|Re: My McGuckin story...|
Message #7 Posted by Ellis Easley on 22 May 2002, 5:29 p.m.,
in response to message #6 by W. Bruce Maguire II
It was in Boulder that I went to McGuckin's. I forget what year. I had the impression there were more stores, is that correct? I remember the scale model of the solar system on the University campus. Also we went to the National Bureau of Standards, watched an orientation film starring Gomez Adams (it was John Astin, but he was smoking a cigar and grinning), looked at the atomic clock, learned that the UTC people (is that the right order? Coordinated Universal Time) use HP atomic clocks to keep all the national clocks in agreement, and how the NBS engineers had been trying for years to prove that a clock with a resonant cavity twice as long should work better, and how WWV uses a local TV signal to synchronize their clock in Fort Collins with the big clock in Boulder (that last one might be in the ARRL Handbook). Also went to NCAR, watched deer from the lunchroom window, saw the two Crays that they had, recognized the model number of IBM computer the Crays used for a disc controller as the same model Tandy used for their corporate mainframe, and saw for myself why those big disks are called washing machines.
Of course, there was a fire burning in the forest.
I pulled one screw from each of a 35, 25, 67, and 34C. They are all the same. I tried swapping them and all combinations I tried worked easily except one of the case screws in my 67 seems to be slightly smaller in diameter, and a bit longer, than the others. What is special about them is that they are made to be self-tapping in plastic and have a three-lobed cross section, one trademark for which is "Plastite". I thought they might be #4 but I found some "store-bought" #4 sheet metal screws which measure .110 inch diameter while the calculator screws overall outer diameters measure from .087 to .091 inches (that is the spacing I can set my dial caliper to, and the screws turn between the jaws without binding). The 97 service manual says the 97 case screws are "#4-20" (they are a lot bigger than the screws in the handhelds) and the screws that thread into the card reader plastic are "#2-28", which does appear to be the same as the handheld case screws. The length is 5/16 inch. The finish is called (I think) "black oxide".
Here's what I would do to avoid losing a screw or risking any damage to a calculator: drill some 1/16 inch holes in a piece of plastic about 1/4 inch thick. It should a similar kind of plastic to a calulator case, not like polyethylene at the rubbery end of the scale or Lucite at the very hard end. The first source that comes to mind is a piece of a TV cabinet. Thread a screw from your calculator into these holes. If you've never turned one of these screws into a fresh hole, you'll be amazed how easily the screw stays upright and cuts a clean thread (if 1/16 is the right size hole!) Then just take the piece of plastic to the store, along with a post-it that says "5/16" "round head" "philips (or is it Posi-Driv?)" "black oxide" and any other things you need from the store while you're out (save a trip!) and try the candidate screws you find in the threads that the HP screw cut. It's normal for Plastite screws to turn in slight fits and starts in a previously cut thread, because of the "tri-lobe" triangular cross section. While you're at it, it would be intersting to see if any sheet metal screws fit in the HP screw threads.
BTW, while I was looking for my #4 sheet metal screws, I found what looked like a regular #6-32 machine screw, but on closer inspection, had a similar triangular cross section, and I remembered that this is called a "thread roller". It is a self-tapping screw that is just slightly tapered at the leading end, doesn't have the deep "drill bit" notch that some other self-tapping machine screws have. At Tandy we used them in "extruded holes" in sheet metal, where some metal is pushed out when a hole is punched, surrounding the hole and providing a region that has room for more thread turns than the sheet metal thickness alone.