Calibrating tool gauges

Knitting needles and crochet hooks made according to the same gauging system and marked with the same gauge number — directly or on the packaging — can nonetheless differ to a perceptible degree in their actual diameters. This variation may be a simple result of careless sorting or otherwise insufficient quality control. However, it also has two significant nonrandom causes. One is that tools made in one country in compliance with its predominant standard, when intended for export, are marked with what is judged to be the nearest equivalent size in the standard of the destination country. Tools are also often labeled with the gauge designations of both countries.

The millimeter frequently appears either as the primary or alternate unit, rounded off to the nearest whole, half, or quarter. These increments are commonly used in countries where manufacturers work directly to a metric gauge (except for the finest-sized steel crochet hooks where the gradation is in tenths or five-hundredths of a millimeter). However, the sizes of hooks and needles produced elsewhere will not necessarily align with a scale divided exactly into quarter millimeters.

For example, I have two crochet hooks of the same highly regarded Japanese brand that are identical except for the size indications on their labels. When measured directly with slide calipers (explained below), the diameter of both hooks is 2.5 mm. One is intended for the domestic and European markets and labeled “4/0 — 2.50 mm.” The other is for export to the US, labeled “C-2 — 2.75 mm.”

Since these markings are on ergonomic handles I sacrificed the one on the latter hook, revealing the metal tool to be embossed “4/0 — 2.5.” The details of the Japanese gauge system are described in a post on the Japanese Knit and Crochet Pattern Help blog which says that a 4/0 hook can also be labeled as 2.25 mm. This means there is a ±10% tolerance in the indication of the actual size, gainsaying the widespread belief that millimeter markings are inherently more accurate than gauge numbers.

The second source of discrepancy between the nominal and actual diameters of hooks and needles is the precision with which the gauges used in their manufacture are calibrated against the underlying standard. (The term “gauge” designates both the measuring tool and the ordered system of numbers and dimensions that it incorporates.) This extends to the gauges commonly marketed to knitters and crocheters, which are typically accurate to about the same ±10% — even when marked in millimeters. (Anyone curious about slide calipers as an alternative, but less interested in background information about them, can skip directly to a how-to discussion below.)

This was a major industrial concern in mid-19th century England, when Imperial units of measurement were still in widespread international use but the push toward global metrication was gaining momentum. A leading participant in the debate, Joseph Whitworth, was among those who convincingly argued that the pivotal issue was the decimal representation of small linear measurements. In 1857, he proposed a standard wire gauge ranging from 0.001 to 0.500 inches, in increments gradually expanding from 0.001″ to 0.025″, with each represented size also serving as its gauge number.

Whitworth’s gauge was never officially adopted but the British Standard Wire Gauge (SWG; discussed in a previous post), finalized in 1883, defined the individual sizes to the nearest 0.001″. He was also concerned with the precise calibration of gauging tools and produced a set of reference gauges in 0.01″ increments between 0.01″ and 0.10″.

Here is one of the individual calibration pieces. It is for a slot gauge (also discussed in the linked post) and the flat block at the right end is normative. (When drawing wire, it is necessary to be able to measure a sample at any point along its length and a side-opening gauge is essential.)

This raises an obvious question about how the calibration gauges were themselves calibrated. Bench micrometers suitable for measuring the exact thickness of such things as Whitworth’s blocks became available early in the 19th century. The first handheld tool capable of the same application was the millimeter-based “calibre à vis et à vernier circulaire” (screw calipers with a circular vernier) patented in 1848 by the Parisian toolmaker Jean Laurent Palmer. It is marked in 0.05 mm increments and the mid-point between each can easily be interpolated, giving the equivalent of 0.001″ resolution.

It is is not known if the crochet hook and knitting needle manufacturers of the day took notice of it but the same tool can also be used to measure the diameters of such implements directly. However, two hands are required to operate a screw micrometer and its adjustment from a small to a large opening is tedious. A slot gauge was, and remains, a more convenient alternative in both regards.

This does not pertain as clearly to the handier slide calipers that were to become ubiquitous in many workshop contexts. Notwithstanding, such devices never found a broad path into the yarncrafter’s toolkit despite the interest in accurately sizing the hooks and needles also found there. One part of the explanation for this lack of uptake is the somewhat non-intuitive aspect of reading measurements on what was once the tool’s only form.

This has a primary scale calibrated in millimeters, with a secondary vernier scale showing additional 0.1 mm increments, in turn subdivided to the nearest 0.02 or 0.05 mm. Vernier calipers of reasonable quality therefore provide the 0.1 mm accuracy needed to match knitting needles to, for example, Frances Lambert’s idiosyncratic gauge numbers (discussed in another previous post), or any other gauge system for which a directly calibrated measuring tool is not at hand.

Anyone with such calipers tucked away somewhere, interested in testing them in this context but uncertain about their operation, will find many tutorial videos explaining how to read a vernier scale (as here). Need for that skill has otherwise been mooted by the subsequent development of analog “dial” and battery-driven “digital” calipers. Both display numerical measurements directly and their use is transparent.

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Worktable notes:

The mismatch in the sizes of the nominally identical crochet hooks noted above could not have been quantified without measuring them directly. Each hook has a profiled head that does not fit through the hole on a knitting needle gauge that corresponds to the diameter of its shaft. A slot gauge can be used for this purpose, but only when there is no need for determining measurements intermediate between two adjacent slots. This limitation also applies to hole gauges, which is why working from Lambert’s gauge numbers also requires a continuously variable measuring tool.

I acquired a pair of high-quality vernier calipers over fifty years ago and remain thoroughly comfortable with them. However, museums commonly forbid metal tools from coming into contact with objects in their collections. When I first needed to deal with this, reasonably accurate plastic calipers were available that showed measurements on a dial with the same 0.05 mm resolution as that of their vernier counterpart.

I still take the plastic ones into museum study facilities but also use them for measuring the older yarncraft tools in my own collection. This includes shepherd’s hooks and other implements where the concept of diameter is either irrelevant or there are no markings that indicate size. Plastic also poses a lesser risk to the surface finishes on some needles and hooks in current production.

Although dial calipers vary in quality, many inexpensive models are more than accurate enough for these purposes. As with the other forms, they commonly have parallel millimeter and inch scales, and care needs to be taken to ensure that the dial is calibrated in millimeters and not decimal inches.

The following photo shows my first vernier pair followed by currently marketed dial and digital calipers, both of plastic. Their direct numerical readouts provide easy means for becoming familiar with such tools, without a learning curve. However, the rightmost digit in any digitally displayed measurement is inherently uncertain and a tool with 0.1 mm resolution has an accuracy of ±0.2 mm. This is clearly noted on the back of the model shown here, which displays three digits. Some digital calipers therefore add a fourth digit, but it is only reliable on finely machined (and correspondingly expensive) metal instruments.

Anyone interested in assessing the utility of such devices will likely find them worth an investment of less than ten USD, EUR, or GPB for the dial model, or the similarly priced digital one ($ € £). There is also a model with plastic jaws and a four-digit display that I have not examined, again at the same cost ($ € £).

Disclaimer: I have provided links to three national points of access to a single online supplier with which I have no association other than as a customer. The same tools and comparable alternatives are available through other channels.