measuring radii on parts when preparing drawings

Steve

Radii can be measured using a radius gauge, obtainable from any trade outlet supplying toolmakers. It's basically a set of sheetmetal fingers with accurate radii blanked, female and male, hinging out of a handle like a set of feeler gauges. If it falls outside that range, I make my own gauges by getting a pair of metalwork dividers (with a fine adjusting thread), set it to the desired radius, and score the radius through a piece of heavy mylar or similar thin rigid plastic. If the radius wraps around 90degrees, you can roughly locate the "tangent points", perhaps by marking the gauge carefully and transferring the marks to the workpiece. If not, you are really into inspired guesswork. For a large radius, one which also spans more than 90 degrees, you could perhaps compute a quarter circumference and wrap a strip of paper that length around it, mark quadrantal points, and use a height gauge, do a bit of trig, rinse and repeat using new quadrant points (one at the horizontal projection of the center inferred from the first iteration.) Or (if it's an accessible silhouette edge) mount the part on a flat surface and rub a machinist's square, sitting on the surface, against the part to make a 'witness' at the vertical tangent point. Then strap the part to the side of (say) a rectangular block, so as to reorient it through 90deg, and repeat. Measure the coordinates of the resulting witnesses with a height gauge.

The easy way is to find someone with a CMM (coordinate measuring machine) but that's not nearly as much fun.

"Andrew Troup" wrote in news:rnjjc.52$g52.658 @news.xtra.co.nz:

Or possibly a comparator, if you can get a clear profile of the radius. How accurately do you need to know the radius?

There are also specific reverse enginering tools and applications for SW. that's how my VAr got to be a VAR. It grew out of the side of their main thing, which is developing their reverse engineering app. I don't actually know much about it, or how useful it is.