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Plug Gauge: Machinery Steel Case-Hardened in Charred Leather (Markham, 1908)
E. R. Markham · Public domain
Forge

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Forge

4. November 2026NO
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Plug Gauge: Machinery Steel Case-Hardened in Charred Leather (Markham, 1908)

A plug gauge checks the size of a hole. Hardened tool steel can creep in size for months, so some makers used machinery steel and case-hardened it instead. E. R. Markham's Tool Making (1908) packs the turned gauge in charred leather in a box, keeps it red hot in the furnace 7 or 8 hours, lets it cool, reheats it in a tube to a low red and plunges it in raw linseed oil. No tempering is needed, and the gauge does not change as it ages. Grinding and lapping are as for any plug gauge. Mild steel stands for his machinery steel; the 3/4 in size is his Fig. 293. Source: E. R. Markham, Tool Making (American School of Correspondence, 1908), pp. 178-182. Picture: his Fig. 295, ring laps in a holder.
Advanced
Two days

Instructions

1

Read Markham

"When it reaches a low red heat, it should be plunged in a bath of raw linseed oil. It will not be necessary to draw the temper, and the danger of alteration as it ages is done away with."
2

Turn, knurl and stamp

Rough out the bar, allowing enough to turn off the decarbonised skin. Turn the handle (B in Fig. 293) to size and knurl it, and turn and finish the neck (C). Mill a flat spot in the middle of the handle and stamp on it the size of the gauge and the name or mark of the part it gauges (some shops stamp on C instead). Stamp before finishing any gauging surface, as driving stamps stretches the steel. Then turn the gauging end (A) .010 to .015 in over the finished size to allow for grinding.

Materials for this step:

Mild Steel BarMild Steel Bar1 bar

Tools needed:

Metal LatheMetal Lathe
Lathe Cutting ToolLathe Cutting Tool
Knurling ToolKnurling Tool
Milling MachineMilling Machine
Steel Letter and Figure StampsSteel Letter and Figure Stamps
HammerHammer
MicrometerMicrometer
Safety GlassesSafety Glasses
3

Pack and carburise

Pack the gauge in a box with charred leather all round it, as for pack hardening, close the box and run it in the furnace for 7 or 8 hours. Then, red hot, take the box out of the furnace and let it cool with the gauge inside. Do not quench straight from the box: Markham's second heat refines the grain, making it as compact as properly hardened tool steel.

Materials for this step:

Charred LeatherCharred Leather1 packing

Tools needed:

Case-Hardening BoxCase-Hardening Box
Muffle FurnaceMuffle Furnace
Safety GlassesSafety Glasses
4

Reheat and quench in oil

Heat the gauge in an ordinary fire, enclosed in a piece of tube so the fire does not touch it. When it reaches a low red heat, plunge it into a bath of raw linseed oil. The temper need not be drawn.

Tools needed:

ForgeForge
Raw Linseed OilRaw Linseed Oil
Safety GlassesSafety Glasses
5

Grind

Grind on dead centres if you can (a worn lathe spindle copies its own error onto the work), with water running on the work to keep it cool; otherwise heat it as little as possible. Use a coarse wheel free from glaze to within .004 in of size, then a finer wheel to .001 in over. Always measure the gauge cold, since a gauge ground to size while warm will be small when it cools.

Tools needed:

Cylindrical GrinderCylindrical Grinder
MicrometerMicrometer
Safety GlassesSafety Glasses
6

Lap to size

Make a lap: a piece of cast iron with a hole bored a trifle larger than the gauge, split, and closed by a screw (Fig. 294); for much gauge work, a holder with split ring laps held by a pointed screw (Fig. 295). Lap the gauge to finished size with flour emery mixed with oil. Lapping warms it too much to caliper, so cool it in a dish of water at the temperature of the room where it will be used (about 70 F), not cold water, before each measurement.

Tools needed:

Cast Iron BarCast Iron Bar
Emery PowderEmery Powder
Machine OilMachine Oil
Glass BeakerGlass Beaker
Distilled WaterDistilled Water
Thermometer - LabThermometer - Lab
MicrometerMicrometer
7

History and honest limits

Markham notes that the creeping of hardened tool steel, put down to a rearrangement of its particles after hardening, led some manufacturers to use machinery steel for gauges. The case-hardened gauge needs no seasoning. He calls the steel "machinery steel"; mild steel bar stands for it here. The size of the box and the furnace are not given; the 3/4 in size is his Fig. 293.

Materials

2

Tools Required

19

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Plug Gauge Set

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