I'm in the process of mounting a DRO on the quill of my Millrite MVI, which
involves replacing the connector block that goes between the quill and the
bronze nut that's moved when the quill feedscrew is turned. To the new
connector block is attached a 0.125" thick mild steel blade that replaces the
indicator and to which the sensor of the DRO will attach.

The connector is attached to the blade with two 10-32 hex socket flat head
machine screws. There is some scraping and hand filing involved in getting the
blade to be parallel to a machined reference surface. The blade moves up and
down parallel to this surface.

So, there is lots of assembling and disassembling of connector from blade, and
the 10-32 screws are very tight, and let go with a crack. So far so good. The
first time I tried to undo the screws after really tightening, one screw came
out without difficulty, but the other one didn't, and didn't feel right. I
figured that the hex socket was rounding out because the steel wasn't quite up
to it. But the socket looked OK. Pressed on. The 3/32" hex wrench (key)
turned 60 degrees and felt soft, but the screw finally let go with the usual
crack.

The hex key had twisted axially into a nice spiral, like a piece of decorative
wrought iron square bar.

Now, I got this US-made hex key set in the late 1950s or early 1960s from a
local hardware store. I'm not completely sure, as things may have gotten mixed
up over the years, but from appearance (these were zinc plated) I think they
were made by Indestro Mfg Co. Never had one of these keys twits before.

The hex socket machine screws are made in India, recently purchased from MSC.

My 1970s Sears Craftsman hex key set has no such problem.

And the Indians are learning how to make steel.

Joe Gwinn

On Feb 26, 6:01*pm, Joseph Gwinn wrote:
In article ,
*JR North wrote:

10/32 hex bolts should not be torqued so tight that even a soft steel
hex wrench should twist. Many times, when a hex fastener is installed
with a powered tool, excessive torque is applied. Hex fasteners need
to be 'cracked' loose with a shock load on the hex driver. Applying
hand twisting torque is almost guarenteed to strip the hex or twist
the hex wrench.

The hex socket screw seems OK, and has been done and undone many times so far.
If I wear the 10-32 screws out, I'll just replace them.

The Craftsman hex key is standing up nicely too. *

The weak link is the old hex key. *I wonder if the plating had something to do
with it, as they may have had to heat the keys to allow hydrogen from plating to
diffuse away, to avoid hydrogen embrittlement. *The Craftsman hex keys are black
oxide finished, as are the hex socket screws.

http://en.wikipedia.org/wiki/Hydrogen_embrittlement

Joe Gwinn



JR
Dweller in the cellar.
On Fri, 26 Feb 2010 18:33:10 -0500, Joseph Gwinn
wrote:

I'm in the process of mounting a DRO on the quill of my Millrite MVI, which
involves replacing the connector block that goes between the quill and the
bronze nut that's moved when the quill feedscrew is turned. *To the new
connector block is attached a 0.125" thick mild steel blade that replaces
the
indicator and to which the sensor of the DRO will attach.

The connector is attached to the blade with two 10-32 hex socket flat head *
machine screws. *There is some scraping and hand filing involved in getting
the
blade to be parallel to a machined reference surface. *The blade moves up
and
down parallel to this surface.

So, there is lots of assembling and disassembling of connector from blade,
and
the 10-32 screws are very tight, and let go with a crack. *So far so good. *
The
first time I tried to undo the screws after really tightening, one screw
came
out without difficulty, but the other one didn't, and didn't feel right. *I
figured that the hex socket was rounding out because the steel wasn't quite
up
to it. *But the socket looked OK. *Pressed on. *The 3/32" hex wrench (key)
turned 60 degrees and felt soft, but the screw finally let go with the usual
crack.

The hex key had twisted axially into a nice spiral, like a piece of
decorative
wrought iron square bar.

Now, I got this US-made hex key set in the late 1950s or early 1960s from a
local hardware store. *I'm not completely sure, as things may have gotten
mixed
up over the years, but from appearance (these were zinc plated) I think they
were made by Indestro Mfg Co. *Never had one of these keys twits before.

The hex socket machine screws are made in India, recently purchased from
MSC.

My 1970s Sears Craftsman hex key set has no such problem.

And the Indians are learning how to make steel.

Joe Gwinn

The Unbrako line was SUPPOSED to do that rather than shearing off or
rounding off, had a demo of that by the counter critter when I got my
set in the late 60s. I've taken out some pretty tight capscrews with
them, but have never had one so torqued up that it pretzeled the
wrench. The alternative is to wedge the hard wrench into the socket,
shear it off flush and have a nice time trying to either remove the
stub or drill out the remains. You might want to look up the torque
limits for that size capscrew.

Stan

stans4 wrote...
I've taken out some pretty tight capscrews with
them, but have never had one so torqued up that it pretzeled the
wrench. The alternative is to wedge the hard wrench into the socket,
shear it off flush and have a nice time trying to either remove the
stub or drill out the remains.

And is that stub hard or hard?

G/f bought a 750 yamaha and when we went to change the steering races we
discovered the remain of a bell-end key in the fork yoke bolt, drills
wouldn't touch it, ended up with dremeloid and a diamond burr and an hour
scooping it out, realised why the previous owner put up with the sloppy
steering....

