Spring calipers are simple yet effective tools used for taking accurate measurements of the inside or outside dimensions of objects, allowing for precise transfers of sizes. Tools like the 79a-6 and similar inside and outside calipers of different sizes, as mentioned in the referenced video snippet, can be very useful. All it takes is willingness and some time to learn how to use them effectively.
These calipers, also known as spring joint calipers, feature a spring that holds the legs open or closed, and an adjustment screw or nut that allows for fine-tuning the distance between the tips. They are used to transfer a measurement to a rule or another measuring instrument, rather than having built-in scales themselves.
Types of Spring Calipers
There are two primary types of spring calipers:
- Outside Calipers: Used for measuring external dimensions, such as the diameter of a rod or the thickness of a block. Their legs curve inwards.
- Inside Calipers: Used for measuring internal dimensions, such as the diameter of a hole or the width of a groove. Their legs curve outwards.
Using Outside Spring Calipers
Here's a general process for using outside spring calipers:
- Identify the Dimension: Determine the external dimension you need to measure (e.g., diameter, width).
- Adjust the Calipers: Loosen the adjustment nut slightly. Place the caliper legs over the object you want to measure. Gently close the legs using the adjustment nut until the tips lightly touch the widest points of the object.
- Find the Light Touch: The key is to apply very light pressure. The calipers should ideally just slide over the object's dimension with minimal friction. You should be able to feel contact, but not force the legs open.
- Remove Carefully: Once set, carefully remove the calipers without changing the distance between the tips.
- Transfer the Measurement: Place the tips of the caliper against a measuring rule (like a steel rule) or other measuring device to read the distance. Align one tip with a major graduation (like the '0' mark) and read the measurement at the other tip.
Practical Tip: Practice makes perfect. As the reference notes, learning to use these tools requires time and willingness. Developing a feel for the correct pressure is crucial for accuracy.
Using Inside Spring Calipers
Using inside spring calipers follows a similar principle but for internal dimensions:
- Identify the Dimension: Determine the internal dimension you need to measure (e.g., diameter of a bore, width of a slot).
- Adjust the Calipers: Loosen the adjustment nut slightly. Insert the caliper legs into the internal space. Gently open the legs using the adjustment nut until the tips lightly touch the innermost points of the dimension.
- Find the Light Touch: The calipers should touch the internal surfaces lightly. When properly set for a hole's diameter, they should be able to pivot slightly within the hole, with the tips maintaining contact at the widest point (the true diameter).
- Remove Carefully: Once set, carefully remove the calipers without disturbing the leg setting. This often involves tilting the calipers slightly while keeping one leg against the surface to reduce the effective length as you extract them.
- Transfer the Measurement: Place the tips of the caliper against a measuring rule or other measuring device to read the internal distance. Align one tip with a major graduation and read the measurement at the other tip.
Key Insight: Unlike vernier or digital calipers that provide a direct reading, spring calipers are primarily transfer tools. Their accuracy depends heavily on the user's skill in setting them correctly and transferring the measurement accurately.
Setting and Adjusting
The spring joint allows the legs to be opened or closed, while the fine adjustment is made using the screw mechanism. Turning the nut clockwise typically closes the legs, and counter-clockwise opens them. Always make adjustments with a light touch against the object or reference dimension.
Summary Table
| Caliper Type | Used For | Leg Shape | How to Set (General) | Measurement Method |
|---|---|---|---|---|
| Outside | External Dimensions | Curves Inwards | Close legs onto the object's widest point. | Transfer to a rule |
| Inside | Internal Dimensions | Curves Outwards | Open legs within the space's widest point. | Transfer to a rule |
Using spring calipers requires developing a feel for the tool, but their simplicity and robustness make them valuable in many workshop and measurement tasks, especially for quickly checking or transferring sizes. As the reference notes, with willingness and practice, anyone can learn to use these effective tools.
