Measuring & Inspection Instruments

Reach versus resolution. A caliper measures outside, inside, depth and step with one tool across a wide range, and reads to 0.01mm on a digital or 0.02mm on a vernier. A micrometer only does one job over a 25mm span, but it reads to 0.001mm and the spindle applies a consistent force through the ratchet, so two people get the same number.

The practical rule: if the drawing tolerance is looser than about ±0.05mm a caliper is fine. Tighter than that and you want a micrometer, because caliper error and operator feel are the same order of magnitude as the tolerance you are chasing.

Two scales. Read the main scale first: the last whole millimetre mark that the vernier zero has passed. Then run along the vernier scale and find the one line that lines up exactly with a line on the main scale — that number is your fraction.

On a 0.02mm vernier the scale is divided into 50, so if the twelfth line coincides you add 12 × 0.02 = 0.24mm. Main scale 34mm plus 0.24 gives 34.24mm. Only one line ever coincides properly; if two look close, your eye is off the perpendicular.

Digital if you are switching between metric and imperial, taking a lot of readings, or you want incremental measurement from an arbitrary zero. It is the fastest to read and the hardest to misread.

Dial where the job involves comparing rather than absolute sizing — a moving needle shows a trend that flickering digits hide. Vernier where batteries are a liability: coolant, grit, heat, or a tool that lives in a box and gets used twice a year. A vernier has nothing to go flat and nothing to fail.

Resolution and accuracy are different numbers and it catches people out. A typical 0–150mm digital caliper displays to 0.01mm but is specified to around ±0.02mm — and that assumes correct measuring force, square jaws and a clean anvil.

In real use, over-tightening the jaws, measuring at the tips instead of down near the beam, or a burr on the workpiece will each cost you more than the instrument's own error. Wipe the jaws, close them and check they zero before every session.

Ingress protection. The first digit is dust, the second is water. IP65 means fully dust-tight and protected against water jets from any direction. IP67 is dust-tight and survives temporary immersion in a metre of water.

For a shop floor the practical difference is small: both will take flood coolant and swarf. IP67 buys you the accident — the instrument that goes in the parts washer or gets left in a tray of suds. If the machine runs wet, it is worth the extra.

A dial test indicator uses a lever with a ball tip that swings through a short arc, typically 0.2mm to 0.8mm of travel. It is the one for clocking a bore true, checking spindle runout or squaring a vice, because the small head gets into places a plunger cannot.

A plunger or dial indicator moves in a straight line with 10mm to 25mm of travel. That is for comparative gauging on a bench or in a fixture — set it against a master, then read the deviation on every part after.

One point worth knowing: a lever indicator reads short if the stylus is not roughly parallel to the surface. At 30° off you lose about 13% of the reading.

Not with a caliper, if the size matters. Caliper inside jaws touch the bore at two points and have a knife edge that never sits on the true diameter, so you read low and you cannot see ovality at all.

A three-point bore micrometer self-centres and gives you a direct reading. A two-point bore gauge is a comparator — you set it on a ring gauge or a micrometer at the nominal size, then read the difference. The two-point method also lets you find taper and ovality by rocking the head and taking readings at different depths and angles.

It is a mechanical edge and centre finder that goes in the spindle like a cutter. Wind onto an edge, a bore wall or the top face and the dial or digital display shows deflection — zero the axis when the needle reads zero and you have your datum.

It replaces three separate tools: an edge finder for X and Y, a wiggler for bore centres, and a Z setter for the top face. Worth it if you set up several jobs a day on a machine without an automated probing cycle. Not worth it if your work is one long production run off fixed stops.

Mitutoyo where the number has to stand up: absolute encoders that keep their origin through a power-off, tight specified accuracy and the widest recognition on an inspection report. It costs more and it is worth it on a QC bench.

Asimeto for the working shop floor — German-designed, specified properly, priced so that losing one is not a disaster. It is where most of our range sits.

Insize when you need something the other two do not list: inside groove calipers, 2-metre beam calipers, blade micrometers, inspection vices. Broad catalogue, sensible money.

On a lot of the range, yes — several manufacturers supply an inspection or calibration certificate as an option, and traceable certification can be arranged on request.

It is not something that can be added automatically at checkout, so call 01732 887334 or put it in the order notes and we will confirm what is available for that specific instrument before it ships.