Why careful measuring pays off
A CAD model is never better than the dimensions it is built on. A hole spacing that is 0.5 mm out means the screws will not go in, and a shaft journal 0.2 mm too large will not go into the bearing. Every such error costs a new test print or a new machining run.
Good dimensions also make the job cheaper. We spend less time interpreting photos and guessing, and more parts are right first time. You do not need to measure everything – but the dimensions that control the fit with other parts should be as accurate as possible. Better to spend an extra twenty minutes measuring than a week waiting for a new test part.
Tools to measure a part for CAD
A digital calliper is the basic tool. Most read to 0.01 mm, but accuracy is typically ±0.02–0.03 mm. That is enough for most plastic parts and spare parts. Use the outside jaws for external diameters, the inside jaws for holes and the depth rod for depths.
An outside micrometer measures with 0.01 mm resolution and better repeatability. It is worth using on shafts, pins and thicknesses that must fit bearings or bushings. For larger holes, a telescopic gauge or a bore gauge gives more reliable values than the calliper's inside jaws.
Add a radius gauge for fillets, a thread gauge for pitch, a square and a protractor. A steel rule and squared paper are needed for the photos. Feeler gauges are useful for measuring clearance in an existing assembly.
Choose datum faces before you measure
First decide which faces are datums – the faces everything else is measured from. Typically this is a flat mating face, a centre hole or an edge that sits against another part. Then measure all positions from the same datum, rather than from hole to hole in a chain. That way errors do not add up.
Hole spacings are easiest to measure as outside edge to outside edge minus one hole diameter, or inside edge to inside edge plus one diameter. Write down both and compare. If they agree within 0.05 mm, you probably have the right centre distance.
Mark on the part with tape or a marker which face is datum A, B and C. Then use the same letters on the sketch. That makes it much easier for us to interpret the dimensions correctly.
Measure three times and think about wear
Measure every important dimension at least three times, in different places and with light measuring pressure. Plastic gives if you press hard with the calliper. If the values differ by more than 0.05 mm, the part is probably skewed, shrunk or worn.
Worn surfaces give wrong dimensions. Measure unworn areas, for example inside a sliding face, or measure the mating part the part has to fit. A shaft in a ball bearing is a good example: the bearing has an exact standard size, the worn journal does not.
Also think about which nominal size the designer probably chose. A measured value of 7.96 mm was almost always 8 mm to begin with. Common standard sizes are shafts of 5, 6, 8, 10 and 12 mm, and the 608 ball bearing measures 8 × 22 × 7 mm. Older parts may be designed in inches: 6.35 mm is 1/4 inch.
Threads, radii and angles
Check the pitch with the thread gauge and measure the major diameter of the thread. An M8 screw typically measures 7.8–7.9 mm over the thread crests and has a 1.25 mm pitch. If no metric gauge fits, it may be an imperial thread (UNC/UNF) or a pipe thread. G 1/4 (BSPP), for example, has a major diameter of about 13.2 mm and 19 threads per inch.
Internal threads are harder to measure. Measure the hole diameter and try a screw whose size you know. Also note the thread depth and whether the hole goes right through.
Radii are measured with the radius gauge: try blades until one sits without a gap of light. Small radii under 1 mm can often be estimated. Angles are measured with a protractor or worked out from two dimensions, for example the height and length of a sloping face.
How to photograph the part for CAD
Good photos replace many measurements. Photograph the part straight from the front, from above and from the side – square to the surface. Stand back a little and zoom in rather than holding the camera close. That reduces perspective distortion and keeps the edges straight.
Place a ruler or calliper in the same plane as the surface you are photographing, ideally on squared paper with 5 or 10 mm squares. The ruler must be at the same height as the feature, otherwise the scale will be wrong. Use even lighting without harsh shadows and a neutral background.
Also take pictures of how the part is fitted, of wear and fractures, and of any markings. A close-up of text or a part number can lead to the original dimensions. The photo gives the shape – the important dimensions still need to be measured.
Dimensioned sketch and common mistakes
A simple freehand sketch is perfectly fine. Draw the part in two or three views and write the dimensions in millimetres. Mark the datums, state which dimensions are critical and which are only approximate. Write “fits a 608 bearing” or “M5 screw passes through” – that kind of information is often more important than an exact number.
Also state the material, if you know it, the temperature and load the part is exposed to and how many you need. Whether the part is to be 3D printed, turned or injection moulded affects how we set the tolerances. Read more in the guide to tolerances and fits.
The most common mistake is measuring a worn or deformed surface and taking it for the original size. The next most common is mixing up radius and diameter, or writing a centre distance after measuring edge to edge. Always state what the dimension refers to: “Ø12”, “R6” or “c/c 40”.
Other mistakes are pressing too hard with the calliper on soft plastic and measuring a part while it is still warm from use. People also forget which side is up in the photos. A tip is to number the dimensions on the sketch and write the values in a separate list. That makes it easy to re-measure a single dimension if something looks odd.
When measuring is not enough
Freeform surfaces, such as an ergonomic handle or a cover with double-curved faces, are hard to measure with hand tools. A 3D scan can then give good data, or we model the shape from photos and a few check dimensions.
Often the simplest option is to send the physical part. We then do the measuring ourselves and can print a test part to check the fit. Read more about spare parts without drawings and reverse engineering existing parts, or send your material for assessment.
Measuring tools at a glance
| Tool | Typical resolution | Used for |
|---|---|---|
| Digital calliper | 0.01 mm (accuracy typically ±0.02–0.03 mm) | External and internal dimensions, depths, hole spacings |
| Outside micrometer | 0.01 mm (finer models 0.001 mm) | Shafts, pins, sheet and wall thickness |
| Telescopic gauge + micrometer | 0.01 mm | Holes and grooves too deep for the calliper |
| Radius gauge | Steps of 0.5–1 mm | Internal and external radii |
| Thread gauge | Metric pitch and threads per inch | Pitch of screws and nuts |
| Feeler gauges | 0.05–0.1 mm per blade | Clearance between parts in an assembly |
The values are guide values and vary between manufacturers and price ranges.
Checklist: data for a CAD model
- Datums marked (A, B, C) on the part and on the sketch
- Critical dimensions measured three times with calliper or micrometer
- Hole spacings checked in two ways
- Threads: major diameter, pitch, depth and whether through or blind
- Radii and angles measured or estimated
- Photos square to the surface, with a ruler in the same plane
- Photo of the part fitted and of any wear or fracture
- Dimensioned sketch in millimetres with critical dimensions marked
- Material, load, temperature and quantity
- Which parts it must fit – with their dimensions
Frequently asked questions
How accurately do I need to measure?
Dimensions that control the fit with other parts should be as accurate as possible, ideally to 0.05 mm. Other dimensions, such as the outer shape and fillets, are often fine to the nearest millimetre. Mark which dimensions are critical, so we know where accuracy is needed.
Are photos enough to make a 3D model?
For simple parts, photos with a ruler in the picture and a few check dimensions are often enough. Photos give the shape, but fit dimensions need to be measured. If the shape is complex, it is better to send the physical part or a scan.
How do I measure a thread?
Measure the major diameter with a calliper and check the pitch with a thread gauge. An M6 screw measures about 5.8–5.9 mm and has a 1 mm pitch. If no metric gauge fits, it may be an imperial or pipe thread.
Can I send you the broken part?
Yes. The pieces of a cracked part are often enough to recreate the whole shape. We measure it and can print a test part to check the fit before final manufacture.