Product type
A micrometer measures one dimension, and it measures it to a finer resolution than any hand tool that reads several. The property that actually earns it its place is the second one: it closes on the part at a measuring force that does not depend on the operator, which is what makes it the instrument a drawing tolerance near its limit gets checked with.
A micrometer measures one dimension, and it measures it to a finer resolution than any hand tool that reads several. The property that actually earns it its place is the second one: it closes on the part at a measuring force that does not depend on the operator, which is what makes it the instrument a drawing tolerance near its limit gets checked with.
The type is usually met as an outside micrometer, measuring a diameter or a thickness between a fixed anvil and a spindle advanced by a calibrated screw. Inside micrometers, depth micrometers and blade micrometers put the same screw to work on a bore, a shoulder or a narrow groove, and a wall-thickness anvil turns an outside instrument onto tube wall. What they all share is the window: one instrument covers a 25 mm band, so a workshop holds a set of sizes rather than a single tool.
Two reading systems are common in general engineering. A digital instrument puts the spindle position on a display and is normally stated to the finer tolerance. A mechanical instrument is read from a thimble and sleeve, needs no power, and is unaffected by being left in a drawer. Both are covered here; the page below is about what decides the specification either way - the range, the resolution, the measuring force and the condition of the faces.
The spindle is advanced by a calibrated screw, so a turn of the thimble moves it a fixed distance and the reading is a length in its own right. Nothing has to be compared against a reference part, which is why the instrument stands alone on a bench and why its accuracy is quoted as a tolerance over the range rather than as a difference from a standard.
The measuring force is set by a ratchet or friction stop in the thimble, which slips once a chosen force is reached. Winding the thimble in by hand instead is the single commonest source of a reading that looks reliable and is not: the faces meet the part differently each time, and on a soft, thin-walled or slightly oval part the spread between a light and a firm touch exceeds the tolerance of the instrument.
Two things decide whether the instrument is still trustworthy, and neither of them appears on a display. The measuring faces wear - carbide is hard, but a rotating part, a casting skin or an abrasive material takes its toll - and a worn face is the usual reason a micrometer that once agreed with a gauge block stops agreeing. The zero setting is the other, which is why a setting rod is supplied above the smallest range and why checking zero is part of using the instrument rather than an occasional task.
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
|---|---|---|
| Instrument type | Outside, inside, depth, blade, or an outside instrument fitted with a wall-thickness anvil | Whether the feature being measured is reachable at all, and whether one instrument can cover the job or several are needed. |
| Measuring range | One 25 mm band per instrument | Which sizes can be measured. A dimension near the bottom of a band is better measured on the instrument that holds it in the middle. |
| Resolution and stated tolerance | The finer digital grade, or the wider mechanical figure | Whether the instrument can demonstrate the drawing tolerance. Resolution alone does not settle this. |
| Reading system | An electronic display, or a graduated thimble and sleeve | How fast the instrument reads, whether it depends on a battery, and how it behaves after long storage. |
| Measuring faces | Carbide, or hardened and lapped steel | How long the instrument holds its accuracy where hard or abrasive parts are measured day after day. |
| Measuring force device | A ratchet stop, or a friction thimble | Whether repeated readings on the same part agree, and whether the instrument can be used on a part that deforms under pressure. |
| Frame and spindle construction | A coated cast frame, a hardened and ground spindle, and a spindle lock | Whether the instrument survives a machine shop, and whether a reading can be held while it is written down. |
| Environmental protection | Sealing against dust and coolant splash, where offered | Whether the instrument can be used at the machine, or has to be kept away from the swarf and mist. |
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|---|---|
| The dimension and its geometry | An outside instrument cannot reach a bore, a shoulder or a groove | Describe the feature, and whether a standard anvil and spindle can get to it. |
| Size band in the batch | One instrument covers 25 mm, so the band decides how many sizes are needed | Give the smallest and largest dimension, not only the nominal size. |
| Drawing tolerance | The stated tolerance has to sit inside the drawing tolerance, with something in hand | Quote the tolerance at that dimension. |
| Resolution required | A display that reads finer than the instrument is stated to hold invites a reading the instrument cannot support | State whether the tolerance needs the finer grade. |
| Material and surface of the part | Soft or thin-walled parts move under the faces; abrasive ones wear them | Describe the material, the surface, and whether the part deforms under light pressure. |
| Environment of use | Swarf, dust and coolant splash decide whether sealing is needed | Say whether the instrument is used at the machine or in an inspection room. |
| Zero checking and handling | Sizes above the smallest range are zeroed against a setting rod, which has to be kept sound | Say whether the instrument is issued to a bench or to individuals, and how it is stored. |
Send what is known even where some of this is still open. A quotation can be issued against a duty description and refined as the gaps are filled.
