- Published: Sunday, 04 December 2011
- Written by Jon Chandler
- Hits: 3798
When laying out panels or circuit boards, accurate measurements are a must. A few inexpensive tools can make excellent results much easier to achieve.
I think we've all used rulers to try and measure parts. For the fuse holder shown, it can be held against the ruler, the width estimated and the marks correctly interpreted to get a decent result. The ruler shown is a high-quality metal ruler and it has metric units on the back. I use a combination of inch and metric units depending on what size something is designed for - for example, in laying out PCB dimensions, I'll use whatever units the enclosure was designed in. Usually, one system will provide nice numbers.
A digital caliper is a far simpler tool for tasks like measuring the diameter of a fuse holder or switch. Just be sure to zero the caliper with the jaws closed before making a measurement and in the case of a threaded component like the fuse holder shown, use the thicker part of the caliper jaw to get the outside dimension. The size is displayed without any need to interpret the scale. Measurements may be displayed in either inches or millimeters. Digital calipers may be purchased for less than $20. An 8" or 10" size may be worth the small extra cost.
The upper jaws on the caliper are used for inside measurements like measuring a mounting hole on a panel.
The length of a component (how far it will extend into an enclosure for example) can be measured with a ruler fairly easily, especially where high accuracy isn't needed. The depth gauge of the caliper may also be used but it can be difficult to measure a wide component because of the narrow dimension of the caliper.
I recently saw a depth gauge for setting router bit and saw blade heights on a pre-Christmas sale for half-price. As shown here, it may be useful for some types of measurements, particularly a large component with an uneven profile.
Rulers and calipers don't help much when it comes to determining clearance over a circuit board inside an enclosure. Drawings provided with enclosures seem to leave off crucial dimensions all-too-often. So how can you measure the room inside an enclosure? Cylinder bore gauges (also known as telescoping gauges) can be a lifesaver. These gauges are shaped like a T. The legs on the cross of the T are spring-loaded. A locking knob on the leg of the T locks the legs in position when they are in the right position.
The gauge is inserted while collapsed through a removable end panel or hole in the enclosure wall, and, when in the right position, the knob is loosened allowing the legs to expand. When the gauge is in the right position, the knob is tightened to lock the legs. The gauge is then removed and measured with a caliper.
The photo shows the gauge being used the height from the top of the PCB mounting posts to the inside cover of the box. This would be impossible to measure accurately with a ruler or caliper because of the overhang where the end panels mount.
Because these gauges only work over a limited range, they are sold in sets of 5 or 6 covering a wide range of sizes. My set is nothing fancy - similar sets are available on ebay for $20 - $30.
A few simple tools make these routine measurements easy. The cost need not be great and the pay-off will be boards that fit in enclosures and parts the fit properly in panels.