Make measuring tools that stay accurate by hardening their working faces to about 60 HRC so they wear slowly, then temper them, and do the final grinding and lapping after hardening, because the quench distorts the part. Lapping removes only about 0.001 mm per hour, so a part should arrive at the lapping stage within a few hundredths of a millimetre of size, not tenths.
Part of Precision Tool Making
The order of hardening, tempering and finishing that lets a gauge, a micrometer face or a scraper keep its size in use.
A Soft Instrument Loses Its Accuracy By Wearing
A measuring face that is soft wears with every use, and the instrument drifts without anyone seeing it. Micrometers says an unhardened face wears in hours of use, and that faces should be case-hardened to at least 60 HRC, with through-hardened tool steel preferred. The same logic applies to anvils, gauges, scraper blades and the working surface of any reference.
Hardening has its own cost. A quench distorts the part, a hardened part is brittle, and a hard surface cannot be cut with a file. So hardening cannot be the last step. The order is: machine near size, harden, temper, then finish by grinding and lapping, which can remove metal from hard steel.
What You Need
- Steel with at least 0.4% carbon to harden at all; tool steels at 0.7-1.2% are the usual choice (Hardening)
- A forge or furnace, a quench tank, and a means of tempering (Tempering)
- Grinding and lapping capability for the finish (Lapping for Ultra-Precision)
- Stock left on every working face for the finishing stage
Method: Harden, Temper, Finish
Step 1: Machine near size with stock left for the finishing stage.
Step 2: Heat uniformly to bright cherry-red, about 760-800°C for 1% carbon steel, checked with a magnet: when it stops attracting, quench.
Step 3: Quench, entering the part in its most symmetric orientation and agitating, to limit warping. Oil suits thicker sections and reduces cracking.
Step 4: Check the hardness with a file: if it skids off, the surface is hard.
Step 5: Temper at once. Tempering gives up a little hardness for toughness. By colour: 220°C faint straw for scrapers and razors, 240°C for engravers and fine gauges, 250°C dark straw for plane irons and chisels (Tempering). The carrier tempers gauge blocks to about 60 HRC.
Step 6: Finish by grinding, then lapping, to final size.
For steel, use the materials article Heat Treatment, and the construction tempering article above. The tempering article in the materials folder is about clay and has nothing to do with steel.
Worked Example
Carrier gauge-block recipe: rough-grind to within 0.05 mm of target, then lap, with lapping removing about 0.001 mm per hour. Removing 0.05 mm at that rate is 0.05 / 0.001 = 50 hours. The rate is the carrier’s figure for fine lapping and a coarser abrasive removes faster, but the sum shows why the grinding stage should leave as little as possible: each hundredth of a millimetre left for the fine stage costs about ten hours.
This is also why hardening must precede finishing and why quench distortion matters: if a quench bends a block by 0.05 mm, you are into days of lapping to remove it. Quench symmetrically and leave only the stock that distortion will cost.
Common Mistakes
- Finishing before hardening. The quench moves the part and the finish is lost.
- Skipping the temper. A glass-hard tool chips or shatters.
- Hardening low-carbon steel. Under about 0.4% carbon it does not harden; use case hardening (Selective Hardening).
- Assuming the temper colour is a measurement. It is a guide.
Safety
Red-hot steel, quench oil and fumes
A quench in oil can flash into flame: use a deep tank with a lid or damp cloth ready, keep the area free of rags and spills, and wear eye protection and tongs. A part that fails in the quench can crack with a loud report and throw fragments. Grinding hardened steel needs a sound wheel: ring-test it, guard it and stand to the side on first start.
What’s Next
- Learn next: Selective Hardening - hard faces on a tough body
- Combine with: Tool Steel - choosing the steel