Table of Contents

Evaluating Process Machinability

The Availability Chart plots the stress ratio, the two spindle ratios, the spindle working temperature ratio and the thermal stress ratio, and can be used to evaluate tool breakage risk during machining and, by the thermal stress ratio, the wear of the cutting edge. The built-in tree Machining State marks a step whose stress ratio reaches 100% as a Stress overload, and a cut whose thermal stress ratio reaches 100% as a Thermal stress overload when no overload and no unevaluated load takes it first (Engineering View Coloring).

A Stress Ratio [StressRatio (API)] below 100% is safe; at or above 100% means bad machining: the stress in the tool body has reached its material's stress line, where a ductile body deforms permanently and a carbide is past the tensile strength its material lists. 100% is the one mark to watch: HiNC computes no number of revolutions a cutter survives above it. Tool breakage also occurs when the Max Spindle Torque Ratio [MaxSpindleTorqueRatio (API)] or the Max Spindle Power Ratio [MaxSpindlePowerRatio (API)] exceeds 100% and persists for longer than one simulation step. If the value significantly exceeds 100% — roughly speaking, above 200% — tool breakage can occur even without sustained duration. HiNC counts that duration for each ratio apart: Spindle Torque Overload Revolutions [SpindleTorqueOverloadRevolutions (API)] and Spindle Power Overload Revolutions [SpindlePowerOverloadRevolutions (API)] are the spindle revolutions of the run of consecutive steps whose torque ratio, or whose power ratio, exceeds 100%.

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

Stress Ratio is the largest principal stress the step's cutting forces cause in the tool body, divided by the stress line 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 outer fibre: a section of the flute takes the area moment of inertia of the cutter's Inner Beam, the flute's bending-equivalent core, with its outer fibre at the flute's own radius, and a section of the neck, the taper or the shank is a solid circle of its own 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 stress line 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): its yield strength (YieldStrength_MPa) where it gives one, the more conservative of its strengths; where it gives none, its tensile strength (TensileStrength_MPa). A carbide, a ceramic, a CBN or a coating has no yield point: its file says so by writing NotDuctile in YieldStrength_MPa, as every shipped cutter material does, the hardened high-speed steels included, and its line is its tensile strength; a material whose yield strength is merely not known leaves the element out and reads its tensile strength too. An insert-end cutter's body is AlloySteel42CrMo unless the cutter names another material: a ductile steel, it gives its yield strength, 930 MPa, and that is its line, not its tensile strength of 1080 MPa. The fracture strength (FractureStrength_MPa) a carbide file also gives — its transverse rupture strength, the outer-fibre stress at which a small test bar of the grade breaks in three-point bending, the median catalogue value of current rod grades of its class — is the line of no ratio: half the test bars break below it, so it is no mark for a cut to stay under. The working margin lies in NC optimization's safety factor: the optimization holds the stress ratio well under 100 % (Tuning Peak Tolerance). 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.

Where on the cutter the ratio sits. Each step also records two sections of the cutter, each as its height above the tool tip and the zone it lies in. Stress Critical Height [StressCriticalHeight_mm (API)] and Stress Critical Zone [StressCriticalZone (API)] place the section whose ratio is the largest: the section the ratio is read at, where the cutter is most likely to break. Stress Onset Height [StressOnsetHeight_mm (API)] and Stress Onset Zone [StressOnsetZone (API)] place the lowest section, counted from the tip up, whose ratio is above 100%: how low on the cutter the overload begins. Where no section is above 100%, the onset height is NaN and the onset zone None; on a step that cuts nothing, or while the ratio is not computed, the critical height and zone read the same. The zone [CutterBeamZone (API)] is one of three: Flute, the core of the cutting part, the top of the flute included; Taper, a non-cutting part of the shank whose radius changes along it; Cylinder, a non-cutting part of one radius, such as a reduced neck or the shank itself. A section where two parts of the shank meet is counted in the part below it, the one it ends, and the top of the flute in the flute. The lowest section of the shank stands at the same height as the top of the flute but has the shank's radius there, and it counts in the part it begins, going up: where it is slim enough to bear more stress than the top of the flute, a critical section at that height is this one, in a taper or a cylinder, and so is an onset there unless the top of the flute is above 100% as well. A critical or onset section in the flute or in the taper points to cutting conditions or an engagement too heavy for the cutter; one in a cylinder points to a stick-out that is too long.

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

The Thermal Stress Ratio [ThermalStressRatio (API)] is the thermal stress over the stress line of the flute material, the line the Stress Ratio divides by: its yield strength where it gives one, else its tensile strength — every shipped cutter material, which has no yield point. The line does not fall with temperature. The thermal stress is ΔT·α·E/(1 − ν): ΔT the flute's temperature under the coating above the ambient, α the flute material's linear thermal expansion coefficient ThermalExpansionCoefficient_dMK in 1e-6/K (µm/(m·K)), E its ElasticModulus_GPa and ν its PoissonRatio — for the shipped WC-Co6-800nm, 6.5 × 10⁻⁶/K, 620 GPa and 0.22, about 5.2 MPa per kelvin of rise. If the Thermal Stress Ratio reaches 100%, thermal plastic deformation of the cutting edge occurs, accelerating wear. Unlike the Stress Ratio, Max Spindle Torque Ratio, and Max Spindle Power Ratio, this is a long-term indicator — reaching 100% does not cause immediate effects. A Thermal Stress Ratio at or above 100% is machining that loses points — a loss of tool life, not a breakage risk as a Stress Ratio at or above 100% is: kept under 100%, the ratio lets the cutter last noticeably longer, and at or above it the wear accelerates. The built-in tree Machining State marks such a cut as a Thermal stress overload, after its overloads (Engineering View Coloring). Outside the entry and exit cuts, such a cut can be optimized, usually by changing the spindle speed by hand (see the note below). In an entry or an exit cut the ratio, as a rule, comes down less: the chip there is unavoidably thin, and a thin chip tends to leave its heat in the cutting edge. How much a given case shows this depends on the edge temperatures the thermal model reaches with the tool material's thermal properties (see the last paragraph of this section): on the ball cutter's layer of the product page's case below, the ratio stayed under 100% in the entry and exit cuts as everywhere else. The product page's case 3-Axis FDAC High Hardness Mold NC Feed Rate Optimization Result Comparison shows the wear side on a ball cutter, with the feed optimized at the programmed spindle speed (a fixed 220 mm/min against a varied feed): a measured wear depth of 50 µm at the fixed feed, 20 µm at the optimized feed.

Note

Thermal Plastic Deformation Solutions

After addressing tool breakage issues, change the spindle speed by hand — as a rule a lower one, which gives the cutting edge time to dissipate heat. HiNC's NC optimization holds the Thermal Stress Ratio under a target only when OptThermalYieldSafetyFactor(API) is set (0 by default: no limit), and it does so by the feed per tooth, the spindle speed staying as programmed; a thin-chip step, such as an entry or an exit cut can make, is the known case it may not bring under (NC Optimization).

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 Stress Ratio obtained by simulating under those conditions usually reaches 100% or more. This is because the manufacturing formulations of tool materials differ from HiNC's conservatively set default values. So ordinary cuts often fall into Thermal stress overload, and that is the leaf's usual look. The thermal properties of the tool material can be adjusted, and an adjustment changes the ratio that the Availability Chart plots, the leaf judges and NC optimization's thermal target holds alike: adjust them against the wear measured on the shop's own cutters, not to empty the leaf.

See Also