drilling and tapping

[Opcom]

I would like to use 1/4-20 brass screws to hold an oscilloscope graticule cover in place. This thing is the escutcheon at the front of the CRT, as on any scope. The panel it is to be fastened to is 1/8" aluminum sheet.

So I have a tap for this 1/4-20 screw size. It says to use a #7 drill. This I do not have. I can not get to the hardware store this week, but this week is the time I have for working on this. I did not realize there are so many sizes of drills. There are 8 sizes between #7 and the 7/32 size I do have. Here are a few.

.201"	# 7	1/4"-20 UNC
.203125"	13/64"	#11 Wood Shank Hole, #20 Pilot, Hard-wood
.213"	# 3	1/4"-28 UNF
.21875"	7/32"	#12 Wood Shank Hole

It is possible I might find a 13/64 around here, even a dull one that could ream out  smaller hole to size.

The force to hold the escutcheon on is very slight. What will happen if I use the 7/32 drill and then the 1/4-20 tap?

The depth of the threads will be less, but strength is not required since the escutcheon weighs about 3 ounces and there will be four 1/4-20 bolts.

The torque used to tighten them will only be enough to flatten a copper split-washer and no more. I could also put some goop on the threads to help keep it from coming loose. There will be a little glob of silicon glue in each corner of the escutcheon to keep it from moving, yet allow it to be removable later for very infrequent service. It is possible but very difficult and inconvenient to put nuts on the backside of the panel.

So, is the 7/32 drill a good idea? bad idea? How bad of an idea?

Thanks,

[KA2DZT]

Try a 3/16" drill which is smaller.  Do a test hole in a scrap piece of aluminum and see if you can tap it.  The 13/64" is your best choice not having a #7

There are lots of drills,  80 number drills, 26 letter drills.  Letter E is the same as a 1/4"

The 7/32" drill may be a little too loose

Fred

[Pete, WA2CWA]

I agree on trying and tapping the 3/16 hole test first. If you use the 7/32 bit and it's winds up being too loose, you're done. I have two drill bit holders. One holds from #1 to #80 and the other from 1/64 to 1/2.

[WA1GFZ]

I just bought a 1/4-20 Tap at Home Depot. The drill bit came with the tap. I think it was around $6. And yes folks it was made in the U.S.A. Had no problem drilling and tapping 3/8 aluminum plate.

[The Slab Bacon]

For low-torque applications, it's basically "horseshoes and hand grenades" anuthing close will usually work. (It doesn't take much to hold a graticule in place) I agree with Fred, I usually use a 3/16" bit for 1/4-20 threads in aluminum.

I have nunber drills 1-60, letter drills A-Z, and 2 sets of fractional drills 1/16" - 1/2". Not to nention fractional drills 9/16" - 1" and a bunch of reamers and it is still not always enough. I still occasionally find my self buying drill bits for oddball sizes that I need. If the hole is going into something round (or close to round) I still find myself occasionally boring it in a lathe to the required size if it is that critical.

[KA2DZT]

Wow!  The Slab Bacon agrees with Fred.

BTW why would you use a 3/16" if you have number drills 1-60??

Fred

[The Slab Bacon]

Wow!  The Slab Bacon agrees with Fred.

BTW why would you use a 3/16" if you have number drills 1-60??

Fred

Because, all of my "precision" drills are in indexes in the cabinet across the shop and there are always a couple or three 3/16 bits in the box of "miscellaneous junk bits" that always sits next to the drill press.............

[k4kyv]

I have a numbered drill index with the complete set of drills, and box full of random fractional sizes.  Under each drill in the numbered index it gives the size of the drill in inches, in decimal fractions. It also has a table that lists the popular screw sizes, 4-32, 4-40, 6-32, 8-32, 10-32, etc., and gives the size drills to use for tapping and for drilling a hole for the screw to pass through.

When drilling a hole for the screw to pass through in some kind of assembly, I often use a size or two larger than recommended, to provide a little leeway in (the usual) case the holes don't exactly line up. I have noticed the pass-through screw holes in a lot of commercial equipment is larger than the recommended size according to the book or my drill index.

[W4NEQ]

If I understand correctly you're trying to tap 1/8" sheet for 1/4 - 20.    As an electronics guy who learned machining, including cnc stuff the hard way, IMHO 1/8" thick is really marginal for a tapped 1/4-20 hole.  If you must use 1/4" - drill 0.250 and use nuts on the back.  If you really don't like that, use 1/4-28 and the correct numbered bit for that tap.  That will at least provide 3.5 threads to hold it.

