Tolerances
ptrotter said:
If a #41 drill were really a better choice then that would have been determined by Boeing or Lockheed or some other entity that builds to very high standards.
Just my opinion
Actually, I published the specs. you are referring to in an earlier post.
This has been worked out before, and the specified hole for a 3/32 rivet is 0.093 to .103 inch diameter
A #41 is 0.096 and a usual tolerance is -0.001 and +0.004 for general machining. So it is well within the approved specifications that Boeing, etc. work to.
This machinist's tolerance of 0.004 is using "jigs and fixtures", so our hand-held drill use
may produce larger holes - how accurately can you hold the drill perpendicular to the work?
http://www.engineersedge.com/drill_sizes.htm
Since the hole does open up during the dimpling process, and the Mil-spec. (and Boeing's spec. probably) is actually for a drilled hole with straight sides, not the sort of stepped hole we make when cold dimpling two sheets, starting with a slightly smaller hole gives you a little more tolerance, moving you into the middle of the range.
Check the image on my web page intro. to make sure you are getting the correct driven head height... it's easy to make an error on double dimpled sheets with the Avery gauge.
http://www.home.earthlink.net/~gilalex/
Real life engineering (well, except for software...Dan...
) is full of tolerances, so we can make things just a little easier by erring on the low side of a hole dimension when we know that hole will get larger with a later step, moving you into the center of the tolerance range.
Does 0.002 make much difference? - maybe not, but is it any harder to do - not really.... so why not let the tolerances work in your favor?
I have tried #42 drills (0.0935) but that size did not let the pilot of my die slip into the holes, and at the -0.001 lower end would fall out of tolerance.
NOTE: I'm not suggesting any extra work here....
gil in Tucson