Drilling Hardened Tool Steel

A hole is the cheapest feature to put in a tool steel part while the steel is still soft, and the most expensive one to put in after hardening. Where a hardened die or insert has to be drilled, the tool material and the cutting speed change with hardness, and the published practice for die steels gives a clear step at 50 HRC. This page sets out the drill parameters for tool steel in both the soft and the hardened condition.

Drill it soft whenever the job allows

Drilling is normally done before hardening. The published parameters for tool steel as a workpiece sit at two hardness levels, 196 HB and 241 HB, and this is an important boundary. A typical tool steel at 241 HB is about 24 HRC, so the general twist drill table covers the soft and the lightly hardened range only, and a die steel at 50 HRC or above is outside it. Beyond that point the choice moves to cobalt high speed steel, to solid carbide, or to a process other than drilling.

Recommended operating parameters for producing holes in tool steel with twist drills, reproduced from Table 6(a) of Drilling, in ASM Handbook, Volume 16, Machining. The two rows are the two hardness levels at which tool steel is listed in the source. HSS is high-speed tool steel. The feed rate is given by the code in the next table, and 118 degrees is the included point angle.

Feed rate for the tool steel rows, reproduced from Table 6(b) of Drilling, in ASM Handbook, Volume 16, Machining. The feed is given per revolution for a two lip drill and rises with the diameter. Where rigidity is low or the cutting fluid is restricted, the source advises lower speeds and lower feeds than these.

The hardness step at 50 HRC on H11 die steel

The clearest published example is on H11 hot work die steel. For H11 at 50 HRC or below, M33, M34 and M36 high-speed tool steel drills of heavy-duty construction give good results. They are run on positive-drive equipment at drill speeds of 7.6 to 9.1 m/min (25 to 30 sfm) and feeds of 0.013 to 0.018 mm/rev (0.0005 to 0.0007 in./rev).

Material harder than 50 HRC required solid-carbide drills with short flutes in the same work. Speeds of 12 to 15 m/min (40 to 50 sfm) and feeds of 0.013 to 0.025 mm/rev (0.0005 to 0.001 in./rev) produced satisfactory results. The drill design also splits at a diameter of about 6.4 mm (1/4 in.). Twist drills with two flutes are used for holes below that size, and straight-flute drills for holes at and above it.

Choosing a carbide grade for a 54 HRC sheet

Carbide grade matters as much as speed at this hardness. Three grades were tested together on H11 steel sheet containing 5 percent chromium and 1.5 percent molybdenum, heat treated to 54 HRC. Two sheets 2.10 mm (0.083 in.) thick were drilled at once with 6.35 mm (0.250 in.) diameter carbide-tip drills, without a cutting fluid.

The softer 6 percent cobalt grade failed through abrasion, excessive burning and chipping, and it broke in the end. The hardest grade, which held only 3 percent cobalt, was too brittle and failed by excessive chipping and breaking during drilling, and it also chipped and broke in handling. The grade that worked was the harder of the two 6 percent cobalt grades, which showed only slight wear after the test and was taken into production. Its hardness came from a finer grain size, which reduced the size of the softer cobalt lakes between the grains.

Composition, hardness and grain size of the three carbide grades tested in drilling H11 steel sheet at 54 HRC, reproduced from the drilling example in Drilling, in ASM Handbook, Volume 16, Machining. C-1 and C-2 carry the same 6 percent cobalt and the same tungsten and carbon levels, and C-2 is the harder of the two because its finer grain size leaves smaller cobalt lakes between the grains.

The lesson carries across the range. At high hardness the strongest binder content is not the answer, and the best grade is usually the finest-grained one within a given cobalt level rather than the one with the least cobalt or the highest hardness number.

Nitrided high-speed steel drills

For the lightly hardened range, a surface treatment can carry an ordinary high speed steel drill further than a change of grade. Blind holes 6.91 mm (0.272 in.) in diameter and 15.75 mm (0.620 in.) deep were drilled in 4335 steel hardened to 48 to 50 HRC. As ground, the drills gave one to four holes per sharpening with a breakage rate of one drill for every 16 to 20 holes. After nitriding, the same drills gave 18 to 20 holes per sharpening and breakage fell to one drill for every 50 to 60 holes.

The drills were standard heavy-duty drills ground from solid heat-treated high speed steel, each with a split point at a positive axial rake of 3 to 4 degrees on the split portion, an included point angle of 135 degrees, a lip relief angle of 5 to 7 degrees, and ground flat 0.8 mm (1/32 in.). Drilling ran at 147 rev/min, a surface speed of 3.20 m/min (10.5 sfm), and a feed of 0.05 mm/rev (0.002 in./rev).

Grades and stock

H11 is the grade the hardened drilling examples are set on, and its composition and working hardness are on the H11 tool steel page. The chromium hot work grade used in most die and mold work is covered on the H13 tool steel machinability page, which separates the annealed case from the hardened one.

Drilling is one cut in a longer sequence, and the turning, milling and reaming speeds for the same grades are collected on the tool steel cutting speeds and feeds page. The wider drill table, in revolutions per minute and feed per revolution by diameter, is on the high speed steel drill speeds and feeds chart.

Nitriding the drill is the same surface treatment that is used on tooling, and the process and the case it leaves are set out on the tool steel nitriding guide. On a hole that is finished by EDM instead of by a drill, the white layer left on the wall is described on the EDM tool steel white layer page.

Before you use these values

This page is a reference summary of published practice and it is not an Aobo Steel specification. The speeds are reproduced from the source tables, and drill design, machine rigidity, hole depth and cutting fluid supply will all move them. Final conditions are trialled on the job.

Source: ASM Handbook, Volume 16, Machining, ASM International, 1989.