Tapped hole margins

Hello all,
I have a design problem requiring a reasonably rigid structure in a
tight space. Forces are not at all large, and structural failure is not
a great threat. However, the parts need to be securely connected to
each other in the sense that we cannot afford to have parts spinning out
of alignment when disturbed. The result is a scientific instrument
package not unlike an indicator holder: it needs to not fall apart every
time it is touched, but does not have to be very strong.
To provide rotational support at the connections[*], I have been
planning for machine screws in pairs. Space is tight, so that has
pushed me toward 4-40, which is a pain to tap. I am thinking of using
one 8-32 screw and making a tight-fitting slot to prevent rotation at
each connection. Any preference or other ideas?
In such a situation, how close to the edge of an Al part would you
consider it "safe" to tap for 4-40 and 8-32, respectively? Again, the
concern is not so much ultimate strength as robustness and common sense.
Ordinarily, I try never to get closer than 0.1" from the OD of the
hole to the edge, and usually think more about staying clear of
parallels, so this is new to me.
Thanks,
Bill
[*
] It pretty much has to be made in pieces. Some of you could probably
remove a joint or two, but as Dirty Harry said "A man's got to know his
limitations." :)
Reply to
Bill Schwab
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If you get the thread-forming ("cold forming"), instead of cutting, taps (available from Enco, etc) then miniature aluminum threads are easy. I do sizes like 0-80 and M1.6 all the time. The only time I've broken a tap is when I dropped it! Different tap drill sizes are required. No chips and no back-and-forth, just screw it straight in and out!
One or maybe two thread heights. Thread height for 4-40 is 1/40" * sqrt (3/4) = 0.022". So somewhere in the 20's of thousandths minimum.
Reply to
Richard J Kinch
Bill, I usually use one thread diameter or 1/2 the head diameter if that's a consideration for a minimal spacing, but you can fudge that a little. RichD, in DRY Atlanta (Spit in this direction. Anything will help)
consider it "safe" to tap for 4-40 and 8-32, respectively?
Reply to
RichD
Bill Schwab wrote in news: snipped-for-privacy@corp.supernews.com:
Press-fit a dowel in one piece and slip fit it in the other, then you can use one larger screw.
Reply to
Anthony
I've gotten away with 0.030" from wall to thread OD using a sharp spiral-point tap but the threads may be visible on the outer surface. It was a lightly stressed radio enclosure, though, with a cover plate to take any bending load if it was dropped. If you tap first, then machine the outside thin you can cut as close as you want to the threads or even cut into them, for instance for a friction drag plate.
4-40 in aluminum isn't that bad. I redesigned a frequently-opened microwave radio chassis with 120 0-80 screws to use 2-56. The shop tapped the panels for about 4 boxes with a thread-forming tap on a Procunier head without breaking one.
Jim Wilkins
Reply to
Jim Wilkins
Rich,
The head diameter is potentially a problem, unless I turn them. I am not sure how to do that. How about tapping a dowel, using a jam nut to limit depth, stick that in a collet in my mill and hold a tool bit in the vise??
Among the other suggestions, it sounds like I can get closer to the edge than I have planned, or go with Anthony's dowel idea.
I had not heard of thread-forming taps. They do not look cheap, but it sounds like a great idea.
Same here (central FL). It's pretty bad when Floridians gather by windows to watch what rain happens to fall :(
Thanks for all for the replies!
Bill
Reply to
Bill Schwab
larger screw and a roll pin?
Stealth pilot
Reply to
Stealth Pilot
There are already good posts regarding the wall thickness and good design practice. I would be cautious, though, about using roll-forming taps if you are by chance tapping cast aluminum (e.g., Mic-6). I haven't had a great deal of experience with these taps, but they are designed to form (displace) the material, and in a more brittle, crystalline matrix there will be a tendency toward fracture rather than flow. In a thin- wall situation, this is not desirable.
Reply to
matt
I do 4-40 all the time. I use these $8 machine taps with "spiral flutes", they look a lot like twist drills. In fact, there are thread drills that will drill the hole and tap it in one pass. They are a little tricky, you have to do the drilling at a lower feed than the threading, so I guess you can't do that on blind holes. Anyway, with the spiral flute taps, you tap the full depth of the hole in one pass, then back all the way out. The chips are sent mostly up the flutes, just like a drill bit. I use "alum-tap" as an aluminum tapping fluid, it is quite amazing. I recently stepped my 4-40 threading up to 1200 RPM, it works even better than 600.
Oh, you can get down to about .1" easily on the 4-40, and about 1/8" for the 8-32.
Jon
Reply to
Jon Elson
It depends on the alloy. The difference between wrought and cast Al is the working of the metal. For non-workable alloys what you say is true. For a cast, but workable alloy, then the tap will compress the metal initially, converting it into wrought aluminum, at least to some level. That should be threadable with a thread forming tap. You have to drill the pilot hole larger to use these taps, and some experimentation with the specific alloy is needed to find what hole gives reasonable tapping forces and good thread engagement.
Jon
Reply to
Jon Elson
I've tapped a pair of 8/32 holes in the end of a 1/2" dia rod so close to the edges that the thread telegraphed to the surface and they held just great. I didn't want the part being atached on the end to be able to rotate. That is the reason for the thing. I had to turn down the diameter of the heads to put the two screws in. :-) ...lew...
Reply to
Lew Hartswick
Proper mechanical design treats threaded fasteners as items to fasten two pieces together, not as locators for parts. If you need to locate two parts in a precision fashion, you need to match drill, ream and dowel pin them, slot and key them or machine mating areas on the parts. Threaded fasteners are supposed to be only used in tension, you see that one violated all the time, too. If you need to support shear forces, you need to put in pins or machine shoulders to take those forces.
Stan
Reply to
stans4
Lew,
Stan makes a very good point about not abusing fasteners, but the fact is, it does work for many such tasks. Maybe post-lathe, I will do a little better job via pins, and I certainly would find a better way for something that had to take real punishment.
Thread-forming taps are now on order. That will hopefully allow me to stay at 4-40, which should get us started w/o a gratuitous redesign.
Thanks to all who replied!
Bill
Reply to
Bill Schwab

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