Tool Life Testing

To compare two grades or parameters fairly, you need a defined end-of-life wear criterion and controlled trials — otherwise "it felt better" is not data. This page sets up a simple, repeatable tool-life test.

Concept

Tool life is the cutting time (or number of parts) until the tool reaches a defined wear limit. A test fixes everything except the one variable being compared (grade, speed, feed), measures the wear land at intervals, and records how long it takes to reach the limit. The Taylor relation captures that higher speed shortens life, but a fair trial is what proves it in your shop.

Why It Matters

Without a defined criterion, two people call "dull" at different points, and comparisons are meaningless. A controlled test lets you justify buying a more expensive grade, or prove that lowering speed actually extends life — with numbers, not opinion.

How It Works

1. Fix the cut: same material, machine, holder, setup, speed and feed.
2. Define the end: e.g. flank wear land reaches 0.3 mm, or finish fails.
3. Run the trial to the limit, recording time/parts.
4. Change only the test variable (grade or one parameter) and repeat.
5. Compare life under identical conditions.

Use the printable wear trial log to record each run.

Worked Example

Compare grades A and B in the same steel turning cut, same speed and feed. Grade A reaches 0.3 mm flank wear after 22 minutes; grade B after 34 minutes. Everything else was held constant, so the ~55% life advantage is attributable to the grade — worth evaluating even at higher cost. (Cost comparison is module M7.)

Running a valid trial

A tool-life trial only means something if everything that could change is held still: same machine, holder, workpiece material, ap and ae, coolant and setup. Then vary one thing — usually speed — and run until the insert reaches a pre-defined wear limit (page 25), recording time or parts to failure. Repeat for at least two or three edges at each speed so an outlier edge does not mislead you. The useful output is a small table: speed vs life, from which you can trade productivity against tool cost (page 29). Do not draw conclusions from a single insert, and do not compare two grades run on different days — the setup itself has changed.

What the life numbers tell you

Read the two results as engineering trade, not as a race. At the locked trial, grade A lasts 22 minutes and grade B lasts 34 — B is about 12/22 = 55% longer for the same cut. Before spending more on B, pair that with the cost-per-part math on page 29: if B's price premium is smaller than the machine downtime and edge-change savings its extra life buys, it wins; if B costs far more than the life saving justifies, A stays. The same logic applies to speed: if raising vc cuts life from 34 to 22 minutes but shortens cycle time enough to make more parts per shift, the faster cut may still win on cost — or may not. The trial gives you the speed–life points; the decision is made by converting them into parts and cost, which is why this page hands off to page 29 rather than declaring the longest-life tool automatically best.

Why you repeat each condition

A single edge can fail early for a reason the test does not control — a soft spot in the bar, a chip jam, a seat that was not fully clean — and that one outlier will falsely condemn a good grade. Running two or three edges at the same speed and grade smooths that scatter: if the first B edge lasts 34 minutes but the second lasts 21, you do not have a clean 55% advantage, you have variation that a third edge must resolve before you buy. This is the practical meaning of "repeat": the trial is comparing distributions, not single lucky runs, and the wear-trial log exists so you can see the spread at a glance rather than remembering one number. When the repeats cluster tightly, the speed–life point is trustworthy; when they scatter widely, the setup or incoming material is not stable enough to justify any grade decision yet — which is itself a finding, and usually points back to machine rigidity or material consistency (pages 21–22) before it points at the tool.

Why the wear criterion must match the part, not just the edge

There is a subtle choice hidden in "stop at 0.3 mm flank wear": the end-of-life limit should be set by what actually makes the part scrap, not by a number from a textbook. For a roughing pass where the finish barely matters, life can legitimately run to a larger wear land before the edge breaks. For a finishing bore held to a tight tolerance, the tool may be "worn out" long before 0.3 mm — because as the flank land grows it pushes the part off size and degrades the finish, and the bore drifts out of tolerance first. This is why the test criterion is chosen from the drawing: if the job is judged on size and surface, the end point is "size out of tolerance" or "finish fails," measured on the part, even though you also track the wear land to explain why. Running a trial to a 0.3 mm land on a precision bore would overstate tool life and approve a tool that has already scrapped its last few parts. Define the failure the shop actually experiences, then measure the wear that causes it — the wear land is the explanatory signal, the part result is the real criterion.

