Quality Scan: Where Is the Tolerancing?
By Edward S. Roth, PE, FSME, LSME CMfgE
Past President (1977-78)
Society of Manufacturing Engineers
The metric system was declared legal in the US by Congress in 1866, but it wasn’t until 1994 that metric was included in the ASME Y14.5M-1994 standard. To this day, metric drawings are still hard to find. Many years ago, manufacturing engineers standardized drills and taps, measuring parts on surface plates, scribing lines on dye-colored parts in X and Y using height gages and angle plates. The line intersections were center punched, then placed in a drill press and drilled, counterbored, or tapped.
Over 90% of all parts have tapped holes in the mating part. The standard tolerance then was ±.010". To use ½" fasteners, a 9/16" drill was used. The difference between a ½" fastener and a 9/16" drill is 1/16" (.062"). This allowed each drilled and tapped hole a 1/32" (.031") diameter tolerance. Quarter inch bolts would use .312" diameter drilled holes to still keep the part tolerance ±.010". This system worked well before machine tools, as the bearing diameter under the hex head bolt is .390". ANSI Y14.5M 1973 changed this by not allowing manufacturing methods on drawings, so drilled holes were defined as 0.562" diameter +.007–000, 4 holes, on a four-hole part, with a .028" diameter positional tolerance on both drilled holes and tapped holes, from datums called primary, secondary, and tertiary. For some still unknown reason, tolerances have since tightened, so that the new norm is .014" or .010" diameter for both ½" and ¼" fasteners. This has been challenge for quality with their Six Sigma limits, and is very costly to manufacture. Don’t’ forget these fasteners are always in tension, and external tooth washers can assure preload.
We need to look at smaller hex fasteners to warrant ±.005" tolerance, for example, a #4 screw at .112" diameter with a bearing diameter of .184" under the head. The difference here is .072". Half of that, .036", will allow the head to cover the hole in the worst case, and half of that .036" is .018” diameter. Since a .014" diameter positional tolerance is equal to ±.005" with .004" extra tolerance. Don’t forget, this is based on a worst case with both the hole and the tapped hole at opposing locations and the bearing diameter at .184". A 34 drill at .111" will drill a .113" hole in metal. But could the lack of manufacturing data hide the drill size and the positional tolerance now used on drawings and be the real cause of the tight tolerances?
Fasteners with head diameters larger than .184" are large head hex screws at .213", hex washer heads at .225", and slotted truss heads at .241"; all increase available tolerance. This begs the question, why are tolerances getting tighter? Countersunk heads at 81° and 100° aren't covered in any of the standards, but are used more than clearance holes in each part. They must be considered as fixed fasteners, with clearance holes in the mating part. There are external tooth countersunk washers to assure preload. Shoulder bolts are available if the fastener is in shear.
The last CAD drawing the author saw was referenced to ANSI Y14.5 1973 and dated 2011. A .562" diameter hole was called out on a four-hole pattern, with tapped holes in the other part. The .562" diameter holes had no size tolerance, and each hole and tap was given a .010" diameter positional tolerance with no material modifier. So the tapped holes made the ½" bolt look to be .510", and the clearance hole to be .552" diameter. There is .042" diameter tolerance wasted here, and had each been given the .031" actually available, the part could have ±.010". Only one datum was identified on each part where there should be three.
ANSI and ASME standards do not explain how to tolerance actual parts, so these standards should be called Dimensioning Engineering Drawings. Real tolerance studies of fasteners up to #10 should be in CAD, available on the internet, or printed off in handbook form. Fasteners larger than #10 don’t need to be called off, because they can be held to greater than ±.010" (.028" diameter). ME
This article was first published in the October 2011 edition of Manufacturing Engineering magazine. Click here for PDF.