Machining Hardened Tool Steel with PCBN Cutting Tools
Tool steel is normally rough machined soft and then hardened, so most of the stock is gone before the steel reaches its working hardness. There are times when hardened material still has to be cut, and polycrystalline cubic boron nitride, known as PCBN, is the tool material that does it. This page gives the published conditions for hardened tool steel, cold work, hot work and high speed, in both a finishing grade and a roughing grade of PCBN.
When hardened tool steel is machined
Rough machining of hardened tool steels from 45 to 65 HRC is not a common use of PCBN because the steel is usually roughed out in the soft state. It becomes convenient when a component has to be refurbished, when a mistake has to be corrected, or when worn threads are being removed from thread rolling dies. Remachining rolls in the steel and paper industries is another example. The harder finishing work is more frequent, and it covers the size and surface finish corrections that were traditionally done by grinding.
Finishing grades of PCBN have a lower CBN content than the roughing grades, with part of the CBN replaced by a ceramic. The slightly reduced toughness is not a disadvantage in a light finishing cut, and these grades give a real alternative to grinding on tool steels, bearing steels and die steels after hardening. A finishing grade has three to five times the wear resistance of a roughing grade in that service, measured on D3 cold work steel at 60 HRC, M2 high speed steel at 62 HRC and hot-work die steel at 50 HRC.
Finishing conditions with a low CBN grade
The table below gives the starting conditions for a low CBN finishing grade. Cutting speeds for finishing run higher than for roughing, and the depth of cut is held light. The value for hardened tool steel work sits at 100 to 200 m/min (330 to 650 sfm) regardless of whether the steel is a cold work, a hot work or a high speed grade, and the depth of cut is the thing that changes with the hardness.
| Material | Hardness | Speed | Feed | Depth of cut | |||
|---|---|---|---|---|---|---|---|
| m/min | sfm | mm/rev | in./rev | mm | in. | ||
| Cold-work tool steels | 58-60 HRC | 100-200 | 330-650 | 0.1 | 0.004 | 0.3 | 0.012 |
| High-speed steels | 60-65 HRC | 100-200 | 330-650 | 0.1 | 0.004 | 0.2 | 0.008 |
| Hot-work die steels | 47-50 HRC | 100-200 | 330-650 | 0.1 | 0.004 | 0.3 | 0.012 |
| Bearing steels | 55-60 HRC | 100-200 | 330-650 | 0.1 | 0.004 | 0.3 | 0.012 |
| Case-hardened steels | 55 HRC | 100-200 | 330-650 | 0.1 | 0.004 | 0.2 | 0.008 |
| Hard facing alloys | 35 HRC | 200-250 | 650-820 | 0.1 | 0.004 | 0.3 | 0.012 |
| Gray cast irons | 200-220 HB | 500-800 | 1650-2600 | 0.1 | 0.004 | 0.3 | 0.012 |
| Sintered irons | 200-220 HB | 300-600 | 980-2000 | 0.1 | 0.004 | 0.2 | 0.008 |
Recommended machining conditions for initial trials with a finishing grade of PCBN cutting tool (DBC50), reproduced from Table 5 of Ultrahard Tool Materials, in ASM Handbook, Volume 16, Machining. The first three rows are the tool steels. Cutting speeds for a finishing grade are higher than for a roughing grade. The source prints 0.12 in. for the hot-work die steel depth of cut, which is a misprint of the metric value beside it.
Roughing conditions with a high CBN grade
Rough machining of hard ferrous materials needs a high CBN content, and round chamfered inserts should be used whenever possible because of their greater geometric strength. Two limits are worth holding on to. High CBN content tools are not suited when the depth of cut falls below 0.5 mm (0.020 in.), and interrupted cuts are not possible on high speed steel at 62 HRC. The table covers turning and milling, and the milling feed column is a feed per tooth.
