Table of Contents

Evaluating Process Machinability

The Availability Chart can be used to evaluate tool breakage risk during machining.

Tool breakage occurs when any of the following exceeds 100% and persists for longer than one simulation step: Yielding Stress Ratio [YieldingStressRatio (API)], Max Spindle Torque Ratio [MaxSpindleTorqueRatio (API)], or Max Spindle Power Ratio [MaxSpindlePowerRatio (API)]. If the value significantly exceeds 100% — roughly speaking, above 200% — tool breakage can occur even without sustained duration. Max Breakage Ratio [MaxBreakageRatio (API)] is the largest of the three that are known on a step, so it exceeds 100% whenever any of them does.

Note: The default simulation uses per-revolution milling mode, where one simulation step equals one spindle revolution.

Yielding Stress Ratio is the largest principal stress the step's cutting forces cause in the tool body, divided by the strength of the tool body's material. HiNC treats the cutter as a round cantilever beam, gripped in the holder with its tip free: a section is bent only by the forces between it and the tip, and a force above a section passes through the tool above it into the holder without bending that section. HiNC does not keep how a step's force is spread along the cutting edge, so it takes the step field Cutting Force Height From Tool Tip CuttingForceHeightFromToolTip_mm(API), the centroid of the engaged cutting edge, as the dividing line: a section below that height is left out, and at each section at or above it, along the flute and the exposed shank, HiNC takes the bending moment of all the step's cutting forces about that section and the stress this causes at the section's diameter. For a section above the top of the flute, where all the force lies below it, that moment is exact. The tip zone (a section closer to the tip than its own radius) is skipped as well; the largest of these stresses is the numerator. The step's absolute force MaxAbsForce_N(API) is therefore not the numerator: the same force gives a higher ratio on a slimmer tool or a longer stick-out, which is why the ratio reflects the tool's actual safety margin better than a force limit. The denominator is the strength of the tool body's material — the flute material of a solid-end cutter, the shank material of an insert-end cutter (the body its inserts sit on) — taken as its yield strength, or as its tensile strength where the material has no yield strength, as carbide has none. When no section can be evaluated — not a milling cutter, no tool-body material or one without a strength value, or no section outside the tip zone at or above the cutting force height — the ratio is not computed rather than reported as zero. The optimization target force OptPreferedForce_N(API) represents the desired MaxAbsForce_N after optimization.

For small tools, breakage is typically caused by insufficient tool force capacity, and breakage is based on the Yielding Stress Ratio.

For small-to-medium tools, breakage is typically caused by insufficient spindle performance, and breakage is based on the Max Spindle Torque Ratio or Max Spindle Power Ratio. When cutting resistance exceeds spindle performance, the machine feed rate remains constant but the spindle speed continuously decreases, causing the feed per tooth to continuously increase, which drives cutting forces to spike until tool breakage or machine stoppage. For the underlying boundary curves and how each ratio's denominator is computed at the current rpm, see Spindle Capability.

Note

Tool Breakage Solutions

Modify the toolpath to reduce cutting width/depth, or use HiNC's optimization feature to adjust feed rates, bringing the Yielding Stress Ratio, Max Spindle Torque Ratio, and Max Spindle Power Ratio below 100%.

Ripple on Curved and Tilted Cuts (Mesh Quantization)

On a nominally constant-engagement cut that is curved or tilted — arc or helical hole milling, ramping, or 5-axis moves — the Yielding Stress Ratio (and the underlying MaxAbsForce_N(API)) can show a small step-to-step ripple even though the theoretical engagement is steady. This is a discretization artifact, not a bug.

Cause. The workpiece is represented by an axis-aligned cubic voxel mesh (see Workpiece Entity Resolution). Orthogonal cubes cannot represent a circle or an inclined face smoothly, so the removed volume and the contact engagement area quantize against the grid from step to step. That quantization shows up as ripple in the per-step peak force, and therefore in the ratio.

What helps. A finer MachiningResolution_mm makes the quantization step smaller, so the ripple amplitude shrinks — at the cost of slower geometry removal (see CPU Usage During Simulation).

What does not help. Switching to fixed-pace / sweeping motion resolution (FixedPace) does not remove this ripple. The ripple comes from the spatial cubic grid, not from the spacing between steps, so changing the step spacing leaves it essentially unchanged.

Getting a smooth curve. There is no built-in filter that smooths step-series curves. If a smooth curve is needed for a report, post-process the exported CSV (WriteStepFiles(API)) yourself — e.g. a moving average. Note that the Yielding Stress Ratio is defined on the per-revolution peak force only (there is no averaged variant); the averaged force fields (Avg…) are inherently smoother when a trend, rather than the breakage peak, is what you need to read.

Thermal Plastic Deformation of Cutting Edge

If the Thermal Yield Ratio [ThermalYieldRatio (API)] exceeds 100%, thermal plastic deformation of the cutting edge occurs, accelerating wear. Unlike the Yielding Stress Ratio, Max Spindle Torque Ratio, and Max Spindle Power Ratio, this is a long-term indicator — exceeding 100% does not cause immediate effects.

Note

Thermal Plastic Deformation Solutions

After addressing tool breakage issues, reduce the spindle speed to allow sufficient time for the cutting edge to dissipate heat.

Note that whether the coolant is properly directed at the cutting edge has a significant impact. If the coolant is not aimed at the cutting edge, it effectively reduces the heat dissipation coefficient.

Tool manufacturers typically provide recommended machining conditions, and the Thermal Yield Ratio obtained by simulating under those conditions usually exceeds 100%. This is because the manufacturing formulations of tool materials differ from HiNC's conservatively set default values. If you consider machining conditions with a Thermal Yield Ratio above 100% to be reasonable, you can adjust the thermal properties of the tool material so that the calculated Thermal Yield Ratio approaches 100%.

See Also