At work, the project I have been working on -- a replacement for the auxiliary or maneuvering stardrive, pretty much -- incorporates subsystems from all over the world: the control system, power amplifier and cabinetry was designed in Japan and built in Brazil; the exciters, the very heart of the electronics, were designed and built in Massachusetts and the output section has a high-power coaxial switch from New Jersey;* a multi-kiloWatt test load built in Ohio and a lovely RF filter and aluminum extrusion frame† made in Italy.
If you're thinking that means at least two different measuring systems -- if not three, there's no telling what those heathens in Massachusetts might be up to -- then you're right; but this is 2019, standard equipment racks everywhere use 19" wide panels in heights that are dimensioned in units of 1.75" which everyone calls "1 RU" (so as to avoid admitting that they're measured in inches), and for everything else, you can look up the conversion online if you can't do it in your head. Modern CNC machine tools move in incredibly fine increments and unless you measure it carefully, you can't tell the difference between something that was milled in millimeter-decimals or done in thousandths of an inch. Right?
Well, usually. That big coaxial switch -- and it's not even that big, compared to the typical sort in the stardrive business -- hangs from a mirror-milled slab of aluminum nearly an inch thick, supported by four nice, fat 3/8"-16 bolts--
That is, it would be, if some machinist in Italy hadn't decided it would be better to be a few thou' under rather than over.
Picture the scene: here's Your Correspondent, having admired the lovely slab of aluminum (vertical, by the way, and at a height to make a nice mirror) and the precise, countersunk holes through it; and having looked over the well-crafted coaxial switch; and having discovered mounting hardware was inadvertently omitted, she has located stainless-steel bolts of the proper thread and length: there she is, awkwardly supporting the heavy, fragile switch in one hand and trying to run a bolt through the shiny slab with the other, and the blamed thing won't fit.
It looked like it would fit. So obviously right I hadn't even questioned it. I set the switch back down on the workbench and checked all four mounting holes with the bolt: it won't go though. Will. Not. Oh, almost. The chamfered end of the bolt kind of fits; but there's no wiggling or lining it up perfectly to get it to pass through, it's a no-go. Got out my cheap plastic dial caliper and measured: the hole is a 32nd under.
One thirty-secondth of an inch less than clearance for a 3/8" bolt and I'm at a dead stop. You can't redrill that with a hand drill; it will stick and bind and chew up the hole, if it doesn't break the drill first. ("Drill bit," most people will call it, which is technically wrong.) I can probably take the slab off the frame and drill it in the press, but even then, odds are good a twist drill will jam up. A step drill would do the job -- if the hole wasn't deeper than the height of the steps, which it is.
Most standard, tapered hand reamers used in electronic work top out at 3/8", a standard bushing size for volume controls and quarter-inch jacks. I've got reamers. I can use them to work the holes to size. But I found the problem a half-hour before quitting time and the reaming will be very, very slow.
Guess what I'll be doing today?
And just as well: the counterbore won't clear a hex-head bolt anyway; it requires a capscrew head. I ordered them yesterday and with luck, they should arrive this morning.
Measure twice, then think it through and measure again. Then cut -- once. Or someone else will have to.
* And I'll take the opportunity for a shout-out to the good people at Myat, who make a wide variety of high-power, arcane coaxial RF transmission line and related components.
† T-slotted aluminum extrusion is one of the more versatile notions ever dreamed up. The most common version is square, with a T-slot down each face and an X-shaped cross-section. A huge variety of fasteners and attachments let you build almost any shape while making only square cuts. Available from multiple makers in inch and metric dimensions, in sizes from miniature up to 3" square and beyond, it's fairly standardized, strong and looks nice. The "frame" I refer to is nearly six feet tall, four feet wide and 30" deep, and arrived flat-packed like an Ikea cabinet. Here's one U. S. source of the extrusions.
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