| Material | Hardness | Speed | Depth of cut | Feed | Comment | |||
|---|---|---|---|---|---|---|---|---|
| m/min | sfm | mm | in. | mm/rev | in./rev | |||
| Turning | ||||||||
| Ni-HARD | 58 HRC | 50 | 165 | 2.5 | 0.100 | 0.35 | 0.014 | Round chamfered inserts preferable |
| White or chill cast iron | 55 HRC | 60-100 | 200-330 | 2.0 | 0.080 | 0.4 | 0.016 | Round chamfered inserts used |
| High-speed steel | 62 HRC | 60-120 | 200-390 | 2.0 | 0.080 | 0.2 | 0.008 | Interrupted cuts not possible |
| Cold-work tool steel | 60 HRC | 80 | 260 | 2.0 | 0.080 | 0.25 | 0.010 | . . . |
| Nickel and chromium irons | 58 HRC | 55 | 180 | 2.0 | 0.080 | 0.3 | 0.012 | . . . |
| Gray cast iron | 220 HB | 600-800 | 2000-2620 | 0.1-2.0 | 0.004-0.080 | 0.1-0.4 | 0.004-0.016 | High cutting speeds and correct workpiece microstructure essential |
| Sintered iron | 200 HB | 300 | 980 | 0.5 | 0.020 | 0.1 | 0.004 | . . . |
| Cobalt-base hard facing alloys | 35 HRC | 200-250 | 650-820 | 1.0 | 0.040 | 0.25 | 0.010 | Feed and speed must not be too low. Not all alloys can be machined successfully. |
| Milling | ||||||||
| Meehanite | 55 HRC | 300 | 980 | 0.5 | 0.020 | 0.5 | 0.020 | Round chamfered inserts (roughing) used |
| Ni-HARD | 58 HRC | 200 | 650 | 1.0 | 0.040 | 0.25 | 0.010 | Round chamfered inserts (roughing) used |
| Cold-work tool steel | 60 HRC | 180 | 590 | 1.0 | 0.040 | 0.2 | 0.008 | . . . |
| Nickel-base hard facing alloys | 35 HRC | 220 | 720 | 0.5 | 0.020 | 0.3 | 0.012 | . . . |
Typical machining parameters for a roughing grade of PCBN cutting tool (Amborite), reproduced from Table 6 of Ultrahard Tool Materials, in ASM Handbook, Volume 16, Machining. The milling feed column is a feed per tooth, in mm/tooth and in./tooth as printed in the source, and not a feed per revolution.
For a hardened tool steel the roughing figures are a 60 HRC cold work grade at 80 m/min (260 sfm) in turning and 180 m/min (590 sfm) in milling, and a 62 HRC high speed grade at 60 to 120 m/min (200 to 390 sfm). Cutting speeds for hardened tool steels generally fall in the range of 50 to 100 m/min (165 to 330 sfm), and with round inserts a depth of cut up to 3 mm (0.120 in.) and a feed up to 0.3 mm/rev (0.012 in./rev) are possible on 60 HRC tool steel. The same 60 HRC cold work grade runs at a 2.0 mm (0.080 in.) depth of cut and a 0.25 mm/rev (0.010 in./rev) feed in the table above.
A roughing grade with a high CBN content runs slowly on purpose. On hard martensitic cast iron the source records a 30 to 60 minute tool life from 55 down to 40 m/min (180 to 130 sfm), where cemented carbide gives a similar life only from 10 to 20 m/min (33 to 65 sfm) and falls away above that as the tool tip softens. Tool steel behaves more like the cast irons than like gray iron on this point, and increases in cutting speed reduce tool life rather than extend it.
Milling hardened tool steel
The fracture toughness of solid PCBN inserts is high enough for rough milling of hardened irons and some hardened steels, and it is the reason the milling figures in the roughing table exist. In a fly-cutting test on D3 cold work tool steel at 58 HRC, with a single 0.8 mm (0.03 in.) nose radius tool in a 200 mm (8 in.) cutter head, the feed rate was raised by 0.1 mm per tooth (0.004 in./tooth) on each pass until the cutting edge fractured. The feed to failure for a solid PCBN insert came out more than twice as high as for conventional ceramic cutting tool materials.
Coolant
Coolant is not generally required for rough machining with PCBN, and it is not recommended for milling, where thermal shock at the edge is the risk. A finishing operation can be run dry, but a flood coolant is recommended and it slightly improves tool life.
PCBN hard machining of tool steel, printable PDF
The finishing and roughing conditions for PCBN cutting tools on hardened tool steel, with the wear data behind them, in one PDF with our contact details.
Grades and stock
The cold work grade that most often reaches a die shop in the hardened state is D2, and its own turning, milling and drilling figures are on the D2 tool steel machining guide. The air hardening counterpart and the way it is machined before and after hardening are on the A2 tool steel machining and fabrication guide.
The hot work grade in the finishing table is covered on the H13 tool steel machinability page, which sets out the annealed and hardened cases separately. Where the hardened cut has to be made with a drill rather than a turning tool, the conditions are on the drilling hardened tool steel page.
Before any of this, the steel has to be hardened, and the cycles are collected on the tool steel heat treatment temperature ranges page. The surface a hard cut leaves is defined on the surface roughness chart, and the damage that can lie under it is treated on the EDM tool steel white layer page.
PCBN hard machining of tool steel, printable PDF
The finishing and roughing conditions for PCBN cutting tools on hardened tool steel, with the wear data behind them, in one PDF with our contact details.
Before you use these values
This page is a reference summary of published practice and it is not an Aobo Steel specification. The conditions are reproduced from the source tables, and the grade of PCBN, the insert geometry, the machine rigidity and the workpiece condition will all move them. Final parameters are trialled on the job.
Source: ASM Handbook, Volume 16, Machining, ASM International, 1989.