A purchase order arrives reading "plug gauge 25 mm, 2 nos". Six decisions have been left to the person who makes it, and each one changes the gauge you receive. The order is not wrong so much as incomplete, and incomplete orders get filled with defaults that may not match your part.
Here is everything a gauge maker needs, and what happens when each item is missing.
1. Nominal size and fit class, not just the diameter. A 25 mm bore might be 25H7, 25H8 or 25G7. The class sets the tolerance band the gauge represents, and those bands come from ISO 286, adopted in India as IS 919: H7 at 25 mm is 0 to +21 µm, H8 is 0 to +33 µm. Order without it and you get the maker's house default, most often H7, which is wrong for you around half the time.
2. Which members you need. GO only, NO-GO only, or a pair. Double-ended gauges put both on one handle and suit bench work. Single-ended pairs suit two operators checking in parallel, and let you replace the GO alone when it wears, which it will do first.
3. Gauge tolerance class. Workshop, inspection or reference. IS 3455 sets out the gauge tolerances and wear allowances that follow from the work tolerance, which is the arithmetic your supplier is applying whether or not the order mentions it. A tighter gauge consumes less of your part tolerance and costs more. If you are holding IT7 or tighter, this matters; on IT9 work you are paying for precision you cannot use.
4. Wear allowance. Applied to the GO member, in the direction of wear, so the gauge starts life slightly inside the limit and remains valid longer. State whether you want it, because a gauge with no wear allowance reaches its limit sooner and a gauge with a large one rejects good parts on day one.
5. Material. Hardened tool steel is the default and correct for most work. Carbide costs roughly two to three times more and lasts five to ten times longer on abrasive materials such as cast iron and glass-filled polymer. On a low-volume steel job, carbide is money you will not recover.
6. Certificate. A NABL-traceable calibration certificate with actual measured sizes and stated uncertainty, or a plain conformance statement. If your customer audits your gauge register, you need the former. Ask for it at the order stage, since certifying afterwards costs more and takes longer.
| Order says | You receive | Risk |
|---|---|---|
| "25 mm plug gauge" | Maker's default class, no certificate | Wrong tolerance band entirely |
| "25H7 GO/NO-GO" | Correct band, default gauge class | Gauge tolerance may eat your budget |
| "25H7 GO/NO-GO, inspection class, tool steel, NABL cert" | What you meant | None worth naming |
Reference temperature. Sizes are defined at 20 °C. If your inspection room runs at 32 °C, say so, because it changes what the certificate means to you in practice rather than what the gauge is made to.
Marking. Ask for the nominal size, class and member marked on the handle. Unmarked gauges get mixed up in a drawer, and a GO and NO-GO of similar size are indistinguishable by eye at a glance.
A standard size in tool steel is usually a stock or short-lead item. Non-standard sizes, tight classes and carbide all extend that, typically to two or three weeks. The expensive path is ordering incompletely, receiving something usable but wrong, and discovering it during a first-article inspection with a customer waiting.
Send the drawing, or at least the feature callout, along with the order. A gauge maker reading "25H7, through hole, 40 mm deep, cast iron, 2000 parts a month" can tell you that carbide pays for itself here and that a 40 mm engagement length needs a full-form GO. That conversation takes five minutes and saves a re-order.
Tell us the feature and the fit when you enquire about plain gauges, or send the drawing through contact and we will specify it back to you before anything is cut.