measuring radii on parts when preparing drawings

Hello,

when somebody gives you an actual part to create accurate drawings of,
what method do you use to obtain the correct radii (and origin points
for those radii) for any curves on the part?

Thanks Steve

 

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.