Chris

[K5WLF]

If I understand correctly you're trying to tap 1/8" sheet for 1/4 - 20.    As an electronics guy who learned machining, including cnc stuff the hard way, IMHO 1/8" thick is really marginal for a tapped 1/4-20 hole.  If you must use 1/4" - drill 0.250 and use nuts on the back.  If you really don't like that, use 1/4-28 and the correct numbered bit for that tap.  That will at least provide 3.5 threads to hold it.

Chris

I'll go along with that, Chris. NC is shaky on 0.125". NF is definitely better. There's a reason why all the AN mil-spec hardware is NF. The other option would be to use Riv-Nuts.

[The Slab Bacon]

1/8" material is definately a bit thin to tap if you are going to 1/4-20 hardware If there is any stress load or large amount of weight hung on them, riv-nuts are definately the way to go, however.......

If all that is being supported is a thin plastic graticule and / or a tin cover, chewing gum would prolly work just fine. Or maybe even 2-sided foam tape, that stuff if pre-cleaned properly will never come off. We jokingly refer to that stuff as "a roll of mounting bracket".

If you feel more confident using hardware fasteners, sheet metal screws would work just fine for this app. and they are designed for thin substrates. When using them in aluminum drill your holes a bit undersized so they get a really good "bite" on the substrate.

Before taking a "desk job" position, fabricating architectural aluminum was something I did on a daily basis.

[WBear2GCR]

I think ur still working on the unremovable front panel with the 'scope hole?

IF it is off and free, I'd opt for a PEM type press fit clinch nut... overkill.

Otoh, why not take a bit of similar scrap and test. The 1/4-20 is a big enough tap so that
you will not break it off when working it into a too small hole.

Otoh, the .0203xxx drill is probably just fine...

Test on scrap. should tell you everything very fast.

Sometimes I use a tapered reamer to bring out a too small hole for tapping... again test on scrap stock.

                     _-_-bear

[WBear2GCR]

Ah yes... the other trick I have used is to take a bit of stock, thicker than the front panel, say 1/4"? Something just large enough to accomodate the space and shape available, this will be tapped for the 1/4-20 and one or two small holes for screws to afix it in position behind the front panel so it will  accept the estucheon screw(s). Those hold-in-place screws can be 6-32 flat heads with nuts and lockwashers on the back, or even 4-40... you make 4 assemblies for this application. The screws or single screw heads that hold it in place can be hidden behind the estucheon plate... the whole piece might be as small as 1/2" long and 3/8" wide and might have a corner cut (for example) to fit where it needs to go... more room behind the panel, go bigger, it's easier to fab that way.

                            _-_-bear

The other option is to reverse that, and put studs in (screws from the back) using the thicker backer stock like the previous idea, and use a nut like Tektronix did on the front... goes behind the panel. you could even fab a square piece with the studs and the hole for the scope tube in the middle, the shape being a mirror of the escutcheon for the front, essentially.

  

[k4kyv]

I drilled and tapped 1/4-20 threads into the bottom plate of the Gates transmitter cabinet to hold the re-located mod reactor and power supply chokes in place.  The original had tapped sleeves riveted into the sheet metal to accommodate the screws, but I just drilled and tapped the thin metal, and still got enough bite to allow me to tighten the screws snugly.  Maybe a little thicker than 1/8", perhaps 5/32".  But the screws, merely to keep the heavy chokes from sliding around, aren't likely to go anywhere and I doubt anyone will ever try to move the transmitter from its vertical position with the transformers and chokes still in place. I stripped out all the iron and took off the door and all removable panels, and it still took 3 people to move the thing out the door.

[W0BTU]

... 4-32, 4-40, ...

4-32? Does that size exist? I have 4-40 and 4-48  taps, but never heard of 4-32.

Anyway, I agree, it's better to use 1/4-28 for that thickness. And use a spiral point tap, they are so much nicer than the old fashioned plug, etc. taps. They push a continuous chop chip right through the hole. That's about all machine shops use anymore.

[KA2DZT]

4-32 is not a size that I ever saw.  Not on the screw chart, so pretty sure no such thing.