Dave H.
--
(The engineer formerly known as Homeless)

"Rules are for the obedience of fools, and the guidance of wise men" -
Douglas Bader

I don't think that zinc plating involves any high temperatures for
electroplating. A prep acid bath may have been part of the process, as a
cleaning step prior to plating.

If surface embrittlement had been a factor, I think cracking, probably
leading to a full break/snap would have taken place.
Most quality hex keys/allen wrenches are already fairly hard, though I would
think of the twisting failure as a feature for tighening fasteners, to let
the user know the screw was as tight as it should be.

I agree with JR, that screws this size shouldn't be torqued to the tighness
of cracking when they let loose.
I prefer to use medium-hold blue threadlocker for small fasteners.. easy
enough to remove for disassembly when required.

In high quality equipment (aerospace), the countersink angles of the screw
head and the base material could create a taper lock, but that's probably
not the case in this situation. Medium hold threadlocker would likely
eliminate a taper lock if applied to the countersink area.

All that may have happened was that the key had a soft spot, or that it was
one of the Unbrako-type keys.

I'd feel fortunate that the key twisted insead of snapping off, which can
result in a very sharp projection which fingers may have come in conact
with.
Additionally fortunate that the screw didn't require drilling to remove it.

--
WB
..........


"Joseph Gwinn" wrote in message
...
In article ,

The hex socket screw seems OK, and has been done and undone many times so
far.
If I wear the 10-32 screws out, I'll just replace them.

The Craftsman hex key is standing up nicely too.

The weak link is the old hex key. I wonder if the plating had something
to do
with it, as they may have had to heat the keys to allow hydrogen from
plating to
diffuse away, to avoid hydrogen embrittlement. The Craftsman hex keys are
black
oxide finished, as are the hex socket screws.

http://en.wikipedia.org/wiki/Hydrogen_embrittlement

Joe Gwinn

In article ,
"Wild_Bill" wrote:

I don't think that zinc plating involves any high temperatures for
electroplating. A prep acid bath may have been part of the process, as a
cleaning step prior to plating.

Plating itself (regardless of the metal being plated) generates hydrogen ions
and drives them into the plated item, all at low temperature. One way to remove
the hydrogen is to heat the plated item up, allowing enough hydrogen to diffuse
away to prevent brittleness. This is done only for critical items.


If surface embrittlement had been a factor, I think cracking, probably
leading to a full break/snap would have taken place.

True. Preventing this bad outcome is why one slightly anneals plated items that
may be vulnerable to hydrogen embrittlement, or doesn't plate in the first place.


Most quality hex keys/allen wrenches are already fairly hard, though I would
think of the twisting failure as a feature for tighening fasteners, to let
the user know the screw was as tight as it should be.

But the Craftsman hex key hasn't twisted yet.


I agree with JR, that screws this size shouldn't be torqued to the tighness
of cracking when they let loose.
I prefer to use medium-hold blue threadlocker for small fasteners.. easy
enough to remove for disassembly when required.

The screw and threaded hole (in 1018 steel) don't seem unhappy.

The intent is accurate fixing of one part with respect to the other. Great
strength isn't required, just great stability. And, it would be a real nuisance
if the screw backed out, as it will take some disassembly to get at these screws.

I may well use locktite (or epoxy) for final assembly.

I designed the assembly to allow for soft soldering, if needed to fix the parts
together stably, but it does not appear that soldering will be needed.


In high quality equipment (aerospace), the countersink angles of the screw
head and the base material could create a taper lock, but that's probably
not the case in this situation. Medium hold threadlocker would likely
eliminate a taper lock if applied to the countersink area.

From the look of the countersunk holes, some taperlocking may be happening.


All that may have happened was that the key had a soft spot, or that it was
one of the Unbrako-type keys.

I buy the soft-spot theory. I think all vendors endeavor to make Unbrako-style
keys, but with varying degrees of success.


I'd feel fortunate that the key twisted insead of snapping off, which can
result in a very sharp projection which fingers may have come in conact
with.
Additionally fortunate that the screw didn't require drilling to remove it.

I don't think there was any danger that the key would snap off. It twisted at
least one full turn axially quite nicely, without pretzeling. Nor did it break
when I twisted it back to more or less straight.

Drilling would be a nuisance, but not that hard because the whole assembly is
easily removed and clamped in the mill vice. A small carbide drill bit would
make short work of the screw.

Joe Gwinn



"Joseph Gwinn" wrote in message
...
In article ,

The hex socket screw seems OK, and has been done and undone many times so
far. If I wear the 10-32 screws out, I'll just replace them.

The Craftsman hex key is standing up nicely too.

The weak link is the old hex key. I wonder if the plating had something to do
with it, as they may have had to heat the keys to allow hydrogen from plating to
diffuse away, to avoid hydrogen embrittlement. The Craftsman hex keys are
black oxide finished, as are the hex socket screws.

http://en.wikipedia.org/wiki/Hydrogen_embrittlement

Joe Gwinn