, and it produces a bright, polished wear land with a built-up edge rather than a crater.
The practical floor is the nose radius. Feed per revolution below roughly half the nose radius is in the rubbing regime for a finishing insert. If the surface finish looks good and the insert dies early, suspect feed before grade.
3. Depth of cut — the nose radius rule again
Depth of cut below half the nose radius concentrates the work at the edge preparation radius, where chip thickness is minimal and the edge is doing more rubbing than shearing. For roughing, keep depth of cut at or above the nose radius.
The other side of the rule is rigidity: depth of cut that is too large for the machine and fixture shows up as chatter, chatter shows up as chipping, and chipping gets blamed on the grade.
4. Stability — the variable nobody measures
Tool overhang, holder type, fixture rigidity and part wall thickness combine into a vibration amplitude that is almost never measured. Two identical inserts on two identical machines can differ three to one in life purely because one holder is 20 mm longer.
A useful test: cut one pass and look at the surface. A regular pattern along the feed direction is a stability signature, not a finish problem. No grade fixes that, and changing grade twice will not either.
5. Coolant — direction and continuity matter more than flow
- Interrupted cuts in cast iron with flood coolant produce thermal cycling and comb cracks perpendicular to the edge. The fix is often less coolant, not more — reduce flood, or switch to air blast, and keep it continuous.
- Stainless benefits from high-pressure coolant through the tool, mainly for chip control: a chip that is re-cut is a second heat cycle on the same edge.
- Titanium should never be cut dry with carbide. Flood is standard.
- Intermittent coolant — on for one pass, off for the next — is worse than no coolant at all, because it adds thermal shock to a cut that was managing without it.
The order to check them
| Symptom on the edge | Most likely variable | What to measure |
|---|---|---|
| Uniform flank wear, life about half expected | Cutting speed | Actual vc from rpm and diameter |
| Bright polished land, built-up edge | Feed too low | Feed per rev against nose radius |
| Chipping along the depth line | Depth of cut | ap against nose radius |
| Regular chatter marks on the part | Stability | Overhang, holder, fixture, wall thickness |
| Comb cracks across the edge | Coolant / thermal cycling | Coolant continuity for the material |
| Crater on the rake face | Speed against coating capability | vc against the coating's rated window |
When the grade genuinely is the answer
- All five variables are inside specification and life is still short — the grade is mismatched to the material condition: cast skin, sand inclusions, forged scale, or a hardened surface layer.
- Interrupted cutting in a tough material — a tougher substrate with a thinner coating.
- Finishing at high speed — harder coating, lower cobalt, sharper edge.
Change one variable at a time, and log cutting speed, feed, depth and edges per insert for twenty edges before drawing a conclusion. Without the log, the conclusion is a guess with a purchase order attached to it.