Fred

[Opcom]

Thanks for all this advice. The 3/16 was too small to let the nose of the tap get in. The 13/64 did OK. There is very little pressure on the threads, only enough to just compress a copper cut-washer. The backside of the escutcheon (graticule cover) has a rubber strip and even with this small pressure is staying firmly positioned.

The bolts are 1/4, but the holes in the escutcheon are more like 3/8", being the same as on a Tektronix scope. This allows for slight inaccuracies I know exist and for me to straighten the escutcheon. The heads of the bolts and the washers under them cover the 3/8' holes in the escutcheon, so it looks like it was made that way. It is not the typical stamped aluminum item from the 500 series, but a black painted casting. It's very light though. The regular plastic graticule from a 500 series will fit under it but I have not decided whether to use one because I would want to illuminate it as done in the scope if it is used. That means two more holes and lamps, etc.

The tube is now mounted and the brass hardware looks good but my camera was dead at the time, pic later..

It is always possible in the future to set blind nuts in the holes if it becomes necessary.

The last thing I need to find is a CRT shield from a 543 or 545 scope. There was no luck trying to modify one from an older scope, just made a mess of it. The shield has the lamps and sockets and IIRC also bolts to the front panel from the back.

[w1vtp]

13/64 is my choice.  13/64ths - 0.2031 minus 0.201 (#7 drill) is 0.0021" larger.  That's not a big difference.  Just go easy on the torque.  It'll be easy to strip it.  Go out and get a numbered drill index.  I have numbered, fractional and lettered sets.

Just don't expect anyone in Home Depot to know what you are talking about.   If it's not the latest app for a smart phone or a gallon of paint you may be having a problem with the youngsters.  An old codger might be able to point you in the right direction - NOT IN HOME DEPOT

Al

[W0BTU]

There are lots of drills,  80 number drills ...

Have you ever seen how tiny a #80 drill is? I have 1-60 and 61-80 drill sets, but seldom have I used anything smaller than a #60.

I forget how small a #80 is, but believe me, if you drop it on the floor, you'll likely never find it. 

And I think it goes without saying that those teeny drills are not recommended for use in a hand drill or conventional drill press, even if you can grip them with your drill chuck.

[Pete, WA2CWA]

Over the years when I needed to tap a hole in a piece of metal and the metal was not thick enough for adequate threads, I would use PEM self-locking Fasteners. Drill the required hole for the fastener and use a hand press to squeeze it into the metal. Looks nice and works well.

http://www.pemnet.com/

[WBear2GCR]

Just for the sake of more complete information "PEM" is a name brand. The generic is "self clinching nut".

My local wholesale screw/nut/bolt supply has them "generic".

You need to be aware that they come in different "depths" and it is best to pick the right one for the
thickness of the metal... Also it is best to use a press to put them in properly... I have used a good vise
to press them too... in a tough spot on thin metal, and soft (aluminum) you can get away with a
visegrip... but be sure to get the thing in flush - that means multiple squeezes...

Oh, they also come in steel, aluminum and stainless steel.

                _-_-bear

[Opcom]

I've seen those in factory built gear and very seldom have any come loose. I did not realize they were that simple to use. the hand press or other press is the trick.

[Pete, WA2CWA]

They even make self-clenching type stand-offs. Several years ago I bought a hand press from Grainger that works very well for putting any of these fasteners in but  a vise works too. Once these fasteners bite into the metal, the only way to get them out is to punch them out. With the things in, you don't have to diddle with washers or nuts especially in hard to get at places.

[The Slab Bacon]

Thanks for all this advice. The 3/16 was too small to let the nose of the tap get in. The 13/64 did OK.

HUH? ? ? ? ? ?    13/64 is only .015 bigger than 3/16. A tap is supposed to have a tapered lead on it (unless you are using a bottoming tap) The tapered lead will cut the hole out to the proper size as you turn it in, especially in soft aluminum. If you are tapping soft / thin aluminum, a tight hole and some good tapping fluid (Alumicut is fantastic) makes for a a really nice looking thread,

Over the years I have reground countless taps to what I needed. (Made bottoming taps out of starter taps and vice-versa) ground "chip breakers" into taps, etc.
 

[Opcom]

it was my only tap in that size so there would be no grinding. It fit or did not fit, it was how it was. pics are here:
http://amfone.net/Amforum/index.php?topic=29823.msg246978#msg246978