These are the types an enquiry for an outside micrometer usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from an outside micrometer |
|---|---|
| Digital calipers | Read several dimensions - outside, inside, depth and step - over a range of sizes, and read directly to a figure. The instrument is closed by hand, so the measuring force depends on the operator, and the tolerance is quoted across the whole beam. Reach for the caliper for the checks that are not near a limit, and for the geometry a micrometer cannot reach. |
| Vernier calipers | The same breadth as a digital caliper without electronics, so no battery and no drift. It has the same hand-closed measuring force, and reading the scale is slower and depends on eyesight. |
| Bore gauges | Measure an internal diameter at depth and centre themselves in the bore, which is what finds the true diameter. An inside micrometer has to be rocked to find the maximum; a bore gauge transfers the measurement to a setting ring or a micrometer instead. |
| Dial indicators | Measure a deviation rather than a size - run-out, flatness, a step, or a change while a machine is running. A micrometer gives one absolute length per reading and says nothing about how the part behaves in motion. |
| Gauge blocks | Not a measuring instrument but the reference the instrument is checked against. A certified set is what establishes whether the micrometer is still reading correctly, and a workshop measuring to a tolerance normally holds one for that purpose. |
| Depth gauges | Built for a bore depth, a slot or a recess, with a base that registers on the surface the depth is measured from. A depth micrometer applies the same screw to the same problem with interchangeable rods, which suits deep or narrow features better than a caliper rod does. |
The boundary that matters most is micrometer against caliper. Where a tolerance sits close to what a hand-closed instrument can hold, the reading has to come from an instrument whose measuring force the operator does not set.
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
Available products
Micrometers
Outside micrometers in two grades: digital styles reading to 0.001 mm, and mechanical styles from 0-25 to 225-250 mm, both with carbide measuring faces and a ratchet stop.
Also in Dimensional Measurement
Every type we quote for under Dimensional Measurement. A type with no link yet is still quoted for - it is described on the Dimensional Measurement overview, and gets a page of its own as soon as a model is published against it.
Frequently asked
It follows from the feature rather than from the accuracy required. An outside micrometer spans a diameter or a thickness from the outside; an inside micrometer expands inside a bore to measure it; a bore gauge measures a bore and transfers the value to a setting ring or a micrometer rather than displaying a length itself. A bore gauge is the quicker instrument for a production bore and an inside micrometer the more direct one for occasional work, and neither substitutes for an outside instrument on a shaft.
Because it is what makes a reading repeatable. A ratchet or friction stop releases at a chosen force, so the faces meet the part the same way each time and a second operator gets the same figure. Without it the reading depends on how firmly the thimble was wound in, and on a soft, thin-walled or slightly oval part that difference is larger than the tolerance of the instrument. That is also why a micrometer and a caliper can disagree on the same part without either being faulty.
It is used to check zero on any instrument whose range does not start at zero, and it is normally supplied with those sizes rather than bought separately. Zero is checked by closing the faces on the rod and confirming that the instrument reads the rod’s own value; a rod that has been dropped, or a face that has picked up a burr, shows up as a zero error before it shows up as a wrong reading on a part. On the smallest range the faces close directly on each other and no rod is used.
The measuring faces, in normal use. Carbide resists wear well, but measuring a rotating part, a casting skin or an abrasive material still dulls and rounds the faces over time, and a worn face is the usual reason an instrument that once agreed with a gauge block stops agreeing. Keeping the faces and the part clean before measuring, using the ratchet stop rather than winding the thimble in, and checking zero against the setting rod are what keep the instrument in service. Where readings are relied on for a customer drawing, a periodic check against a certified reference is what demonstrates that they still mean something.
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