Machining Cost Calculator (Cycle Time)

The first surprise in any honest cycle time breakdown is how little of it involves metal being removed. On a typical prismatic part in a machining centre, cutting is often a quarter to a half of the cycle, and the rest is rapid moves, tool changes, probing, dwell, and the operator opening the door.

This has a direct consequence for where effort is worth spending. If cutting is 25% of the cycle, then making every cut instantaneous - which is not available - would improve the cycle by 25%. Meanwhile reducing the tool count by three, or fixturing two parts instead of one, can easily do better than that and costs nothing per part thereafter.

It is not an argument against getting feeds and speeds right; wrong ones break tools and scrap parts, which are expensive in their own way. It is an argument against treating cutting parameters as the main lever when they are usually the smaller one.

In 1907 Frederick Taylor published the relationship that still governs this: cutting speed times tool life to some power is a constant. V x T^n = C. The exponent n depends on the tool material and is small - about 0.125 for high speed steel, 0.25 for carbide, 0.4 to 0.5 for ceramics.

A small exponent means a brutal trade. With carbide at n = 0.25, doubling the cutting speed divides tool life by two to the fourth power - a factor of sixteen. Even a modest 20% speed increase roughly halves the life. The relationship runs the other way too: slowing down 20% roughly doubles the life, which is why a struggling operation often improves dramatically from a small reduction.

This is also why the "just run it faster" instinct is dangerous with carbide specifically and much safer with high speed steel. At n = 0.125 the same doubling of speed costs a factor of 256 in life - high speed steel simply cannot be pushed, which is part of why it was displaced. But within its band it is far more forgiving of a wrong guess.

Going faster saves machine time and costs tool changes. Both are money, so somewhere the two curves cross and there is a speed that minimises cost per part.

The classical result is neat: the economic tool life depends only on the Taylor exponent, the time a tool change takes, and the cost of a cutting edge expressed in minutes of shop rate. T = (1/n - 1) x (change time + tool cost / rate). The part being made does not appear in it at all. For carbide at n = 0.25 that is three times the sum of the change time and the edge cost in minutes, which for typical numbers lands somewhere between 20 and 80 minutes of tool life.

Most shops run slower than that, and the reason is psychological rather than economic. A tool that wears out in fifteen minutes is visible, annoying and feels wasteful. A cycle that runs three minutes longer than it needs to is invisible. The cost of the second is usually larger.

The saving grace is that the cost curve near the optimum is flat. Being 10% off the economic speed costs almost nothing, which means precision here is not required - only avoiding being badly wrong in either direction.

Setup is a fixed cost per batch, so its contribution per part is inversely proportional to the batch. That single fact dominates cost estimating for anything below a few hundred pieces.

A part with a two minute cycle and an hour of setup costs about three and a half dollars each in hundreds, and about twenty dollars each in fives. The machining is identical. Nothing about feeds, speeds, tooling or programming moved; only the divisor did.

It follows that for short runs, effort spent on setup reduction beats effort spent on cycle time by a wide margin - soft jaws that repeat, a fixture that locates without indicating, tool assemblies left preset. And it follows that quoting a short run at a long run's per-part price is one of the more reliable ways to lose money.