Evaluating the methods for measuring large radii
December 9, 2011
Large-radius bends are used in many places, such as appliance handles and automobile components. For measuring large-radius bends, fabricators have at least three options: go/no-go fixtures, entering the bent part’s measurements into a CAD program and using it to calculate the radius, and a depth gauge.
Tubing bent to a large radius is all around us. Take a look at the handles on a new refrigerator or the trim on an automobile—chances are you will see a few large-radius bends. These bends can be formed on many types of bending machine, such as a roll bender, stretch bender, or maybe a very large wipe bender. Regardless of how they are made, it’s necessary to inspect the bent tube for quality control purposes. One critical dimension to be measured is the bend radius.
When it comes to tight-radius bending (a bend in which the centerline bend radius is less than 3 times the tube’s diameter, or CLR < 3xD), most fabricators pay little attention to the actual numerical value of the formed radius. For example, if you bend a 1-inch tube on a 2.50-in. centerline radius, and the actual measured centerline radius is 2.54 in., would you take the time to recut the bend die? Probably not. It’s much easier to make adjustments elsewhere to compensate for any radial growth.
However, when bending large radii, it’s necessary to pay close attention to the bent tube. Large radii usually are cosmetic and span large distances. Thus, the bend radius must stay within a defined window. Defining the window is the first step.
How much tolerance is reasonable? If a drawing specified a bend radius of 100 in., a tolerance of ±1⁄8 in. would be impossible to hold. In fact, it would be nearly impossible to measure. If the span were short, for example 12 in., you could get away with 100 in. ±20 in. (see Figure 1).
After establishing the tolerance, it’s necessary to make the measurements. In the case of a large bend radius, fabricators have a few simple choices.
This is a hard fixture custom-made to match the component. You drop the part into the fixture; if the part fits, it’s a go. If it doesn’t, it’s a no-go.
It should be noted that the gauge doesn’t actually measure the part’s radius. The fixture has stops at various positions designated by the OEM along the length of the tube. As long as the tube fits into the gauge, it’s a good part. If interference from the stops prevents the component from fitting into the gauge, the part must be reworked or scrapped.
A simple means of measuring a large radius is to take a few measurements, put the data into a CAD system, and then have the CAD system draw an arc through the data points. No more than three data points are required.
First, measure the H dimension using a pin gauge or caliper (see Figure 2). Then take three data points— at the endpoints of H and one between the endpoints—and draw a similar arc in a CAD program. The CAD system can then render the radius (R).
The quickest way to measure a radius over a fixed length is to use a depth gauge designed specifically for this process (see Figure 3).
You place the yoke’s feet against the tube’s surface, then advance the probe until it contacts the tube. The depth gauge uses the three contact points to determine the radius.