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?
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.
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* 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.
BUILDING A 1:1 BALUN
4 years ago
15 comments:
If I understand your situation, how about you run a 3/8-16 tap thru those holes in the plate, thread the bolts thru, and cinch them down tight with the nuts on the back. Or not, can't see exactly whats on the back.
Ooh, neat stuff! Send me (lemme see, that's six sides to a cube and four sides per face, right?) 6x4=24 of everything. And one of the cup--er, tool--holders. Fun times coming!
What!? Someone didn't use Potrzebie units?
I usually go direct to 8020.net - the originals...
JimBob: Alas, through-holes in the plate and a tapped holes in the switch. Might speed the reaming to run a tap through first, though!
Chuck, thank you for that link!
Obviously, the optimum solution is to skinny the bolts just a little, over on the shop lathe. There is a shop lathe-right?
Can you polish or machine down the bolt shanks enough for a slip fit?
Tolerance stacks are a bitch when they go in the wrong direction...
Um, rprentiss196 and Rick T, I'm not sure you understand how this works -- the bolts go through clearance holes in the plate and thread into tapped holes in the body of the switch. If they are "slimmed down," the threads will not engage properly. This is very, very bad, and gets expensive quickly.
OK, I was envisioning bolts with machined threads smaller than the shank (the threaded portion fits in the back plate but the shank does not). If the threads don't fit at all then Ouch.
Murphy and his many cousins around the world strikes again.
You were thinking of "stripper bolts."
In fact, I did end up tapping the holes, but only as a way of hogging out aluminum prior to reaming. It helped but it was still slow and awkward. But the bolts fit through the holes now!
That's odd. Clearance drill (W or X) for 3/8 is pretty darn close to 10mm. I can't imagine Luigi running out of 10mm drill bits.
Ahh, 10mm is tap drill for oddball M11 thread and 9.5 and 9.9 are clearance taps for M9.
You likely got burned because all of his drills were for the tap diameter of M10 and clearance drills would have been too big.
DRP, I'm not sure these holes were actually drilled; they were awfully pretty. The entire plate was kind of casually mirror-finished, with corners smoothly rounded on a tiny radius and two rows of overlap marks on the largest flat surfaces. I think all of the work was done on a CNC machine and possibly without changing cutters.
What I ended up doing was chewing away a lot of material with a 3/8"-16 tap, running a hand-held step bit in as far as possible from both sides, using a taper reamer that maxed out at 3/8" to remove nearly all of the remainder and then finishing by working a 3/8" twist drill in a hand chuck through each hole. Deburred with one of those little swivel deburring tools, brushed remaining chips away, degreased (Tap Magic is your friend, until later when it isn't) and cleaned up with a soft cloth and cotton swabs. The inside of the holes is no longer mirror-bright but they're pretty good. And it only took a couple of hours from getting tools out to putting them away and sweeping the floor.
If you did this by hand, consider using a drill motor next time you need to tap a part. Best not to attempt this with an old style single speed unit, though. If possible, I use a socket extension with the appropriate size drive (1/4, 3/8) to match the tap, with a socket reversed in the chuck, or just chuck up the tap shank if it is long enough. There are square (eight point) sockets that might match the size of the tap base, but those aren't common.
Avoid the temptation to drive the tap too far without reversing, if it is a deep hole.
I've broken taps by hand, but not using this method, even tiny ones. Not clear why.
Will, this assembly included a tuned RF filter of very precise characteristics and fairly fragile concentric line. All the trouble I went through in resizing the holes was so as to *not* use power tools on it. Vibration would be bad, and the ways in which a fat 3/8"-16 tap or 3/8" drill being run by a motor could get hung up in that thick slab make my skin crawl.
In general, I would no more drive a tap with a drillmotor than I would walk blindfolded across a busy freeway. I'm sure there are people who could do so without breaking anything but I am not one of them.
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