Tapping Tool Steel by Hardness Range

Tapping is the operation that stops first when a tool steel part gets hard, because a tap combines a cutting edge with a long, thin body that has to carry the torque. The published practice for tapping steel splits into three hardness ranges, and every part of the tap changes across them, from the grade of high speed steel in the tap to the percentage of full thread the hole is prepared with.

Three ranges from 5 to 55 HRC

The three ranges come from a tap manufacturer test series on 11 steels spanning 5 to 55 HRC, run to find the effect of hardness on tapping. The recommendations that came out of it are reproduced below. They are worth reading as a single system rather than as six separate rows, because the tap design, the tap material, the tap hardness, the surface treatment, the thread percentage and the speed all move together.

Recommended practice for tapping steels in three ranges of hardness, reproduced from Table 2 of Tapping, in ASM Handbook, Volume 16, Machining. (a) A special tap should have four flutes, a 3 degree positive hook angle, thread relief and a 3 to 4 thread chamfer. (b) High-speed steels similar in total alloy content to the steels listed can be substituted. (c) Or another oxidizing process.

What each range changes

Up to 30 HRC a standard tap of general-purpose high speed steel is the most economical choice. Steels in the T1, M1, M7 and M10 group are named for it, at a minimum tap hardness of 62 HRC and a steam oxide surface, with 75 percent of full thread prepared in the hole and a speed up to 30 m/min (100 sfm).

From 30 to 40 HRC the tap becomes a modified standard or a special, and the tap material moves up to M2 and M3 at a minimum hardness of 64 HRC. The surface treatment changes to nitride plus steam oxide, the thread preparation drops to 60 percent of full thread, and the speed falls to 6 to 14 m/min (20 to 45 sfm).

From 40 to 55 HRC every item is at its most severe. The tap has to be a special, with four flutes, a 3 degree positive hook angle, thread relief and a 3 to 4 thread chamfer. The tap material moves to the highly alloyed T15, M3, M33 and M42 grades at a minimum hardness of 66 HRC, the surface is again nitride plus steam oxide, and only 55 percent of full thread is prepared. The speed is down to 0.15 to 3.0 m/min (0.5 to 10 sfm), so a tap in this range runs slowly on purpose and a machinist who treats it like a soft-steel tap will break it.

One point runs through the whole table. As the workpiece hardness rises, the tap is asked to remove less material. The thread percentage falls from 75 to 55 percent, which lowers the torque on the tap and gives the harder grades a chance to survive.

Nominal tapping speeds for carbon and low-alloy steel

The table below is the wider speed list for high speed steel taps on carbon and low-alloy steels, and it shows the same fall in speed with hardness across a range of common grades. It is a useful yardstick for the softer end of tool steel work, which taps closer to medium-carbon steel than to the hardened range above.

Nominal speeds for tapping carbon and low-alloy steels with high speed steel taps, reproduced from Table 3 of Tapping, in ASM Handbook, Volume 16, Machining. Each steel listed stands for a group of similar steels. For steel up to 375 HB the source gives M1, M7 and M10 taps, and for steel of 375 HB and higher it gives M3 and M40. The 45 to 48 HRC rows are the hardened end of the table and they show how sharply the speed falls.

Grades and stock

The high speed steels named in the uppermost range are the highly alloyed grades that hold their hardness at the cutting edge, and the M42 family is set out on the M35 tool steel page together with the related cobalt grades. The wider family of high speed tool steels is on the high speed steels page.

The steam oxide treatment named in the first two ranges is covered with its effect on tool life on the steam treatment tool life improvement page. Where the surface treatment is nitriding instead, the process and the case it leaves are on the tool steel nitriding guide.

The hole is prepared before the tap goes in, and the drill size that leaves the right amount of material for a given thread is on the tap drill size chart. The wider drilling conditions for tool steel, soft and hardened, are on the drilling hardened tool steel page.

Before you use these values

This page is a reference summary of published practice and it is not an Aobo Steel specification. The recommendations are reproduced from the source table, and tap geometry, hole depth, lubrication and machine control will all move them. Final conditions are set on the job.

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