Measuring parts
Read drawings, measure real hardware, and locate holes from useful references.
Start with the that make a part fit: its mounting holes, the surfaces that touch other parts, its position, and the space around it. Decorative grooves can wait. A simple model with the right hole spacing is enough to start designing a mount.
Use a supplier drawing when one exists, and compare it with the hardware you actually have. Similar-looking , hubs, and rails can have different patterns or revisions.
Read a drawing
A drawing usually shows several views of the same part. A section view shows a cut through it, exposing features hidden inside. A centerline marks the center of a hole, shaft, or symmetric feature.
Start by finding the units and overall dimensions. Then identify the face against which your custom part will sit. This is a useful : a reference from which you locate other geometry.
Common markings include:
| Marking | Meaning |
|---|---|
| Ø8 | An 8 mm diameter, if the drawing uses millimeters |
| R3 | A 3 mm radius; the full circle would have a 6 mm diameter |
| 4× | The same feature occurs four times |
| THRU | The opening passes through the full part |
| M4 × 0.7 | A metric thread with 4 mm nominal diameter and 0.7 mm pitch |
| PCD or BCD | Pitch-circle or bolt-circle diameter, measured through hole centers |
A thread label is not a -hole size. An M4 accepts an M4 screw; a plate through which that screw passes freely needs a suitable clearance hole. Use the drawing and the chosen fit.
Check which view defines a dimension. A 16 mm hole spacing may be measured horizontally between centers, rather than diagonally or around a circle. Do not infer a missing dimension from how large the drawing looks on your screen.
Use a ruler and calipers
A ruler is useful for checking a part's overall size and making rough layouts. Calipers measure outside sizes with their large jaws, inside sizes with their small jaws, and depths with a depth rod. They are useful for checking stock, shaft diameters, and simple mounting geometry.
Before measuring:
- Clean the measuring faces and the part.
- Close the calipers gently and check zero.
- Confirm the displayed units.
- Hold the jaws square to the feature, with light, repeatable pressure.
- Repeat the measurement after removing and repositioning the tool.
Do not write down more decimal places than your measurement can justify. Calipers may display them, but a soft foam ball changes shape when you squeeze it. Use light pressure and note how you held flexible parts while measuring.
Keep calipers away from powered . Measure stationary parts with power disconnected and stored energy controlled.
Find hole spacing
The distance between hole centers is often the dimension you need, even though the centers are empty space. For two equal round holes, you can measure the total outside span and subtract one hole diameter.
Suppose two 4.5 mm holes have an outside-to-outside span of 44.5 mm:
You can also measure the gap between the closest edges and add one diameter. A 35.5 mm inside gap gives the same 40 mm spacing. Comparing the methods helps expose a poor measurement.
For unequal diameters and , measured along the line joining their centers:
These methods depend on reaching the correct edges. Small, chamfered, or inaccessible holes are harder to measure accurately. Prefer the manufacturer's center coordinates when available; use a test print or drilled template to check an uncertain pattern before making an expensive plate.
Locate a pattern
Choose one and two perpendicular directions on the mounting face. Record every hole center from those references. This is easier to reconstruct than a chain of measurements taken from changing edges.
For the practice plate, put the origin at its center:
| Feature | X | Y | Diameter |
|---|---|---|---|
| Left hole | −20 mm | 0 mm | 4.5 mm |
| Right hole | 20 mm | 0 mm | 4.5 mm |
The plate edges are at X = −35 and 35 mm, and Y = −20 and 20 mm. You can describe the same pattern from the lower-left corner, but do not mix coordinates from the two origins in one .
Photograph the measured face and mark the references on the image. Include connector positions, protruding screws, and any shoulder that stops the custom mount from sitting flat.
Measure the installed space
Leave room around the part as well as for the part itself. A motor needs space for its wires and . You need to reach a hub’s connectors, and a may have a flange wider than its housing.
Measure the hardware in the state in which it will be used. Include the plug, , screw head, retaining collar, or belt flange. Also reserve the space needed to insert a tool and remove the component.
For a moving part, check how far it travels and what it passes along the way. Check cables where that movement pulls them tightest or bends them most. Leave a smooth bend rather than a sharp fold.
Check the CAD model
In Onshape, select the relevant edge, face, or pair of entities and inspect the measurement readout. Check a known distance immediately after importing a part. An inch-to-millimeter error changes linear size by a factor of 25.4.
Make a short measurement sheet with the part number, units, reference face, dimensions, and any uncertainty. Then use Hardware and imports to create or insert a model. Keep uncertain dimensions easy to change instead of hiding them inside several unrelated sketches.