Tool Steel Grindability and Grinding Wheel Selection
Choosing a grinding wheel for tool steel starts with the grade, because the alloy content decides how hard the steel is to grind. This page gives the grindability group for the grades used in mold, tool and die work and the wheel properties that go with each group in surface grinding and cylindrical grinding, covering abrasive type, grit size, wheel hardness, bond, wheel speed, table speed, cross feed and infeed. The groups run from easy, where the steel grinds freely, to difficult, where the grade holds enough carbide to resist abrasive cutting. This is a reference summary of published practice and not an Aobo Steel specification.
Grindability groups by grade
Find the grade you grind in the chart and note the group it falls into. That group then sets every wheel property in the two tables below, because the same four bands run across all three charts in the source.
The groups run from easy to difficult. Water hardening and low alloy grades sit in the easy band and grind freely, air hardening and medium alloy grades sit in the middle, and the high alloy cold work and high speed grades sit in the hard and difficult bands, where the wheel runs softer, finer and slower to keep the surface cool.
Grindability rating chart for selected grades of steel
| Grindability group | Tool steel grades in that group |
|---|---|
| Easy | W1, W2, L6, 4140, 4150, 8620, O1, O6, A6, S1, S5, S7, H11, H12, H13, H19, H21, A8 |
| Medium | A2, M1, M7 |
| Hard | CPM10V, D2, D3, D7, T1, T5, M1, M2, M3, M4, M7, M42 |
| Difficult | T15 |
Source table 16.1, printed page 118.
Surface grinding wheel selection
The surface grinding chart gives the wheel for each group and the table speed, cross feed and infeed that go with it. The abrasive moves from semi-friable on the easy group to friable on the harder groups, the grit becomes finer, and the wheel speed drops on the difficult group.
Grinding wheel selection guide for surface grinding
| Wheel property | Easy | Medium | Hard | Difficult |
|---|---|---|---|---|
| Abrasive | Semi-friable | Friable | Friable | Friable |
| Grit size | 36-60 | 36-60 | 46-60 | 80-100 |
| Hardness | H-J | G-H | F-G | F |
| Bond | Vitrified | Vitrified | Vitrified | Vitrified |
| Wheel speed | 5500-6500 | 5500-6500 | 5500-6500 | 3000 |
| Table speed | 50-75 ft (15.2-22.9 m) per minute | 50-75 ft (15.2-22.9 m) per minute | MAX. | MAX. |
| Cross feed | 1/32-1/16 in (0.794-1.588 mm) | 1/32-1/16 in (0.794-1.588 mm) | 1/64-1/32 in (0.397-0.794 mm) | Wheel width |
| Infeed | 0.001-0.005 in (0.0254-0.1270 mm) | 0.001-0.003 in (0.0254-0.0762 mm) | 0.001-0.005 in (0.0254-0.1270 mm) | 0.0005-0.001 in (0.0127-0.0254 mm) |
Source table 16.2, printed page 119.
Cylindrical grinding wheel selection
The cylindrical chart uses the same four groups with a finer grit range and higher table speeds than surface grinding. Cross feed and infeed are the same for the first three groups and open up on the difficult group.
Grinding wheel selection guide for cylindrical grinding
| Wheel property | Easy | Medium | Hard | Difficult |
|---|---|---|---|---|
| Abrasive | Semi-friable | Friable | Friable | Friable |
| Grit size | 60-80 | 60-80 | 60-80 | 80-100 |
| Hardness | K-L | J-K | H-J | F-H |
| Bond | Vitrified | Vitrified | Vitrified | Vitrified |
| Wheel speed | 5500-6500 | 5500-6500 | 5500-6500 | 3000 |
| Table speed | 90-150 ft (27.4-45.7 m) per minute | 90-150 ft (27.4-45.7 m) per minute | 100-150 ft (30.5-45.7 m) per minute | 100-150 ft (30.5-45.7 m) per minute |
| Cross feed | 1/32-1/16 in (0.794-1.588 mm) | 1/32-1/16 in (0.794-1.588 mm) | 1/32-1/16 in (0.794-1.588 mm) | 1/16-1/8 in (1.599-3.175 mm) |
| Infeed | 0.001-0.002 in (0.0254-0.0508 mm) | 0.001-0.002 in (0.0254-0.0508 mm) | 0.001-0.002 in (0.0254-0.0508 mm) | 0.0005-0.001 in (0.0127-0.0254 mm) |
Source table 16.3, printed page 119.
Reading the three charts
The values are reproduced as printed. Table speed is given in feet or metres per minute, and cross feed and infeed are given in inches with the source’s millimetre conversion printed under each inch figure. The wheel speed row is labelled RPM in the source and printed without a unit.
Where the source prints MAX. for table speed, no figure is given. On the difficult group of the surface chart the cross feed is set by the width of the wheel instead of by a number.
Two printing points are kept as they stand. The millimetre conversion under the 1/16 to 1/8 in cross feed in the cylindrical chart is printed as 1.599 to 3.175 mm, and the two infeed conversions in the difficult column are printed as 0.0127 to 0.0254 mm.
The source chart prints M1 and M7 in both the medium group and the hard group and prints T5 twice in the hard group. Each grade is listed once in each group where the source shows it, and the repeated T5 entry is not carried over here.
Before you grind
Grinding damage in hardened tool steel comes from heat. A wheel that is too hard for the grade, a grit that is too fine, or a pass that is too heavy raises the surface temperature until the steel scorches, softens or cracks. Settle the wheel from these charts before the damage appears, then keep the speeds and feeds steady. Wet and dry grinding is a shop decision, and the source warns that a heavy wet pass can quench the surface just as a dry pass can burn it. Its closing advice is to ask your own grinding wheel supplier what data they can provide for the wheel you run.
Wheel choice also depends on the condition of the steel, and the bar we ship is annealed, as described on the annealed condition of tool steel page. Cutting data for the same grades is on the D2 machining guide and the A2 machining guide, cutting ratings are on the tool steel machinability rating chart, and the finish left behind by grinding or by EDM is covered on the surface roughness chart and the EDM white layer page. Hardness figures and their conversion are on the hardness conversion calculator.
Reference data compiled from Heat Treatment, Selection, and Application of Tool Steels (William E. Bryson, 2nd edition, Hanser), tables 16.1, 16.2 and 16.3, printed pages 118 and 119.