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."

Bill Schwab writes:

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?

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!

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?

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.

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)

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?

Bill Schwab wrote in
:


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?


Press-fit a dowel in one piece and slip fit it in the other, then you can
use one larger screw.


--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
better idiots.

Remove sp to reply via email

On Oct 31, 12:51 am, Bill Schwab wrote:
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
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

Rich,

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.

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.



RichD, in DRY Atlanta (Spit in this direction. Anything will help)

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

On Wed, 31 Oct 2007 08:09:35 GMT, Anthony
wrote:

Bill Schwab wrote in
:


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?


Press-fit a dowel in one piece and slip fit it in the other, then you can
use one larger screw.

larger screw and a roll pin?

Stealth pilot

On Oct 30, 9:51 pm, Bill Schwab wrote:
Hello all,

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.

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.

Bill Schwab wrote:
Space is tight, so that has
pushed me toward 4-40, which is a pain to tap.
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.


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?
Oh, you can get down to about .1" easily on the 4-40, and about 1/8" for
the 8-32.

Jon

matt wrote:
On Oct 30, 9:51 pm, Bill Schwab wrote:

Hello all,


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.


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.
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

Bill Schwab wrote:
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

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...

On Oct 30, 10:51 pm, Bill Schwab wrote:
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."

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

Lew,

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...

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