Why even a "failed" trial is still useful

A trial that does not clearly favour one grade is not a wasted run. If grades A and B both scatter between 21 and 34 minutes and overlap heavily, that negative result is itself actionable: it tells you the grade difference is small relative to the shop's normal variation, so buying the premium grade is not justified on life alone — spend elsewhere. If a speed increase barely changes life, the cut is not heat-limited at that speed and you have headroom to raise productivity. If life collapses the moment you increase vc, you have located the edge of the working window and should not push past it. The Taylor curve on which this page rests is not a single data point but a slope, and you only learn the slope by deliberately running a couple of speeds and watching how fast life drops. Treat every run — even the inconclusive ones — as adding a point to that picture, and record it on the trial log. The worst outcome is not a trial that fails to pick a winner; it is making the decision with no trial at all, which is where "it felt better" and an unjustified price premium come from.

Why you measure wear at intervals, not only at the end

If you run the edge until it finally breaks and then record the time, you have learned almost nothing useful. Measuring the wear land at intervals — say after every fixed number of parts — gives you the growth curve: a gentle, steady flank land that accelerates near the end is normal gradual wear, and the test can be stopped at the defined criterion before the edge collapses. A land that grows with a sudden step, or a crater that deepens between readings, reveals the dominant mechanism and warns you the edge is about to fail catastrophically (page 26). It also lets you compare two grades on the shape of wear, not just the final number: one tool may last 22 minutes but wear steadily, another may look durable then break with little warning, and for an unattended machine the steady, predictable wear is preferable even at the same average life. The interval readings are what turn a single "how long did it last" into an understanding of how the edge approaches its limit, and they feed the diagnosis on page 27. This is exactly what the wear-trial log is designed to capture — several points, not one.

Why the fastest cut is rarely the cheapest

The Taylor relation this page rests on says higher speed shortens life sharply — a modest speed increase can halve the edge life — but a new operator often reasons the other way: "run faster, finish the part sooner." The trial exists to expose the hidden cost. Running faster raises the metal-removal rate and shortens cycle time, which saves machine and labour cost per part, but it also burns through inserts faster and triggers more tool changes, each costing a few minutes of spindle-down time and a fresh edge. The optimum is not at maximum speed (too many changes) nor at minimum speed (cycle too long) but somewhere in between, where the sum of tool cost plus machine cost per part is lowest. This is exactly the cost-per-part curve on page 29, and it is why a life trial that only records "which speed lasts longest" is incomplete — you must also record the cycle time, so the slower, longer-life cut can be weighed against the faster, shorter-life one on cost rather than on durability alone. The shop decision almost never picks the edge that lasts longest; it picks the speed that puts the cheapest good part on the bench, which the trial makes visible.

Why a classroom trial cannot replace the catalog on day one

It is worth being clear about what a shop trial can and cannot prove. A simple comparison of two grades on one machine, one material and one cut gives you a trustworthy answer for that condition — but it does not reproduce the manufacturer's catalog data, which is built from many controlled trials under standard conditions. On day one of a new material, the catalog is still the right starting point for vc and fz, because it encodes years of results across machines and setups; your own trial refines that number to your specific machine, coolant and workholding, rather than replacing it. This is why the page frames the trial as refining a starting point, not as discovering the cutting speed from scratch. A shop that runs a quick trial, records the life at the catalog speed, then adjusts once or twice, lands on a faster and more durable setting than one that either trusts the catalog blindly or guesses from experience. The catalog and the trial work in sequence: catalog starts you in the window, trial proves and tunes it for your shop.

Why record the machine and setup, not just the life

The wear-trial log captures more than minutes to failure because a life number is meaningless without the conditions that produced it. A 34-minute result on a rigid machine with through-coolant and a short overhang does not transfer to a floppy knee mill with flood coolant and a long bar, and if you record only the time, the next person who reads the log will apply a number that no longer holds. This is why the log asks for machine, holder, overhang, coolant, ap, ae and material condition alongside the wear reading: it makes the trial repeatable on your own floor and lets you tell when a life change came from the tool rather than from a setup that quietly drifted. It also feeds page 30's tool management: a grade that proves itself on one machine may not transfer to another, and the recorded conditions are what tell you whether it can. A life number without its setup is an anecdote; with its setup, it is a datum you can build on.

A tool-life test only means something if the conditions are controlled. One variable changed at a time, a fixed feed and depth, and recorded VB values let you compare grades honestly. A "feels faster" trial that mixes speed, feed and fixturing cannot be repeated, and the shop ends up with a parameter set that happened to work once.

Common Mistakes

• Changing more than one variable. The result is confounded. Fix: isolate the test variable.
• No written wear criterion. Comparisons drift. Fix: define the end-of-life measure.
• One trial only. Variation. Fix: repeat and average.

A valid trial fixes material, feed, depth, holder and coolant while varying one speed, and records both VB and part size. Run to a defined wear criterion rather than until the edge looks bad, and repeat at least a few edges so scatter — not one lucky insert — sets the life number.

Practice